Title of Invention	A METHOD OF PREPARING A POLYCRYSTALLINE BLOCK
Abstract	The invention relates to a method of preparing a polycrystaline block of a halde of formula AeLn4X(3+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl. Br or 1, and A represents one or more alkali metals such as K. Li, Na, Rb or Cs, e, which may be zero, being less than or equal to 3f, and f being greater than of equal to 1, having a low water and oxyhalide content, comprising a step of heating a mixture of, on the one hand, at feast one compound having at least one Ln-X bond and, on the other hand, a sufficient amount of NH4X in order to obtain the desired oxyhalide content, said step resulting in a molten mass comprising the rare-earth halide, said heating step being followed by a cooling Step, and the heating step, after having reached 300ºC, never going back down below 200ºC before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having rennarkable scinimation properties to be grown.

Title of Invention

A METHOD OF PREPARING A POLYCRYSTALLINE BLOCK

Abstract

The invention relates to a method of preparing a polycrystaline block of a halde of formula AeLn4X(3+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl. Br or 1, and A represents one or more alkali metals such as K. Li, Na, Rb or Cs, e, which may be zero, being less than or equal to 3f, and f being greater than of equal to 1, having a low water and oxyhalide content, comprising a step of heating a mixture of, on the one hand, at feast one compound having at least one Ln-X bond and, on the other hand, a sufficient amount of NH4X in order to obtain the desired oxyhalide content, said step resulting in a molten mass comprising the rare-earth halide, said heating step being followed by a cooling Step, and the heating step, after having reached 300ºC, never going back down below 200ºC before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having rennarkable scinimation properties to be grown.

