CA1060762A - Method and apparatus for growing hg12 crystals - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improved method and apparatus for growing mercuric iodide (HgI2) crystals which combines known vapor growth techniques to produce larger, more uniform, and higher perfection crystals for use in X-ray, nuclear, or other radiation detectors. The improved method combines the following individual crystal growth techniques:
(a) growth at low supersaturations and/or undercooling, .DELTA.T;
(b) pulling the crystal so that the pulling rate is equal to growth rate in the gradient and the crystal thus grows at a constant .DELTA.T; and (c) oscillating the temperature of the source TS between TS1 > TC and TS2 < TC where TC is the temperature of the growing crystal.

Description

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METHOD AND APPARATUS FOR GROWING HgI2 CRYSTALS
BACKGRO~ND OF_ THE INVE~TION

This invention relates to mercuric iodide crystals, and more particularly to an imprGved method and apparatus for growing such crystals thereby producing larger, more :
unifonm, and higher perfection crystals having particular application in radiation detectors.
Mercuric iodide (HgI2) crystals have been proposed for use in high-atomlc number, room temperature operable lo radiation detectors. The main difficulties with HgI2, however, is associated with structural imperfections, etc., and much effort ha~ been recently directed to providing higher purity raw material and/or techniques for growing the crystals. Such prior efforts are exemplified by articles in the Journal of Crystal Growth, 24/25, 205-211 ~1974) by M. Schieber, et al; 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 ZO H. Cho, et al; and Crystal Growth Conference, Supplement to Physics and Chemistry of Solids, Pergammon Press, pp. 475-482 (1967j by H. Scholz, et al.

SUMMARY OF THE INVENTION
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Nucleation and growth of radiatlon detector quality mercuric iodide (HgI2) crystals are accomplished by this invention. Broadly, the improved method of growing HgI2 crystals constitutes a combination of known individual vapor growth techniques: (a) growth by low supersaturations and/or undercooling, ~T; (b) pulling the crystal so that the pulling rate is equal to growth rate in the gradient and the crystal thus grows at a constant ~T; and (c) osc~llating the temperature of the source TS between TSl ~ TC and TS2 ~ TC where TC is the temperature of the growing crystal. Crystals grown by the improved and combined growth method of this invention are larger, more uniform, and of higher perfection than crystals grown by any of the individual tech~iques taken separately. -Thus, the improv~d HgI2 crystal growth method and apparatus combines into one operation or technique three principles of vapor growth which in the past have been used separately.
Therefore, it is an ob~ect o this invention to provide an improved method and apparatus for grow~ng HgI2 -crystals.
A further ob~ect of the invention is to provide a method and apparatus for growing larger, more uniform, and higher perfection HgI2 crystals for utilization in ~ -rad~ation detectors.
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Another ob3ect of the invention is to provide an improved HgI2 cyrstal growth method and apparatus which combines three previously known crystal growth techniques resulting in crystal superior to those produced by the individual techniques.
Another ob3ect of the invention is to provide an impraved HgI2 crystal growth method which combines into one technique individual techniques composed of: growth by low supersaturations or undercoolings; growth by pulling the crystal; and growt~ by ~emperature oscillation.
Other ob3ects of the invention will become readily apparent from the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 schematlcally illustrates the apparatus and method of the invention for growing HgI2 crystals; and Figure 2 graphically illustrates the spectrum of americium 241 (241A~) detected with HgI2 crystal grown by the impr~ved method of this invention.

DESCRIPTION OF THE INVENTION
The present invention is directed to an improved method and apparatus which combines three individually known techniques for growing mercuric iodide (HgI?3 crystals. Basically, the invention involves the combining of low supersaturations or undercoolings, pulling ~he 1C~j0 7 ~ ~
crystal, and temperature oscillation. More specially, the invention involves placing source material of HgI2 powder in one end of a longi~udinally extending ampoule.
The ampoule is placed in a horizontally extending furnace which is divided into two distinct temperature zones, i.e., a source zone and a crystal zone, with a relatively short transition therebetween. The HgI2 crystal forms at the end of the ampoule opposite the source msterial ' as the crystal end i~ very slowly m3ved away from the transition of the furnace (at the rate of crystal growth).
The crystal zone teKperature i~ maintalned constant within a range of lOO-C to 130C whereas the source zone tempera-ture is varied between two temperatures, the first of which is on the order of 1C above the crystal zone and the second a little less than that therebelow. This establishes alternate growth and etch (reevaporization) cycles which favor the nucleation of a single nucleus.
The temperature oscillations in the source zone are important during the early stages of the process and can be discontinued when the dominant seed has grown to millimeter size.
Prior to describing the details of the improved method and apparatus for carrying out the method, which.
as pointed out above, involves the combining of t~ree individ~al crystal growth methods, shese individual methods or techniques will first be described; namely, (a) gr~wth - 11Nj0 7~ ~
at low supersaturations and/or undercoolings, QT;
(b) pulling the crystal so that the pulling rate i8 equal to gr~wth rate in the gradient and the crystal thus grows at a constant ~T; and (c) oscillating the temperature of the source TS between TSl ~ TC and TS2 ~ TC where TC is the tempera~ure of the growing crystal.
a. Low Supersaturations or Undercoolings. It is known fro~ the article in J. Crystal Growth, 24/25, 205-211 - (1974~, that HgI2 can be grown statically from the vapor phase at large values of undercoolings of ~T
of about 40C (e.g., TS ~ 140 and TC - 100-C).
One of the complicating factors which occurs during crystal grcwth of HgI2 is the phase trans-formation of:

