DE2629650A1 - METHOD AND DEVICE FOR GROWING HGI DEEP 2 CRYSTALS - Google Patents

Description

United States Energy Research and Development Administration, Washington, D.C. 20545, U.S.A.

Method and device for growing HgI ₂ ~ crystals

The invention relates to crystals of mercury iodide and more particularly to a method and apparatus for growing such crystals to make larger, more uniform ones and to create more perfect crystals which are particularly useful in radiation detectors.

Mercury iodide crystals have been proposed for use in high atomic number radiation detectors operable at room temperature. The main difficulties with HgI ₂ , however, are its structural imperfections, etc., with much effort being made to provide higher purity raw material and / or methods of crystal growth. For example, in this context, reference is made to the following articles: Journal of Crystal Growth, 24/25, 205-211 (1974) by M. Schieber UA; Philips Tech. Rev., 28, 316-319 (1967) by H. Scholz; Acta Electronica, 17, 69-73 (1974) by H. Scholz; IEEE Transactions on Nuclear Science NS-22, No. 1 (1975) by ZH Cho UA; Crystal Growth Conference,

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Supplement to Physics and Chemistry of Solids, Pergammon Press, pages 475-482 (1967) by H. Scholz et al.

The nucleation and growth of mercury iodide (Hgl_) crystals of radiation detector quality is achieved by the present invention. In general, the process according to the invention for the growth of HgI “crystals forms a combination of known individual evaporation growth processes: a) growth through slight supersaturation and / or undercooling, ΔΤ; b) pulling the crystal in such a way that the pull rate is equal to the growth rate in the gradient and the crystal thus grows with a constant ΔΤ; and c) oscillation of the temperature of starting material (source) T between T ..> T and T 2 <T, where T is the temperature of the growing crystal. Crystals made by the improved combined growth process of the present invention are larger, more uniform, and of higher perfection than crystals made by any of the individual processes separately. Accordingly, the method according to the invention and the device according to the invention for HgI ₂ crystal growth combine in one operation or method three principles of evaporation growth which have been used separately in the past.

It is an object of the present invention to provide a method and an apparatus for growing HgI crystals, in particular in such a way that larger, more uniform and more perfect HgI ₂ crystals are produced which can be used in radiation detectors.

The invention has also set itself the goal of an improved HgI - crystal growth process and a device for Implementation of this procedure provide the three known Crystal growth processes are combined, in which way a crystal results that is superior to a crystal which can be produced by the individual processes.

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The invention further aims to provide an HgI ₃ crystal growth process which combines the following individual processes in a single process: growth at low supersaturations or undercooling; Growth by pulling the crystal; and growth by temperature oscillation.

Further preferred embodiments of the invention emerge in particular from the claims and from the description of embodiments based on the drawing; in the drawing shows:

1 schematically shows the device according to the invention for performing the method according to the invention for Growth of Hglj crystals;

Fig. 2 is a graphical representation of the spectrum of

241
Americium 241 (Am), determined with a Hgl_ crystal which was produced by growth according to the method according to the invention.

The invention relates to a method and an apparatus which uses three individually known methods _{to grow mercury iodide (HgI 3} ) crystals. In principle, the invention comprises the combination of low supersaturations or undercooling, pulling of the crystal and temperature oscillation. In particular, the invention provides that the starting material (source material) of Hgl_ powder is arranged in one end of a longitudinally extending ampoule. The ampoule is placed in a horizontally extending oven which is divided into two different temperature zones, namely a starting material zone (source zone) and a crystal zone, with a relatively short transition between them. The Hgl "crystal forms at the end of the ampoule opposite to the starting material when the crystal end is moved very slowly away from the transition in the furnace (at the rate of crystal growth). The crystal zone temperature becomes within a range

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kept constant from 100 ° C to 130 ° C, whereas the source zone temperature is varied between two temperatures, the first temperature on the order of 1 C above the crystal zone, while the second is slightly less than this. This creates alternating growth and growth Etching (re-evaporation) cycles, which favor the nucleation (nucleation) of a single nucleus (nucleus). the Temperature oscillations in the starting material zone (source zone) are important during the early stages of the procedure and can then be terminated when the dominant vaccinee has grown to millimeter size.

