Lodewijk van den Berg

Enhancement of mercuric iodide detector performance through increases in wafer uniformity by purification and crystal growth in microgravity

Wafers from mercuric iodide crystals grown in microgravity on two occasions have previously been found to be characterized by a higher hole mobility-lifetime product, which enables energy dispersive radiation detectors with superior resolution. In the present work, we have identified the specific structural modifications that are responsible for this enhanced performance. As a result of this study, the performance of terrestrial wafers also has been improved but not yet to the level of wafers grown in microgravity. High resolution synchrotron x-ray diffraction images of a series of wafers, including those grown both in microgravity and on the ground, reveal two principal types of structural changes that are interrelated. One of these, arrays of inclusions, affects performance far more strongly than the other, variation in lattice orientation. Inclusions can be formed either from residual impurities or in response to deviations from ideal stoichiometry. The formation of both types is facilitated by gravity...

Unattended Radiation Sensor Systems for Remote Terrestrial Applications and Nuclear Nonproliferation

The design of instrumentation for remote sensing presents special requirements in the areas of power consumption, long‐term stability, and compactness. At the same time, the high sensitivity and resolution of the devices needs to be preserved. This paper will describe several instruments suitable for remote sensing developed under the sponsorship of the Defense Threat Reduction Agency (DTRA). The first is a system consisting of a mechanical cryocooler coupled with a high‐purity germanium (HPGe) detector. The system is portable and can be operated for extended periods of time at remote locations without servicing. The second is a hand‐held radiation intensity meter with high sensitivity that can operate for several months on two small batteries. Intensity signals above a set limit can be transmitted to a central monitoring station by cable or radio transmission. The third is a small module incorporating one or more high resolution mercuric iodide detectors and front end electronics. This unit can be operat...

Mercuric iodide x-ray and gamma-ray detectors for astronomy

The recent technological developments and availability of mercuric iodide detectors have made their application for astronomy a realistic prospect. Mercuric iodide, because of its high resistivity and high density, can be used in a variety of astronomy instrumentation where high spectral resolution, low noise levels, stability of performance, resistance to damage by charged particles and overall ruggedness are of critical importance. X-ray detectors with areas of 12 to 100 mm square and 1 mm thickness have absorption efficiencies approaching 100% up to 60 keV. The spectral resolution of these detectors ranges from 400 eV to 600 eV at 5.9 keV, depending on their area, and the electronic noise threshold is less than 1.0 keV. Gamma ray detectors can be fabricated with dimensions of 25 mm x 25 mm x 3 mm. The spectral resolution of these detectors is less than 4% FWHM at energies of 662 keV. Because of the high atomic numbers of the constituent elements of the mercuric iodide, the full energy peak efficiency is higher than for any other available solid-state detector that makes measurements up to 10 MeV a possibility. The operation of gamma ray detectors has been evaluated over a temperature range of -20 through + 55 degrees Celsius, with only a very small shift in full energy peak observed over this temperature range. In combination with Cesium Iodide scintillators, mercuric iodide detectors with 25 mm diameter dimensions can be used as photodetectors to replace bulky and fragile photomultiplier tubes. The spectral resolution of these detectors is less than 7% FWHM at 662 keV and the quantum efficiency is larger than 80 % over the whole area of the detector.

Fabrication and performance of mercuric iodide pixellated detectors

The radiation detection efficiency and spectral resolution of mercuric iodide detectors can be improved significantly by increasing the volume of the detectors and by using a pixellated anode structure. Detector bodies with a thickness of nominally 10 mm and an active area of approximately 14 mm x 14 mm have been used for these experiments. The detectors were cut from single crystals grown by the physical vapor transport method. The cut surfaces were polished and etched using a string saw and potassium iodide solutions. The Palladium contacts were deposited by magnetron sputtering through stainless steel masks. The cathode contact is continuous; the anode contacts consist of an array of 11 x 11 pixels surrounded by a guard ring. The resistance between a pixel and its surrounding contacts should be larger than 0.25 Gohm. The detector is mounted on a substrate that makes it possible to connect the anode pixels to an ASIC, and is conditioned so that it is stable for all pixels at a bias of -3000 Volts. Under these conditions the spectral resolution for Cs-137 gamma rays (662 keV) is approximately 5% FWHM. When depth sensing correction methods are applied, the resolution improves to about 2% FWHM or better. It is expected that the performance of the devices can be improved by the careful selection of crystal parts that are free of structural defects. Details of the fabrication technologies will be described. The effects of material inhomogeneities and transport properties of the charge carriers will be discussed.

Spectral performance of mercuric iodide gamma-ray detectors at elevated temperatures

The effects of elevated temperatures on the spectral performance of a planar mercuric iodide (HgI2) gamma ray detector were evaluated at 25°C, 35°C, 45°C and 55°C using two test isotopes. 137Cs at 662 keV and 241Am at 59 keV. Spectral analysis was used to determine the spectral parameters (i.e. %FWHM of the full energy peak, the peak channel position and the peak to background ratio). Spectral performance degraded slightly with increasing test temperatures, but recovered on returning to ambient conditions. The results demonstrate that temperature excursions up to 55°C minimally degrade the spectral performance of mercuric iodide detectors.

