shape of the measured spectrum, a result of the energy dependence of absorption/scattering on materials present as described above.
There are circumstances in which the effects of shielding and the requirement to determine specific isotopes of an elemental source would require the use of a high-purity germanium detector. Some low-level emitters, such as enriched uranium, can be detected passively through other, normally associated isotopes, such as U-238 or U-232. The gamma rays emitted specifically from U-235 are low in energy and easily shielded, or are so weak in intensity as to be difficult to detect under most field constraints. Thus, enriched uranium is often detected passively through detection and identification of gamma rays from associated isotopes, if present. Otherwise, active interrogation methods can be employed, typically using neutrons or high-energy gamma rays as the interrogating medium, that cause the U-235 in the sample to fission and thus emit neutrons and other measurable gamma rays. However, such interrogating systems tend to be rather large and expensive and present radiation safety issues. In short, several options are currently available for making radioisotopic measurements. The specific system used must be optimized for the specific situation and associated concept of operations. Ongoing research and development aims to yield new materials and techniques, which will provide some relief in some scenarios. This development should be encouraged and funded. However, there is much that can be done by designing appropriate systems with detectors currently available.7,8
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