Bismuth tri-iodide (BiI3), a wide band-gap semiconductor, demonstrates many of the material properties necessary for
high resolution room temperature gamma-ray spectroscopy. These material properties include high density, large bandgap,
and high atomic number. The theoretical intrinsic photopeak efficiency of BiI3 is approximately 2-3 times higher
than CdZnTe over the range of 200-3000 keV. BiI3 has a theoretical intrinsic photopeak efficiency of 19% at 662 keV,
compared to CdZnTe which has a theoretical intrinsic photopeak efficiency of 13% at 662 keV. A modified vertical
Bridgman growth method is being used to grow large, greater than 100 mm3, single BiI3 crystals. Growth parameter
optimization has demonstrated that single crystals can be obtained with temperature gradients of 10°/cm or 15o/cm and a
growth rate of 0.5 mm/hr, or with a temperature gradient of 10o/cm and a growth rate of 1 mm/hr. Polycrystalline
material results from all other growth parameter combinations. X-ray diffraction spectra are used to determine if the
crystals are single crystals or polycrystalline. UV-VIS spectra analysis has revealed that the band-gap of BiI3 is 1.72 eV.
The resistivity of the crystals has been determined by generating I-V curves to be on the order of 108-109 Ω-cm. Zone
refining is being performed to increase the purity of the starting material and the resistivity of the crystals. Detectors
have been fabricated with both gold and palladium electrodes.
Iodine-based compound semiconductors may allow one to build a portable gamma-ray spectrometer with improved
efficiency and energy resolution compared to many other portable spectrometer devices. Iodine-based semiconductors
have a wide band gap that allows these detectors to operate without any cooling mechanism. Bismuth iodide (BiI3), lead
iodide (PbI2) and mercuric iodide (HgI2) have theoretical gamma-ray detection efficiencies approximately 2-3 times
higher than CdZnTe, the current compound semiconductor material proposed for use in several homeland/national
security applications, over the range of 200-3000 keV. At 662 keV, BiI3, HgI2 and PbI2 have theoretical intrinsic
photopeak efficiencies of 16.8%, 19.3% and 19.9%, respectively, while CdZnTe has a photopeak efficiency of 9.03%. In
addition, gamma-ray spectrometers made from iodine-based compound semiconductor materials have demonstrated
energy resolutions (FWHM) less than 2% at 662 keV. A 2% FWHM represents a significant improvement over many of
today's scintillator-based radiation detectors used for homeland/national security purposes. We present some
fundamental challenges in working with iodine-based semiconductors, including crystal growth issues and properties of
the materials limiting radiation detector size, and the need for advanced electrode designs. Finally, we present
elementary measurements illustrating the detection capabilities of iodine-based compound semiconductor materials.
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