Before the details of the method according to the invention and the device for carrying out 'the method are described the individual processes are described - which in combination result in the invention; These are a) Growth with low levels of supersaturation and / or hypothermia, AT; b) pulling the crystal in such a way that the pulling speed is equal to the growth rate in the gradient and the crystal thus grows at a constant δΤ; c) Oscillation of the temperature of the source T between TS> T and T <T, where T is the temperature of the growing Crystal is.

a. Low oversaturation or hypothermia:

From the article in Journal of Crystal Growth, 24/25, 205-211 (1974) it is known that HgI _{2 can be} generated statically from the vapor phase with large values of supercooling of δΤ of approximately 40 ° C (for example T = 140 and T " = 100 ° C) can be grown.

One of the complicating factors during the crystal growth of HgI ₂ is the phase transformation of:

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127 ° C
red HgI ₃ * ~ yellow

Therefore, if you keep T> 127 C, a lot of yellow nuclei (nuclei) will be produced. Even if the growth experiments are carried out below 127 ° C, ie both T and T are less than 127 ° C, there is still a noticeable rate of nucleation and metastable yellow HgI ₀ growth. Therefore decrease at values of ΔΤ «. Crystals grown at 40 C have a chance of growing yellow HgI ₀ , which destructively _{transforms into red HgI 0} when T is cooled to room temperature. A practical recommendation is to grow crystals at very low aT's of about 0.5 C <ΔΤ c 5 C and thus grow at very low growth rates of about 100 micrometers per day. Crystals that grow very slowly have a much higher degree of perfection and a much higher degree of purity. The latter can be deduced from the fact that the so-called high purity HgI ₀ , originally considered free of carbon impurities, leaves a black, low pressure residue when all of the material evaporates and as a. Crystal is condensed.

b. Pulling the crystal:

It is known from the above-mentioned Journal of Crystal Growth article that pulling the growing crystal in a shallow gradient causes the rates of pull, evaporation and growth to equalize; accordingly, pulling allows large single crystals to be grown. This can either be achieved in a two-zone oven with the corresponding temperatures of T ₀ and T such that T = T "- T, or in a three-zone oven with a triangular temperature profile, where T and T rise and fall of the oven with the same result

of T = T ~ T. Again it was shown that only with S C

very low values of δΤ pulled crystals monocrystalline and are nuclear detector quality. Pulling the crystal alone without oversaturation or hypothermia is enough does not stop the growth of good quality detector crystals

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to ensure.

Temperature oscillation:

From the "capillarity" theory of nucleation (cf. "Condensation and Evaporation" by JP Hirth UA, Macmillan Company, New York, 1963) it is known that the critical radius (r) of a readily formed nucleus is inversely proportional to its Is vapor pressure (P). It therefore appears possible to control the nucleation process and to promote the nucleation of only a single nucleus by re-evaporating the nuclei formed. By oscillating their temperature between growth and re-evaporation, the smaller and less perfect nuclei, ie those with higher energies, are evaporated again, whereas the larger and more perfect nuclei survive the re-evaporation process. The application of these principles to the evaporation growth of HgI ₂ was first made by H. Scholz at Philips Aachen (see the Acta Electronica article mentioned above), who previously applied these principles to the growth of other crystals such as GaP, τ-Fe-O or NiFe 0. grown by chemical vapor transport. Others had also used the temperature oscillation method for the growth of CuS and CoS ~ for the hydrothermal synthesis of Se. The production of crystals from yttrium phosphate arsenate and vandate from flux by superimposing temperature fluctuations on a linear cooling curve has already been carried out. Scholz (see the Acta Electronica article) notes that the only way to produce single crystals of HgI ₂ by the temperature oscillation method is to use a vertical furnace with a radial gradient.

Experiments carried out to confirm the present invention have, however, produced single crystals of HgI ₂ in a horizontal system according to the invention and all crystals produced in this way were of detector quality, although those that grew with a smaller δΤ were of better quality.