Full Text	1 PREPARATION OF RARE-EARTH HALIDE BLOCKS Rare-earth halides (in what follows, (n is used to demote a rare earth), especially when they are doped with cerium, and In particular cerium-doped l.nBr3 5 and cerrum-doped LnC13, have very useful scintillation properties especially for applications in nuclear imaging and in spectroscopy (positron emission tomography or PET, gamma camera, oil prospecting and the like). To obtain these properties satisfactory, it is necessary for these compounds to be obtained in the form of large crystal. Generally, these crystals are single crystals. In certain 10 particular cases, they may be polycrystals, within which the crystals have one dimension of the order of one or more centimeters. However, rare earth halides are highly hygroscopic compounds that react with water and with 3ir as soon as they are heated, forming very stable oxyhalides. It has in general been considered that oxyhalide contents of the order of 0.1% by weigh were acceptable, the 15 crystals obtained with these contents being sufficiently transparent in appearance. In addition, certain crystals, such as Csl:Tl, accommodate high oxygen contents (for example around 0.2% of CsOH) as far as the scintillation properties are concerned. Now, the Applicant has discovered that the scinrtillation properties, especially the luminous efficiency, that is to say the number of UV-visible photons 20 emitted per Mev of energy of an incident particle, of rare-earth hairdes can be drastically improved by towering the oxyhalide content in a rare-earth halide cryatal below this value. The Applicant therefore sought to develop manufacturing methods that result in rare-earth halides that are as pure 33 possible (especially as regards 25 oxygen), that is to say the water content or which is very much less than 0.1% by weighl and the oxyhalide content of which is less than 0.2% by weight, and even less than 0.1% by weight or indeed less than 0.05% by weight. Moreover, means have to be found for preserving (for example over several months) and handling, these halides that allow this purity to be maintained This is because the growth of 30 the crystals (generally single crystals) is usually earned out in batch mode, which involves phases of patting them into storage and of removing them from storage which phases are conducive to contamination of the rare-earth halide by the water 2 and oxygen of air in addition, it is very difficult to produce an installation for preparing a rare-earth halide (as raw material for growing crystals, generally single crystals) that does not itself introduce a small quantity of water or oxygen resulting in the 5 formation of an undesirable oxyhalide. This is because any installation is always imperfectly impermeable and also always contains a small quantity of adsorbed water, so that partial contamination is usual in this kind of preparation, and a high degree af oxidation by the impurities of the gaseous environment is generally expected, most particularly at high temperatures such as above 300ºC. The 10 invention also provides a solution from this standpoint since the method according to the invention resutts in a very pure rare-earth halide, even with an installation initially containing water, whether adsorbed, absorbed or in condensed phase, and even in the presence of a reason able amount of water and oxygen in the atmosphere during the heating leading to melting. 15 The Applicant has discovered that crystals manufactured according to the indention can even have melting points substantially different from those mentioned in the literature, this being interpreted as an effect owing to the high purity of the crystals (especially a low oxychloride content) obtained thanks to the invention. Thus, an LaCl3 crystal produced according to the invention has a 20 crystallization temperature of 880ºC, whereas the values published by the prior art are spread between 852 and 860ºC. Likewise, an LaBr3 crystal manufactured according to the invention has a crystallization temperature of 820°C, whereas the values published in the prior art are between 777ºC and 789ºC. The invention especially makes it possible to prepare single crystals having 25 a particularly low scintillation decay time. The advantage of this is that it is desirable to have crystals whose scintillation peaks have the lowest possible decay time, as in this way the time resolution is improved. To make this measurement, the light intensity of the main peak is recorded over time. Thus, the invention allows the production of single crystals whose decay time of the main 30 component is less than 40, and even less than 30 and even less than 20 nanoseconds. For the purpose of the present invention, X represents a halogen atom chosen from Cl, Br and l The present invention does not relate to rare-earth fluorides because they are not hygroscopic and because the if chemistry is highly specific. The single crystals prepared according to the invention also have a particularly low energy resolution, especially less than 5%, or even less than 4% 5 or even less then 3.5% The following conventional methods may be used to prepare a rare-earth halide. 1. vacuum dehydration of LaX3(H2O), at 80ºC but this method gives LaOX contents that are too high and results in crystals of low quality; 10 2. chlorination. of said La2O3 with HCl gas above 500ºC. this method is dangerous as it requires the use of large amounts of HCl gas - a toxic gas - and it is also vety difficult to ensure on an industrial scale that the chlorination reaction is complete; 3. dehydration of LaX3(HzO)7 in gaseous HX. This method is also dangerous 15 because of the large amount of HX used; and 4 reaction of an La2O3 powder with gaseous NH4C1 at aboul 340ºC: this is very difficult to ensure on an industrial scale that the chlarination reaction is complete. The document "The ammonium-bromide route to anhydrous rare earth bromides MBr3, Journal of the Less-Common Metals, 127 (1987) 155-160 20 teaches the preparation of a rare-earth halide/ammonium bromide complex and its themial decompostion al less than 20ºC/hour in order to form a rare-earth halide. without ever reaching the melting point. By operating in this way, the halide retains a high specific surface area - greater than 0.1 m2/g - propitious to moisture absorption and to oxychloride formation. The fact of working at below 400ºC 25 greatly limits the problems of corrosion of materials, and this is one of the reasons why it is preferred in the prior art to use such low temperatures. The prior art making use of compounds of the NH4X type does not generally heat beyond 300 or 400ºC, as at higher temperatures NH4X disappears by sublimation acid the rare-earth halide becomes particularly sensitive to oxidation from the traces of water 30 and oxygen that are present in the gaseous environment. As documents of the prior art, mention may also be made of WO 016O944, WO0160945 and US 6451106. 4 The invention solves the abovementioned problems. The invention makes il possible to obtain a very pure rare-earth halide in the form of a polycrystailine block, especially one having a rare-earth oxyhalide content of less than 0.2% by weight or even less than 0.1% by weight or even less than 0.05% by weight or 5 even less, than 0.02% by weight, and a water content of less than 0.1 % by weight. The method of preparation according to the invention camprises a step of healing a mixture of, on the one hand, at least one compound having at feast one Ln-X bond and, on the other hand. NH4X, in which Ln represents a rare earth and X is chosen from Cl, Br and I, said compound and NH4X possibly beirrg combined, 10 at least partially, within a complex, said step resulting in a molten phase comprising the Intended halide, followed by a cooling step resulting in at feast one solid block comprising said halide The NH4X, by reacting with the oxyhalides, acts-as oxygen scavenger and consequently strips the rare-earth halide of its oxychlorides, given that these oxychlorides may come from the reaction between 15 the water that is absorbed by the rafe-earth halide. and the rave-earth halide during the heating. This purification takes place according to the principle, of the following reaction. LnOX + 2 NH4X - LnX3 + H2O + 2 NH3 20 The method according to the invention makes it possible in particular to prevent water, present in the mixture or the crucible or the apparatus, in adsorbed. absorbed or complexed form, from combining permanently with the rare-earth halide into the rare-earth oxychloride. Thus, the method according to the invention 23 results in a final block having much less oxyhalide than the same method without the initial NH4X- in particular, this may be observed with an installation difficult to strip of tts adsorbed water, that is to say an installation that usually results in a high oxyhalide content (for example at least 0.2% oxyhalide) in the final halide even when no oxyhalide is Intentionally put into the starting mixture (or a very low 30 content, ie. less than 100 ppm by weight) and even in the presence of the usual water and oxygen contents in the gaseous environment for this kind of manufacture. 5 The polycrystalline block obtained according to the invention is very pure. The invention combines, in a single heating step, the oxygen scavenging action, conferred by the presence of the ammonium halde. and the fart ol immediately proceeding to melting the rare-earth halide so as to drastically reduce its specific 5 surface area, thereby making it less moisture-sensitive wrile it is being stored and handled. The halide is therefore purified in a first stage and then melted in a second stage so as to become much less sensitive to oxidation by water and oxygen, these first and second stages being carried out within one and the same heating step, which means that, once the mixture has reached the temperature of 10 300ºC, its temperature does not return to room temperature or even to a temperature below 200ºC before the desired rare-earth halide has been melted. This preparation of the block according to the invention is carried out in an inert or neutral atmosphere (for example In nitrogen or argon), but this atmosphere may even contain relatively large amounts of water and oxygen, that is to say in such a 15 way that the sum of the water and oxygen masses in the gaseous atmosphere is less, than 200 ppm by weight. In general, dating production of the block according to the invention the water content of the inert atmosphere ranges from 10 to 180 ppm by weight and the oxygen content of the atmosphere ranges from 0.5 to 2 ppm by weight. 20 Because of its low speciffc surface area compared with a powder, the block absorbs fewer impurities from the air (moisture and oxygen) and consequently can be stored and handled while retaining a very high pure state. Under these conditions, this block can be used for preparing crystals (generally single crystals) of rare-earth halides that are very pure and of high quality. 25 The invention also relates to a method of preparing the blocks according to the invention in a carbon-rich crucible. Whereas according to the prior art, such as for example the article by P. Egger et al., J.Crystal Growth 200 (1999) 515-520, the growth of Ba2Y12Er2Cl7 (0 30 compositions forming the subject of the present invention are advantageously melted in a crucible that is rich in carbon, as is the case with glassy carbon, in order to produce the block according to the invention. 8 The rare earths Ln to which the present invention relates are those of Column 3 (according to the new notation) of the Periodic Tabfe of the Elements, inducting Sc, Y. La, and the lantharudes from Ce to Lu. More particularly involved are the halides of Y. La. Go and Lu. which may especially be doped with Ce or Pr. 5 The rare-earth halides more particularly involved in being manufactured in block form according to the present invention may be represented by the general formula AeLnX^i31+B in which Ln represents one or more rare earths, X represents One or more halogen atoms chosen from Cl, Br or 1, and A represents one or more alkali metals such as K, Li, Na, Rb or Cs, e and f representing values such that. 10 - e, which may be zero, is less than or equal to 3f; - 1 is greater than or equal to 1. The method according to the invention is more effective the lower the atomic number of X. Thus, the effectiveness of the method seconding to the invention in reducing the oxyhalide content in the final black increases, depending 15 on the nature of X, in the following order: I <: br cl. the method according to indention is more effective larger ionic radius of ln thus effectiveness invention in reducing oxyhalide content final block increases depending on nature following order: sc lu y ce> 20 The rare-earth halides more particularly involved are especially the following: - ALn2X7 in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl, Br or I, A representing an afkali metal such as Rb and Cs; 25 - LaCl3, which may eapecially be doped with 0.1 to 50% by weight of CeCl3: - LnBr3, which may especiallv be doped with 0.1 to 50% by weight of CeBr3; - LaBr3, which may especially be doped with 0.1 to 50% by weight of 30 CeBr3; - GdBrs, which may especiatiy be doped with 0.1 to 50% by weight of 9 The mixture may also contain water, either in free form or in complexed form, for example complexed with the rare-earth halide. Surprisingly, the amount of water may be very high without this resulting in a higher oxyhalide content being obtained in the final polycrystalline block according to the invention, as long as the 5 rnixture contains a sufficient amount of NH4X. The mixture may even include, for example, up to 20% by weight of water, or more. it may also include, for example, less than 16% by weight of water, or even less than 5% by weight of water. The mixture of, one the one hand, at least one compound having at least 10 one Ln-X bond and, on the other hand. NH4X, these two compounds being, where appropriate, at least partially in complexed form, contains sufficient NH4X for the final block to comprise less than 0.2% by weight of rare-earth oxyhalide, or even less than 0.1% by weight of rare-earth oxyhalide, or even Jess than 0.05% by weight of rare-earth oxyhalide, or even less than 0.02% by weight of rare-earth 15 oxyhalide. Preferably, the Ln atoms in the compound are linked only to X atoms or oxygen atoms or A atoms. This is particularly the case for a complex of formula LnX3.(NH4X)x in which the Ln atoms are linked only to X atoms. Here, it is considered that no atom belonging to NH4X is linked to the Ln atoms. 20 Preferably, an amount of NH4X which is at least the sum of the following two quantities: - A) a number of moles of NH4X equal to one and preferably three times the number of moles of Ln that are not linked to an oxygen; - B) is a number of moles of NH4X equal to three times and preferably five 25 times the number of moles of oxygen atoms linked to Ln, is introduced into the mixture. in particular, it is possible to introduce, into the mixture, an amount of NHsX which is at least the sum of the two following quantities: - A) a number of moles of NH4X equal to three times the number of moles 30 of Ln that are not linked to an oxygen; B) a number of moles of NH4X equal to five times the number of moles of oxygen atoms linked to Ln. 10 it is clearly understood that if NH4X is counted for the purpose of calculating A), the same NH4X must not be counted for the purpose of calculating B), and vice versa If the mixture contains no oxygen linked to Ln, the amount of NH4X in the case of B) by itself is zero 5 For the purpose of the present invention, it is considered that the number of moles of oxygen atoms finked to Ln is identical to the number of moles of oxyhalide of formula LnOX as obtained by the dissolution method described below. It is therefore easy to calculate, from the mass of oxyhalide obtained by the dissolution method, the number of moles of oxygen atoms linked to Ln by 10 assuming that the oxyhalide has the formula LnOX. If A (generally Rb or Cs) is present, because this atom has a very low tendency to combine with oxygen, its presence is not involved in the calculations of the amounts of NH4X, It will be clearly understood that, to calculate this amount, it is necessary to take account of all the NH4X molecules present in the mixture, whether or not this 15 NM4X is completed for example with a rare-earth halide. The mixture may comprise a mixture of the compound having at least one Ln-X bond and NH4X. This complex may, for example, be prepared by wet chemistry, according to the following principle: a ram-earth salt, such as a rare-earth oxide or a hydrated rare-earth 20 halide is firstly dissolved in the corresponding hydroacid (that is to say HCl if it is desired to obtain a chloride, HBr if if is desired to obtain a bromide). At this stage, AX (A generally being Rb or CS) is added if a halide containing A is desired. Preferably, added to the ammonium halide solution are 1 to 4 mofes of ammonium halide per mole of rare- 25 earth halide, so as to obtain a solution, tfit is desired finally to obtain a halide of the rare-earth Ln doped with another rare earth Ln' (in fact doped with a halide of Ln') such