Mercuric iodide is a material used for the fabrication of the sensing element in solid state X-ray and gamma ray detecting instruments. The operation of the devices is determined to a large degree by the density of structural defects in the single crystalline material used in the sensing element. Since there were strong indications that the quality of the material was degraded by the effects of gravity during the growth process, a research and engineering program was initiated to grow one or more crystals of mercuric iodide in the reduced gravity environment of space. A special furnace assembly was designed which could be accommodated in a Spacelab rack, and at the same time made it possible to use the same growth procedures and controls used when growing a crystal on the ground. The space crystal, after the flight, was subjected to the same evaluation methods used for earth-grown crystals, so that comparisons could be made

Schnepple, W. F.

Vandenberg, L.

English

NASA Technical Reports Server

GRckJTH OF MERCURIC IODIDE (HgI,) FOR NUCLEAR RADIATION DETECTORS 
L. van den Berg and W. F. Schnepple 
EG&G Energy Measurements 
Santa Barbara Operations 
130 Robin Hill Road 
Goleta, California 93117, USA 
February 6, 1987 
I 269 
, PRECEDING PAGE BLANK NOT FILMED 
https://ntrs.nasa.gov/search.jsp?R=19890010922 2020-03-20T03:54:51+00:00Z
1. I m c T I o N  
Mercuric Iodide is a material used for the fabrication of the sensing element 
in solid state x-ray and gama ray detecting instruments. 
the use of mercuric iodide in such instrumentation is that the systems are 
able to operate at room temperature, compared with systems based on germanium 
and silicon which need cooling to liquid nitrogen temperature. 
The advantage of 
The proper operation of the devices is determined to a large degree by the 
density of structural defects in the single crystalline material used in the 
sensing elements. 
material was degraded by the effects of gravity during the growth process 
(mechanical slippage and irregular convection) a research and engineering 
program was initiated to grow one or more crystals of mercuric iodide in the 
reduced gravity environment of space. 
which could be accoarmodated in a Spacelab rack, and at the same time made it 
possible to use the same growth procedures and controls used when growing a 
crystal on the ground. 
Since there were strong indications that the quality of the 
A special furnace assembly was designed 
The space crystal, after the flight, was subjected to the same evaluation 
methods used for earth-grown crystals, so that comparisons could be made. 
2 70 
2. PREPARA'JDRY GROUND-BASED RESEARCH 
It is usually easy to suggest in general terms the possible benefits of an 
experiment in the reduced gravity of space flight. More effort is required to 
document with experimental results that the damaging effects of gravity on the 
structural quality of a crystal are indeed certain or at least highly 
probable. 
Crystals of mercuric iodide are grown by physical vapor transport in a closed, 
evacuated ampoule. 
clump of heated source material and travel as vapor to the growing crystal. 
The transport in normal gravity is partially by diffusion and partially by 
convection, and as long as these transport mechanisms are constant with time, 
no negative effect on the crystal quality is expected to exist. 
arise when the convection transport becomes irregular with time, because the 
surface of the growing crystal will then be exposed to varying 
supersaturations of mercuric iodide vapor and growth irregularities will 
result. 
diffusion flow. 
This means that mercuric iodide molecules sublime from a 
Problems will 
In the absence of gravity, transport will be exclusively by regular 
No technique has been developed at this point to measure in detail existing 
convection currents on the ground, and the only way to obtain information 
about the effects of the convection currents on the structural quality is by 
actually doing the experiment. 
A different situation exists with respect to the statement that the crystal 
will slip by the force of its own weight, and thereby increase the density of 
defects. 
structure and the critical resolved shear stress (CRSS) of the crystalline 
material was studied in cooperation with a group at the University of 
California in Santa Barbara. 
2. 
scale, a crystal of approximately 1 an3 can be subject to slippage 
To obtain quantitative data concerning this effect the dislocation 
This work has been reported in References 1 and 
The results of the measurements indicated that, expressed on a macroscopic 
27 1 
perpendicular to the c-direction under the effect of its own weight when it is 
at the growth temperature. 
measurements at different temperatures. The curves indicate how the CRSS of a 
sample changes with temperature and that no irregular hardening effects take 
place during the test. It should also be realized that the samples used for 
these measurements m y  already have been strain-hardened by the cutting and 
polishing process required to shape the samples. 
Figure 1 shows a series of yield strength 
272 
F i g .  1 
273 
3. CRYSTAL GRmTH 
Furnace and Growth Amp0 ule Development 
To accommodate the Spacelab I11 facilities, a special furnace has been 
designed which reflects the basic elements of the usual ground-based furnaces 
(Ref. 3). A sketch of this prototype furnace is shown in Figure 2. 
