CALCULATED PERFORMANCE FOR NEW RADIATION DETECTOR DESIGNS

Radiation detectors with capability to detect and identify radiation at distances approaching 100 meter and more will enhance the capability to counter terrorist scenarios involving radioactive or special nuclear materials (SNM). Long-distance detection capability, especially when coupled with directional sensitivity or imaging features can support more efficient search of large areas where radioactive materials may be hidden and can improve monitoring of choke points. A program of Monte Carlo calculations to support development of new detectors for these applications is underway. One of two goals is to develop a database that will guide detector design and support estimation of detector performance for a variety of detector types. Radiation arriving at the detector plane will be binned according to energy so that performance of detector designs with sensitivity in different energy ranges can be estimated. The second goal is to model the response of specific detector designs at various distances considering propagation of radiation from the source to the detector, interactions within the detector, background, and surrounding material configurations including packing and shielding. Calculations have been made for unclassified uranium and plutonium metal sources, both bare and surrounded by simulated high explosive and a stainless steel shell. Fission-related emissions from the enclosed source are more intense because reflected neutrons induce additional fissions within the source. Emissions of discrete gamma rays associated with radioactive decay are modeled separately from fission-related emissions. Radiations arriving at the detector can be separated into two groups, one for radiation that reaches the detector plane without further interaction after leaving the source and another for radiation that undergoes one or more interactions between the source and the detector. In an imaging detector, radiation arriving directly from a compact source may appear in a single pixel, like a headlight, while radiation interacting near the source may be analogous to foggy glow, and ground-scattered radiation analogous to road glare extending from source to detector. Background may affect all detector pixels nearly uniformly. Calculated results relevant to current detector concepts will be presented. For example, counting times can be estimated for various source strengths and configurations.