Purification of hgl2 for growth of nuclear detector quality crystals

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of, or under, Contract AT(29-1)-1183, with the United States Energy Research and Development Administration (formerly the United States Atomic Energy Commission).

This invention relates to the growth of mercuric iodide crystals, and more particularly to a process for purification of mercuric iodide to be utilized as a source material for the growth of such crystals.

Since the advent of semiconductor detectors, efforts have been directed to the development of detector materials having a sufficiently high atomic number so as to overcome the low photoelectric efficiency that results from low atomic number detector materials. Compounds such as CdTe and, more recently, Hgl_ have been explored. While 1 Z mercuric iodide (Hgl?) promises to be one of the most potentially useful, high atomic number, room temperature operable radiation detector materials structural imperfections due primarily to high impurity concentrations have led to problems of carrier trapping and detrapping and of detector fabrication. While substantial effort has been directed to methods for growing Hgl? crystals, relatively little effort has been directed to the development of high purity Hgl^ raw material to be used for growth of detector quality Hgl^ crystals. Technical literature directed to purification of the HgI2 raw material is

- Mexemplified by an article by S. P. Swierkowski, et al, Appl. Phys. Ltrs. 23, 281, 1973, and an article by M. Schieber, et al, Journal of Crystal Growth 24/25, 205-211, 1974. As noted in the Journal of Crystal Growth article, in establishing the detector quality of a given crystal, experiments indicate that purity is a more critical requirement than physical perfection, and a higher degree of purification of both solvent and raw materials would lead to high quality detector crystals.

SUMMARY OF THE INVENTION

The present invention is directed to a process for the purification of Hglj utilized as raw material for growing crystals of detector quality. Broadly, the process involves a combination of three stages composed of synthesis of Hgl? from Hg and Ig, repeated sublimation, and zone refining. The results of spectrochemical analysis of Hglj after each of these stages verifies that the overall process produces the optimum purification of the raw material.

Therefore, it is an object of this invention to provide a process for producing high purity Hgl? raw material.

A further object of the invention is to provide a process for producing high purity material of HgI2 which can be utilized as a source material for the growth of detector quality crystals.

¾Another object of the invention is to provide a process for producing high purity HgI2 taw material by a combination of stages comprising synthesis of HgI2 from Hg and 1%, repeated sublimation, and zone refining.

Other objects of the invention will become readily apparent from the following description.

DESCRIPTION OF THE INVENTION

The invention is directed to a process for producing high purity mercuric iodide (Hgl^ material to be utilized 10 for growth of detector quality crystals. The purification process is basically composed of the following three sequential purification stages: (a) synthesis of Hgl^ from high purity mercury and repurified iodine, (b) repeated sublimation, and (c) zone refining.

While each of the three purification stages have been used Individually, as set forth in the above-referenced Journal of Crystal Growth article, to provide source material for the growth of HgI2 crystals, it has been found that the highest purity of HgI2 raw material to be 20 used for growth of detector quality crystals is produced by the combinational sequence of these three stages when used as set forth above. Tests, as described in greater detail hereinafter, have clearly illustrated that the herein disclosed sequence of purification stages provides a major improvement in the purity of the Hgl^ raw materials, thereby providing increased efficiency of radiation detectors using Hgl^ crystals made from the improved purity raw materials.

The first stage of the process, the synthesis of

Hgl^, is done in the following way: mercury of 99,999 percent purity and iodine, purified by repeated sublimation until no red-colored residue Is left behind, are melted together in stoichiometric amounts (200.59 gr mercury and 253.808 gr iodine) in an evacuated (torr of 10^ to 10 reaction tube or ampoule of Pyrex or quartz (tube volume of 400 to 500 ml) at a temperature of 260*0 for 4 hours. The temperature can vary from 260 to 300*0 and the time from one-half to six hours. A slight excess of iodine, amounting to about one percent by weight (0.1 to 1.5 wt7.) is added to compensate for vaporization loss, which may occur prior to sealing the tubes when evacuated with a vacuum pump. While the repeated sublimation iodine process is known, it is briefly described as follows: The iodine is introduced in an ampoule similar to the one used 20 for repeated sublimation of Hgl?. Best results are obtained when iodine Is introduced in the ampoule in an absence of -2 air in a glove box. The ampoule is evacuated to 10 to “6 torr and introduced in the furnace. The temperature of the hot zone is about 50 to 100*C and the ^ΔΤ of 20 to 50*0. The number of sublimation cycles is determined by the fact that no red-colored residue is left in the ampoule.

