HgI2 Detectors Research Articles

Analysis of HgI2 And PbI2 Crystals and Detectors by Particle-Induced X-Ray Emission (PIXE) and Ion Backscattering Spectroscopy (IBS)

ABSTRACTThe Ion Micro-Analysis Group (IMAG) in Livermore conducts quantitative trace elemental analysis with PIXE and depth profiling with IBS using an MeV ion microbeam. The system has the capability to produce two-dimensional trace element and IBS images. PIXE analyses have been conducted on HgI2 and PbI2 crystals and detector materials in order to identify and quantify near surface trace contaminants. IBS measurements have been conducted to investigate elemental depth distributions in various materials. The results of measurements on several different samples are reported and a discussion of factors affecting quantitative in vacuo microanalysis of these materials is presented.

Mercuric Iodide X- and Gamma-Ray Deteciors for Space Applications

ABSTRACTRecently, substantial progress has been made in the development of HgI2 detectors and associated electronics for space instrumentation. The many technical challenges in the instrument development involving spectroscopy performance, power consumption, weight, size, operation over wide ranges of temperature and ambient gas pressure, and others have been overcome. Descriptions of instrumentation under development such as: x-ray and alpha backscattering spectrometer, scanning electron microscope and particle analyzer, Mössbauer spectrometer, and gamma-ray spectrometer are given.

Use of the EGS4 Monte Carlo code to evaluate the response of HgI2 and CdTe detectors for photons in the diagnostic energy range

Abstract We present the results of the detailed simulation of the response of a HgI 2 crystal in terms of efficiency, energy and space resolutions versus photon energy in the diagnostic energy range 20–100 keV. Some configurations of CdTe crystals for positron emission tomography are also evaluated.

HgI2 detectors for Pu contamination monitoring and radiological protection measurements

HgI2 detectors developed from solution-grown crystals show good-detection properties (resolution, efficiency, stability) for X-rays and low-energy gamma rays (E < 50 keV). These room temperature spectrometers have the same efficiency and a better resolution than NaI(Tl) ones in the 10–30 keV energy range and are especially well suited for Pu contamination measurements by detection of the emitted XL. Two examples of devices based on these HgI2 detectors are shown: (1) a compact probe for Pu detection in wounds which gives also information on the depth of the contamination, and (2) an array of 16 × 1 cm2 HgI2 detectors for Pu contamination of soil or wet surfaces where alpha detection is not valid. A tentative use of HgI2 detectors as compact ionization chambers working in the current mode is also described.

Properties of lead iodide semiconductor radiation detectors

Semiconductor radiation detectors have been fabricated from melt grown crystals of lead iodide (PbI2) and the performance of these detectors as room temperature X-ray spectrometers has been measured. These detectors exhibit good energy resolution (915 eV FWHM for the 5.9 keV peak of 55Fe at 20°C). Preliminary results indicate they are more stable than HgI2 detectors and capable of operating at temperatures over 100°C.

Development of mercuric iodide X-ray and alpha backscattering spectrometers for space applications

An alpha particle instrument in combination with a room temperature mercuric iodide X-ray spectrometer can provide a complete and detailed in situ chemical analyses of extraterrestrial bodies. Conceptual designs of the instrument will be presented. One of the important questions about the resistance of HgI2 detectors to the radiation damage will be addressed. Our experimental results of exposure of HgI2 detectors to proton fluxes up to 1012 protons/cm2 show no degradation in the detector performance.

Practical application of HgI2 detectors to a space-flight scanning electron microscope

Mercuric iodide X-ray detectors have been undergoing tests in a prototype scanning electron microscope system being developed for unmanned space-flight. The detector program has met with considerable success, although not all goals have yet been met. This success has been the result of carefully addressing the issues of geometric configuration in the SEM, compact packaging that includes separate thermoelectric coolers for the detector and FET, X-ray transparent hermetic encapsulation and electrical contacts, and a clean vacuum environment.

