E. Cross

CHARACTERIZATION OF LEAD IODIDE FOR NUCLEAR SPECTROMETERS

Abstract We report on the results of a number of investigations into the material properties of lead iodide and their relation to x- and gamma-ray spectrometers. The effectiveness of zone refining as determined by inductively coupled plasma optical emission spectroscopy is demonstrated. We show that zone refining is effective in producing lead iodide with a higher degree of purity in terms of extrinsic dopants and we determine the segregation coefficients for a number of these impurities. Low temperature photoluminescence also indicates an improvement in the material properties of the purified lead iodide. The chemical etching characteristics, including etch rates, of lead iodide are presented for a number of etching solutions. Triple axis x-ray diffraction measurements have been employed to determine the structural perfection of the lead iodide after diamond sawing and etching and recovery of the crystallinity of the material is seen after Nal etching.

Lead Iodide X-Ray and Gamma-Ray Spectrometers for Room and High Temperature Operation

In this study the authors report on the results of the investigation of lead iodide material properties. The effectiveness of a zone refining purification method on the material purity is determined by ICP-MS and ICP-OES and correlated to the electrical and physical material properties. They show that this zone refining method is very efficient in removing impurities from lead iodide, and they also determine the segregation coefficient for some of these impurities. Triple axis X-ray diffraction (TAD) analysis has been used to determine the crystalline perfection of the lead iodide after applying various cutting, etching and fabrication methods. The soft lead iodide crystal was found to be damaged when cleaved by a razor blade, but by using a diamond wheel saw, followed by etching, the crystallinity of the material was much improved, as observed by TAD. Low temperature photoluminescence also indicates an improvement in the material properties of the purified lead iodide. Electrical properties of lead iodide such as carrier mobility, were calculated based on carrier-phonon scattering. The results for the electrical properties were in good agreement with the experimental data.

The investigation of custom grown vertical zone melt semi-insulating bulk gallium arsenide as a radiation spectrometer

Vertical zone melt (VZM) bulk GaAs boules have been zone refined (ZR) and zone leveled (ZL) to reduce EL2 deep donor levels and impurity concentrations with the intent of improving properties for gamma ray detectors. ZR and ZL GaAs boules had background impurity levels and deep donor EL2 concentrations near or below detectable limits. The crystal mosaic of the material at locations near the seed end was slightly superior to commercial liquid encapsulated Czochralski (LEC) material, and nearly equivalent to commercial vertical gradient freeze (VGF) material. The crystal mosaic in ZL material degraded towards the tail end. The homogeneity of the electrical properties for the ZL and ZR VZM material was inferior compared to commercially available bulk GaAs material. Post growth annealing may help to homogenize some electrical properties of the material. The charge collection efficiency of the ZR GaAs detectors was only 30% maximum, and only 25% maximum for the ZL GaAs detectors. Resulting gamma ray spectra was poor from detectors fabricated with the ZL or ZR VZM material. Detectors fabricated from material that was both ZR and ZL did not demonstrate gamma ray resolution, and operated mainly as counters. The poor spectroscopic performance is presently attributed to the inhomogeneity of the electrical properties of the ZR and ZL GaAs materials. Comparisons are made with detectors fabricated from VGF SI bulk GaAs.

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

Abstract 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.

MULTICHANNEL CDZNTE GAMMA RAY SPECTROMETER

A 3 cm{sup 3} multichannel gamma spectrometer for DOE applications is under development by Digirad Corporation. The device is based on a position sensitive detector packaged in a compact multi-chip module (MCM) with integrated readout circuitry. The modular, multichannel design will enable identification and quantitative analysis of radionuclides in extended sources, or sources containing low levels of activity. The MCM approach has the advantages that the modules are designed for imaging applications, and the sensitivity can be arbitrarily increased by increasing the number of pixels, i.e. adding modules to the instrument. For a high sensitivity probe, the outputs for each pixel can be corrected for gain and offset variations, and summed digitally. Single pixel results obtained with discrete low noise readout indicate energy resolution of 3 keV can be approached with currently available CdZnTe. The energy resolution demonstrated to date with MCMs for 511 keV gamma rays is 10 keV.