The crystal growing ampoule, located in the center of the furnace, has a 
cylindrical shape. 
internally it forms a pedestal on which the crystal grows. 
fits over a metal support tube, inside of which a Peltier cooler is installed 
to provide cooling to the growing crystal. 
The bottom surface of the ampoule is indented so that 
The indentation 
The thermal profile around the ampoule necessary for crystal growth is 
provided by two independently controlled heating coils. 
ampoule is heated by means of a circular coil which is covered with a metal 
heat equalization ring. 
equally distributed amount of heat along the vertical walls of the ampoule. 
The combined heat inputs create a temperature profile which has a minimum at 
approximately half the height of the ampoule. At this level the crystal 
growth source material accumulates when the center of the pedestal is not 
cooled by the cold sting. Nucleation and crystal growth can be initiated by a 
combination of a reduction in the ampoule bottom temperature and activation of 
the Peltier cooler. 
The lower part of the 
A helically wound vertical coil provides basically an 
On the tope of the helical coil a reflective shield is installed to minimize 
heat losses. 
quality, so that the growing crystal can be observed through a microscope. 
This furnace assembly is covered with a bell jar of high optical 
274 
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Fig. 2 
SOURCE 
150.0 
0 107.5 
U 
30.0 
D I S T R I B U T I O N  
AMPOULE WALL A T  
SOURCE MATERIAL 
Fig.  4 
275 
The temperature profile along the walls of the ampoule used to grow the 
crystals on the ground is created by a combination of radiation and convection 
effects. During an experiment in the Spacelab, the driving force for 
convection in the air between the bell jar and ampoule will be reduced. 
addition, the furnace assembly will assume different orientations with respect 
to the prevalent acceleration vector. 
changed gravity conditions a series of experimental simulations was performed 
whereby the space between the bell jar and ampoule was evacuated and the whole 
furnace assembly was positioned upside down. 
the heat supply around the top of the ampoule is not sufficient when there is 
not a continuous convective supply of hot air from the lower part of the 
furnace which collects in the upper part of the bell jar and keeps the top of 
the ampoule hot. 
heater on the inside surface of the reflecting dome (not shown in Figure 2). 
The power input to this heater is slaved to the power controls for the helical 
heating coil. 
In 
In order to evaluate the effects of 
These experiments showed that 
This problem was corrected by installation of a lamellar 
The normally used ground-based growth ampoules have smooth walls and a flat 
pedestal. In the initial phase of the crystal growth procedures, the 
poly-crystalline charge is evaporated from the bottom of the ampoule to the 
minimum temperature zone on the vertical wall, and subsequently a seed is 
nucleated by reducing the pedestal temperature and activating the Peltier 
cooler. 
to perform these activities on the ground before the flight. 
ampoules with source and seed in place were subjected to vibration tests, 
simulating take-off and re-entry vibrations, to ascertain the adherence of 
source and seed to the glass surface. 
source material started to come loose at certain critical vibration 
intensities. 
fingers on the part of the inside wall where the source is located and a 
dovetail cavity in the pedestal to anchor the seed (see Figure 3). 
ampoule is positioned in the furnace with the section with glass fingers at 
the rear. The resistance of the furnace windings has been adjusted in that 
area so that an asymmetric cold region is created for the source material to 
collect. The seed is nucleated inside the dovetail cavity and is grown until 
it completely fills and extends somewhat beyond the cavity. 
prepared in this way are again subjected to qualification testing before 
flight. 
In order to maximize the crystal growth time in space it was decided 
The resulting 
Both the grown seed and parts of the 
The ampoule design was therefore modified to incorporate glass 
The 
The ampoules 
276 
The growth sequence is fully automated and consists of three essential parts: 
heat-up, growth and cool-down (see Figure 4). 
temperatures at the different control points in the furnace (source, heat 
distribution ring, cooling sting) are increased in a controlled way to values 
which are optimal for growth. The growth phase is then started by gradually 
decreasing the pedestal temperature, primarily by reducing the heat input to 
the base heater. During the growth phase it is possible, when necessary, to 
oscillate the temperature of the source material so that periodic growing and 
etching of the crystal occurs to obtain more stable crystal growth and to 
avoid spurious nucleation. 
early stages of the growth of mercuric iodide crystals (Ref. 4 and 5). 
During the heat-up phase the 
This is a technique usually employed during the 
When the source material has almost been depleted or when the experiment time 
has elapsed, the system is gradually cooled down to ambient temperature and is 
returned to its storage container for re-entry. 
277 
4.  SPACE EXPERIMENT 
Notwithstanding the efforts made to predetermine the proper growth conditions 
on the ground (elimination of convection around the ampoule by evacuation of 