Correlation between mercuric iodide detector performance and crystalline perfection

X-ray, neutron and gamma ray diffraction rocking curves; X-ray topography; microhardness; and optical microscopic measurements have been performed directly on several mercuric iodide (Hgl2) nuclear radiation detectors fabricated from single crystals grown from the vapor phase. Two types of detectors were measured: spectrometer types (grades A and B), which had resolutions of 5–10% for the 662 keV photopeak of 137Cs, or radiation counters (grades C and D), where the spectral resolution ranged from 11% to no resolution. A good correlation has been found between the detector grade and the full width at half maximum (FWHM) of both the X- and gamma ray rocking curves (i.e., the higher the detector grade (A or B), the narrower the FWHM of the diffraction peak). X-ray topography also correlated with well both the FWHM of the diffraction X-ray rocking curve and the detector grade. The uniformity of the microhardness of the HgI2 detectors was found to be proportional to the nuclear performance of the detector. The better spectrometer-grade detectors were softer and much more uniform in microhardness than the most inferior detectors. The better detectors were also found to have much smoother surfaces than the poorer detectors, as observed by optical microscopy studies.

I(t), I(V) and surface effect studies of vapor grown and solution grown HgI2 detectors

The experiments we have carried out during the last years show that solution grown HgI2 crystals exhibit a lower μτ product than vapor grown crystal, despite a crystalline perfection and a purity at least as high, or even higher. They also show different surface properties. From the current response I(t) following a voltage step, and from the I(V) characteristic analysis, we determine the trapping levels Et present in the sample and show the influence of the contact deposition technique and surface treatments on the electrical field gradient nearby the electrode. A nitric acid treatment increases the blocking effect of the cathode contact on solution grown crystal, and therefore improves their detection performance for low energy X-rays and gamma rays.

Photoluminescence variations associated with the deposition of palladium electrical contacts on detector-grade mercuric iodide

Specimens of mercuric iodide with evaporated semitransparent palladium contacts have been studied using low-temperature photoluminescence spectroscopy. Distinct differences were found between spectra taken from beneath the Pd contacts and those taken from regions on the HgI2 sample that were masked during the Pd deposition, indicating that contact fabrication can change the defect structure near the contact/substrate interface. Comparison of the spectra from spots beneath the contacts with spectra from bulk material specimens and HgI2 detectors graded in terms of their nuclear detection performance suggests that the processing steps used to deposit electrical contacts and the choice of contact material may have a significant influence on detector performance.

Time-of-flight x-ray photoconductivity of HgI2

Time-of-flight photoconductivity measurements were performed on HgI2 using a penetrating, pulsed x-ray source, simulating the operation of photoconductive x-ray detectors. By examining a variety of HgI2 samples, a wide range of electron and hole mobilities were observed, but in all cases hole transport was highly localized, limiting the collection of the photocarriers in HgI2 detectors. The intrinsic photocarrier generation and recombination processes differed from classical Onsager and Langevin mechanisms observed in low-mobility photoconductors.

Energy resolution of HgI2 detectors of x-ray and gamma quanta

This article reports on the results of an investigation of the spectrometric characteristics of uncooled semiconductor detectors on the basis of mercuric iodide. We determined the limitations which are imposed upon the energy resolution by noise of various origins and by the inhomogeneity of the crystal in the spectrometry of quanta in the energy range of 10/sup 3/ keV. The energy resolution of the detector was determined from the response function as the width of the total absorption peak on half its full height.

Dark current transients in HgI2 single crystals used as γ- and x-ray spectrometers

Transient characteristics of dark currents have been measured in HgI2 single crystals as functions of voltage and temperature. The existence of a hole trapping center, 0.7 eV deep and of density ∼1015 cm−3, was established, which effectively lowers the hole drift mobility by about seven orders of magnitude. The samples were found to be slightly extrinsic n type, the Fermi level being 0.9 eV deep. The room-temperature electron density was about 2×103 cm−3. The meaning of these characteristics to the performances of HgI2 γ- and x-ray spectrometers is discussed. Particularly, it is suggested that lowering of the HgI2 detector operation temperature to about 0 °C will improve its energy resolution capabilities.

Mercuric Iodide Photodetectors for Scintillation Spectroscopy

We have measured the responses to 137Cs (662 keV) of both a 1-inch-diam by 2-inch-thick NaI(Tl) scintillator optically coupled to a 1-inch-diam by 800-μm-thick mercuric iodide (HgI2) photodetector, and a 1-cm-diam by 1-cm-thick CaWO4 scintillator coupled to a 1.3-cm-diam by 600-μm-thick HgI2 photodetector. Best spectral resolution to 137Cs was 7.8% FWHM for the NaI(Tl)-HgI2 and 12.5% FWHM for the CaWO4-HgI2 detectors; peak-to-valley ratios were 26:1 and 16:1, respectively. HgI2 detectors operate at room temperature and their use in scintillation spectroscopy presents the ultimate miniaturization of scintillation detectors, limited mainly by the size of the scintillation crystal.

