Growth of mercuric iodide (HgI2) for nuclear radiation detectors

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

Spacelab 3 vapor crystal growth experiment

The Space Shuttle Challenger, with Spacelab 3 as its payload, was launched into orbit April 29, 1985. The mission, number 51-B, emphasized materials processing in space, although a wide variety of experiments in other disciplines were also carried onboard. One of the materials processing experiments on this flight is described, specifically the growth of single crystals of mercuric iodide by physical vapor transport

Energy Resolution Enhancement of Mercuric Iodide Detectors

A pulse processing technique has been developed which improves the gamma-ray energy resolution of mercuric iodide detectors. The technique employs a fast (100 ns) and a slow (6.4 µs) pulse height analysis to correct for signal variations due to variations in charge trapping. The capabilities of the technique for energy resolution enhancement are discussed as well as the utility of the technique for examining the trapping characteristics of individual detectors. We have achieved an energy resolution of 2.6% FWHM at 662 keV with an acceptance efficiency of 100% from a mercuric iodide detector which gives 8.3% FWHM using standard techniques

Advances in the development of encapsulants for mercuric iodide X-ray detectors

Advances in the development of protective impermeable encapsulants with high transparency to ultra-low-energy X-rays for use on HgI_2 X-ray detectors are reported. Various X-ray fluorescence spectra from coated detectors are presented. The X-ray absorption in the encapsulants has been analyzed using characteristic radiation from various elements. Results suggest that low-energy cutoffs for the detectors are not determined solely by the encapsulating coatings presently employed but are also influenced by the front electrode and surface effects, which can affect the local electric field or the surface recombination velocity. An energy resolution of 182 eV (FWHM) has been achieved for Ni L lines at 850 eV. Improved detector sensitivity to X-ray energies under 700 eV is demonstrated

Modulsystem zum Aufbau von Bioreaktoren

The description relates to a module system for the construction of bioreactors which have graduated diameters within a specific range and whose height-diameter ratio can have values of stirred reactors or also values of column-shaped reactors. The invention is characterized by the provision of a base for all diameters, said base permitting the connection of stir drives as well as equipment for the dispersion and/or distribution of gas and/or liquids, and a contour for the fluid-tight connection to a corresponding flange on a reactor vessel component having one of the graduated diameters, whereby the reactor vessel components also has a flange at its other end permitting the connection of another reactor vessel component or it acts as a termination element

Large-area mercuric iodide photodetectors

This article discusses the limits of the active area of mercuric iodide photodetectors imposed by the size of available crystals, electronic noise, and the uniformity of charge carrier collection. Theoretical calculations of the photodetector electronic noise are compared with the experimental results. Different entrance contacts were studied including semitransparent palladium films and conductive liquids. HgI/sub 2/ photodetectors with active area up to 4 sq cm are matched with NaI(Tl) and CsI(Tl) scintillation crystals and are evaluated as gamma radiation spectrometers

Preliminary studies of charge carrier transport in mercuric iodide radiation detectors

Mercuric iodide single crystals have been grown by static and dynamic sublimation methods. Characteristics of contacts and detector capacitance have been studied by photon excitation methods. Gamma and x-ray spectrometry has been carried out with completed detectors showing resolutions comparable to the best results published to date. A measurement of hole trapping length has been made from the spectral shapes observed and has been found to be approximately 0.3 mm. Transient waveform analysis with alpha-particle excitation shows hole mobilities of approximately 3 cm/sup 2//V-sec for a highly purified crystal and 0.05 for an expected less pure crystal. Electron mobilities of 120 cm/sup 2// Vsec are observed. An attempt is made to explain the observed tansient waveforms in terms of a single dominant trap model, With only partial success. Due to the strongly excitonic character of the material, it is proposed that the unfamiliar observations made regarding transport properties with the HgI/sub 2/ detectors studied may be due to exciton dissociation under high electric fields, to long exciton lifetimes and to interactions between excitons and trapping centers in the material. (auth

Influence of temperature upon dislocation mobility and elastic limit of single crystal HgI/sub 2/

The practical importance of studying mechanical properties and dislocation structure of HgI/sub 2/ is reviewed briefly. Specifically, the performance of single crystal HgI/sub 2/ radiation detectors is evidently sensitive to crystalline imperfections; the dislocation structure, in turn, can be altered during detector fabrication, depending upon the mechanical properties of the crystal and the stresses to which the crystal is subjected. The influence of temperature upon dislocation mobility and plasticity in vapor-grown crystals of mercuric iodide is examined. Dislocation mobiity is determined by measuring the lengths of the longest arms of dislocation etch pit rosettes on (001) surfaces following microhardness indentation and chemical etch. Measurements were made in the range from room temperature to the phase transition temperature of 127/sup 0/C. Dislocation mobility was found to be an increasing function of temperature, with the effect accelerating as the phase transition is approached. Increasing temperature was also found to lower the critical resolved shear stress for plastic deformation on slip on (001) planes. In these contexts, the vapor-grown crystals are clearly softer at their elevated growth temperatures. The results are discussed in terms of a dislocation model involving soft and hard glide dislocations