Dosimetry of mixed beta and gamma radiation in work areas

The mixed beta and gamma radiation fields from distributed sources of high-energy beta emitters, such as 106Ru-106Rh and 144Ce-144Pr, encountered during maintenance of nuclear fuel reprocessing plants, require accurate control of any personnel occupancy times. The dose rates may be high and the beta-to-gamma dose rate ratio can vary widely. Our goal was to develop a reliable, portable survey instrument that can provide simultaneous information about the beta and gamma-ray dose rates in the 10-3 to 103 rad/h range in mixed fields. A thin walled, double, concentric ionization chamber survey instrument was built and tested in the field. This instrument is equipped with a temperature-compensated, solid-state electrometer and can measure the beta and gamma-ray dose rates simultaneously. A plastic scintillation detector survey instrument is now under development, because more information can be obtained on the mixed beta and gamma radiation field, e.g. the average and maximum energies of the principal beta emitter. This system depends on an electronic version of the absorption method originally developed for sample purity analysis in the laboratory. The results of evaluations and tests with both systems are discussed. (author)