Computer Simulation of Semiconductor Dosimeters

A previous Monte Carlo program, ISIDE, for the simulation of semiconductor gamma ray detectors, has been extended to a new code, PHANTOM, which can be used either for calculating the gamma ray exposure from the output of semiconductor detectors working in pulse mode or for determining the isodose curves in a cylindrical phantom exposed to a disk gamma ray source. The results are used to evaluate the exposure from the spectra recorded by a HgI2 detector in the energy range 10 KeV - 1 MeV. A good agreement with experimental results either for phantoms and for semiconductor detectors is obtained.

Excitation and Detection of High Energy Characteristic X-rays (20-90 KeV) Using a Novel Radiometric Technique*

Recently, we have developed a new radioisotope excited technique for x-ray fluorescence which employs “direct” beta particle excitation to produce characteristic x-rays for elemental analysis. We have also studied this technique with various other isotopes with different decay modes. But in all these previous works, we have only studied the x-ray energy region below the Bak lines (≤ 35 KeV), because of the poor efficiency of the Si(Li) semiconductor at higher energies.In this work we have employed a planar Ge detector, which has about a 100% efficiency in the range of about 20 to 100 KeV. We have also compared this detector with a HgI2 detector, even though its resolution and efficiency is far less than the planar Ge detector.

Gamma-Ray Detectors for Intelligent, Hand-Held Radiation Monitors

Small radiation detectors based on HgI2, bismuth germanate (BGO), plastic, or NaI(Tl) detector materials were evaluated for use in small, lightweight radiation monitors. The two denser materials, HgI2 and BGO, had poor resolution at low-energy and thus performed less well than NaI(Tl) in detecting low-energy gamma rays from bare, enriched uranium. The plastic scintillator, a Compton recoil detector, also performed less well at low gammaray energy. Two small NaI(Tl) detectors were suitable for detecting bare uranium and shielded plutonium. One became part of a new lightweight hand-held monitor and the other found use as a pole-mounted detector for monitoring hard-to-reach locations.

Crystal growth and applications of mercuric iodide

This paper reports recent progress in the use of HgI2 in high energy resolution X-ray and gamma ray spectrometers which operate at room temperature, and reviews the present state of the art of HgI2 technology. Purification of starting materials, methods of crystal growth, detector fabrication, and characterization methods used for HgI2 are described, and some applications of HgI2 detectors to various device systems are given.

New thermal-neutron solid-state electronic detector based on HgI2 single crystals

The use of HgI2 as a thermal-neutron solid-state electronic detector, in particular its application for neutron diffractometry, is demonstrated for the first time. A single crystal HgI2 detector is used to count prompt gamma emissions (0.2–5 MeV) from (n,γ) nuclear reactions in Gd or Cd foils. The neutron counting efficiency depends on the HgI2 detector thickness. For a 1-mm thickness of HgI2 the efficiency is about 10% compared to the efficiency of a 10BF3 gas detector.

Present Performances and Potential Applications of Mercuric Iodide Detectors

The need for room temperature detectors capable of high detection efficiency for x-ray and γ-ray spectroscopy has long been recognized. This has resulted in substantial work on potentially suitable high-Z semiconductors. Detectors made from mercuric iodide have proved to be the most attractive candidates, particularly for low-energy x-ray applications. Such detectors are now commercially available. Mercuric iodide exhibits the wide-band gap required for room temperature operation and the high atomic number (Z) required for efficient photons detection. However, in common with other II-VI compound semiconductors, poor hole collection results from deep hole traps. Fortunately, for low-energy x-rays incident on the negatively-biased contact, the signal is dominated by electron collection. Therefore in this case, very good energy resolution can sometimes be obtained, opening up the possibility of wide-ranging applications for x-ray fluorescence spectroscopy. Good resolution, long-term stability, and a lack of polarization effects all represent the objectives of work on these detectors; these factors and their control will be discussed here. The performance of recent detectors has been discussed elsewhere. The paper will focus on three aspects of development of HgI2 detectors: i) Methods of purification and crystal growing. Several methods are used and these will be discussed. ii) Fabrication of detectors. iii) Applications and performance of detectors. Mercuric iodide detectors are described here both from the point of view of fabrication methods and from the user's point of view. The paper highlights the performance achieved at different photon energies.