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

Abstract

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-driven convection during growth. As the level of inclusions is reduced, through growth from material of higher purity, through the achievement of balanced stoichiometry, or by suppression of convection mixing during crystal growth, the hole mobility-lifetime product is enhanced in spite of an accompanying decreased uniformity in lattice orientation. Sixfold enhancement in the performance of x-and ␥-ray detectors has been accomplished to date. Further augmentation in performance appears likely.

Structural high-pressure evolution of the tetragonal low-pressure polymorph of Mn3O4

“…The decreasing c/a ratio indicates a reduction of the tetragonality with increasing pressure. The structure refinements show that the uniaxial elongation of the Mn 3+ O 6 octahedra gets reduced which is in full agreement with the previously reported regularization as reported from structure modelling [1]. The Mn 3+ O 6 polyhedral compressibility is also compatible with the macroscopic compression behavior thus explaining the compressional anisotropy.…”

“…Due to the Jahn-Teller effect of the Mn 3+ cations, the octahedra are uniaxially elongated which results, as a consequence of the cooperative effect between edge-sharing octahedra, in a cubic-to-tetragonal symmetry breaking. Previous highpressure X-ray powder-diffraction experiments [1] confirmed a transition to a marokite-type high-pressure polymorph of orthorhomic symmetry that occurs between 10 and 12 GPa. Moreover the EDX data yield a bulk modulus of 137±3.8 GPa for a second-order BirchMurnaghan (BM) equation of state (EoS).…”

“…Of all compounds, HgI 2 has been the most studied. This is due to actual use of the red modification as detector material for x-rays and γ-rays at room temperature 1 . At ambient conditions, two unstable polymorphs exist in addition to the thermodynamically stable red form: an orange phase with a layered structure exhibiting stacking disorder 2 , and a yellow modification 3 related to the structure of the high temperature yellow phase.…”

A solid-state 199Hg NMR study of mercury halides

“…Mercuric iodide is used as a room-temperature X-and γ-ray detector [20,21] and HgBr x I 1-x has been investigated as a photodetector [22]. Acousto-optical devices containing mercury compounds are used in military, space, and commercial applications, including the detection of targets and of chemical and biological agents.…”

Coplanar-grid electrode design of HgI2 detector by finite elements simulation

“…High-energy radiation detectors have high resolution ability to γ-ray and X-ray at room temperature, which were fabricated with HgI 2 [4][5][6][7]. Single-crystal HgI 2 detectors have demonstrated excellent energy resolution and detection efficiency, and the performance of polycrystalline film has been approaching that of single-crystal over the last few years [8][9][10]. Because of the lower fabrication cost for large area detectors and the more practical experiences gained with HgI 2 materials in space and commercial applications, polycrystalline α-HgI 2 films are thought to be one of the promising direct converters in X-ray digital radiography [11][12][13][14][15][16].…”