as cerium, all that is required is to introduce, during dissolution in the hydroacid, the desired proportion of Ln' (for example, 10% of CeX3(H2O)7 into a solution obtained from 30 LaX3(H2O) if it is desired finally to obtain anhydrous LnX3 doped with 10% of CeX3). The solution is then dried in an oven or by any suitable means. The salt obtained, of formula LnXz . (NH^X)X where x = 3.5, is stable and can be stored in seated containers. The mixture of, on the one hand, at least one compound having at least one 5 Ln-X bond and. on the other hand, NH4X is then subjected to a heat treatment. For this heat treatment, the mixture is generally placed in a crucible, which may be made of platinum, carbon, such as graphite, or molybdenum or tantalum or boron nitride or silica. The crucible may also be made of graphite coated with pyrolytic carbon or made of graphite coated with silicon carbide ot made of graphite coated 10 with boron nitride. Preferably, for the melting, a crucible allowing the block to be demolded cold is used, To produce the blocks according to the invention, it is preferable to use a crucible made of a material containing at least 20% carbon by weight. Such a material may, for example, be carbon or graphite, or amorphous carbon (or glassy carbon) or graphite coated with, pyrolytic carbon (also glassy 15 carbon) or graphite coated with silicon carbide, or graphite coated with boron nitride (possibly pyrolytic). The crucible may therefore be coated with a layer of pyrolytic carbon The material may comprise, on the one hand, a graphite substrate and, on the other hand, a coating, it being possible for this coating to be made of pyrolytic carbon or of silicon carbide or of boron nitride (possibly 20 pyrolytic). The coating serves especially to block the pores in the graphite. The crucible is then positioned in a sealed furnace, the atmosphere from which is purged in order to render it inert, for example purged under a low vacuum and then flushed with a stream of dry nitrogen. The temperature of the furnace is then progressively raised up to at least 400° C Ths water from the complex is 25 eliminated and then NH4X sublimes and is deposited on the cool downstream parts of the furnace. It is important for the mixture to be protected from the ambient air and to be completely in an inert atmosphere, especially above 30CTC and preferably above 200ºC. This is why the potential air intakes into the installation are located beyond the point where the NH4X is deposited so that air cannot get 30 back into the mixture being purified. Owing to the fact that the NH4X is generally present in excess in the mixture, the actual temperature of the mixture generally presents a temperature 12 hold corresponding to the temperature at which the NH4X is eliminated, even if the programmed temperature is constantly increasing. In the case of NH4CI, this temperature hold lies between 300 and 400ºC. This applies not only if the NH4X is initially in free form but also if it is in complexed form. Because the heated mass 5 contains much less NH4X after this temperature hold, it might be expected that the mixture would then be easily oxidized by the impurites present in the gaseous environment (presence of water and oxygen), and to be more so the high the temperatures (at this stage, the temperature of the heated mass is generally above 300ºC). The Applicant has discovered that this is not the case and that it is to possible to control the oxidation of the rare-earth halide After the temperature hold during which the NH4X is given off, the temperature must then be rapidly increased up to a temperature sufficient to melt the desired rare-earth halide (for example, 880ºC in the case of LaCl3). After the temperature hold at which the NH4X is given off, generally between 300 and 400uC, the is mixture, already converted compared with the starting material (since it has lost the NH4X). may be heated at a rate of greater than 50°C/hour and even greater than 100uC/hour and even greater than 150ºC/hour and even greater than 200ºC/hour, In general, the heating rate is less than 600ºC/hour owing to the fact that it is generally necessary to protect the materials of the installation according to 20 their thermal-shock resistance. When the heated material is molten, it is preferred to maintain a temperature above the melting point for at least one hour, generally between one and six hours. As regards heating the mixture, once this is at a temperature above 300ºC, its temperature is not brought back down to room temperature, or even to a 25 temperature below 200° C, before the desired rare-earth halide has reached the melting point. It is preferable to heat the mixture until melting in a single heating step, without lowering the temperature, even momentarily, before the molten mass comprising the molten halide is obtained. The entire heating step (from room temperature up to melting) may generally be carried out in less than 10 hours, or 30 even less than 6 hours or even fess than 4 hours. The molten mass can then be rapidly cooled. A block of anhydrous rare-earth halide is thus recovered, this comprising less than 0.1% by weight of water 13 and less than 0.2% by weight of rare-earth oxyhalide, or even less than 0.1% by weight of rare-earth oxyhalide, or even less than 0 05% by weight of rare-earth oxyhalide or even less than 0,02% by weight of rare-earth oxyhalide. This block is easy to handle and to store. In general, blocks of at least 1 g per unit, or indeed at 5 least 109 per unit, or indeed at least 50 g per unit or indeed at feast 500 g per unit may be produced. These blocks generally have a bulk density of at least 75%. or indeed at least 80%, or indeed at least 85% of the theoretical density, it being understood that the theoretical density is that corresponding to the same, material without any porosity. The block according to the invention is poly crystalline and 10 contains a multitude of grains, each of which is a small single crystal. A block generally contains at least 100 grams and even at least 1000 grains. No grain of the block represents more than 10% of the entire mass of the block. The ammonium halide condensed on the cool downstream parts of the furnace may be at least partly reused, for exampte in the method according to the 15 invention To measure the centerl of oxyhalides in a rare-earth halide, all that is required is to separate them using, water (for example at room temperature) since the oxyhatides are insoluble in water whereas the halides are soluble The oxyhalides may be recovered by filtration, for example over a polypropylene (PP) 20 filter and then dried at 120ºC. If the halide includes A (generally Rb or Cs), this method results in the dissolution of AX since A does not form an oxyhalide. This method, called the "dissolution method" of the 'method of insolubles", results, even in the presence of A in the halide, in a determination ot the content of oxyhalide of formula LnXO. 25 The block according to the invention may be used as raw material for growing crystals (generally single crystals) using known techniques such as Bridgman growth or Kyropoulos growth or Czochralski growth, or growth using the gradient freeze method These single crystals are very pure and can be used as scintillates material. This crystal preparation is carried out in an inert atmosphere so (nitrogen or argon for example), but this atmosphere may contain relatively large amounts of water and oxygen, that is to say in such a way that the sum of the water and oxygen masses in the gaseous atmosphere is fees than 200 pprn by 14 weight. In general, during production of the crysial (generally a single crystal), the water content of the inert atmosphere ranges from 10 to 180 ppm by weight and the oxygen content of the atmosphere ranges from 0.5 to 2 ppm by weight Because pf the low surface area of the block or blocks used as raw material 5 and also because this surface ares increases during the rise in temperature up to melting, the final single crystal is very pure and has a remarkable scintillation efficiency. Thus, the invention also relates to a single crystal of formula AeLnfX^r+xxx the symbols of which have the meanings given above, said single crystal comprising less than 0.2% and even less than 0.1 % or indeed less than 0,05% or 10 indeed less than 0,02% by weight of rare-earth oxyhalicde. This applies especially when Ln is chosen from La, Gd, Y, Lu and Ce, and when X is chosen from Cl and Br Reference may more particularly be made to the following single crystals: - those of general composition Ln1-xCexBr3 in which Ln is chosen from lanthanides or mixtures of lanthanides of the group La Gd, Y and Lu, especially 15 chosen from lanthanides or groups of lanthanides of the group La and Gd, and in which x is the molar degree of substitution of Ln wtth cerium, where x is greater than or equal to 0.01 mol% and strictly less than 100 mol%, - those of general composition Ln1xCe2Cl3, in which Ln is chosen from tenthanides or mixtures of lanthianides of the group Y, La, Gd and Lu, especially 20 from elements or mixtures of elements of the group La, Gd and Lu, and in which x is the molar degree of substitution of Ln with cerium, where x is greater than or equal to 1 mol% and strictly less than 100 mol%. The above mentioned growth methods may result in a large single crystal, that is to say one at least 1 cm3. or indeed at least 10 cm3 and even at least 25 200 cm3 in size. This single crystal may then be cut to the sizes suitable for the desired applications. The single crystal according to the invention, because of its high purity, has a particularly high luminous efficiency, One way of measuring this luminous efficiency is to measure it relative to that of a crystal of Nal doped with 600 ppm by 30 weight of Tl lodide, the energy resolution of which at 622 keV is 6.8%, the integration time being 1 us and the radioactive source being 137Cs at 622 keV. The coupling between the crystals (Nal or rare-earth halide) and the photomultiplier 15 takes place by means of a silicons grease that is transparent up to 320 nm. Of course, the exit face of the Nal turned toward the photo multiplier is polished. Under these measurernent conditions, the invention makes it possible to obtain luminous efficiencies of at least 90% of that of the Tl-Nal crystal, and in any case 5 greater than those obtained on crystals not according to the invention. The crystal or single crystal may especially be produced in a crucrbfe made of platinum or graphite or graphite coated with pyrolytic carbon fn the examples that follow, the energy resolution was measured in the following manner a 10x10x5 mm piece is cut from the single crystal. All the faces 10 of the piece apart from one of the large 10x10 mm faces are left as cut and the face with which the photo multiplier (PMT) couples is polished. The crystal is wrapped in several thickness of PTFE (Teflon) tape except on the face that is coupled to the PMT. The crystal is prepared in a glove box, the dew point of which is below- 40ºC. 15 Example 1. anhydrous LaCl3 433 g of La2O3 were dissolved by 1330 ml of 37% HCI diluted in 2450 mi of water. 497 g of NH4CI were added. Next, the water and HCI in excess were evaporated by heating to. 100ºC in order to obtain an LaCl3(NH4Cl)3.5 complex, which 20 contained 0.7% by weight of water using the Karl Fischer measurement. The LaCl3-(NH4Cl)3.5 complex was a compound that did indeed have at least one Ln-X bond, since it contained La-CI bonds. It was also itself a mixture within the meaning of the invention, comprising, on the one hand, a compound having an Ln-X bond and, on the other hand. NH4X (in thrs case NH4Cl). Moreover, within this 25 mixture, the amount of NH4X is such that the ratio of the number of moles of NH4X to the number of moles of Ln not linked to oxygen is 3.5, which corresponds to a preferred ratio according to the invention. Moreover, it was unnecessary to include in our calculations for NH4X being introduced in the case of oxygen linked to Ln since the starting mixture did not contain this type of bond. 30 By heating at a rate of 200°C/hour from room temperature up to 950°C, 200 g of the complex were then decomposed with sublimation of the NH4CI and melted in a crucible made of graphite coated with pyrolytic carbon. said crucible itself being 16 placed in a seated silica lube with a nitrogen flush. The nitrogen atmosphere contained about 50 ppm by weight of water and between 1 and 2 ppm oxygen by weight. The oxycnloride content in the final block, measured by dissolution, was 0 01% by weight The water content was less than 0.1% by weight (the detection 5 limit of the method used) The block obtained had a mass of 651 g Example 2 (comparative example): anhydrous LaCI3 The procedure was exactly as in the case of example 1, except that the compiex was replaced with an anhydrous LaCI3 powder whose oxychloride content was 10 less that 0.02%, the size of the particles was submilimetric and the water content was not detectable by Karl Fischer. The oxychloride content in the final block, measured by dissolution, was 0.23% by weight. The water content was less than 0.1% by weight. 15 Example 3: anhydrous LaBr3 :CG 300 g of La2O3 were dissolved by 630 ml of 47% HBr diluted in 2330 ml of water. 662 g of NH4Br were added. The solution obtained was filtered over PP. The solution was then dried with a rotary evaporator In a 101 flask. The complex obtained, of formula LaBr3.(NH4Br)3.5, contained 0,23% water by weight measured 20 using Kart Fischer. Next. 142,6 g of this complex was removed and doped with 0,5% by weigh! of (NH4Br)3.5CeBr3, and this was heated at 200°C/h in a stream of nitrogen in a graphite crucible. The temperature was held at 660ºC for 4 h 30. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The melted pellet weighed 76.61 g and 25 contained only 0,035% oxybromide LaOBr (measured by the method of insolubles). The water content was also less than 0.1 % The hydrostatic density of this block, measured by immersion in hexane, was 3pproximately 4 92 glcm3, i.e 87% of the theoretical density, thereby proving goud densification 30 This melted block was then used for growth in a Bridgman furnace in a graphite crucible under a stream of nitrogen. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of 17 oxygen. The crystal obtained was dear and free of white oxybromide inclusions and free of bubbles. The oxybromide content of this crystal was 0.05% by weight. More than 80% of the mass of this cryslal was suitable for use as a scintiflator. 5 Example 4: anhydrous La Br3 from a wet complex The complex LaBr3(NH14Br)3.5 prepared as in the preceding example was used, but wetted so that it contained 14.7% by weight of water measured by Karl Fischer. 124 g of this mixture (complex + water) were removed and heated at 200ºC/h in a stream of nitrogen in a graphite crucible up to 860°C. The 10 temperature was held at 860°C for 4 h 30 The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The melted pellet weighed 64,1 g and contained only 0,034% by weight of oxybromide (measured by the method of insolubles). The water content was less than 0.1% by weight. 15 Example 6: anhydrous GdBr3 271.2 g of Gd2O3 were dissolved by 796 g of 48% HBr diluted in 430 g of water Next. 661 2 g of NH4Br and 855 g of water were added. The solution obtained was filtered over PP. The solution was then dried in a rotary evaporator in a 10 1 flask. 20 1164 g of the complex (NH4Br)4.5 GdBr3 were then obtained. The complex obtained contained 6.3% water measured by Karl Fischer. Next, 254.7 g of this complex were removed and heated at 200ºC/h in a stream of nitrogen in a graphite crucible. The temperature was held at 815°C for 1 h 30. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by 25 weight of oxygen The sintered but unmelted pellet weighed 104.9 g. It was therefore a pulverulent solid that was returned to the ambient conditions. The furnace was recharged with 92.7 g of the above sintered pellet and this was heated at 200°C/h under a stream of nitrogen in a graphite crucible. The temperature was held at 840cC for 1 h 30. The nitrogen atmosphere contained 30 approximately 50 ppm by wefght of wafer and between 1 and 2 ppm by weight of oxygen. The melted pellet weighed 92.7 g and contained 0 65% by weight of 18 GdOBr (measured by the method of in solubles), thereby demonstrating the fact that returning the block to room temperature before melting is contraindicated. Example.7;anhydrous GdBr3 5 The complex (NH4Br) 5GdBr3 as prepared according to the preceding example was used for this test. The complex obtained contained 6.3% by weight of water measured by Karl Fischer. Next, 245.7 g of this complex were removed and healed al 200ºC/h in a stream of nitrogen in a graphite crucible up to 840r'C. The temperature was held at 840°C for 1 h 30. The nitrogen atmosphere contained 10 approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The meited pellet weighed 105.3 g and contained only 0.038% by weight of oxybromide GdOBr (measured by the method of insolubles). This result is all the more exemplary owing to the fact that gadolinium is a heavy (so-called yttric) rare earth whose bromides are very sensitive to hydration. 