The goal of this project is the use of a novel semiconductor photodetector utilizing large mercuric iodide photodetectors coupled to highly optimized cesium iodide (thallium) (CsI(Tl)) scintillators that will both increase the energy resolution and decrease the size and weight of detector systems assembled as the ultra-compact hand held units for assaying low-grade radioactive materials to facilitate cleanup activities and facility transition. This effort is motivated by the limitations such as inferior energy resolution for photomultiplier tubes (PMTs) and silicon devices, and by disadvantages such as large size, voltage regulation required, and magnetic field sensitivity for PMTs, and by the bulky apparatus associated with cryogenic cooling for direct gamma-detectors such as germanium. Recently fabricated 1/2-inch diameter cesium iodide/mercuric iodide (CsI/HgI2) scintillator pairs have produced better and more reproducible results than ever reported for room temperature scintillation spectroscopy. Additionally, HgI2 semiconductor photodetectors can enable extremely compact and rugged geometry and packaging. The Phase I work will include the selection of an appropriate HgI2 crystalline material grown in a crystal laboratory, and the fabrication of large-area photodetectors with diameters from 1 to 2 inches using a new structure that will optimize the geometry, packaging, and coupling of the scintillator crystals for compactness, efficiency, and light collection. Scintillator/ photodetector pairs will be constructed based on these designs, and evaluated in the laboratory using optimized preamplification electronics. Finally, the feasibility of these detectors will be tested for use in nondestructive assay by making realistic measurements of Special Nuclear Material at Sandia National Laboratory. A successful Phase I effort will resolve all of the questions concerning the implementation of the new devices as the detector in new ultra-compact, high-resolution scintillation spectrometers. A prototype spectrometer using this technology will then be fabricated in a future Phase II effort. The novel new detector can eventually become a PMT replacement with widespread use in a large number of terrestrial and space applications.Anticipated Results /Potential Commercial Applications as described by the awardee:A successful Phase I effort will resolve the issues concerning the viability of large-area (1 to 2 inch) HgI2 photodetectors for use in high resolution, compact scintillation spectrometers. This will be the critical component enabling the design, fabrication, and assembly in Phase II of a prototype, fieldable, ultra-compact scintillation spectrometer for facilities cleanup and transition. Such devices are sought throughout the gamma-ray spectroscopy community and would find widespread use in space/vacuum instruments, high energy physics research, health physics, personnel monitoring, medical research, and other commercial applications.
