Gamma-ray Detector Performance Research Articles

Dislocation reduction in an MOVPE-grown CdTe/Si epilayer by ex-situ annealing and its effect on the performances of gamma ray detectors fabricated

We studied ex situ thermal cycle annealing on metalorganic vapor phase epitaxy (MOVPE) grown CdTe on (211) Si substrates for dislocation density reduction and its effect on the performance of gamma-ray detector fabricated. The ex situ annealing was performed by varying temperatures from 600 °C to 1000 °C for different anneal durations and cycles varied from 1 to 7 in a hydrogen environment. Gamma-ray detector was fabricated in a p-CdTe/n-CdTe/n+-Si heterojunction diode structure. Dislocation densities were evaluated by the etch pit density (EPD) technique. Dislocation densities were decreased when the annealing temperature was increased above 800 °C. This is due to the annealing enhancing dislocations’ glide motion, which annihilate and reduce their density. The device property could be improved by subjecting the annealing during the early stage of the growth. Devices subjected to annealing showed lower dark current and improved gamma detection property when compared to the devices that were not subjected to annealing during their fabrications.

Distribution of the high resistivity region in CdZnTe and its effects on gamma-ray detector performance

The effect of the location of the high resistivity region on gamma-ray detector performance within the crystal boule is investigated for 10% zinc with 1.5% excess Te. By varying the indium-doping concentration in several CdZnTe boules, the region of high resistivity is seen to move along the vertical length of the crystal. The variation of the zinc concentration within the crystal boule is compared with the location of the high resistivity region along the length of the crystals. The concentration of zinc is extracted from Fourier transform infrared (FTIR) measurements, and the segregation coefficient is calculated using data obtained from the CdZnTe crystals. The zinc distribution is plotted in terms of the location along the crystal length in order to correlate the concentration with detector performance. Radiation spectra obtained from the 122-KeV gamma rays using a 57Co source reveal a strong dependence between detector performance and the relative location of the high resistivity region within the crystal. Initial results suggest that there are three semi-distinct regions along the length of the boule that give very different characteristics, where it can be said that the best detector performance is in the middle region with a 6% resolution of the 122-KeV peak, which is quite good for test detectors without a guard ring such as these. It is determined that this middle region has a zinc concentration of ∼9–11%, which varies slightly from the original concentration of 10%. The differences in the performance characteristics are discussed, and defect distribution within the crystal as the main source of the variation is suggested. Also, based on the results, it is believed that the role of indium is essentially to compensate for the vacancies in the crystal and, therefore, is secondary to the crystalline properties and impurities within the boule. Overall, it is believed that crystalline defects and inclusions play a greater role in determining the performance characteristics of CdZnTe radiation detectors.

Defects in CZT crystals and their relationship to gamma-ray detector performance

This paper reviews some of the progress obtained in the understanding of defects in detector grade cadmium zinc telluride material (CZT). Several techniques have been utilized to elucidate some of the issues related to compositional uniformity, effects of precipitates, grain boundaries, and surface defects related to mechanical and chemical treatments. In few cases, special mapping capabilities had to be developed to allow correlations with detector performance.

The comparison on the performance of a gamma-ray spectrometer with the variation of Pt(Au)/CdZnTe/Pt(Au) interface

Cd 1− x Zn x Te grown by high-pressure Bridgman method (HPB) has been known to be a good candidate for gamma-ray detection material. The metal contacting conditions are very important for the performance of gamma-ray detector. Therefore, it is essential to find proper contact technology. For these purposes, various contact methods (electroless deposition, thermal vacuum evaporation) and materials (Au, Pt) are attempted. From the measurement of current–voltage characteristics, gamma-ray spectroscopy and auger electron spectroscopy (AES), the physical properties of metal contact on the CdZnTe are analyzed and contact methods compared with one another. Electroless Pt deposition is found to have better properties than others in many respects.

CdZnTe material uniformity and coplanar-grid gamma-ray detector performance

"Electron-only" detection techniques such as the coplanar-grid method are effective in overcoming some of the charge transport problems of CdZnTe and, consequently, have led to large-volume gamma-ray detectors with good energy resolution while operating at room temperature. A requirement for the success of these techniques is uniformity in electron generation and transport. Once large inhomogeneities in these properties caused by grain boundaries and other large-scale crystal defects are eliminated through simple material screening techniques, small variations remain and limit the gamma-ray energy resolution. In this paper we present the results from our characterization and analysis of these nonuniformities in commercially available CdZnTe, and we establish the relationship between the nonuniformities and the ultimate coplanar-grid gamma-ray detector performance through experimental measurements.