15 Example 8 (comparative example): single crystal from LaCl3 powder The same batch of anhydrous LaCl3 powder was used as that used for example 2 for 8ridgman furnace growth in a graphite crucible in a stream of nitrogen. The nitrogen atmosphere contained approximately 50 ppm by weight of water and 20 between 1 and 2 ppm by weight of oxygen. The crystal obtained had many white oxychloride inclusions and bubbles organized in the form of filaments along the pulling axis. The oxychloride content of this crystal was 0,25% by weight Approximately 90% of the mass of this crystal was unsuitable for use as a scintilator. 25 Example 9: anhydrous RbGd2Cl7 138.2 g of Rb2CO3 were dissolved by 242 g of 37% HCI diluted in 165 g of water. The solution obtained was filtered over PP. Next. 433.8 g of Gd2O3 were dissolved by 750 g of 37% HCI diluted in 482 g of water. After complete dissolution, the 30 filtered rubidium solution was added. Finally. 576.2 g of NF-UCI and 881 g of water were added. The solution obtained was filtered over PP The pH was - 0.32 and the density of the solution was 1.24. The solution was then dried in s rotary 19 evaporator in a 10 1 flask. 1227 g of (NH4CI)9Rb-Gd2Cl7 were then obtained. Next, 142 6 g of this complex were removed and heated at 200°C/h in a stream of nitrogen in a graphlte crucible up to 660ºC. The temperature was held at 660ºC at for 4 h 30. The nitrogen atmosphere contained approximately 50 ppm by weight of 5 water and between 1 and 2 ppm by weight of oxygen. The melted pellet contained only less than 0.05% by weight of GdOCI (measured by the method of insolubles). Example 10: synthesis from LaOBr The following mixture was produced in a glassy carbon crucible; 0,5374 g of 10 LaOBr. 1.3585 .g of NH4Br (i.e. 5.5 moles) and 10,0673 g of the (NH4Br)3.5LaBr3 complex. The mixture was heated at a rate of 200°C/h up to 830ºC. with a hold at this temperature for 2 h. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The insoluble content in the final block was 0.19% by weight. 15 Example 11: LaCl3 single crystal A 1 kg block of LaCI3 containing 10% CeCI3 by weight was used, this being manufactured according to the invention and having an LaOCI content of less than 0.05% by weight. This block was then used for Bridgman-type growth in a graphite 20 crucible. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The crystal obtained was very clear. Its oxychloride content measured by the method of insolubles was less than 0.05%. Next, a piece measuring 10x10x5 mm was cut from this crystal and Us scintillation efficiency compared with a piece of Nal:TI (Nal doped with 600 ppm by 25 weight of Tl iodide) using the following protocol: Photomuitiplier: Hamamatsu R-1306: Reference Nal crystal 50 mm in diameter and 50 mm in length. Integration lime: 1 us Radioactive source: 137Cs at 622 keV. 30 The light emission from me LaCl3 crystal was 93% of that of the Nal reference crystal. Its energy resolution was 3.6% The main component of the decay time of the scintillation was 27 nanoseconds. 20 Example 12 (comparative example): LaCl3 single crystal 1 kg of commercial LaCl3 and CeCl3 powders (LaOX and water contents of example 2) were used. The mass of CeCl3 represented 10% of the mass of the 5 blend of these two powders. They were melted in a graphite crucible and underwent growth of the Kyropouios (KC 01) type. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The crystal obtained was slightly milky. Its content of fnsolubles was 0.1% by weight. Next, a 10x10x5 mm piece was cut from this crystal and its 10 scintillation yield compared with a Nal:TI using the same protocol as in the previous example. The light emission from the LaCl3 crystal was 83% of that of the Nal reference crystal. Its energy resolution was 3.9%. Example 13 (comparative example): LaCl3. single crystal 15 A crystal was produced by Bridgman-type growth in a silica crucible in accordance with the teaching of the publication IEEE Transactions on Nuclear Science: "Scintillation properties of LaCl3 crystals: Fast, efficient and High Energy resolution scintilfators". The mass of CeCl3 represented 10% of the mass of the mixture before growing the crystal. Next, a 10x10x5 mm piece was cut from this crystal 20 and its scintillation yield compared with a Nal Tl piece using the same protocol as in the two previous examples. The light emission from the LaCl3 crystal was 87% of that of the Nal reference crystal. Its energy resolution was 4.2% Example 14: LaBr3 single crystal 25 Three 1 kg blocks of LaBr3 doped with 0.5% by weight of GeBr3 were used, each block manufactured according to the invention and such that the LaOBr content was 30 obtained was very clear. The oxychloride content of this block could not be measured by the method of insolubles Next, a 10x10.x 5mm piece was cut from 21 this crystal and its scintillation yield compared with a Nal:Tl piece according to the following protocol: - Photomultiplier Hamamatsu R-1306 - Reference: Nal:Tl crystal (Nal doped with 600 ppm by weight of TI iodide) 5 50 mm in diameter and 50 mm in length; - The energy resolution of this reference crystal was 6.8% on the 137Cs line: - the measured crystals were wrapped in Teflon and coupled to the photo multiplier (PMT) using a sificone oil (EDM fluid 200); - Integration time: 1 us 10 - Radioactive source: 137Cs at 622 keV The light emission from the LaBr3 crystal was 147% of that of the Nal reference crystal. Its energy resofution was 4.2% The main component of the decay time of the scintillation was 39 nanoseconds. 15 Example 15 (comparative example): LaBr3 single crystal A crystal obtained by Bridgman-type growth in a silica crucible according to the teachings of the publication "Applied Physics Letters of 03 September 2001 (Vol. 79, No. 10)" was compared with the previous trials. This crystal also contained 0.5% by weight of CeBr3. Next, a 10x10x5 mm piece was cut from this 20 crystal and its scintiflation yield compared with a Nal'TI piece using the same protocol as in the previous example. The crystal was slightly milky. The light emission from the LaBr3 crystal was 102% of that of the Nal reference crystal. The main component of the decay time of the scintillation was 38 nanoseconds. Example 16: LaCl3 single crystal Three 1 kg blocks of LaCl3 doped with 5% by weight of CeCI3 were used, each manufactured according to the invention and such that the LaOCl content was 22 measured by the method of insolubles. it was less than 0.05% by weight. Next, a 10x10x5 mm piece was cut from this crystal and its scintillation yield compared with a NaNTI piece according to the following protocol. - Photomultipfier: Hamamatsu R-1306 5 - Reference: Nsl;TI crystal (Nal doped with 600 ppm by weight of Tl iodide) 50 mm in diameter and 50 mm in length; - The energy resolution of this crystal was 6.8% on the 137Cs line; - the measured crystals were wrapped in Teflon and coupled to the photomultipller (PMT) using a silicone oil (EDM fluid 200); 10 - Integration lime: 1 us - Radioactive source: 137Cs at 622 keV The light emission from the LaCl3 crystal was 98% of that of the Nal reference crystal Its energy resolution was 4.6%. The main component of the decay time of the scintillation was 28 nanoseconds. 15 Example 17: anhydrous LaCl3 This example was as in example 1 except that the blocK was prepared in 3 platinum crucible. The final block stuck to the crucible and was much more difficult to remove from the mold than in the case of the graphile crucible coated with 20 pyrolytic carbon. 23 CLAIMS 1. A method of preparing a polycrystalline block of at least 10 g of a halide of formula AeLn,X(3ftc) in which Ln represents one or more rare earths, X represents one or nioce halogen atoms chosen from Cl, Br or I, and A represents one or 5 more alkali metals such as K, Li. Na, Rb or Cs. e. which may be zero, being less than or equal to 3f. and f being greater than or equal to 1, said block comprising less than 0.1% by weight of water and less than 0.2% by weight of a rare-earth oxyhalide, characterized in that it comprises a step of heating a mixture of, on the one hand, at least one compound having at least one Ln-X bond and, on the 10 other hand, a sufficient amount of NH4X in order to obtain the desired oxyhalide content, said compound and NH4K possibly being combined, at least partially, within a complex, said step resulting in a molten mass comprising the rare-earth halide of formula AeLnrXj(3+t;), said healing step being to flowed by a cooling step after the molten mass has been obtained, and said heating step, after having 15 reached 300ºC never going back down below 200ºC before said moltert mass has been obtained, 2. The method as claimed in the preceding claim, characterized in that the compound having at least one Ln-X bond is of formula ArLnsOuXr+3s-2u in which A, X and Ln have the meanings given above, r,s and u representing integer or 20 noninteger values that meet, cumulatively, the following conditions: r ranging from 0 to 2s, s being greater than or equal to 1, u ranging from 0 to s, which compound may or may not be complexed with water or with NH4X, 25 3, The method as claimed in eiher of the preceding claims, characterized in that the Ln atoms of the compound are finked to X or oxygen or A atoms. 4, The method as claimed in the preceding claim, characterized in that the heating step is carried out without lowering the temperature before the molten mass is obtained. 30 5. The method as claimed in one of the preceding claims, characterized in that the heating step includes a temperature hold owing to the elimination of the NH4X in gaseous phase. 24 6. The method as claimed in the preceding claim, characterized in that the heating step is carried out with a rate of temperature rise of greater than 50ºC/hour after said temperature hold. 7 The method as claimed in the preceding claim, characterized in that the 5 healing step is carried out with a rate of temperature rise of greater than 100°C/hour after said temperature hold. 8 The method as claimed in the preceding claim, characterized in that the heating step is carried out with a rate of temperature rise of greater than 150ºC/hour after said temperature hold. 10 9. The method as claimed in one of the preceding claims, characterized in that the heating step lasts less than 10 hours. 10. The method as claimed in the preceding claim, characterized in that the heating step lasts less than 6 hours, 11. The method as claimed in the preceding claim, characterized in that the 15 heating step lasts less than 4 hours 12. The method as claimed in one of the preceding claims, characterized in that the heating step is carried out in an inert gas atmosphere, the water and oxygen contents of which are such that the sum of the water and oxygen masses in the gaseous atmosphere is less than 200 ppm by weight. 20 13. The method as claimed in the preceding claim, characterized in that the water content of the inert atmosphere ranges from 10 lo 180 ppm by weight and the oxygen content of the atmosphere ranges from 0,5 to 2 ppm by weight. 14. The method as claimed in one of the preceding claims, characterized in that the amount of NH4X is at least the sum of the following two quantities: 25 - A) a number of motes of NH4X equal to one times the number of moles of Ln that are not linked to an oxygen; - B) is a number of moles of NH4X equal to three times the number of moles of oxygen atoms linked to Ln. 15 The method as claimed in the preceding claim, characterized in that the 30 amount of NH4X is at least the sum of the two following quantities: - A) a number of moles of NH4X equal to three times the number of moles of Ln that are not linked to an oxygen: 25 - B) a number of moles of NH4X equal to five times the number of moles of oxygen atoms linked to Ln. 16. The method as claimed in one of the preceding claims, characterized in that the heating step is carried out in a crucible made of a material containing at least 5 20% carbon by weight 17 The method as claimed in the preceding claim. characterized in that the crucible is made of carbon or glassy carbon or graphite. 13. The method as claimed in either of the preceding two claims, characterized in that the crucible is coated with a layer of pyrolylic carbon. 10 19. A polycrystaltine block of at least 1 g of halide of formula AgLn1X(3f+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl, Br or I. and A represents one or more alkali rnetais such as K, Li, Na, Rb or Cs, e and f representing values such that: e, which may be zero, is less than or equal to 3f; 15 f is greater than or equal to 1; comprising less than 0.1% by weight of water and less than 0.2% by weight of rare-earth oxyhalide 20. The block as claimed in the preceding claim, characterized in that It comprises less than 0.1% by weight of rare-earth oxyhalide. 20 21. The block as claimed in the preceding daim. characterized in that it comprises less than 0.05% by weight of rare-earth oxyhalide. 22 The block as claimed in the preceding claim, characterized in that it comprises less than 0.02% by weight of rare-earth oxyhalide. 23. The block as claimed in one of the preceding block claims, characterized in 25 that it weighs at least 10 g. 24. The block as claimed in the preceding claim, characterized in that it weighs at least 50 g. 25. The block as claimed in one of the preceding block claims, characterized in that it has a bulk density of at least 75% of the theoretical density corresponding 30 to the same material with no porosity. 26. The block as claimed in one of the preceding block claims. characterized in that Ln is La or Ce and X is Cl or Br. 26 27. The block as claimed in one of the preceding block claims, characterized in that none of its grams represents more than 10% of the mass of the entire block. 28. The method as claimed In one of claims 1 to 18, characterized in that the block is one of those of claims 19 to 27. 5 29 A block that can be obtained by the method of one of claims 1 to 18 and 28. 30. A method of preparing a crystal comprising the melting of at least one block of one of the preceding block claims. 31. The method as claimed in the preceding claim, characterized in that the crystal is a single crystal. 10 32, A single crystal obtained by melting a block of one of the preceding block claims. 33. A single crystal of formula A>LnrX(3f+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl, Br or I. and A represents one or more alkali metals such as K, Li. Na. Rb or Cs, e and f 15 representing values such that: - e, which may be zero, is less than or equal to 3f, f is greater than or equal to 1; comprising less than 0.1% by weight of rare-earth oxyhalide. 34. The single crystal as claimed in the preceding claim, characterized in that its 20 oxyhalide content is less than 0.05% by weight. 35. The single crystal as claimed in the preceding claim, characterized in that its oxyhalide content is less than 0.02% by weight. 36. The single crystal as claimed in one of the preceding single-crystal claims, characterized in that Ln is chosen from La. Gd. Y, Lu and Ce, and X is chosen 25 from Cl and Br. 37. The single crystal as claimed in one of the preceding single-crystal claims. characterized in that its volume is at least 10 cm3. 38. The single crystal as claimed in one of the preceding single-crystal claims, characterized in that its luminous efficiency is at least 90% of that of a Wai crystal 30 doped with 600 pptn by weight of Ti iodide, the energy resolution of which at 622 keV is 6.8%, the integration lime being 1 us and the radioactive source being 137Cs at 622 keV. 27 39. The single crystal as claimed in one of the preceding single-crystal claims, characterized in that is energy resolution is less than 5%. 40. The single crystal as claimed in the preceding claim, characterized in that its energy resolution is less than 4% 5 41. The single crystal as claimed in the preceding claim, characterized in that its energy resolution is less than 3.5% 42. The single crystal as claimed in one of the preceding single-crystal claims. characterized in that the time for the main component to decay is less than 10 43. The single crystal as claimed in the preceding claim, characrerized in that the time for the main component to decay is less than 30 nanoseconds. 44. The single crystal as claimed in the preceding claim, characterized in that the time for the main component to decay is less than 20 nanoseconds. The invention relates to a method of preparing a polycrystaline block of a halde of formula AeLn4X(3+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl. Br or 1, and A represents one or more alkali metals such as K. Li, Na, Rb or Cs, e, which may be zero, being less than or equal to 3f, and f being greater than of equal to 1, having a low water and oxyhalide content, comprising a step of heating a mixture of, on the one hand, at feast one compound having at least one Ln-X bond and, on the other hand, a sufficient amount of NH4X in order to obtain the desired oxyhalide content, said step resulting in a molten mass comprising the rare-earth halide, said heating step being followed by a cooling Step, and the heating step, after having reached 300ºC, never going back down below 200ºC before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having rennarkable scinimation properties to be grown.

