PROCESS FOR PRODUCING A HALIDE MONOCRYTAL WITH A POLYGONAL SECTION

Abstract

Process for the crystal growth of a single crystal made from a halide material comprising at least one element from the rare earth group, in a vertical melting / recrystallization furnace, in which said melting / recrystallization is carried out from a powder of the material constituting the crystal in an ampoule having a polygonal section over at least part of its length, such that the single crystal finally obtained has this same polygonal section.

Description

Title of the invention: METHOD FOR PRODUCING A HALIDE MONOCRYTAL WITH A POLYGONAL SECTION

[0001] The invention relates to a method for manufacturing a single crystal made of a halide material, in particular for application as a scintillator material.

[0002] Scintillant materials are widely used for the detection of gamma or X-rays as well as energetic particles such as alpha particles, neutrons, etc.

[0003] Scintillation is based on the physical principle of converting the energy of incident photons, X-rays, gamma rays, or energetic particles into light in or near the visible range. In particular, inorganic scintillators are preferably monocrystalline. A monocrystalline allows for easier extraction of light due to the absence of internal scattering through grain boundaries, heterogeneities, and other defects present in polycrystalline materials that impact the scintillation mechanism. A state of good crystallinity (without crystal defects or impurities) is necessary because it determines the most efficient scintillation mechanisms.

[0004] Furthermore, these non-monocrystalline materials are, for most known compositions, too opaque beyond a certain thickness, which is otherwise insufficient to sufficiently stop incident energetic particles or photons. Generally, a monocrystalline scintillator material is therefore preferred if not mandatory for such applications. The detection of gamma photons, alpha or beta particles, charged particles or neutrons is of major interest in many applications in nuclear medicine, fundamental physics, for security scanners for baggage (detection gates), continuous control of containers and trucks at borders, geophysical diagnostic systems in mines and for oil drilling, etc.

[0005] The classic approach of the person skilled in the art seeking to manufacture a single crystal is to carry out crystallization from a molten bath whose composition corresponds to the stoichiometry of the targeted crystal. The single-crystalline material obtained can also be called an ingot.

[0006] The growth of single crystals from melts can be carried out both in the laboratory and on an industrial scale by a few well-known and mastered methods of the art and often named after the name of their inventor: Czochralski, Kyropoulos, vertical or horizontal Bridgman, in particular vertical Bridgman-Stockbarger or horizontal Bridgman / Bagdassarov or Slab / Exchange of heat, Stepanov method also known as EFG (Edge-Feed-Growth), TGT (Temperature Gradient Technique).

[0007] The following reference publications describe such methods, which are otherwise well known to those skilled in the art of crystal growth:

[0008] G. Dhanaraj, K. Byrappa, V. Prasad, M. Dudley. Springer handbook of crystal growth. 01 Jan 2010. 001:10.1007 / 978-3-540-74761-1

[0009] Th.Duffar. Crystal Growth Processes Based on Capillarity: Czochralski, Floating Zone, Shaping and Crucible Techniques. J. Wiley-Blackwell. 2010. DOI: 10.1002 / 9781444320237.chô

[0010] The general characteristic of all these methods is the slow solidification of the melts under a thermal gradient, most often from a seed, leading to a single structure of a crystal comprising a minimum of crystallographically disoriented grains or blocks and in particular a single large monocrystal.

[0011] Due to the need to promote the unique crystallization mechanism during the In a crystal growth process, the liquid / solid interface is placed under a controlled thermal gradient, generally aimed at being as strong as possible. This thermal gradient is also responsible for the segregation of impurities at the interface and is therefore also a fundamental reason for the appearance of mechanical stresses in a solidified material.

[0012] Crystal ingots obtained by all the methods mentioned above have in the raw state a more or less cylindrical shape (apart from the Stepanov and EFG methods) for the usable part (apart from the crystal seed and heel) due to the rotational symmetry of the growth elements, regardless of whether rotation during growth is applied (Czochralski, for example) or not (Bridgman-Stockbarger Vertical). Deviations from the circular shape are mainly due to differences in growth rates along the axes or to the growth mechanism at the periphery (face growth), in an uncontrolled manner, and are generally considered as factors of disturbance of the growth process or at least as disadvantages compared to the desired result.