The mercury and iodine mixture is best introduced in the reaction tube prior to sealing, and melting is carried out in a glove box under a continuous flow of dry argon (flow rate of 0.5 to 2 liters/min.). The term glove box utilized here is briefly described as a closed box of polyethylene which is operated from the outside with gloves. The box is sealed from the atmosphere and a vacuum or a continouous flow of an inert gas controls the desired atmosphere under working conditions. After 10 melting of the mixture, the tube is cooled slowly (2 to 6 hours) to room temperature (22“C) and then placed in a gradient furnace, where the hot zone is about 100°C (60 to llO’C) in order to sublime the excess iodine. While gradient furnaces are known, one utilized herein is briefly described as a furnace having a uniform temperature zone in the center and a temperature gradient between the center and the end zones of the furnace where the cold zone has a temperature of about 25 to 40°C.

The synthetic material formed in accordance with 20 the above-described stage is then subjected to the second stage of the process, repeated sublimation. Briefly, the repeated sublimation process? is carried out as follows: The material is introduced in an ampoule of 200 to 50® ml, • 2 -6 evacuated to a vacuum of 10 to 10 , sealed; placed into a hot zone of 100. to 140^eC and a cold zone of 60 to 80°C. A black residue is left behind in the hotter part of the ampoule.

While the purified material is in the colder part, the purified material is removed and reintroduced in a new ampoule and the whole procedure is repeated again and again. The number of cycles of repeated sublimation may vary between 6 and 30, this variation in cycles being determined by the amount of the residue which is still left behind.

The resublimed material is then introduced in a

Pyrex or quartz tube of 1/2 inch diameter and 6 inches . long, for example, evacuated and refilled with one-half atmosphere of argon, and subjected to the third stage of the process, zone refining. The argon may vary from .1 to .9 atmosphere, or nitrogen or helium may be used in place of argon.

The zone refining stage of the purification process constitutes a vertical zone refining process where two narrow (1 to 5) zones, separated 2 inches from each other, move downard at a rate of 0.2 inches/hour. Movement rates 20 of 0.01 to 2 inches/hour are acceptable. The molten zone refining apparatus is briefly described as follows: Two holders allow a vertical ampoule to stay in place while one or several nickel-chromium coils of about 3/4 to 1 inch diameter and .1 to .2 inch wide can move along the ampoule at variable rates of Ο.ΟΙ^ίο 2 inches/hour. Each coil is heated electrically and is followed by a concentric air cooler which narrows the width of the molten zone.

In a test conducted, the coil temperature was not controlled, but was in the neighborhood of 700°C. The molten zone of Hglg, however, was only about 259*C to, say, 300^eC (259*C being the melting point of Hglg). Thus, the ingot of the Hglg remains solid while the material where the hot coil passes is molten. The total number of zone passes varies from 30 to 100, the number being determined by the amount of black material which segregates usually at the bottom of the ampoule. In each case, a black-colored, impurityrich, bottom part of the molten ingot is obtained. Vertical rather than horizontal zone refining is preferred for the following reason: Hglg has a high-vapor pressure and . horizontal zone refining causes the appearance of both vapor and molten phases which prevent the dissolution of the Impurities in the liquid phase and which is moved towards the ends of the ampoule.

In some cases, spectrochemical analysis could not detect certain impurities prior to the third stage, zone 20 refining; but after increasing the content of the impurity by zone refining, the specific impurity could be detected. The efficiency of purification of each of the three stages of the mercuric iodide raw material purification process of this invention is summarized in the following semiquantitative spectrochemical analysis results. Table I gives the results of comparison of the content of impurities of the original iodine and mercury and the synthetic mercuric iodide made in accordance with this invention. The units in Table I indicate parts per million (ppm).