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The combined waxing process according to the invention can - as shown in FIG. 1 - be summarized as a two-zone horizontal oven 10 in which (in a closed tube or an ampoule 11) HgI ₂ vapor from a hot sublimation zone 12 to a cold condensation zone 13 the ampoule 11 is transported, which is evacuated and can be made of quartz or Pyrex, for example, and has a diameter of 2 inches and a length of 16 inches. For example, the oven 10 may have an interior chamber or heating box 14 approximately 2 feet long and 2 1/4 inches in diameter and mounted on a slow moving translation table 15. The furnace 10 includes a source material zone 16 and a crystal zone 17, the zones being heated by electrical heating coils 18 and 19, respectively. The source material zone 16 is controlled by an oscillation temperature controller 20, whereas the crystal zone 17 is controlled by a temperature controller 21 with a power supply (not shown) in communication with the heating coils 18 and 19. (Note: the temperature profile of furnace 10 is shown immediately above the drawing of the furnace in Figure 1 above.)

The translation table 15 is at a variable speed having translational drive device 22, for example of the screw type, · arranged, wherein this table is actuated by a device not shown, and further the rotation of the drive device 22 moves the table 15 along such that the oven 10 moves with respect to the ampoule 11, which is provided with a fixed support 24 connected rod 23 is held. The furnace 10 is constructed so that the zones 16 and 17 are independent are maintained at a separate uniform temperature by the respective control devices 20 and 21 with only a very short (1/2 inch) transition zone between the two zones. There were different ones Transitional zone temperatures used, but they change usually in the range of 100 ° C to 130 ° C. The temperature difference & T between zones 16 and 17 can usually be up to 40 C, but must be in the order of 1 ° C

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or the like. For crystals of detector quality / which are im Furnace 10 can be manufactured according to the method of the invention. The HgIp source or source material 25 (and the Crystal 26) are located in an evacuated ampoule 11. The ampoule 11 is located inside the chamber 14 of the furnace 10 held in a fixed position with respect to the moving furnace by a rod 23. The furnace movement speed must match with the rate of evaporation of the starting material 25 and the rate of growth of the crystal 26; accordingly becomes the location of the vapor-solid interface of the growing Crystal 26 held firmly in the same position in furnace 10. This then allows the crystal 26 to grow and for example at a fixed temperature (102 C) and a fixed δ Τ (4 C). For example, a linear Evaporation rate from 1 mm per day to 3 mm per day of the starting material the furnace moving speed and the crystal growth rate be maintained at similar rates of 1 to 3 mm per day.

In addition to pulling, the growth method according to the invention provides as well as the furnace according to the invention, the oscillation of the source material temperature, which is mainly during the Initial stages is important to periodically turn to any Crystal zone 17 forming nucleus-forming seedlings (nuclei) to grow and re-evaporate (etch). Typical aT's for growth and re-evaporation are of the order of 1 to 10 C. The method according to the invention allows that 1 to 5 dominant germs survive for eventual growth. Typical sizes of those grown by this process Crystals are 1/4 χ 1/4 χ 1/8 inches, although the size range is from 1/4 χ 1/8 χ 1/32 to 1/2 χ 1/2 χ 1/4 inches may vary wherever the requirements for detector quality crystals are still met.

Initially, the quartz or pyrex glass ampoules 11 become chemical cleaned, rinsed in alcohol and cleaned before loading

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HgI ₀ raw material 25 (10 to 40 g) baked out. The loaded ampoules typically contain 30 g of HgI ₀ and are then

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evacuated to a pressure of 10 torr or less and then sealed, for example with a flame. The evacuation range of the ampules can be satisfactorily in the range of 10 to 10 torr. At this time, the crystal zone 13 of the ampoule is also thoroughly heated by the flame to drive out any Hgl _o scattering powder, etc. The ampoule 11 is then introduced into the heating chamber 14 of the furnace and held therein by rod 23, as described above.

By using the cycling or temperature oscillation method, better control over the Number of small seed crystals reached, which arise during the spontaneous nucleation during the growth phase. Usually an excess of the nuclei undergoes nucleation during initial growth and they often belong to the yellow metastable ß-phase, although the nucleation temperatures are considerably below the solid-state oc-ß phase transition temperature must be kept at 127 ° C. By reversing the sign of ΔΤ to cause the crystal etch and by selection With appropriate cycle conditions, the yellow germs and smaller red germs can be selectively removed while still always a dominant red seedling remains for eventual growth into one or more larger crystals. The re-evaporation cycle can then be terminated when one (or a few) desired extremely perfect dominant germ grown to millimeter size is. Typical oscillation periods for the two inch oven described above are 1 to 30 minutes, although in one of the larger four inch horizontal ovens used in the practice of the invention, periods of 90 minutes of the same Success were used.