[0013] The Stepanov and EFG methods can produce crystals with fairly complex shapes but for metals or chemically simple oxide compositions (A12O3, Si, Ga2O3 etc). In particular, for the solidification of more complex oxide crystalline phases (for example Bi4Ge30i2) or even halides, the EFG method does not allow the growth of good quality crystals, certainly due to a differentiated segregation effect of the main compounds in the narrow feed channels of the molten bath. The EFG method is also difficult to adapt to the growth of mechanically fragile crystals.

[0014] Furthermore, the final elements obtained from these cylindrical ingots must most often have the shape of slabs or other parallelepiped shapes, in particular rectangular parallelepipeds, and more generally a polygonal section, to gain in cutting efficiency of the final parts, for example rectangular pixels for the PET (Positron Emission Tomography) application. The cylindrical shapes of crystals conventionally initially obtained lead to heavy losses of materials and time due to cutting and machining to obtain the desired final shape. In addition, some crystals are brittle and do not withstand the machining process well. They can therefore often crack during this process, which considerably reduces yields.

[0015] For example, if we consider a single crystal whose usable part, as obtained after growth, has a diameter of 5 cm and a length of 10 cm, it is possible to obtain detectors of diameter 5 cm and length 5 cm by a simple cross section. However, for certain applications, for example medical or geophysical systems, crystals in the form of slabs 5 cm long and whose section is square, with 2.5 cm on each side, can be sought. From a cylindrical ingot previously described, only two of these detectors can be extracted, which means that only half of the single-crystal material can be exploited, with a lot of waste.

[0016] The present invention aims to remedy the preceding problems and in particular to propose a method for manufacturing a single crystal whose dimensions are adapted for its use, without excessive loss of material or risk of deterioration of the crystal during a possible phase of modification of the shape thereof.

[0017] To this end, the subject of the present invention relates in particular to a method for the crystal growth of a single crystal made from a halide material comprising at least one element from the rare earth group, in a vertical melting / recrystallization furnace comprising the following steps: a. a powder of the material constituting the crystal is introduced into an ampoule, preferably in the form of a sintered powder, possibly in the presence of a seed of said crystal placed at the bottom of the ampoule, b. said bulb is placed in said vertical furnace, comprising at least two independent zones heated by heating elements according to a temperature gradient, including an upper zone, in which the heating elements are adjusted in such a way that the temperature is higher than the melting temperature of the material constituting the crystal, and a lower zone in which the heating elements are adjusted in such a way the temperature is lower than said melting temperature, said zones being separated by an insulating ring or baffle, c. we translate said temperature gradient, i. either by the translation of said bulb, the oven being fixed, ii. either by translation of said oven, the bulb being fixed, iii. either by controlling the temperatures of the heating elements;

[0018] the translation speed of the gradient being adapted to allow the growth of said single crystal in said bulb, a. the furnace is cooled and a single crystal is recovered in said bulb,

[0019] said method being characterized in that said bulb has a polygonal section over at least part of its length, such that the single crystal finally obtained has this same polygonal section.

[0020] Advantageously, said crystal growth method is a method chosen from the vertical Bridgman techniques, in particular Brigman-Stockbarger, Temperature Gradient Technique, preferably vertical Bridgman or Bridgman-Stockbarger.

[0021] For the purposes of the invention, the term “rare earth” means a group consisting of the fifteen elements of the lanthanides, scandium and yttrium.

[0022] The method according to the invention is characterized in that said bulb has a polygonal section, over at least part of its length, such that the single crystal finally obtained has this same polygonal section.

[0023] It is thus possible to recover a single crystal of polygonal section, adapted to its potential and different applications and uses.

[0024] According to other advantageous but non-limiting embodiments of the present invention, which can be combined with each other where appropriate: - The said bulb has a square section. - The said bulb has a hexagonal section. - The said bulb has a polygonal section over its entire part passing through between said two independent zones during said translation and preferably has a polygonal section over its entire length. - The heating elements are arranged and configured in such a way that the temperature is uniform around the periphery of said bulb at the level of the insulating ring. - Heating elements are arranged opposite the flat faces of said bulb. - The heating elements are arranged at equal distance from the flat faces of the said bulb. - The heating elements are electrical resistors, halogen lamps or laser diode arrays, preferably halogen lamps. - In said vertical furnace, the independent upper and lower zones are divided into sub-zones, each zone being independently temperature-controlled to control the thermal gradient within the furnace. - The material constituting the monocrystal is chosen from the group consisting of CLLB (Cs2LiLa(Br)6:Ce), LaBr3:Ce, LaBr3:Ce:Sr, La(Br,I)3:Ce, La(Br,I)3:Ce:Sr, CLLBI (Cs2LiLa(Br,I)6:Ce), CeBr3:Ca,Ce(Br,I)3:Ca, SrI2:Eu, SrI2:Eu:Sm, preferably CLLBI or CLLB.

