A generalized expression for charge carrier transport and absorption-limited sensitivity of x-ray photoconductors is derived by analytically solving the continuity equation for both holes and electrons considering the drift of electrons and holes in the presence of deep traps The normalized sensitivity equation is applied to stabilized a-Se and HgI2 In the latter case, the sensitivity model is fitted to published data to determine the electron and hole ranges (64×10−6 cm2/V and 7×10−8 cm2/V, respectively) in screen-printed polycrystalline HgI2

Charge collection and absorption-limited sensitivity of x-ray photoconductors: Applications to a-Se and HgI2

Present status of room temperature semiconductor detectors

Recent advances in X-ray photoconductors for direct conversion X-ray image detectors

Crystal growth and applications of mercuric iodide

A review is given of room temperature semiconductors used as nuclear radiation detectors including GaAs, CdTe and HgI2. Some new applications are described in the field of dosimetry, spectroscopy and gamma or X-ray imaging.

Some new developments in the field of high atomic number materials

The influence of native defects in detector grade mercuric iodide crystals has been investigated by doping with either mercury or iodine. Measurements have been made of mobility-lifetime products and of thermally stimulated conductivity in undoped, Hg doped, and I2 doped crystals. The results are discussed in terms of different trapping mechanisms.

Native Defect Compensation in HgI2 Crystals

Thick (~l cm) mercuric iodide gamma-ray detectors have been produced which show spectroscopic qualities at moderate detector biases (~5 kV) comparable to those of thin spectrometers. Efficiency measurements indicate that the entire volume of the detectors is active. Spectra resolutions of less than 10% have been obtained for gamma-ray energies above 1 MeV. Short charge collection times have produced the best results. Measurement of crystal charge transport properties is discussed. A small amount of bias conditioning is necessary for best performance. Operating parameters of the detectors have been investigated.

Recent Developments in Thick Mercuric Iodide Spectrometers

Mercuric iodide is a high-density high-Z semiconducting material useful for gamma ray detection. This makes it convertible to a thermal neutron detector by covering it with a boron-rich material and detecting the 478-keV gamma rays resulting from the /sup 10/B(n,/spl alpha/)/sup 7/Li/sup */ reaction. However, the 374 barn thermal capture cross section of /sup nat/Hg, makes the detector itself an attractive absorber, and this has been exploited previously. Since previous work indicates that there are no low-energy gamma rays emitted in coincidence with the 368-keV capture gamma from the dominant /sup 199/Hg(n,/spl gamma/)/sup 200/Hg reaction, only the 368-keV capture gamma is seen with any efficiency with a relatively thin (few millimeter) detector. In this paper, we report measurements of neutrons via capture reactions in a bare mercuric iodide crystal and a crystal covered in /sup 10/B-loaded epoxy. The covered detector is an improvement over the bare detector because the presence of both the 478and 368-keV gamma rays removes the ambiguity associated with the observation of only one of them. Pulse height spectra, obtained with and without lead and cadmium absorbers, showed the expected gamma rays and demonstrated that they were caused by neutrons.

Neutron detection with mercuric iodide

Development of mercuric iodide nuclear detectors in our laboratory has concentrated on crystalline perfection, optimization of electronic properties, and device fabrication. Increased crystalline perfection can be obtained with better crystal growth temperature control combined with slightly modified growth parameters. Adjustment of the stoichiometry of the crystal growth source material has increased the mobility-trapping time (??) product for the holes. Preliminary experiments with different contact materials and contacting techniques gave ambivalent results.

Recent Improvements in HgI2 Detectors

Single crystals of CdZnTe of dimensions 10 times 10 times 3 mm3 and HgI2 of dimensions 10 times 10 times 2 mm3 were fabricated into planar detectors. Four sets of detectors were then exposed to simulated solar proton events of varying magnitude at the Kernfysisch Versneller Instituut in the Netherlands. Events were simulated by generating protons over the energy range 60 MeV to 200 MeV in 10 energy bands which approximated the spectral shape of the August 1972 solar particle event. One pair of detectors (a CdZnTe and a HgI2) was exposed to an integral fluence of 108 protons cm-2, a second to 109 protons cm-2, a third to 1010 protons cm-2 and the fourth to 1011 protons cm-2. The latter corresponds to an absorbed dose in silicon of 100 krad or in SI units, 1 kGy. The detectors were characterized both before and after the irradiations in terms of background count rate and spectral performance using 55Fe, 109Cd, 241Am, 57Co and 137Cs radioactive sources. Typical pre-irradiation FWHM energy resolutions of 3 keV and 4 keV were recorded at 60 keV for the CdZnTe and HgI2 detectors, respectively. It was found that HgI2 is very radiation tolerant showing little degradation in energy resolution or charge collection efficiency (CCE) for input fluences up to 1011 protons cm-2. CdZnTe, on the other hand, showed a clear degradation in energy resolution and CCE for input fluences above 109 protons cm-2. For example, the energy resolution degraded by a factor of 3 between 109 and 1010 protons cm-2 while the CCE decreased by a factor of 2.

L. van den Berg

Papers

Native Defect Compensation in HgI2 Crystals

Recent Developments in Thick Mercuric Iodide Spectrometers

Neutron detection with mercuric iodide

Recent Improvements in HgI2 Detectors

Assessment of the Radiation Tolerance of CdZnTe and HgI $_{2}$ to Solar Proton Events