red HgI2 ' yellow HgI2 Therefore, hold~ng TS ~ 127C will produce many yellow nuclei. Even if growth experiments are performed below 127C, i.e., both TS and T~ are s~aller than 127-C, there is still an appreciable rate of nucleation and growth of the metastable yellow HgI2~ Therefore, crystals grown at values of ~T ''40-C
will decrease the chances to grow yellow HgI2, which transfonms destructively to red HgT2 when TC is cooled down to room te~perature. A practical recommendation is to grow crystals at very low ~Ts of about O.5-C ~ ~T ~ 5-C, and hence grow at very 1U~ 7~'~
low growth rates of about 100 ym/day. Very slowly grown crystals prove to have a much higher perfection and also a much higher purity. The latter can be ascertained from the fact that so-called highly purified ~gI2, originally considered to be free of carbon impurities, still leaves a black low-pressure residue when all the ~aterial has been evaporated and condensed as a single crystal.
b. lling the Crystal: It is also known from the above-cited J. Crystal Growth article that pulling the growing crys~al in a shallow gradient causes an equalization of the rates of pulling, evaporation, -and gr~wth; thus, pulling allows the growth of-large single crystals. This can be achieved in either a two-zone furnace with the re~pective temperatu~es of TS and TC such that aT ~ TS ~ TC- or in a triangular shaped te~perature profile, three-zone furnace where TS and T are in the ascending and descending gradients of the furnace with the same result of ~T ~ T - Tc.
Again, it has been shown that only crystals pulled at very low values of ~T are single crystalline and of - -nuclear detector quality. Thus, pulling the srystal alone, without supersaturation or undercoolings, is not sufficient to ensure the growth of good quality detector crystal~.

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c. Temperature Oscillation: It is known from the "capillarity" theory of nucleation (see "Conden~ation and EvaporatLon" by J. P. Hirth, et al, the Macmillan Company, New York, 1963) that the critical radius (r*) of readily formed nucleus is inversely proportional to its vapor pressure (P). Therefore, it appears possible to control the nucleation process and promote the nucleation of only one single nucleus if one reevaporate~ the formed nuclei. By oscillating their temperature between growth and reevaporation, the smal~r and the more imperfeQt nuclei, i.e., those having higher energies, will reevaporate, while the larger and more perfect ones will survive the reevaporation process.
The first to apply these principles to vapor growth of HgI2 was H. Scholz of Philips Aachen (see above-sited Acta Electronica article), who had applied them earlier to the growth of other crystals such as GaP, Y-Fe203, or NiFe204 grown by vapor chemical transport. Others have also used the temperature oscillation method for the growth of CuS and CoS2, for the hydrothermal synthesis of Se.
Also, producing crystals of yttrium phosphate arsenate and vanadate from flux by superimposing temperature fluctuations on a linear cooling curve has been previously carried out. Scholz (see the ~ 7~ ~
Acta Electronica article) indicated that the only way one can produce single crystals of HgI2 by the temperature oscillation method is by using a vertical furnace with a radial gradient.
However, exper~ments carried out to verify the present invention have produced slngle crystals of HgI2 in a horizontal system according to the invention and ~11 crystals thus produced were of detector quality, although those grown at smaller ~T were of better quality~