TABLE I

<td> 10</td><td> Element</td><td colspan="3"> Red Color Iodine After Residue 99.99999% Repeated After Sub- Pure Original Sublimation limation Mercury Iodine n 30 ״ of Iodine Metal</td><td> Synthetic Hgl₂</td>

<td></td><td> C</td><td> 2</td><td> Trace</td><td> 10</td><td> --</td>

<td></td><td> Ag</td><td> * —</td><td> --</td><td> 3</td><td> --</td>

<td></td><td> Si</td><td> — —</td><td> --</td><td> 5 . 1</td><td> 1</td>

<td></td><td> Fe</td><td> 10</td><td> 5</td><td> 50</td><td> 1</td>

<td></td><td> Mg</td><td> --</td><td> --</td><td> 5 1</td><td> --</td>

<td></td><td> Ca</td><td> — —</td><td> --</td><td> 50</td><td></td>

<td></td><td> Bi</td><td> ' --</td><td> --</td><td> --</td><td> --</td>

<td></td><td> Al</td><td> 2</td><td> --</td><td> 10 --</td><td> --</td>

<td></td><td> Na</td><td> 1</td><td> — -</td><td> 10</td><td> --</td>

<td> 20</td><td> Ni</td><td> 5</td><td> --</td><td> 10</td><td> --</td>

<td></td><td> Pb</td><td> --</td><td> — -</td><td> - — — —</td><td> --</td>

<td></td><td> Ba</td><td> --</td><td> --</td><td> --</td><td> — —</td>

<td></td><td> B</td><td> --</td><td> Trace</td><td> 50</td><td> 5</td>

<td></td><td> Cr</td><td> 1</td><td> 5</td><td> 100</td><td> --</td>

<td></td><td> Mn</td><td> 1</td><td> --</td><td> 30</td><td> --</td>

<td></td><td> Sn</td><td> - -</td><td> — —</td><td> 20</td><td> — —</td>

Table II shows the efficiency of zone refining as a purification stage in the process. The multiple-zone refining or melting process produces a segregation of impurities in the bottom of the ingot, which is much above what could be detected in the bulk or original material and in the top of the ingot. The results given in Table II are in ppm after 92 zone refining passes of original Baker-produced Hglj taw material. Baker material being defined herein as that Hgl^ taw material known as Baker's 10 Spectrochemically Analyzed” material.

TABLE II

<td></td><td> Element</td><td> Original Baker Material</td><td> Zone Refined ׳ Bottom</td><td> Zone Refined Top</td>

<td></td><td> Cu</td><td> 4</td><td> 1000</td><td> 1</td>

<td></td><td> Ag</td><td> 1</td><td> 50</td><td> 1</td>

<td></td><td> Si</td><td> 2</td><td> 100</td><td> 8</td>

<td></td><td> Fe</td><td> 3</td><td> 50</td><td> 3</td>

<td></td><td> Mg</td><td> 3</td><td> 50</td><td> 2</td>

<td></td><td> Ca</td><td> 1</td><td> 100</td><td> 1</td>

<td> 10</td><td> Bi</td><td> --</td><td> 100</td><td> 1</td>

<td></td><td> Al</td><td> --</td><td> 100</td><td> 1</td>

<td> -</td><td> Na</td><td> Trace</td><td> 50</td><td> 3</td>

<td></td><td> Nt</td><td> --</td><td> 1</td><td> --־</td>

<td></td><td> Pb</td><td> — —</td><td> 10</td><td> --</td>

<td></td><td> Ba</td><td> * —</td><td> 1</td><td> --</td>

<td></td><td> B</td><td> — *</td><td> 100</td><td> 3</td>

<td></td><td> Cr</td><td> --</td><td> 2</td><td> --</td>

<td></td><td> Mn</td><td> --</td><td> 5</td><td> — —</td>

<td></td><td> Sn</td><td> --</td><td> 7</td><td> --</td>

The combination of the three stages of the prior known purification methods has proven to be superior over each one separately, as can be seen in Table III, wherein the Impurities detected therein are identified, and where B » Baker Analyzed, RS « repeated sublimed, ZR ״ zone refined, N ״ the number of cycles or passes,

Il.

and SY = synthetic Hgl made in accordance with the 2 three-stage operational sequence of this invention.

