Investigation of Lead Iodide Crystals for use as High Energy Solid State Radiation Detectors
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INVESTIGATION OF LEAD IODIDE CRYSTALS FOR USE AS HIGH ENERGY SOLID STATE RADIATION DETECTORS DOMINIQUE C. DAVIDt, R. B. JAMESt, H. FEEMSTERt, R. ANDERSONt, A. J. ANTOLAKt, D.H. MORSEt, A. E. PONTAUt, H. JAYATIRTHA§, A. BURGER§, X. J. BAO', T. E. SCH-LESINGER0, G. S. BENCH', and D. W. HEIKKINEN* tSandia National Laboratories, Livermore, CA 94450 § Fisk University, Nashville, TN 37208 •TN Technologies, Round Rock, TX 78664 0 Carnegie Mellon University, Pittsburgh, PA 15213 •: Lawrence Livermore National Laboratory, Livermore, CA 94450 ABSTRACT Significant developments have occurred in the technology of room-temperature PbI2 nuclear sensors which lead to some improvements in the detection of high energy gamma-rays. Discussion of crystal growth, purification, monitoring purification, and detector processing are reviewed as they relate to device performance. INTRODUCTION State-of-the-art room-temperature gamma radiation sensors with improved energy resolutions have an enormous range of applications, such as monitoring nuclear proliferation, space exploration, environmental remediation and safety, and industrial areas. Presently, commercial x-ray and gamma-ray nuclear devices are fabricated using silicon and germanium semiconductors. Yet, these particular semiconductors exhibit limitations for military and commercial applications. Usage where size or portability is an essential issue necessitates physically more rugged and stable material with higher cross section for photon interaction, larger band-gap for low dark current, and room-temperature operation. New materials like mercuric iodide (HgI2) and cadmium telluride room-temperature detectors are presently used in various commercial applications. Yet, significant limitations also exist in these nuclear sensors materials. Because of its lower band-gap energy in comparison to other materials, cadmium telluride (CdTe) exhibits relatively higher dark current and has a lower photoelectric efficiency. On the other hand, HgI2 sensors exhibit problems with polarization, a destructive phase transition at high temperatures, and are chemically reactive. In contrast, lead iodide (PbI2) room-temperature semiconductors are known to have extraordinary photoelectric efficiencies for gamma-rays while its 2.5 eV band-gap promises high resistivity and thus low background noise during detector operation. High purity crystals have been fabricated and used as x-ray and gamma-ray devices. Nevertheless, significant problems continue to exist in the material due to low charge carrder collection efficiency which directly results in low energy resolution for a measured monoenergetic source. The low efficiency is probably due to the presence of impurities introduced during crystal growth and device fabrication. These impurities result in broadening of the photopeak due to charge carrier trapping, so that the amount of charge collected is no longer uniquely proportional to the energy of the incident photon. Attempts to correlate detector performance with levels of organometallic analytes that act
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