ABSTRACT
Another way to differentiate between these natural progenies and other α-emitters is to employ an α-spectrometer using a solid-state semiconductor detector (NCRP, 1985; Knoll, 2000) or perhaps a liquid scintillation counter (McDowell and McDowell, 1994; Kessler, 1989). Technically, this is no longer a gross α-measurement. It also presupposes that the energies of the natural emitters do not interfere with other α-emitters of interest to the investigator. The signals from semiconductor detectors are proportional to the energy of the α-particles. Most long-lived α-emitters have energies below 6 MeV, whereas the daughters of radon and thoron decaying by α-emission do so with equal or greater energies. It is therefore possible to gate the associated electronics or specify energy regions of interest to count only the pulses from low-energy α-emissions. This method works better when detecting 235U but may not work well with measurements of 241Am and 239Pu due to the higherenergy α-emissions of these nuclides and the greater energy overlap with radon or thoron daughters. The reader should be aware of the limitations of the instruments being used especially when using energy discrimination techniques to avoid interference from radon and/or thoron.