The improved combined growth method of the present ~nvention can be summarized, as illustrated in Figure 1, as a ~wo-z~ne horizon~al furnace, generally indicatet at 10, in w~ich is transported (in a closet tube or ampoule 11) HgI2 vapor from a hot-~ubliming region 12 to a cool-condensing region 13 of the ampoule 11, which is evacuated and may be . TM
constructed of quartz or Pyrex, for example, having a diameter of 2 inches and a length of 16 inches. Tbe furnace 10, for example, may have an interior chamber or heating box 14 about 2 feet long and 2 1/4 ~nches in diameter, and is mounted in a slow motion trans-lation table generally indicated at 15 The furnace 10 comprises a source zone 16 and a crys~al zone 17, the zones being heated by electric heating coils 18 and 19, respectively. Source zone 16 is controlled iV'7~
by an oscillating temperature controller 20~ while the crystal zone 17 is controlled by a ~emperature controller 21, it being understood that an appropriate power supply (not shown) ~6 connected ~o heating coils 18 and l9. (Note: The temperature profile of furnace lO is drawn immediately above the drawing of the furnace at the top of Figure l.) Tran~lation table 15 i8 mounted on a variable speed tran3lation drive mechanism 22, of thescre~ type, for example, act~vated by apparatus not shown, whereby rotation of the drive mechanism 22 move~ the table 15 therealong, such that the furnace lO moves with respect to ampoule ll ~hich is held by a rod 23 connected to a f~xed support 24. The furnsce lO ~s construc~ed sc that zones 16 a~d 17 can be i~dependently ma~ntained by the respective controller~ 20 and 21 at separate, uniform temperature with only a fairly short (l/2 ~nch) tran~ition reg$on between the two zones. Various zone temperatures have been u~ed, bu~ they normally vary in the range of 100C ~o 130-C. The temperature difference, ~T, between zo~e~ 16 and 17 which usually ca~ be as large as 40C, ~Nst be on the order of l~C or 80 for dete~tor grade cry~tals produced in the fur~ce lO in aocordance with tha i~proved co~bi~ed ~ethod. The ~gI ~ource msterial

- 2 25 (and the crystal 26) ~8 conta~ned in evacuated ampoule ~1. The ampoule ll, placed inside cha~ber 14 of ~urnace lO
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is held fixed by rod 23 relative to the moving furnace.
The furnace movement speed must be matched to the source material 25 evaporation rate and the crystal 26 growth rate; t~us, the position of the vapor solid interface of the growing crystal 26 ix held fixed in the same position in the furnace 10. This, then, allow~ the crystal 26 to grow, for example, at a fixed temperature (102C) and a fixed ~T (4C). For example, with a source material linear evaporation rate of lmm/day to 3mm/day the furnace movement speed and the crystal growth rate should be kept at similar rate~ of 1-3mm/day.
In addition to pull~ng, the improved growth method a~d furnace also applies the oscillation of the source ~emperature9 which is important mainly during t~e early stages in order to periodically grow and Peevaporate (etch) a~y nucleated seeds that fonm in the cyrstal zone 17. Typical ~Ts ~r growth and reevaporation are on the order of 1 to lO-C. By the method of this invention, one ~o five dominant seeds are allowed to survive for eventual growth. Typical sizes of crystals grown by this method are 1/4 x l/4 x 1/8 inches, although the size range may vary from 1/4 x 1/8 ~ 1/32 to l/2 x 1/2 x 1/4 inches and still fulfill the requirements for detector quality crystals.
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Initia$1y, the quartz or Pyrex glass ampoules 11 are chemically cleaned, rinsed in alcohol, and baked ~n 1C~j0 7~'~
an oven prior to be~ng charged with the purified HgI2 - raw material 25 (10 to 40 grams). The loaded ampoules typically containing 30 grams of ~gI2 are then evacuated to a pressure of lO torr or less and sealed off, such as with a torch. The evacuation range of the ampoules may satisfactorily range from 10 to 10 torr. At this time, the crystal region 13 of the ampoule is also thoroughly heated by the torch to drive off any stray HgI2 powder, etc. The ampoule 11 is then loaded in the heating chamber 14 of furnace 10 and supported therein by rod 23 as a~ove described.
By using the technique of cycling or tempexature oscillation, a better control over the number of small seed crystals produced by spontanesus nucleation during the growth phase is also achieved. Ordinarily, during ~he initial growth, an excess of seeds nucleate and are frequently of the yellow, metastable ~-phase, even though seed temperatures may be held considerably below the solid stage a_~ phase transition temperature of 127C. By reversing the sign of ~T to cause crystal etching, and by choosing proper cycling conditions, the yellow seeds and smaller red seeds can be removed selecti~ely, while still leaving a dominant red seed for eventual growth into one or several larger crystal~. The reevapora~ion cycle can be discontinued when one (or a few) desired high perfection dominant seed has grown to millimeter slz~. Typical oscillation per~ods for the 2-inch furnace, above described, are 1 to 30 minutes, although in one of the larger 4-inch horizontal furnaces, used in verifying this invention, period~ of 90 minutes were used with equal ~uccessO
Even with relatively rapid growth (103 to 105 ~m/day) and ~Ts of 5 to lO-C~ the crystals obtained by thi~
method ha~e yielded good detector qualities. H~wever, at mNch slower growth rates (102 to 10 ~m/day) and lower ~Ts (0.5 to 5-C), the detector properties are mNch lmproved.
Figure 2 clearly illustrates the advance provided by this invent~on wherein a spectrum of 241Am detected with a HgI2 crystal grown by the imprvved growth method is shown.
It has thus been shown that the prese~t lnYention provides a method and apparatus which combines into one techni~ue three principles of vapor growth which prsduces more perfect, purer9 and better nuclear resolution cryQtals tha~ those produced by using the three principles separately~
As pointed out above, the improved crystal growth , method of this invention produees crystals which have part~cular application as X-ray and/or nuclear detectors.
While part~cular parameters, materials, etcO, u~ed and a par~icular embodimen~ of the furnace have been illustrated and/or described, modifications will become apparent to tho~e skilled in this art, and it i8 intended to cover in the appended claims all such modifications a~ come within the ~pirit and scope of this invention.