the space between the ampoule and the bell jar), the temperatures again had to 
be adjusted to obtain crystal growth in space. 
that during the first 14 hours in space no growth was observed. 
temperature difference between the source and the pedestal had to be increased 
by 1.6OC to initiate growth. 
This accounts for the fact 
The 
As growth continued, it became clear that much higher temperature gradients 
could be applied than is usually done on the ground, without disturbing the 
stability of the crystal surfaces or without causing any new nucleation. 
Using the higher temperature gradients it was therefore possible to obtain 
higher growth rates by diffusion transport alone than is usually seen on the 
ground with both diffusion and convection transport, as shown in Figure 5. 
278 
GROUND CRYSTAL GROWTH RUN, SEP-OCT 1983 
SEE n W E  
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279 
5. CRYSTAL ANALYSIS 
Gama Ray Rocking Curves 
Since mercuric iodide is selected for radiation measurements because of its 
high absorption, it is impossible to use regular x-rays to do transmission 
measurements on pieces thicker than a few hundred urn. 
structural quality of a crystal approximately 1 cm thick it is necessary to 
use higher energy gama rays emitted by artificial sources. 
these kind of sources is that the radiation emitted by them is highly 
monochromatic, so that the diffraction peaks have a Gaussian shape. 
To evaluate the 
The advantage of 
Rocking curve measurements on the space crystal have been performed at the 
University of Missouri Research Reactor in Columbia, Missouri, where high 
intensity isotope sources can be produced and the equipnent to do this type of 
measurements is available (Ref. 6). 
6b. 
indicating that the crystal really consists of three sections at slight angles 
with respect to each other. 
other hand shows only the one peak expected of a single crystal without major 
grain boundaries. 
evidenced by the width and asymnetrical shape of the peak. 
The results are shown in Figures 6a and 
The rocking curve of an earth-grown crystal shows several peaks, 
The rocking curve of the space crystal on the 
The space crystal however shows signs of internal strain as 
Electronic Measurements 
The operation of radiation detectors is critically dependent on the transport 
properties of the electronic charge carriers created by the incident 
radiation. A good measure of the quality of the material is therefore provided 
by the values of the electron and hole mobilities and lifetimes (trapping 
times). Because of the high resistivity of the material these values can most 
adequately be determined by evaluation of the nuclear spectral response of a 
detector structure as a function of applied bias. Details of this procedure 
have been described in the literature (Ref. 7). 
280 
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The data obtained for detectors made from the space crystal and the maximum 
values found for ground-based detectors are as follows: 
Ground-based 
Crystals (max.) 
Space Crystal 
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42 
One can see that the electronic properties of the space crystal are 
significantly improved over ground-based crystals, especially with respect to 
the holes, which is the most critical factor for detector operation. 
considering these results it should be realized that the maximum values quoted 
for the ground-based crystals are for different detectors and are not found 
together in one single detector. 
I 
When 
282 c - '-\, 
SUMMARY AND mCLus1ms 
The experiment to grow a crystal of mercuric iodide in space performed 
exceptionally well, so that the stated experimental objectives were fulfilled. 
That the experiment was successful the very first time around is, in our 
opinion, for a large part due to the extensive pre-flight ground-based 
research and operational experiments. 
The space growth conditions showed that the stability of the crystal surfaces 
is much higher than on the ground where convection is present. 
the temperature gradient in the furnace could be increased so that the growth 
rate with diffusion transport alone was still larger than on the ground. 
As a result 
The structural quality of the space crystals, as measured by gama ray rocking 
curves, is superior to ground-based crystals. As a result, the electronic 
transport properties of the material were significantly improved. 
283 
The investigators want to acknowledge the technical and programmatic support 
of the NASA Spacelab Payload Project Office. Especially the efforts by 
Messrs. R. Ruff and R. Porter have been critical to the success of this 
experiment. 
284 
References 
1. T. W. James and F. Milstein, J. Mat. Sci. - 16 (1981) 1167. 
2. F. Milstein, et al., Nucl. Inst. Meth. 213 (1983) 65. 
3. W. F. Schnepple, et al., J. of Spacecraft and Rockets - 16 (1979) 144. 
4. M. Schieber, et al., J. of Crystal Growth 33 (1976) 125. -
5. L. van den Berg, et al., J. of Crystal Growth - 42 (1977) 160. 
6. W. B. Yelon, et al., J. Appl. Physics - 52 (1986) 4604. 
7. A. J. Dabrowski, Advances in X-ray Analysis - 29, edited by John C. Russ, 
Charles S. Barrett, and Donald I. Leyden (Plenum Publishing Corporation, 
1982) 185. 
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Growth of mercuric iodide (HgI2) for nuclear radiation detectors

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