High resolution synchrotron X-radiation diffraction imaging of crystals grown in microgravity and closely related terrestrial crystals

Irregularities in three crystals grown in space and four terrestrial crystals have been compared by high resolution monochromatic synchrotron x-radiation diffraction imaging. For two of the materials, mercuric iodide and lead tin telluride, features consistent with the presence of additional phases in terrestrial samples have been suppressed in the comparable crystals grown in microgravity. Comparison of the images of highly purified terrestrial mercuric iodide with those of lower purity space and terrestrial material suggests specific detector performance models. These models ascribe the improved performance of detectors made from space-grown mercuric iodide to reduction in a widely dispersed impurity phase rather than to extreme macroscopic lattice regularity. While the general grain structure of lead tin telluride is not strongly affected by growth in microgravity, the subgrain uniformity of the space crystal is substantially higher than that of the comparable terrestrial crystal. The greater uniformity is associated with suppression of the second phase that appears to be characteristic of the terrestrial crystal examined.

Characterization of strain and crystallographic defects in HgI 2 single crystals

We investigated bulk-grown HgI2 crystals to better understand the nature of crystallographic defects and strain/stress in different growth regions of the crystal and their affect on the performance of HgI2-based radiation detectors. Double-axis and triple-axis high-resolution x-ray diffraction were used to characterize the mosaic structure and strain in HgI2. Rocking curves revealed significant mosaic spreading in <110> growth regions exhibiting X-defects versus X-defect-free <100> growth regions. Both <110> and <100> growth regions exhibited little strain (~0.01%). We report the narrowest rocking curves (~ 9 arcsec) to date on HgI2 as a result of the resolution of the instrument (~ 6 arcsec). Raman spectroscopy was used collaboratively to confirm little residual stress in the crystals. We developed a growth rate ratio (chi) and show this geometric model used to describe crystal shape and regions of <100> and <110> growth. Optical characterization of X-defects are presented and discussed. Further the influence of crystallographic defects and strain on radiation detector performance are discussed.

Conductivity variations in mercuric iodide detectors

When bias is applied to a mercuric iodide detector with planar contacts, placed in a light-tight enclosure, the leakage current initially increases with increasing bias, as can be expected for a semiconductor detector. At some value of the bias however, the current starts to decrease sharply, and reaches very low values when the bias is further increased. This phenomenon of negative differential conductivity has been observed previously in high-bandgap semiconductors with deep electron and hole traps. This combination of traps can provide an effective recombination channel which reduces the number of free electrons and holes at high electric fields.When bias is applied to a mercuric iodide detector with planar contacts, placed in a light-tight enclosure, the leakage current initially increases with increasing bias, as can be expected for a semiconductor detector. At some value of the bias however, the current starts to decrease sharply, and reaches very low values when the bias is further increased. This phenomenon of negative differential conductivity has been observed previously in high-bandgap semiconductors with deep electron and hole traps. This combination of traps can provide an effective recombination channel which reduces the number of free electrons and holes at high electric fields. It is suggested in this paper that mercuric iodide contains a set of traps with the required properties in the form of doubly ionized mercury interstitials and iodine vacancies. These traps are incorporated in the single crystals by the processing methods used to obtain the pure material and by the presence of mercurous iodide in the synthesized material. The main advantage of this behavior is the apparent low leakage current of the detectors at high fields (1000 V/mm) at ambient temperatures. The disadvantage is that carriers generated by ionizing radiation will also be subject to this recombination process, so that complete charge collection may be impaired. Additional details of this hypothesis will be presented for discussion.

Measurement of Charge Carrier Properties in Mercuric Iodide

The transport properties of electrons and holes in mercuric iodide determine the performance of detectors. The relatively low levels of these properties limit the volume of detectors with high spectral resolution that can be fabricated and leads to long pulse collection times. These transport parameters can be increased by optimizing the material processing, the crystal growth, and the detector fabrication. This paper will present our own results obtained by analysis of individual pulse shapes. The values for the mobilities generally measured are between 60-90 cm2/Vsec for electrons and 2-4 cm2/Vsec for holes. These results will be compared with the values published by other investigators. Possible reasons for the differences in the reported values will be discussed and explanations will be suggested.

Keys to the Enhanced Performance of Mercuric Iodide Radiation Detectors Provided by Diffraction Imaging

High resolution monochomatic diffraction imaging is playing a central role in the optimization of novel high energy radiation detectors for superior energy resolution at room temperature. In the early days of the space program, the electronic transport properties of mercuric iodide crystals grown in microgravity provided irrefutable evidence that substantial property improvement was possible. Through diffraction imaging, this superiority has been traced to the absence of inclusions. At the same time, other types of irregularity have been shown to be surprisingly less influential. As a result of the knowledge gained from these observations, the uniformity of terrestrial crystals has been modified, and their electronic properties have been enhanced. Progress toward property optimization through structural control is described.