The effect of elemental and hydrocarbon impurities on mercuric iodide gamma ray detector performance

Mercuric iodide is a room temperature semiconductor material that is used for gamma ray and x-ray radiation detection. Mercuric iodide is synthesized from mercuric chloride and potassium iodide and is then purified by a series of melts and sublimation steps and by zone refining. The mercuric iodide is grown into crystals and platelets and then fabricated into detectors. Elemental contamination may be a determining factor in the performance of these detectors. These contaminates may be present in the starting material or may be introduced during, or be unaffected by, the purification, growth or fabrication steps. Methods have been developed for the analysis of trace levels of elemental contamination. Inductively Coupled Plasma/Mass Spectroscopy (ICP/MS), Inductively Coupled Plasma/Optical Emission Spectroscopy (ICP/OES) and Gas Chromatography/Mass Spectroscopy (GC/MS) are used to determine sub ppm levels of many trace elemental impurities. Trace levels of many elemental impurities in the raw mercuric iodide are significantly reduced during the purification and zone refining processes. Though the levels of impurities are reduced, poor performing mercuric iodide detectors have contamination levels remaining or reintroduced which are higher for Ag, Al, Ca, Cu, Mg, Mn, Na, Pb and Zn than detectors with good gamma ray response. This paper will discuss the analytical methodology, the effects of purification on impurity levels, and the correlation between detector performance and impurity levels.

Electrodrift purification of mercuric iodide for improved gamma-ray detector performance

The primary problems with mercuric iodide for use in room-temperature radiation detectors are the low yield of high quality devices and the problems with detector stability. It is believed that one of the prime causes of these failures is the presence of certain detrimental impurities. Previous experiments have demonstrated that these impurities can be introduced during processing and that they can strongly affect detector performance. Significant effort has been expended to develop and utilize various mercuric iodide purification schemes. While these techniques are effective to some degree, the level of impurities in detector materials is still higher than that found in more established semiconductors. In this work we developed and investigated a new purification scheme that is based on the mobility of many of the impurities in mercuric iodide under an applied electric field. In this “electrodrift” process undesirable metallic impurities are removed from a portion of a bulk mercuric iodide charge to produce higher purity material for detector crystal growth. The lower level of impurities results in fewer charge-trapping centers and therefore there is the potential for significant improvements in the carrier lifetimes and detector stability. This paper reports on some preliminary results of the effectiveness of this method.

Defects in Silver-Doped Mercuric Iodide Crystals and their Effect on X-Ray and Gamma-Ray Detector Performance

ABSTRACTThe processing steps associated with purification of source material, crystal growth, and attachment of electrical contacts can introduce defects into mercuric iodide (HgI2) that degrade the performance of detectors. We have employed low-temperature photoluminescence (PL) spectroscopy to study radiative recombination centers in the interfacial region between a thin semitransparent film of silver and mercuric iodide. The Ag film was found to introduce a new broad emission band centered at 5490 Å in the photoluminescence spectrum of HgI2. This PL feature can be used as a signature to identify the existence of Ag as a contaminant in HgI2crystals and detectors. Experiments were also conducted on mercuric iodide surfaces that had been doped with silver, and the results showed that Ag is a rapid diffuser in bulk HgI2. Detectors with silver electrodes were also fabricated and tested using an americium-241 gamma-ray source. Large increases in the leakage currents were observed for the Ag-doped HgI2devices, indicated that Ag impurities are electrically active in HgI2. These measurements show that silver is unacceptable as an electrode material for mercuric iodide x-ray and gamma-ray detector applications. In addition, they reveal that caution must be taken during handling of mercuric iodide source material, crystals, and detectors to avoid contact with silver, silver compounds, or with any material that contains silver as a contaminant.

Timing techniques for improving thick HgI 2 gamma-ray detector performance

Spectroscopy in thick HgI 2 γ-ray detectors is limited by low hole mobility, short hole trapping times, and nonuniform internal electric fields. Estimates of ne, the change deposited by a γ-ray absorption event, thus depend upon the event location d. Determining d then allows a more precise estimate of ne than is possible from collected charge alone. We theoretically examine the concept of timing the duration of the electron current flow to determine d and show how d is employed to produce an accurate ne value. A modeling study shows that 1 to 2 cm thick HgI 2 detectors are capable, in principle, of 1% resolution at 662 keV with photopeak efficiencies of 6–12%.