[0025] The invention also relates to a bulb for the crystallization of a crystal in a vertical furnace, in particular a vertical Bridgman furnace, said bulb having a polygonal section, over at least part of its length.

[0026] Said bulb is advantageously of square or hexagonal section.

[0027] Said bulb is advantageously made of quartz.

[0028] The invention thus resides in a technological solution making it possible to grow crystals whose section differs from the classic circular shape, using at low cost the vertical crystal growth technique known as Bridgman and in particular Bridgman-Stockbarger.

[0029] It consists more precisely of the modification of the following elements of the Bridgman-Stockbarger method, advantageously in combination:

[0030] - a. Modification of the shape of the crucible

[0031] The Bridgman-Stockbarger method is a method involving the use of a crucible taking the shape of an ampoule, which implies that the final ingot takes the internal shape of the crucible. Therefore, the first modification according to the invention consists of reshaping the crucible (ampoule) to directly obtain the desired final shape of the crystal.

[0032] - b. Rearrangement of heating elements

[0033] In the standard Bridgman-Stockbarger technique, the heating elements and in general the thermal configuration have rotational symmetry, which allows homogeneous heating of the crucible along all its sides. However, the use of a polygonal crucible, for example square, does not allow this homogeneity because certain parts of the crucible (or the molten mass contained therein) will be closer to the heating elements (such as its corners) and therefore hotter, while other parts (such as the lateral parts of the polygon) will be comparatively colder. The rearrangement and remodeling of the thermal installation and in particular of the heating elements is advantageously implemented according to the invention to ensure uniform and in particular isothermal heating according to the straight sections of said bulb in a horizontal plane corresponding to its shape.

[0034] This gives us, compared to conventional crystal growth techniques of the vertical Bridgman, Bridgman-Stockbarger or TGT type, the following advantages:

[0035] - The crystals produced by the technique according to the invention have a clear shape with a geometry other than cylindrical.

[0036] - It is possible to produce from such crystals final elements in the form of slab with a better yield of crystal growth. For the example given previously, the growth of a crystal with a length of 10 cm and a square section of a little more than 5 cm (to take into account the cutting) makes it possible to obtain 8 detector crystal elements with a square section of 2.5 cm on each side and a length of 5 cm (instead of two from the traditional cylindrical ingot previously described) and thus to multiply by 4 the yield of each crystal growth.

[0037] - The method allows the manufacture of final elements and detectors of any shape, especially parallelepiped, especially rectangular, especially for crystals that do not easily tolerate cutting and machining steps.

[0038] - In some cases, the method allows the phases to be completely eliminated machining, or even shaping, because the crystals acquire a clear shape from their growth phase. Simple polishing can be advantageously implemented to smooth the flat surfaces of the crystal.

[0039] The present precedent is applicable to the crystal growth of any crystal, in particular of single crystals usually obtained by the Bridgman-Stockbarger technique, in particular with congruent fusion. Examples include, but are not limited to, the following scintillating single crystals and their derivatives: LaBr3:Ce, LaBr3:Ce:Sr, La(Br,I)3:Ce, La(Br,I)3:Ce:Sr, CeBr3:Ca, Ce(Br,I)3:Ca, SrI2:Eu, SrI2:Eu:Sm and compositions exhibiting non-congruent fusion: CLLB ((Cs2LiLa(Br)6:Ce), CLLBI (Cs2LiLa(Br,I)6:Ce), CLLBC (Cs2LiLa(Br,Cl)6:Ce), CLYC (Cs2LiY(Cl)6:Ce), obtained in particular in the presence of a self-flux (excess LiCl, LiBr or LiI in the composition of the crystal) or a flux (e.g. NaCl, NaCl, NaI).

[0040] The Li and / or Li-containing components advantageously comprise an amount enriched in 6Li, with reference to its natural abundance (approximately 7.6%).

[0041] The implementation of a method according to the present invention is particularly advantageous for obtaining single crystals used in the field of scintillation and in particular for the detection of gamma or X rays as well as energetic particles such as alpha particles, neutrons, etc.