Material analysis and characterization on zone refined and zone leveled vertical zone melt GaAs for radiation spectrometers

Abstract GaAs is a wide band gap (1.42 eV) semiconductor that has shown promise as a room temperature operated γ-ray detector. A practical γ-ray detector would be large in volume, hence the resistivity of the material must be high to ensure large depletion volumes and low leakage currents. Commercially available semi-insulating (SI) bulk GaAs is compensated by a balance between native defect deep donors (EL2) and residual dopant impurities. The high concentrations of electrically active deep and shallow levels are believed to contribute to electric field distortions observed in γ-ray detectors fabricated from SI bulk GaAs. Hence, the controlled reduction of native defects and contaminant impurities may yield improved bulk GaAs for γ-ray detectors. Custom grown vertical zone melt (VZM) bulk GaAs is presently under investigation as an alternative material for room temperature operated γ-ray detectors. The VZM technique allows for zone refinement (ZR) and zone leveling (ZL) of the GaAs ingots. Custom growth of the material allows for controlled changes in the bulk crystal, including deliberate reductions in EL2 and impurity concentrations. Comparisons are made to commercial vertical gradient freeze (VGF) and liquid encapsulated Czochralski (LEC) bulk GaAs.

Optical and electrical characterization of copper- and chlorine-doped cadmium zinc telluride

The authors have used low-temperature photoluminescence spectroscopy and photo-induced current transient spectroscopy to study the properties of copper-doped Cd{sub 1{minus}x}Zn{sub x}Te with x {approx} 0.1 and chlorine-doped Cd{sub 1{minus}x}Zn{sub x}Te with x = 0.2, 0.35 and 0.5. The current-voltage characteristics and detector response were also measured. They observed variations in charge collection and resistivity in the Cu-doped samples which was correlated with variations in PICTS spectra. The Cl-doped material was found to have insufficient resistivity for detector operation.

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

Abstract 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.

Electron paramagnetic resonance investigation of donor impurities in CdZnTe grown by different techniques

Cadmium zinc telluride (CdZnTe) is being developed for room- temperature x-ray and gamma ray detectors. Identification and control of point defects and charge compensators are currently important issues. We have used electron paramagnetic resonance (EPR) and photo-induced EPR to evalute shallow-donor defects in CdZnTe crystal grown by two different techniques. Samples grown by the high-pressure Bridgman technique and a crystal grown by horizontal Bridgman at IMARAD and doped with indium were included in this study. Prior to the EPR investigations, we performed liquid-helium photoluminescence (PL) in order to examine the radiation recombination paths and identify the presence of other defects in these crystals. Spectra were obtained showing sharp excitonic lines, shallow and deep DAP emission bands, and a deeper 1.1 eV emission. The PL data help define the optical excitation range used in photo-EPR measurements. The photo-EPR data obtained from our samples is used to determine the concentration of isolated donor centers, while the EPR signal present under no illumination gives a measure of the net compensation. We also report the excitation wavelength dependence of the isotropic EPR signal from the shallow donors.

Structural defects and impurities on CZT crystals

Various types of precipitates and grain boundaries have been studied in Cd1-xZnxTe (CZT). In this study we used elemental analysis methods such as scanning electron microscopy (SEM), microprobe analysis, inductively coupled plasma mass spectroscopy (ICP/MS) and the new laser ablation ICP/MS methods. Transient charge technique was applied for the first time of CZT crystals for evaluating the electrical transport properties of semiconductors. Another method, IR transmission spectroscopy, enables us to evaluate the microstructure defects and then to correlate this with impurity level and electrical properties in order to have a better understanding of the requirements to improve the yield for large volume CZT spectrometers. We have evaluated crystals from the former Soviet Union, which have high concentration of defects. Precipitates and grain boundaries rich with carbon were observed in CZT crystals. Electrical transport properties such as (mu) (tau) (mobility-lifetime product) were measured and correlated with the chemical physical defects, as observed by IR transmission. On crystals rich with many microstructures, as shown by IR transmission, lifetimes below 1 microsecond(s) were measured, compared with 5 - 15 microsecond(s) on the detector grade materials. SEM and microprobe analysis performed on the precipitates gave high values of carbon. However, using laser ablation ICP/MS, a value in the range of 200 - 800 ppm for carbon was measured.