The Cog-Beta Ray Spectrometer for Mixed Field Beta/Photon Dosimetry

Beta radiation is now recognised as a significant radiation safety problem and several international conferences have recently been devoted to the problems of mixed field beta/photon dosimetry. Conventional dosimetry applies algorithms to TLD multi-element badges which attempt to extract dose information based on the comparison of TL signals from 'thick/thin' and/or 'bar/filtered' elements. These may be grossly inaccurate due to inadequate or non-existent knowledge of the energy spectrum of both the beta radiation and the accompanying photon field, as well as other factors. The operation is discussed of a beta ray energy spectrometer based on a two-element, E × dE, detector telescope intended to support dose algorithms with spectral information. Beta energies are measured via a 5 cm diameter × 2 cm thick BC-404 plastic scintillator preceded by a single, 100 μm thick, totally depleted, silicon dE detector. Photon events in the E detector are rejected by requiring a coincidence between the E and the dE detectors. The spectrometer is capable of measuring electron energies from a lower energy coincidence threshold of approximately 125 keV to an upper limit of 3.5 MeV. This energy range spans the great majority of beta-emitting radionuclides in nuclear fields.