[0042] Among the preferred materials for such an implementation, we can notably cite CLLB ((Cs2LiLa(Br)6:Ce), CLLBI (Cs2LiLa(Br,I)6:Ce), and in particular the compositions CLLBI, Cs2LiLai_xCex(BriyIy)6, where 0 <x<0,2 et 0<y<0,l.

[0043] For example, obtaining CLLBI phase crystals according to the invention comprises melting and recrystallization from the components: CsBr, LaBr3, CeBr3 and Lil in the absence of Lal3 or Cel3.

[0044] The advantages of the present invention are illustrated by the following exemplary embodiments, it being understood that the present invention is not limited to these examples, in any of the aspects described.

[0045] In these examples, two Ce-doped CLLB crystals (hereinafter referred to as CLLB) having respectively square and hexagonal sections are synthesized according to the present method.

[0046] The crystal growth of CLLB is carried out from powders introduced into a vacuum-sealed ampoule to which a heat treatment is applied in a Bridgman-Stockbarger type furnace. At the end of this heat treatment, the ampoule is recovered and cut in order to extract the crystal obtained. The seed and the heel of the ingot are then cut. The synthesis of the single crystal is finalized by polishing the sides and in particular the optical face of the crystal. The last step consists of measuring the scintillation performance of the crystal.

[0047] More precisely, the single crystals are obtained according to the succession of the following steps:

[0048] CLLB powders are obtained chemically from the following materials and materials, mixed in the proportions leading to the formation of said CLLB, for example according to the proportions described in application WO2014 / 167262, as reported in table 1 below:

[0049] [Tables 1] Material Chemical formula Lanthanum bromide LaBr3 Cesium bromide CsBr Lithium bromide LiBr Cerium bromide CeBr3

[0050] The anhydrous powders are mixed and placed in a quartz crucible and then heat-treated in a furnace under an inert atmosphere of a rare gas such as argon up to the formation temperature of the CLLB halide material, but without going as far as its melting. The CLLB powder obtained, partially sintered, is then ground in a glove box.

[0051] The CLLB powder is introduced from a glove box into the two quartz ampoules shown in [Fig.l] attached.

[0052] As shown in [Fig.l], the bulbs 20 and 21 according to the invention are not cylindrical but have a polygonal section on at least the portion of the bulb involved in the effective growth of the single crystal, that is to say at least on the part of the bulb whose internal walls will be in contact with the useful part of the growing crystal.

[0053] The bulbs can be made of quartz glass to ensure optimal detachment of the crystal, in particular by prior deposition of a thin layer of carbon deposit on the internal walls of the bulb. The deposition can be ensured for example by means of an electric discharge, the graphite electrode creating a cloud of plasma, under low pressure of an Argon atmosphere.

[0054] The bulb 20 according to the first embodiment has a square section over substantially its entire length while the bulb 21 according to the second embodiment has a hexagonal section over the portion of its length over which the useful part of the single crystal will be recovered at the end of melting.

[0055] These embodiments are contrary to the conventionally used bulbs, of cylindrical shape. It is in fact generally accepted that crystal growth of the vertical Bridgman type, in particular Bridgman-Stockbarger, must be carried out in bulbs having this geometry, in particular to avoid angular zones generating stresses during crystallization and consequently more likely to generate cracks in the crystal finally obtained.

[0056] The CLLB powder is then sintered, applying a thermal cycle to the prepared ampoule. A quartz stopper then allows the ampoule to be sealed after extraction of the atmosphere under vacuum.

[0057] The ampoule thus synthesized, containing the sintered CLLB powder, is then introduced into the central part of a Bridgman-Stockbarger furnace.

[0058] In a known manner, as illustrated by [Fig.2], the vertical Bridgman furnaces 100 are furnaces composed of two independent heating zones delimited by refractory materials 5, and generally reflecting in the infrared, including a high zone 1, in which the temperature is higher than the melting temperature of the compound from which it is sought to make a single crystal (CLLB here) and a low zone 2 in which the temperature is lower than said melting temperature. These two heating zones are separated by an insulating ring 3 called a baffle, also made of refractory material. The temperature control in the two zones is obtained by a plurality of heating elements 7 placed specifically in said two zones in order to obtain a uniform temperature at any point of the perimeter of the bulb and at a given height of the furnace, in particular at the level of the insulating ring, that is to say near the liquid-solid interface of crystal growth. Preferably, these heating elements are halogen lamps having a maximum emission at a wavelength between 0.5 and 1.5 micrometers, for example at 1.1 micrometers. The heating elements 7 are connected to thermocouples (not shown) for their control. A translation mechanism (not shown) connected to a pull rod 4 makes it possible to move the growth bulb from the upper zone 1 to the lower zone 2.