Claims (11)

What we claim is:

1. An improved method for growing mercuric iodide crystals comprising the steps of: growing the crystals at low supersaturations and undercoolings (.DELTA.T); pulling the crystals such that the pulling rate is equal to growth rate in a temperature gradient and the crystal grows at a constant .DELTA.T; oscillating the temperature of the source (TS) between TS1 > TC and TS2 < TC where TC is the temperature of the growing crystal.

2. The method defined in Claim 1, wherein the step of growing the crystals at low supersaturations and undercoolings (.DELTA.T) is carried out with a .DELTA.T of about 0.5°C to 10°C.

3. The method defined in Claim 1, wherein the step of pulling the crystals is carried out in a two-zone furnace wherein .DELTA.T = TS - TC.

4. The method defined in Claim 1, wherein the step of pulling the crystals is carried out in a three-zone furnace where TS and TC are in the ascending and descending gradients of the furnace with .DELTA.T = TS - TC.

5. The method defined in Claim 1, wherein the step of oscillating the temperature of the source (TS) is carried out in a vertical furnace.

6. The method defined in Claim 1, wherein the step of oscillating the temperature of the source (TS) is carried out in a horizontal furnace; wherein temperature TC is maintained constant within a range from about 100°C
to about 130°C; and wherein temperature TS ranges from about 1°C above to about 1°C below temperature TC.

7. The method defined in Claim 1, additionally including the steps of containing purified mercuric iodide to be crystallized by placing purified mercuric iodide powder in a cleaned ampoule, evacuating the ampoule, sealing of the ampoule so as to retain the powder in the evacuated ampoule, and fixedly securing the ampoule in a movable two-zone furnace such that the furnace is movable with respect to the ampoule.

8. The method defined in Claim 7, wherein the contained mercuric iodide power is in an amount of about 30 grams, wherein the ampoule is evacuated to about 10-5 torr, and wherein the sealing of the ampoule is carried out by heating with a torch.

9. The method defined in Claim 1, wherein the step of oscillating the temperature of the source (TS) is carried out for time periods ranging from 1 to 90 minutes causing repeated crystal seed growth and reevaporation thereby promoting only growth of dominant crystal seeds.

10. An apparatus for growing HgI2 crystals comprising:
a horizontally extending furnace, said furnace being divided into two distinct temperature zones and having a central chamber extending through said zones, temperature controller means for independently controlling the temperature of each said two temperature zones such that one zone is held constant while the other zone is varied in temperature, said furnace being mounted on a variable speed translation drive mechanism, an evacuated ampoule containing purified HgI2 raw source material located in said central chamber of said furnace and fixedly secured such that said two temperature zones of said furnace are movable with respect thereto, so that activation of said variable speed translation drive mechanism moves said furnace with respect to said ampoule such that the temperature of the ampoule is varied in accordance with the temperature of the temperature zone of the furnace within which it is located, thereby causing a decrease or increase in temperature of the HgI2 raw source material causing growth of reevaporation of crystals formed in said ampoule.

11. The apparatus defined in Claim 10, wherein the ampoule is constructed from material selected from the group consisting of quartz and PyrexTM.