Full Text

1 PREPARATION OF RARE-EARTH HALIDE BLOCKS
Rare-earth halides (in what follows, (n is used to demote a rare earth), especially when they are doped with cerium, and In particular cerium-doped l.nBr3 5 and cerrum-doped LnC13, have very useful scintillation properties especially for applications in nuclear imaging and in spectroscopy (positron emission tomography or PET, gamma camera, oil prospecting and the like). To obtain these properties satisfactory, it is necessary for these compounds to be obtained in the form of large crystal. Generally, these crystals are single crystals. In certain
10 particular cases, they may be polycrystals, within which the crystals have one dimension of the order of one or more centimeters. However, rare earth halides are highly hygroscopic compounds that react with water and with 3ir as soon as they are heated, forming very stable oxyhalides. It has in general been considered that oxyhalide contents of the order of 0.1% by weigh were acceptable, the
15 crystals obtained with these contents being sufficiently transparent in appearance. In addition, certain crystals, such as Csl:Tl, accommodate high oxygen contents (for example around 0.2% of CsOH) as far as the scintillation properties are concerned. Now, the Applicant has discovered that the scinrtillation properties, especially the luminous efficiency, that is to say the number of UV-visible photons
20 emitted per Mev of energy of an incident particle, of rare-earth hairdes can be drastically improved by towering the oxyhalide content in a rare-earth halide cryatal below this value.
The Applicant therefore sought to develop manufacturing methods that result in rare-earth halides that are as pure 33 possible (especially as regards
25 oxygen), that is to say the water content or which is very much less than 0.1% by weighl and the oxyhalide content of which is less than 0.2% by weight, and even less than 0.1% by weight or indeed less than 0.05% by weight. Moreover, means have to be found for preserving (for example over several months) and handling, these halides that allow this purity to be maintained This is because the growth of
30 the crystals (generally single crystals) is usually earned out in batch mode, which involves phases of patting them into storage and of removing them from storage which phases are conducive to contamination of the rare-earth halide by the water
2
and oxygen of air
in addition, it is very difficult to produce an installation for preparing a rare-earth halide (as raw material for growing crystals, generally single crystals) that does not itself introduce a small quantity of water or oxygen resulting in the 5 formation of an undesirable oxyhalide. This is because any installation is always imperfectly impermeable and also always contains a small quantity of adsorbed water, so that partial contamination is usual in this kind of preparation, and a high degree af oxidation by the impurities of the gaseous environment is generally expected, most particularly at high temperatures such as above 300ºC. The
10 invention also provides a solution from this standpoint since the method according to the invention resutts in a very pure rare-earth halide, even with an installation initially containing water, whether adsorbed, absorbed or in condensed phase, and even in the presence of a reason able amount of water and oxygen in the atmosphere during the heating leading to melting.
15 The Applicant has discovered that crystals manufactured according to the
indention can even have melting points substantially different from those mentioned in the literature, this being interpreted as an effect owing to the high purity of the crystals (especially a low oxychloride content) obtained thanks to the invention. Thus, an LaCl3 crystal produced according to the invention has a
20 crystallization temperature of 880ºC, whereas the values published by the prior art are spread between 852 and 860ºC. Likewise, an LaBr3 crystal manufactured according to the invention has a crystallization temperature of 820°C, whereas the values published in the prior art are between 777ºC and 789ºC.
The invention especially makes it possible to prepare single crystals having
25 a particularly low scintillation decay time. The advantage of this is that it is desirable to have crystals whose scintillation peaks have the lowest possible decay time, as in this way the time resolution is improved. To make this measurement, the light intensity of the main peak is recorded over time. Thus, the invention allows the production of single crystals whose decay time of the main
30 component is less than 40, and even less than 30 and even less than 20 nanoseconds. For the purpose of the present invention, X represents a halogen atom chosen from Cl, Br and l The present invention does not relate to rare-earth
fluorides because they are not hygroscopic and because the if chemistry is highly
specific.
The single crystals prepared according to the invention also have a particularly low energy resolution, especially less than 5%, or even less than 4% 5 or even less then 3.5%
The following conventional methods may be used to prepare a rare-earth halide.
1. vacuum dehydration of LaX3(H2O), at 80ºC but this method gives LaOX contents that are too high and results in crystals of low quality;
10 2. chlorination. of said La2O3 with HCl gas above 500ºC. this method is
dangerous as it requires the use of large amounts of HCl gas - a toxic gas - and it is also vety difficult to ensure on an industrial scale that the chlorination reaction is complete;
3. dehydration of LaX3(HzO)7 in gaseous HX. This method is also dangerous 15 because of the large amount of HX used; and
4 reaction of an La2O3 powder with gaseous NH4C1 at aboul 340ºC: this is very difficult to ensure on an industrial scale that the chlarination reaction is complete.
The document "The ammonium-bromide route to anhydrous rare earth bromides MBr3, Journal of the Less-Common Metals, 127 (1987) 155-160 20 teaches the preparation of a rare-earth halide/ammonium bromide complex and its themial decompostion al less than 20ºC/hour in order to form a rare-earth halide. without ever reaching the melting point. By operating in this way, the halide retains a high specific surface area - greater than 0.1 m2/g - propitious to moisture absorption and to oxychloride formation. The fact of working at below 400ºC 25 greatly limits the problems of corrosion of materials, and this is one of the reasons why it is preferred in the prior art to use such low temperatures. The prior art making use of compounds of the NH4X type does not generally heat beyond 300 or 400ºC, as at higher temperatures NH4X disappears by sublimation acid the rare-earth halide becomes particularly sensitive to oxidation from the traces of water 30 and oxygen that are present in the gaseous environment.
As documents of the prior art, mention may also be made of WO 016O944, WO0160945 and US 6451106.
4
The invention solves the abovementioned problems. The invention makes il
possible to obtain a very pure rare-earth halide in the form of a polycrystailine
block, especially one having a rare-earth oxyhalide content of less than 0.2% by
weight or even less than 0.1% by weight or even less than 0.05% by weight or
5 even less, than 0.02% by weight, and a water content of less than 0.1 % by weight.
The method of preparation according to the invention camprises a step of healing a mixture of, on the one hand, at least one compound having at feast one Ln-X bond and, on the other hand. NH4X, in which Ln represents a rare earth and X is chosen from Cl, Br and I, said compound and NH4X possibly beirrg combined,
10 at least partially, within a complex, said step resulting in a molten phase comprising the Intended halide, followed by a cooling step resulting in at feast one solid block comprising said halide The NH4X, by reacting with the oxyhalides, acts-as oxygen scavenger and consequently strips the rare-earth halide of its oxychlorides, given that these oxychlorides may come from the reaction between
15 the water that is absorbed by the rafe-earth halide. and the rave-earth halide during the heating. This purification takes place according to the principle, of the following reaction.
LnOX + 2 NH4X - LnX3 + H2O + 2 NH3
20
The method according to the invention makes it possible in particular to prevent water, present in the mixture or the crucible or the apparatus, in adsorbed. absorbed or complexed form, from combining permanently with the rare-earth halide into the rare-earth oxychloride. Thus, the method according to the invention
23 results in a final block having much less oxyhalide than the same method without the initial NH4X- in particular, this may be observed with an installation difficult to strip of tts adsorbed water, that is to say an installation that usually results in a high oxyhalide content (for example at least 0.2% oxyhalide) in the final halide even when no oxyhalide is Intentionally put into the starting mixture (or a very low
30 content, ie. less than 100 ppm by weight) and even in the presence of the usual water and oxygen contents in the gaseous environment for this kind of manufacture.
5
The polycrystalline block obtained according to the invention is very pure. The invention combines, in a single heating step, the oxygen scavenging action, conferred by the presence of the ammonium halde. and the fart ol immediately proceeding to melting the rare-earth halide so as to drastically reduce its specific 5 surface area, thereby making it less moisture-sensitive wrile it is being stored and handled. The halide is therefore purified in a first stage and then melted in a second stage so as to become much less sensitive to oxidation by water and oxygen, these first and second stages being carried out within one and the same heating step, which means that, once the mixture has reached the temperature of
10 300ºC, its temperature does not return to room temperature or even to a temperature below 200ºC before the desired rare-earth halide has been melted. This preparation of the block according to the invention is carried out in an inert or neutral atmosphere (for example In nitrogen or argon), but this atmosphere may even contain relatively large amounts of water and oxygen, that is to say in such a
15 way that the sum of the water and oxygen masses in the gaseous atmosphere is less, than 200 ppm by weight. In general, dating production of the block according to the invention the water content of the inert atmosphere ranges from 10 to 180 ppm by weight and the oxygen content of the atmosphere ranges from 0.5 to 2 ppm by weight.
20 Because of its low speciffc surface area compared with a powder, the block
absorbs fewer impurities from the air (moisture and oxygen) and consequently can be stored and handled while retaining a very high pure state. Under these conditions, this block can be used for preparing crystals (generally single crystals) of rare-earth halides that are very pure and of high quality.
25 The invention also relates to a method of preparing the blocks according to
the invention in a carbon-rich crucible. Whereas according to the prior art, such as for example the article by P. Egger et al., J.Crystal Growth 200 (1999) 515-520, the growth of Ba2Y12Er2Cl7 (0 30 compositions forming the subject of the present invention are advantageously melted in a crucible that is rich in carbon, as is the case with glassy carbon, in order to produce the block according to the invention.
8
The rare earths Ln to which the present invention relates are those of Column 3 (according to the new notation) of the Periodic Tabfe of the Elements, inducting Sc, Y. La, and the lantharudes from Ce to Lu. More particularly involved are the halides of Y. La. Go and Lu. which may especially be doped with Ce or Pr.
5 The rare-earth halides more particularly involved in being manufactured in
block form according to the present invention may be represented by the general formula AeLnX^i31+B in which Ln represents one or more rare earths, X represents One or more halogen atoms chosen from Cl, Br or 1, and A represents one or more alkali metals such as K, Li, Na, Rb or Cs, e and f representing values such that.
10 - e, which may be zero, is less than or equal to 3f;
- 1 is greater than or equal to 1.
The method according to the invention is more effective the lower the atomic number of X. Thus, the effectiveness of the method seconding to the invention in reducing the oxyhalide content in the final black increases, depending
15 on the nature of X, in the following order: I <: br cl. the method according to indention is more effective larger ionic radius of ln thus effectiveness invention in reducing oxyhalide content final block increases depending on nature following order: sc lu y ce> 20 The rare-earth halides more particularly involved are especially the
following:
- ALn2X7 in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl, Br or I, A representing an afkali metal such as Rb and Cs;
25 - LaCl3, which may eapecially be doped with 0.1 to 50% by weight of
CeCl3:
- LnBr3, which may especiallv be doped with 0.1 to 50% by weight of CeBr3;
- LaBr3, which may especially be doped with 0.1 to 50% by weight of 30 CeBr3;
- GdBrs, which may especiatiy be doped with 0.1 to 50% by weight of
9
The mixture may also contain water, either in free form or in complexed
form, for example complexed with the rare-earth halide. Surprisingly, the amount
of water may be very high without this resulting in a higher oxyhalide content being
obtained in the final polycrystalline block according to the invention, as long as the
5 rnixture contains a sufficient amount of NH4X.
The mixture may even include, for example, up to 20% by weight of water, or more. it may also include, for example, less than 16% by weight of water, or even less than 5% by weight of water.
The mixture of, one the one hand, at least one compound having at least
10 one Ln-X bond and, on the other hand. NH4X, these two compounds being, where
appropriate, at least partially in complexed form, contains sufficient NH4X for the
final block to comprise less than 0.2% by weight of rare-earth oxyhalide, or even
less than 0.1% by weight of rare-earth oxyhalide, or even Jess than 0.05% by
weight of rare-earth oxyhalide, or even less than 0.02% by weight of rare-earth
15 oxyhalide.
Preferably, the Ln atoms in the compound are linked only to X atoms or oxygen atoms or A atoms. This is particularly the case for a complex of formula LnX3.(NH4X)x in which the Ln atoms are linked only to X atoms. Here, it is considered that no atom belonging to NH4X is linked to the Ln atoms. 20 Preferably, an amount of NH4X which is at least the sum of the following two
quantities:
- A) a number of moles of NH4X equal to one and preferably three times the number of moles of Ln that are not linked to an oxygen;
- B) is a number of moles of NH4X equal to three times and preferably five 25 times the number of moles of oxygen atoms linked to Ln,
is introduced into the mixture. in particular, it is possible to introduce, into the mixture, an amount of NHsX which is at least the sum of the two following quantities:
- A) a number of moles of NH4X equal to three times the number of moles 30 of Ln that are not linked to an oxygen;
B) a number of moles of NH4X equal to five times the number of moles of oxygen atoms linked to Ln.
10
it is clearly understood that if NH4X is counted for the purpose of calculating A), the same NH4X must not be counted for the purpose of calculating B), and vice versa If the mixture contains no oxygen linked to Ln, the amount of NH4X in the case of B) by itself is zero
5 For the purpose of the present invention, it is considered that the number of
moles of oxygen atoms finked to Ln is identical to the number of moles of oxyhalide of formula LnOX as obtained by the dissolution method described below. It is therefore easy to calculate, from the mass of oxyhalide obtained by the dissolution method, the number of moles of oxygen atoms linked to Ln by 10 assuming that the oxyhalide has the formula LnOX. If A (generally Rb or Cs) is present, because this atom has a very low tendency to combine with oxygen, its presence is not involved in the calculations of the amounts of NH4X,
It will be clearly understood that, to calculate this amount, it is necessary to take account of all the NH4X molecules present in the mixture, whether or not this 15 NM4X is completed for example with a rare-earth halide.
The mixture may comprise a mixture of the compound having at least one Ln-X bond and NH4X. This complex may, for example, be prepared by wet chemistry, according to the following principle:
a ram-earth salt, such as a rare-earth oxide or a hydrated rare-earth
20 halide is firstly dissolved in the corresponding hydroacid (that is to say
HCl if it is desired to obtain a chloride, HBr if if is desired to obtain a
bromide). At this stage, AX (A generally being Rb or CS) is added if a
halide containing A is desired. Preferably, added to the ammonium