[0059] As shown schematically in [Fig.2] attached, the crystal growth technique carried out in this type of furnace can be broken down into several stages, each illustrated by a schematic representation in [Fig.2].

[0060] Step 1: The bulb 20 (or 21), loaded with the sintered powder 8 of CLLB is placed on the drawing rod 4, in such a way that its lower part 6, which advantageously comprises a crystalline seed of CLLB, is arranged at the level of the ring 3.

[0061] Step 2: Heating the upper zone 1 of the furnace by the heating elements 7 to a temperature T higher than the melting temperature Tfusion, allows the melting of the powder 8 and the passage to the liquid state 9 of the charge of the bulb 20 (21).

[0062] Step 3: The bulb is translated towards the lower zone 2, where T < Tfusion, which allows the liquid to crystallize with the appearance of a crystal 10, the crystallization zone being located substantially in the middle of the baffle 3. The translation speed of the bulb is chosen between 0.5 mm / h and 1.0 mm / h depending on the ingot size.

[0063] Step 4: The translation of the bulb towards the lower zone, where T < Tfusion, stops when all of the liquid has crystallized. The last step consists of cooling the assembly to room temperature and opening the bulb 20, 21 by cutting it using a blade or the diamond wire of a saw to recover the single crystal 12.

[0064] According to an advantageous characteristic of the present invention, the heating elements 7 are rearranged to ensure homogeneous heating at all points of the bulb and therefore of the newly formed crystal to avoid excessive stresses, as illustrated by [Fig.3] commented on below. These heating elements 7 can be of any known type: heating resistors, halogen lamps, laser / diode networks. In the case of vertical Bridgman, Bridgman-Stockbarger, or TGT furnaces, the use of halogen lamps facilitates obtaining such homogeneity because it allows easy rearrangement of the heating elements within the furnace compared to the conventional configuration.

[0065] In the attached [Fig. 3], the positioning of the heating elements with respect to the walls of the square-section bulb is shown in section, so as to obtain at any point of this section plane a homogeneous temperature at any point of the bulb. More precisely, the heating elements are preferably arranged opposite the flat faces of the square-section bulb 20.

[0066] Other crystals of different compositions and shapes were synthesized by a method according to the present invention, as reported in Table 2 below:

[0067] [Tables2] Example Crystal Composition Flux Section 1 CLLB LiBr square 2 CLLB LiBr hexagonal 3 CLLBI Lil square 4 CLLBI Lil hexagonal 5 CLLBC LiCl hexagonal 6 CLYC LiCl square 7 Srl2:Eu Fluxless square 8 Srl2:Eu:Sm Fluxless hexagonal 9* CeBr3:Ca LiBr hexagonal 10* Ce(Bro.97do.o3):Ca Lil square 11 LaBr3:Ce LiBr hexagonal 12 LaBr3:Ce:Sr LiBr hexagonal 13 La(Br0.98,10.02)3:Ce Lil square 14 LaBr3:Ce NaBr hexagonal 15 La(Br0.98,10.02)3:Ce Nal square

[0068] *The crystals were synthesized in a quartz ampoule with a protective carbon layer on the inner surface.

[0069] The single crystals finally obtained are not cracked and have a polygonal shape perfectly matching that of the internal part of the bulbs used. In addition, the side walls of the single crystals, synthesized in contact with the quartz walls of the bulb, appear completely smooth, without the need for machining.

[0070] The photographs of the crystals thus obtained are shown in [Fig. 4] attached to this description. It can be seen that their section corresponds perfectly to that of the internal part of the bulb and that their walls are smooth and transparent and do not present any defects.

[0071] Scintillation performance measurements of the obtained polygonal CLLB crystals are carried out and compared with those of cylindrical crystals obtained by a conventional method.

[0072] For this purpose, a pulse height analyzer was used to measure the scintillation performance of crystals under gamma radiation. Such an instrument records electronic pulses of different heights from particle and event detectors, digitizes the pulse heights, and records the number of pulses of each height in registers or channels, thus recording a "pulse height spectrum."