Even with relatively rapid growth (10 ³ to 10 ⁵ / </ day)

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and aT's of 5 to 10 C showed that by this method

crystals obtained have good detector qualities. At essential

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slower growth rates (10 to 10 microns / day) and

lower Δ T's (0.5 to 5 ° C), the detector properties are greatly improved.

Figure 2 clearly illustrates that provided by the present

Invention made progress; Namely, Fig. 2 shows a

241
Spectrum of Am., Which is determined by the detector effect of a

HgI crystal was determined by the inventive Procedure has grown.

It can thus be seen that the present invention provides a method and an apparatus which, in a method combines three principles of vapor growth, resulting in more perfect, purer and better core dissolution Crystals result when they can be obtained separately using the three principles.

As already mentioned, the crystal wax process according to the invention Crystals are generated that can be used particularly as X-rays and / or nuclear detectors.

Although specific parameters, materials, etc. are shown and / or described for a specific embodiment of the furnace but modifications can be made without departing from the scope of the invention.

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Claims (11)

Expectations 1.Method of growing mercury iodide crystals,
characterized by the combination of growing the crystals at low supersaturations and supercooling (aT) / pulling the crystals such that the pulling rate is equal to the growth rate in a temperature gradient and the crystal grows with a constant ΔΤ, oscillating the temperature of the starting material (T) between T ₁ > T and T _co <T, where T is the temperature of the alternating crystal. 2. The method according to claim 1, characterized in that the step of crystal growth at low supersaturations and undercooling (δΤ) with an AT of approximately 0.5 C.
up to 10 ° C. 3. The method according to claim 1, characterized in that the step of pulling crystals is carried out in a two-zone furnace where ^ T = T - T. 4. The method according to claim 1, characterized in that the step of crystal pulling is carried out in a three-zone furnace becomes where T and T are in rising and falling Gradients of the furnace are, with & T = T - T. o C 5. The method according to claim 1, characterized in that the step of oscillating the temperature of the source material (T_) is carried out in a vertical furnace. 6. The method according to claim 1, characterized in that the step of oscillating the temperature of the source material is carried out in a horizontal furnace, wherein the
Temperature T is maintained constant within a range of about 100.degree. C. to about 130.degree. C., and further wherein the temperature T _{c is} in the range of about 1.degree. C. above to about 1.degree. C. below the temperature T. 609883/0900 7. The method according to claim 1, characterized in that the purified mercury iodide to be crystallized in the form of purified mercury iodide powder is placed in a cleaned ampoule, the latter being evacuated and sealed so as to retain the powder in the evacuated ampoule, and finally with the ampoule firmly in place a movable two-zone oven is arranged such that the oven is movable with respect to the ampoule. 8. The method according to claim 7, characterized in that the enclosed mercury iodide powder an amount of is approximately 30 grams with the vial evacuated to approximately 10 torr, and ultimately the seal of the Ampoule is carried out by heating it with a flame. 9. The method according to claim 1, characterized in that the step of oscillating the temperature of the source T is performed for periods of time ranging from 1 to 90 minutes, causing repeated seed growth and re-evaporation causes, whereby only the growth of the dominant crystal nuclei is promoted. 10. Apparatus for growing mercury iodide crystals characterized with an oven divided into two temperature zones with a central chamber extending through the zones by a temperature control device for independently controlling the temperature in each of the two zones such that one zone is kept constant while the other zone is changed in temperature, and wherein further, the furnace is mounted on a variable speed translational drive device and wherein an evacuated ampoule containing purified HgI source raw material is disposed in the central chamber of the furnace and is fixed in a fixed position in such a way that the two temperature zones of the furnace can be moved with respect to wherein activating the variable speed translational drive device with respect to the furnace the ampoule is moved so that the temperature of the ampoule 60S883 / 090Q according to the temperature of the temperature zone of the furnace / within which it is located is changed, causing the temperature of the Hgl ~ raw source material to drop or rise causing the crystals formed in the ampoule to grow or re-evaporate. 11. The device according to claim 10, characterized in that the ampoule consists of quartz or Pyrex. 609883/0900 JH Blank page