halide solution are 1 to 4 mofes of ammonium halide per mole of rare-
25 earth halide, so as to obtain a solution, tfit is desired finally to obtain a
halide of the rare-earth Ln doped with another rare earth Ln' (in fact
doped with a halide of Ln') such as cerium, all that is required is to
introduce, during dissolution in the hydroacid, the desired proportion of
Ln' (for example, 10% of CeX3(H2O)7 into a solution obtained from
30 LaX3(H2O) if it is desired finally to obtain anhydrous LnX3 doped with
10% of CeX3).
The solution is then dried in an oven or by any suitable means. The salt obtained, of formula LnXz . (NH^X)X where x = 3.5, is stable and can be stored in seated containers.
The mixture of, on the one hand, at least one compound having at least one 5 Ln-X bond and. on the other hand, NH4X is then subjected to a heat treatment. For this heat treatment, the mixture is generally placed in a crucible, which may be made of platinum, carbon, such as graphite, or molybdenum or tantalum or boron nitride or silica. The crucible may also be made of graphite coated with pyrolytic carbon or made of graphite coated with silicon carbide ot made of graphite coated
10 with boron nitride. Preferably, for the melting, a crucible allowing the block to be demolded cold is used, To produce the blocks according to the invention, it is preferable to use a crucible made of a material containing at least 20% carbon by weight. Such a material may, for example, be carbon or graphite, or amorphous carbon (or glassy carbon) or graphite coated with, pyrolytic carbon (also glassy
15 carbon) or graphite coated with silicon carbide, or graphite coated with boron nitride (possibly pyrolytic). The crucible may therefore be coated with a layer of pyrolytic carbon The material may comprise, on the one hand, a graphite substrate and, on the other hand, a coating, it being possible for this coating to be made of pyrolytic carbon or of silicon carbide or of boron nitride (possibly
20 pyrolytic). The coating serves especially to block the pores in the graphite.
The crucible is then positioned in a sealed furnace, the atmosphere from which is purged in order to render it inert, for example purged under a low vacuum and then flushed with a stream of dry nitrogen. The temperature of the furnace is then progressively raised up to at least 400° C Ths water from the complex is
25 eliminated and then NH4X sublimes and is deposited on the cool downstream parts of the furnace. It is important for the mixture to be protected from the ambient air and to be completely in an inert atmosphere, especially above 30CTC and preferably above 200ºC. This is why the potential air intakes into the installation are located beyond the point where the NH4X is deposited so that air cannot get
30 back into the mixture being purified.
Owing to the fact that the NH4X is generally present in excess in the mixture, the actual temperature of the mixture generally presents a temperature
12
hold corresponding to the temperature at which the NH4X is eliminated, even if the programmed temperature is constantly increasing. In the case of NH4CI, this temperature hold lies between 300 and 400ºC. This applies not only if the NH4X is initially in free form but also if it is in complexed form. Because the heated mass 5 contains much less NH4X after this temperature hold, it might be expected that the mixture would then be easily oxidized by the impurites present in the gaseous environment (presence of water and oxygen), and to be more so the high the temperatures (at this stage, the temperature of the heated mass is generally above 300ºC). The Applicant has discovered that this is not the case and that it is
to possible to control the oxidation of the rare-earth halide
After the temperature hold during which the NH4X is given off, the temperature must then be rapidly increased up to a temperature sufficient to melt the desired rare-earth halide (for example, 880ºC in the case of LaCl3). After the temperature hold at which the NH4X is given off, generally between 300 and 400uC, the
is mixture, already converted compared with the starting material (since it has lost the NH4X). may be heated at a rate of greater than 50°C/hour and even greater than 100uC/hour and even greater than 150ºC/hour and even greater than 200ºC/hour, In general, the heating rate is less than 600ºC/hour owing to the fact that it is generally necessary to protect the materials of the installation according to
20 their thermal-shock resistance. When the heated material is molten, it is preferred to maintain a temperature above the melting point for at least one hour, generally between one and six hours.
As regards heating the mixture, once this is at a temperature above 300ºC, its temperature is not brought back down to room temperature, or even to a
25 temperature below 200° C, before the desired rare-earth halide has reached the melting point. It is preferable to heat the mixture until melting in a single heating step, without lowering the temperature, even momentarily, before the molten mass comprising the molten halide is obtained. The entire heating step (from room temperature up to melting) may generally be carried out in less than 10 hours, or
30 even less than 6 hours or even fess than 4 hours.
The molten mass can then be rapidly cooled. A block of anhydrous rare-earth halide is thus recovered, this comprising less than 0.1% by weight of water
13
and less than 0.2% by weight of rare-earth oxyhalide, or even less than 0.1% by weight of rare-earth oxyhalide, or even less than 0 05% by weight of rare-earth oxyhalide or even less than 0,02% by weight of rare-earth oxyhalide. This block is easy to handle and to store. In general, blocks of at least 1 g per unit, or indeed at 5 least 109 per unit, or indeed at least 50 g per unit or indeed at feast 500 g per unit may be produced. These blocks generally have a bulk density of at least 75%. or indeed at least 80%, or indeed at least 85% of the theoretical density, it being understood that the theoretical density is that corresponding to the same, material without any porosity. The block according to the invention is poly crystalline and
10 contains a multitude of grains, each of which is a small single crystal. A block generally contains at least 100 grams and even at least 1000 grains. No grain of the block represents more than 10% of the entire mass of the block.
The ammonium halide condensed on the cool downstream parts of the furnace may be at least partly reused, for exampte in the method according to the
15 invention
To measure the centerl of oxyhalides in a rare-earth halide, all that is required is to separate them using, water (for example at room temperature) since the oxyhatides are insoluble in water whereas the halides are soluble The oxyhalides may be recovered by filtration, for example over a polypropylene (PP)
20 filter and then dried at 120ºC. If the halide includes A (generally Rb or Cs), this method results in the dissolution of AX since A does not form an oxyhalide. This method, called the "dissolution method" of the 'method of insolubles", results, even in the presence of A in the halide, in a determination ot the content of oxyhalide of formula LnXO.
25 The block according to the invention may be used as raw material for
growing crystals (generally single crystals) using known techniques such as Bridgman growth or Kyropoulos growth or Czochralski growth, or growth using the gradient freeze method These single crystals are very pure and can be used as scintillates material. This crystal preparation is carried out in an inert atmosphere
so (nitrogen or argon for example), but this atmosphere may contain relatively large amounts of water and oxygen, that is to say in such a way that the sum of the water and oxygen masses in the gaseous atmosphere is fees than 200 pprn by
14
weight. In general, during production of the crysial (generally a single crystal), the water content of the inert atmosphere ranges from 10 to 180 ppm by weight and the oxygen content of the atmosphere ranges from 0.5 to 2 ppm by weight
Because pf the low surface area of the block or blocks used as raw material
5 and also because this surface ares increases during the rise in temperature up to
melting, the final single crystal is very pure and has a remarkable scintillation
efficiency. Thus, the invention also relates to a single crystal of formula AeLnfX^r+xxx
the symbols of which have the meanings given above, said single crystal
comprising less than 0.2% and even less than 0.1 % or indeed less than 0,05% or
10 indeed less than 0,02% by weight of rare-earth oxyhalicde. This applies especially
when Ln is chosen from La, Gd, Y, Lu and Ce, and when X is chosen from Cl and
Br Reference may more particularly be made to the following single crystals:
- those of general composition Ln1-xCexBr3 in which Ln is chosen from lanthanides or mixtures of lanthanides of the group La Gd, Y and Lu, especially
15 chosen from lanthanides or groups of lanthanides of the group La and Gd, and in which x is the molar degree of substitution of Ln wtth cerium, where x is greater than or equal to 0.01 mol% and strictly less than 100 mol%,
- those of general composition Ln1xCe2Cl3, in which Ln is chosen from tenthanides or mixtures of lanthianides of the group Y, La, Gd and Lu, especially
20 from elements or mixtures of elements of the group La, Gd and Lu, and in which x is the molar degree of substitution of Ln with cerium, where x is greater than or equal to 1 mol% and strictly less than 100 mol%.
The above mentioned growth methods may result in a large single crystal, that is to say one at least 1 cm3. or indeed at least 10 cm3 and even at least
25 200 cm3 in size. This single crystal may then be cut to the sizes suitable for the desired applications.
The single crystal according to the invention, because of its high purity, has a particularly high luminous efficiency, One way of measuring this luminous efficiency is to measure it relative to that of a crystal of Nal doped with 600 ppm by
30 weight of Tl lodide, the energy resolution of which at 622 keV is 6.8%, the integration time being 1 us and the radioactive source being 137Cs at 622 keV. The coupling between the crystals (Nal or rare-earth halide) and the photomultiplier
15
takes place by means of a silicons grease that is transparent up to 320 nm. Of course, the exit face of the Nal turned toward the photo multiplier is polished. Under these measurernent conditions, the invention makes it possible to obtain luminous efficiencies of at least 90% of that of the Tl-Nal crystal, and in any case 5 greater than those obtained on crystals not according to the invention.
The crystal or single crystal may especially be produced in a crucrbfe made of platinum or graphite or graphite coated with pyrolytic carbon
fn the examples that follow, the energy resolution was measured in the following manner a 10x10x5 mm piece is cut from the single crystal. All the faces
10 of the piece apart from one of the large 10x10 mm faces are left as cut and the face with which the photo multiplier (PMT) couples is polished. The crystal is wrapped in several thickness of PTFE (Teflon) tape except on the face that is coupled to the PMT. The crystal is prepared in a glove box, the dew point of which is below- 40ºC.
15
Example 1. anhydrous LaCl3
433 g of La2O3 were dissolved by 1330 ml of 37% HCI diluted in 2450 mi of water. 497 g of NH4CI were added. Next, the water and HCI in excess were evaporated by heating to. 100ºC in order to obtain an LaCl3(NH4Cl)3.5 complex, which
20 contained 0.7% by weight of water using the Karl Fischer measurement. The LaCl3-(NH4Cl)3.5 complex was a compound that did indeed have at least one Ln-X bond, since it contained La-CI bonds. It was also itself a mixture within the meaning of the invention, comprising, on the one hand, a compound having an Ln-X bond and, on the other hand. NH4X (in thrs case NH4Cl). Moreover, within this
25 mixture, the amount of NH4X is such that the ratio of the number of moles of NH4X to the number of moles of Ln not linked to oxygen is 3.5, which corresponds to a preferred ratio according to the invention. Moreover, it was unnecessary to include in our calculations for NH4X being introduced in the case of oxygen linked to Ln since the starting mixture did not contain this type of bond.
30 By heating at a rate of 200°C/hour from room temperature up to 950°C, 200 g of the complex were then decomposed with sublimation of the NH4CI and melted in a crucible made of graphite coated with pyrolytic carbon. said crucible itself being
16
placed in a seated silica lube with a nitrogen flush. The nitrogen atmosphere contained about 50 ppm by weight of water and between 1 and 2 ppm oxygen by weight. The oxycnloride content in the final block, measured by dissolution, was 0 01% by weight The water content was less than 0.1% by weight (the detection 5 limit of the method used) The block obtained had a mass of 651 g
Example 2 (comparative example): anhydrous LaCI3
The procedure was exactly as in the case of example 1, except that the compiex was replaced with an anhydrous LaCI3 powder whose oxychloride content was 10 less that 0.02%, the size of the particles was submilimetric and the water content was not detectable by Karl Fischer.
The oxychloride content in the final block, measured by dissolution, was 0.23% by weight. The water content was less than 0.1% by weight.
15 Example 3: anhydrous LaBr3 :CG
300 g of La2O3 were dissolved by 630 ml of 47% HBr diluted in 2330 ml of water. 662 g of NH4Br were added. The solution obtained was filtered over PP. The solution was then dried with a rotary evaporator In a 101 flask. The complex obtained, of formula LaBr3.(NH4Br)3.5, contained 0,23% water by weight measured
20 using Kart Fischer. Next. 142,6 g of this complex was removed and doped with 0,5% by weigh! of (NH4Br)3.5CeBr3, and this was heated at 200°C/h in a stream of nitrogen in a graphite crucible. The temperature was held at 660ºC for 4 h 30. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The melted pellet weighed 76.61 g and
25 contained only 0,035% oxybromide LaOBr (measured by the method of insolubles). The water content was also less than 0.1 %
The hydrostatic density of this block, measured by immersion in hexane, was 3pproximately 4 92 glcm3, i.e 87% of the theoretical density, thereby proving goud densification
30 This melted block was then used for growth in a Bridgman furnace in a graphite crucible under a stream of nitrogen. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of
17
oxygen. The crystal obtained was dear and free of white oxybromide inclusions and free of bubbles. The oxybromide content of this crystal was 0.05% by weight. More than 80% of the mass of this cryslal was suitable for use as a scintiflator.
5 Example 4: anhydrous La Br3 from a wet complex
The complex LaBr3(NH14Br)3.5 prepared as in the preceding example was used, but wetted so that it contained 14.7% by weight of water measured by Karl Fischer. 124 g of this mixture (complex + water) were removed and heated at 200ºC/h in a stream of nitrogen in a graphite crucible up to 860°C. The 10 temperature was held at 860°C for 4 h 30 The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The melted pellet weighed 64,1 g and contained only 0,034% by weight of oxybromide (measured by the method of insolubles). The water content was less than 0.1% by weight.
15
Example 6: anhydrous GdBr3
271.2 g of Gd2O3 were dissolved by 796 g of 48% HBr diluted in 430 g of water Next. 661 2 g of NH4Br and 855 g of water were added. The solution obtained was filtered over PP. The solution was then dried in a rotary evaporator in a 10 1 flask.
20 1164 g of the complex (NH4Br)4.5 GdBr3 were then obtained. The complex obtained contained 6.3% water measured by Karl Fischer. Next, 254.7 g of this complex were removed and heated at 200ºC/h in a stream of nitrogen in a graphite crucible. The temperature was held at 815°C for 1 h 30. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by
25 weight of oxygen The sintered but unmelted pellet weighed 104.9 g. It was therefore a pulverulent solid that was returned to the ambient conditions. The furnace was recharged with 92.7 g of the above sintered pellet and this was heated at 200°C/h under a stream of nitrogen in a graphite crucible. The temperature was held at 840cC for 1 h 30. The nitrogen atmosphere contained
30 approximately 50 ppm by wefght of wafer and between 1 and 2 ppm by weight of oxygen. The melted pellet weighed 92.7 g and contained 0 65% by weight of
18
GdOBr (measured by the method of in solubles), thereby demonstrating the fact that returning the block to room temperature before melting is contraindicated.
Example.7;anhydrous GdBr3