[0073] The scintillation intensity was recorded at room temperature in a glove box using a 137Cs gamma source at 662 keV. A Photonix APD avalanche photodiode (type 630-70-72-510) without a window, under 1600 V of voltage and cooled to 250 K, was used as a photodetector. The output signal was amplified with shaping time conditions of 6 ps by an ORTEC 672 spectroscopic amplifier. In order to maximize light collection, the samples were wrapped in Teflon powder and then compressed (according to the technique described in J.T.M. de Haas and P. Dorenbos, IEEE Trans. Nucl. Sci. 55, 1086 (2008)), except for the cleaved face intended for coupling with the photodiode. When exposed to a high-energy source, the crystal produces photons that are detected by a photomultiplier regardless of the photon's wavelength. The detector used is sensitive from UV to IR and can count each photon.In this way, a scintillation histogram is obtained, with on the abscissa values proportional to the quantity of emitted light detected by the optical device (measured with a 137Cs isotopic source with an Advanced Photonix APD 630-70-72-510 detector, said detector being at a temperature of 270K), and on the ordinate the number of gamma photon interaction events with the scintillator. According to this experiment, the more the scintillation peak is observed with a high number of channels, the higher the number of photons emitted per pulse.

[0074] It was thus possible to verify that the scintillation performances of the crystals obtained by the process according to the invention, and of polygonal section, present performances comparable to those obtained for CLLB crystals, which allows us to conclude that there is an identical crystalline quality.

Claims

1. Claims A method of crystallizing a single crystal made from a halide material comprising at least one element from the rare earth group, in a vertical melting / recrystallization furnace, comprising the following steps: a) a powder of the material constituting the crystal is introduced into an ampoule, preferably in the form of a sintered powder, possibly in the presence of a seed of said crystal placed at the bottom of the ampoule, b) said bulb is placed in said vertical furnace, comprising at least two independent zones heated by heating elements according to a temperature gradient, including an upper zone, in which the heating elements are adjusted in such a way that the temperature is higher than the melting temperature of the material constituting the crystal, and a lower zone in which the heating elements are adjusted in such a way that the temperature is lower than said melting temperature, said zones being separated by an insulating ring or baffle, c) we translate the temperature gradient, i. either by translation of said bulb, the oven being fixed, ii. either by translation of said oven, the bulb being fixed, iii. either by controlling the temperatures of the elements heating; the translation speed of the gradient being adapted to allow the growth of said single crystal in said bulb, d) the furnace is cooled and a single crystal is recovered from said bulb, said method being characterized in that said bulb has a polygonal section over at least part of its length, such that the single crystal finally obtained has this same polygonal section and in that the vertical furnace comprises heating elements uniformly surrounding the bulb, and in that the crystal is selected from the group consisting of CLLB (Cs2LiLa(Br)6:Ce), LaBr3:Ce, LaBr3:Ce:Sr, La(Br,I)3:Ce, La(Br,I)3:Ce:Sr, CLLBI

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10. (Cs2LiLa(Br,I)6:Ce), CeBr3:Ca,Ce(Br,I)3:Ca, SrI2:Eu, SrI2:Eu:Sm, preferably CLLBI or CLLB. Crystal growth method according to claim 1 wherein the crystal growth is obtained using a technique chosen from vertical Bridgman, in particular Brigman-Stockbarger, TGT (Temperature Gradient Technique). Method according to one of claims 1 or 2, in which said bulb has a square section. Method according to one of claims 1 or 2, in which said bulb has a hexagonal section. Method according to one of the preceding claims, in which said bulb has a polygonal section on at least its part passing between said two independent zones during said translation. Method according to one of the preceding claims, in which the heating elements are arranged and configured in such a way that the temperature is homogeneous around the periphery of said bulb at the level of the insulating ring. Method according to one of the preceding claims, in which heating elements are arranged facing the flat faces of said bulb. Method according to one of the preceding claims, in which the heating elements are arranged at equal distance from the flat faces of said bulb. Method according to one of the preceding claims, in which the heating elements are electrical resistors, halogen lamps or laser diode arrays. Method according to one of the preceding claims, in which the independent upper and lower zones are divided into sub-zones, each zone being independently temperature-controlled to control the thermal gradient within the furnace.

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