NOTE: Table III appears on page 12.

While a cursory review of the repeated sublimation (RS) column indicates that the material does not contain impurities in an amount that is detectable by the spectrochemical analysis, Table ITT shows that, after repeated sublimation, both Baker's (B) and synthetic (SY) Hg^ material, if subjected to the additional purification stage 10 of zone refining, the material is cleansed of impurities which can then be detected in the bottom or in the top part of the ingot. Impurities whose coefficient of segregation (K) is smaller than 1 are segregated in the bottom part of the ingot; while impurities where K is larger than 1 are segregated in the top of the ingot.

It has thus been shown the combination and operational sequence of the three-stage purification process of this invention; namely, synthesis, repeated sublimation, and zone refining, produce a high purity Hgl2 raw material 20 that is particularly effective as the source material for the growth of detector quality crystals, by either the vapor-growth or solution-growth techniques. Thus, by use of the process of this invention for preparation of highpurity Hg^ raw materials growth capabilities of highpurity nuclear, X-ray, and infrared solid state semiconductor spectrometer crystals has been, substantially advanced.

TABLE

B, RS(N 30), ZR(N » 50) SY, RS(H 30 ־), ZR(N 92 ־)

Element B B, RS(N 30) Bottom Top Center SY SY, RS(N 30) Bottom Top Center

<td></td><td> Cu</td><td> 4</td><td> a</td><td> 1</td><td></td><td> a</td><td> ' »</td><td> a</td><td> 5</td><td> 5</td><td> a</td>

<td></td><td> Ag</td><td> 1</td><td> י.</td><td> 1</td><td> a</td><td> *</td><td> a</td><td> V</td><td> Trace</td><td> Trace</td><td> a</td>

<td></td><td> Si</td><td> 2</td><td> 7</td><td> 2</td><td> 2 ,</td><td> 3</td><td> 1</td><td> 1</td><td> 2</td><td> 7</td><td> 2</td>

<td></td><td> Fe</td><td> 3</td><td> 1</td><td> 5</td><td> a</td><td> a</td><td> 1</td><td> a</td><td> 3</td><td> 5</td><td> a</td>

<td> 1 H kA</td><td> Mg</td><td> 3</td><td> 1</td><td></td><td> a ,</td><td> a</td><td> •</td><td> •</td><td> 1</td><td> 1</td><td> 1</td>

<td> N 1</td><td> Ca</td><td> 1</td><td></td><td> *</td><td> e</td><td> a t</td><td> a</td><td> a</td><td> 1</td><td> Trace</td><td> a</td>

<td></td><td> Bi</td><td> a</td><td> «V</td><td> 1</td><td> a</td><td> a</td><td> a</td><td> a</td><td> a</td><td></td><td> a</td>

<td></td><td> Al</td><td> •</td><td> a</td><td> a</td><td> a</td><td> *</td><td> a</td><td> a</td><td> Trace</td><td> a</td><td> a</td>

<td></td><td> Na</td><td> Trace</td><td> •</td><td> 1</td><td> *</td><td> Trace</td><td> a</td><td> a</td><td> 2</td><td> 1</td><td> a</td>

<td></td><td> Pb</td><td></td><td> a</td><td> 2</td><td> «1</td><td> 1 a</td><td> a</td><td> a</td><td> 5</td><td></td><td> a</td>

<td></td><td> B</td><td></td><td> a</td><td> a</td><td> Trace</td><td> Trace</td><td> 5</td><td> Trace</td><td> 5</td><td> 4 4</td><td> a</td>

<td></td><td> Sn</td><td></td><td> a</td><td> 2</td><td> a</td><td> a</td><td> •</td><td> a</td><td> 5</td><td> a</td><td> a</td>

While the three־stage purification process described herein has been applied only to Rgl? raw material, it might be used for purification of other source material, such as germanium (Ge) (for intrinsic high-purity germanium detectors), cadium-tellurium (CdTe), and other crystals for spectrometric detectors.

While particular examples, parameters, etc., have been described for carrying out the invention, modifications and changes will become apparent to those skilled in the art, and it is intended to caver in the appended claims all such modifications as come within the spirit and scope of this invention.