5 The complex (NH4Br) 5GdBr3 as prepared according to the preceding example was used for this test. The complex obtained contained 6.3% by weight of water measured by Karl Fischer. Next, 245.7 g of this complex were removed and healed al 200ºC/h in a stream of nitrogen in a graphite crucible up to 840r'C. The temperature was held at 840°C for 1 h 30. The nitrogen atmosphere contained 10 approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The meited pellet weighed 105.3 g and contained only 0.038% by weight of oxybromide GdOBr (measured by the method of insolubles). This result is all the more exemplary owing to the fact that gadolinium is a heavy (so-called yttric) rare earth whose bromides are very sensitive to hydration.
15
Example 8 (comparative example): single crystal from LaCl3 powder The same batch of anhydrous LaCl3 powder was used as that used for example 2 for 8ridgman furnace growth in a graphite crucible in a stream of nitrogen. The nitrogen atmosphere contained approximately 50 ppm by weight of water and 20 between 1 and 2 ppm by weight of oxygen. The crystal obtained had many white oxychloride inclusions and bubbles organized in the form of filaments along the pulling axis. The oxychloride content of this crystal was 0,25% by weight Approximately 90% of the mass of this crystal was unsuitable for use as a scintilator.
25
Example 9: anhydrous RbGd2Cl7
138.2 g of Rb2CO3 were dissolved by 242 g of 37% HCI diluted in 165 g of water. The solution obtained was filtered over PP. Next. 433.8 g of Gd2O3 were dissolved by 750 g of 37% HCI diluted in 482 g of water. After complete dissolution, the 30 filtered rubidium solution was added. Finally. 576.2 g of NF-UCI and 881 g of water were added. The solution obtained was filtered over PP The pH was - 0.32 and the density of the solution was 1.24. The solution was then dried in s rotary
19
evaporator in a 10 1 flask. 1227 g of (NH4CI)9Rb-Gd2Cl7 were then obtained. Next, 142 6 g of this complex were removed and heated at 200°C/h in a stream of nitrogen in a graphlte crucible up to 660ºC. The temperature was held at 660ºC at for 4 h 30. The nitrogen atmosphere contained approximately 50 ppm by weight of 5 water and between 1 and 2 ppm by weight of oxygen. The melted pellet contained only less than 0.05% by weight of GdOCI (measured by the method of insolubles).
Example 10: synthesis from LaOBr
The following mixture was produced in a glassy carbon crucible; 0,5374 g of 10 LaOBr. 1.3585 .g of NH4Br (i.e. 5.5 moles) and 10,0673 g of the (NH4Br)3.5LaBr3 complex. The mixture was heated at a rate of 200°C/h up to 830ºC. with a hold at this temperature for 2 h. The nitrogen atmosphere contained approximately 50 ppm by weight of water and between 1 and 2 ppm by weight of oxygen. The insoluble content in the final block was 0.19% by weight. 15
Example 11: LaCl3 single crystal
A 1 kg block of LaCI3 containing 10% CeCI3 by weight was used, this being
manufactured according to the invention and having an LaOCI content of less than
0.05% by weight. This block was then used for Bridgman-type growth in a graphite
20 crucible. The nitrogen atmosphere contained approximately 50 ppm by weight of
water and between 1 and 2 ppm by weight of oxygen. The crystal obtained was
very clear. Its oxychloride content measured by the method of insolubles was less
than 0.05%. Next, a piece measuring 10x10x5 mm was cut from this crystal and Us
scintillation efficiency compared with a piece of Nal:TI (Nal doped with 600 ppm by
25 weight of Tl iodide) using the following protocol:
Photomuitiplier: Hamamatsu R-1306:
Reference Nal crystal 50 mm in diameter and 50 mm in length.
Integration lime: 1 us
Radioactive source: 137Cs at 622 keV.
30 The light emission from me LaCl3 crystal was 93% of that of the Nal reference crystal. Its energy resolution was 3.6% The main component of the decay time of the scintillation was 27 nanoseconds.
20
Example 12 (comparative example): LaCl3 single crystal
1 kg of commercial LaCl3 and CeCl3 powders (LaOX and water contents of
example 2) were used. The mass of CeCl3 represented 10% of the mass of the
5 blend of these two powders. They were melted in a graphite crucible and
underwent growth of the Kyropouios (KC 01) type. The nitrogen atmosphere
contained approximately 50 ppm by weight of water and between 1 and 2 ppm by
weight of oxygen. The crystal obtained was slightly milky. Its content of fnsolubles
was 0.1% by weight. Next, a 10x10x5 mm piece was cut from this crystal and its
10 scintillation yield compared with a Nal:TI using the same protocol as in the
previous example. The light emission from the LaCl3 crystal was 83% of that of the
Nal reference crystal. Its energy resolution was 3.9%.
Example 13 (comparative example): LaCl3. single crystal
15 A crystal was produced by Bridgman-type growth in a silica crucible in accordance with the teaching of the publication IEEE Transactions on Nuclear Science: "Scintillation properties of LaCl3 crystals: Fast, efficient and High Energy resolution scintilfators". The mass of CeCl3 represented 10% of the mass of the mixture before growing the crystal. Next, a 10x10x5 mm piece was cut from this crystal
20 and its scintillation yield compared with a Nal Tl piece using the same protocol as in the two previous examples. The light emission from the LaCl3 crystal was 87% of that of the Nal reference crystal. Its energy resolution was 4.2%
Example 14: LaBr3 single crystal
25 Three 1 kg blocks of LaBr3 doped with 0.5% by weight of GeBr3 were used, each block manufactured according to the invention and such that the LaOBr content was 30 obtained was very clear. The oxychloride content of this block could not be measured by the method of insolubles Next, a 10x10.x 5mm piece was cut from
21
this crystal and its scintillation yield compared with a Nal:Tl piece according to the following protocol:
- Photomultiplier Hamamatsu R-1306
- Reference: Nal:Tl crystal (Nal doped with 600 ppm by weight of TI iodide) 5 50 mm in diameter and 50 mm in length;
- The energy resolution of this reference crystal was 6.8% on the 137Cs line:
- the measured crystals were wrapped in Teflon and coupled to the photo multiplier (PMT) using a sificone oil (EDM fluid 200);
- Integration time: 1 us
10 - Radioactive source: 137Cs at 622 keV
The light emission from the LaBr3 crystal was 147% of that of the Nal reference crystal. Its energy resofution was 4.2% The main component of the decay time of the scintillation was 39 nanoseconds.
15 Example 15 (comparative example): LaBr3 single crystal
A crystal obtained by Bridgman-type growth in a silica crucible according to the teachings of the publication "Applied Physics Letters of 03 September 2001 (Vol. 79, No. 10)" was compared with the previous trials. This crystal also contained 0.5% by weight of CeBr3. Next, a 10x10x5 mm piece was cut from this
20 crystal and its scintiflation yield compared with a Nal'TI piece using the same protocol as in the previous example. The crystal was slightly milky. The light emission from the LaBr3 crystal was 102% of that of the Nal reference crystal. The main component of the decay time of the scintillation was 38 nanoseconds.
Example 16: LaCl3 single crystal
Three 1 kg blocks of LaCl3 doped with 5% by weight of CeCI3 were used, each manufactured according to the invention and such that the LaOCl content was 22
measured by the method of insolubles. it was less than 0.05% by weight. Next, a 10x10x5 mm piece was cut from this crystal and its scintillation yield compared with a NaNTI piece according to the following protocol.
- Photomultipfier: Hamamatsu R-1306
5 - Reference: Nsl;TI crystal (Nal doped with 600 ppm by weight of Tl iodide)
50 mm in diameter and 50 mm in length;
- The energy resolution of this crystal was 6.8% on the 137Cs line;
- the measured crystals were wrapped in Teflon and coupled to the photomultipller (PMT) using a silicone oil (EDM fluid 200);
10 - Integration lime: 1 us
- Radioactive source: 137Cs at 622 keV
The light emission from the LaCl3 crystal was 98% of that of the Nal reference crystal Its energy resolution was 4.6%. The main component of the decay time of the scintillation was 28 nanoseconds.
15
Example 17: anhydrous LaCl3
This example was as in example 1 except that the blocK was prepared in 3 platinum crucible. The final block stuck to the crucible and was much more difficult to remove from the mold than in the case of the graphile crucible coated with
20 pyrolytic carbon.
23 CLAIMS
1. A method of preparing a polycrystalline block of at least 10 g of a halide of formula AeLn,X(3ftc) in which Ln represents one or more rare earths, X represents one or nioce halogen atoms chosen from Cl, Br or I, and A represents one or 5 more alkali metals such as K, Li. Na, Rb or Cs. e. which may be zero, being less than or equal to 3f. and f being greater than or equal to 1, said block comprising less than 0.1% by weight of water and less than 0.2% by weight of a rare-earth oxyhalide, characterized in that it comprises a step of heating a mixture of, on the one hand, at least one compound having at least one Ln-X bond and, on the
10 other hand, a sufficient amount of NH4X in order to obtain the desired oxyhalide content, said compound and NH4K possibly being combined, at least partially, within a complex, said step resulting in a molten mass comprising the rare-earth halide of formula AeLnrXj(3+t;), said healing step being to flowed by a cooling step after the molten mass has been obtained, and said heating step, after having
15 reached 300ºC never going back down below 200ºC before said moltert mass has been obtained,
2. The method as claimed in the preceding claim, characterized in that the compound having at least one Ln-X bond is of formula ArLnsOuXr+3s-2u in which A, X and Ln have the meanings given above, r,s and u representing integer or
20 noninteger values that meet, cumulatively, the following conditions:
r ranging from 0 to 2s, s being greater than or equal to 1, u ranging from 0 to s, which compound may or may not be complexed with water or with NH4X,
25 3, The method as claimed in eiher of the preceding claims, characterized in that the Ln atoms of the compound are finked to X or oxygen or A atoms. 4, The method as claimed in the preceding claim, characterized in that the heating step is carried out without lowering the temperature before the molten mass is obtained.
30 5. The method as claimed in one of the preceding claims, characterized in that the heating step includes a temperature hold owing to the elimination of the NH4X in gaseous phase.
24
6. The method as claimed in the preceding claim, characterized in that the heating step is carried out with a rate of temperature rise of greater than 50ºC/hour after said temperature hold.
7 The method as claimed in the preceding claim, characterized in that the 5 healing step is carried out with a rate of temperature rise of greater than
100°C/hour after said temperature hold.
8 The method as claimed in the preceding claim, characterized in that the heating step is carried out with a rate of temperature rise of greater than 150ºC/hour after said temperature hold.
10 9. The method as claimed in one of the preceding claims, characterized in that the heating step lasts less than 10 hours.
10. The method as claimed in the preceding claim, characterized in that the heating step lasts less than 6 hours,
11. The method as claimed in the preceding claim, characterized in that the 15 heating step lasts less than 4 hours
12. The method as claimed in one of the preceding claims, characterized in that the heating step is carried out in an inert gas atmosphere, the water and oxygen contents of which are such that the sum of the water and oxygen masses in the gaseous atmosphere is less than 200 ppm by weight.
20 13. The method as claimed in the preceding claim, characterized in that the water content of the inert atmosphere ranges from 10 lo 180 ppm by weight and the oxygen content of the atmosphere ranges from 0,5 to 2 ppm by weight. 14. The method as claimed in one of the preceding claims, characterized in that the amount of NH4X is at least the sum of the following two quantities:
25 - A) a number of motes of NH4X equal to one times the number of moles
of Ln that are not linked to an oxygen;
- B) is a number of moles of NH4X equal to three times the number of moles of oxygen atoms linked to Ln.
15 The method as claimed in the preceding claim, characterized in that the 30 amount of NH4X is at least the sum of the two following quantities:
- A) a number of moles of NH4X equal to three times the number of moles of Ln that are not linked to an oxygen:
25
- B) a number of moles of NH4X equal to five times the number of moles
of oxygen atoms linked to Ln.
16. The method as claimed in one of the preceding claims, characterized in that
the heating step is carried out in a crucible made of a material containing at least 5 20% carbon by weight
17 The method as claimed in the preceding claim. characterized in that the
crucible is made of carbon or glassy carbon or graphite.
13. The method as claimed in either of the preceding two claims, characterized in
that the crucible is coated with a layer of pyrolylic carbon. 10 19. A polycrystaltine block of at least 1 g of halide of formula AgLn1X(3f+e) in which
Ln represents one or more rare earths, X represents one or more halogen atoms
chosen from Cl, Br or I. and A represents one or more alkali rnetais such as K, Li,
Na, Rb or Cs, e and f representing values such that:
e, which may be zero, is less than or equal to 3f; 15 f is greater than or equal to 1;
comprising less than 0.1% by weight of water and less than 0.2% by weight of
rare-earth oxyhalide
20. The block as claimed in the preceding claim, characterized in that It
comprises less than 0.1% by weight of rare-earth oxyhalide. 20 21. The block as claimed in the preceding daim. characterized in that it
comprises less than 0.05% by weight of rare-earth oxyhalide.
22 The block as claimed in the preceding claim, characterized in that it
comprises less than 0.02% by weight of rare-earth oxyhalide.
23. The block as claimed in one of the preceding block claims, characterized in 25 that it weighs at least 10 g.
24. The block as claimed in the preceding claim, characterized in that it weighs at least 50 g.
25. The block as claimed in one of the preceding block claims, characterized in that it has a bulk density of at least 75% of the theoretical density corresponding
30 to the same material with no porosity.
26. The block as claimed in one of the preceding block claims. characterized in that Ln is La or Ce and X is Cl or Br.
26
27. The block as claimed in one of the preceding block claims, characterized in that none of its grams represents more than 10% of the mass of the entire block.
28. The method as claimed In one of claims 1 to 18, characterized in that the block is one of those of claims 19 to 27.
5 29 A block that can be obtained by the method of one of claims 1 to 18 and 28.
30. A method of preparing a crystal comprising the melting of at least one block of one of the preceding block claims.
31. The method as claimed in the preceding claim, characterized in that the crystal is a single crystal.
10 32, A single crystal obtained by melting a block of one of the preceding block claims.
33. A single crystal of formula A>LnrX(3f+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl, Br or I. and A represents one or more alkali metals such as K, Li. Na. Rb or Cs, e and f
15 representing values such that:
- e, which may be zero, is less than or equal to 3f,
f is greater than or equal to 1; comprising less than 0.1% by weight of rare-earth oxyhalide.
34. The single crystal as claimed in the preceding claim, characterized in that its 20 oxyhalide content is less than 0.05% by weight.
35. The single crystal as claimed in the preceding claim, characterized in that its oxyhalide content is less than 0.02% by weight.
36. The single crystal as claimed in one of the preceding single-crystal claims, characterized in that Ln is chosen from La. Gd. Y, Lu and Ce, and X is chosen
25 from Cl and Br.
37. The single crystal as claimed in one of the preceding single-crystal claims. characterized in that its volume is at least 10 cm3.
38. The single crystal as claimed in one of the preceding single-crystal claims, characterized in that its luminous efficiency is at least 90% of that of a Wai crystal
30 doped with 600 pptn by weight of Ti iodide, the energy resolution of which at 622 keV is 6.8%, the integration lime being 1 us and the radioactive source being 137Cs at 622 keV.
27
39. The single crystal as claimed in one of the preceding single-crystal claims, characterized in that is energy resolution is less than 5%.
40. The single crystal as claimed in the preceding claim, characterized in that its energy resolution is less than 4%
5 41. The single crystal as claimed in the preceding claim, characterized in that its energy resolution is less than 3.5%
42. The single crystal as claimed in one of the preceding single-crystal claims. characterized in that the time for the main component to decay is less than
10 43. The single crystal as claimed in the preceding claim, characrerized in that the time for the main component to decay is less than 30 nanoseconds.
44. The single crystal as claimed in the preceding claim, characterized in that the time for the main component to decay is less than 20 nanoseconds.

The invention relates to a method of preparing a polycrystaline block of a halde of formula AeLn4X(3+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms chosen from Cl. Br or 1, and A represents one or more alkali metals such as K. Li, Na, Rb or Cs, e, which may be zero, being less than or equal to 3f, and f being greater than of equal to 1, having a low water and oxyhalide content, comprising a step of heating a mixture of, on the one hand, at feast one compound having at least one Ln-X bond and, on the other hand, a sufficient amount of NH4X in order to obtain the desired oxyhalide content, said step resulting in a molten mass comprising the rare-earth halide, said heating step being followed by a cooling Step, and the heating step, after having reached 300ºC, never going back down below 200ºC before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having rennarkable scinimation properties to be grown.

Documents:

01238-kolnp-2005-abstract.pdf

01238-kolnp-2005-claims.pdf

01238-kolnp-2005-description complete.pdf

01238-kolnp-2005-form 1.pdf

01238-kolnp-2005-form 3.pdf

01238-kolnp-2005-form 5.pdf

01238-kolnp-2005-international publication.pdf

1238-KOLNP-2005-(05-03-2007)- CLAIMS.pdf

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Patent Number

255304

Indian Patent Application Number

1238/KOLNP/2005

PG Journal Number

07/2013

Publication Date

15-Feb-2013

Grant Date

11-Feb-2013

Date of Filing

28-Jun-2005

Name of Patentee

SAINT-GOBAIN CRISTAUX ET DETECTEURS

Applicant Address

18, AVENUE D'ALSACE, F-92400 COURBEVOIE, FRANCE

Inventors:

#	Inventor's Name	Inventor's Address
1	ILTIS, ALAIN	7,RUE DES CORVEES, F-77690 MONTIGNY SUR LOING. FRANCE

PCT International Classification Number

C09K 11/00

PCT International Application Number

PCT/FR2003/003556

PCT International Filing date

2003-11-13

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	02/14856	2002-11-27	France