Beta dose rate evaluations with a Geiger-Mueller pancake detector

Methods for estimating dose rates to the 70 μm layer of the human dermis from external beta (β) emitters with a Geiger-Mueller (GM) pancake detector have been investigated through experimental evaluation. First, dose rates were measured for two separate groups of sources by use of an extrapolation chamber on the first group and by use of the computer code VARSKIN on the second group. Second, the response of a GM pancake probe with a 16 mesh stainless steel screen was measured on each of these sources in a configuration which is typically used during field survey activities. The results were compiled in the form of a response curve showing the dose rate response of the GM pancake as a function of average beta energy. This curve provides useful data pertaining to the energy dependency of a standard GM pancake probe when converting beta measurements to dose rates. Third, a special system for measuring β dose rates was developed by making measurements with a series of three sequential thicknesses of plastic placed over the probe on each of the sources. These measurements were made with the intent to derive a calculational technique by which dose rates could be accurately assessed with a pancake detector with no prior knowledge of the beta spectra being measured. Measurements were made with plastic filter thicknesses of 6.5, 13.5, and 44.3 mg cm^-2 thereby breaking the source beta spectra into three consecutive energy bands. A multivariate regression was performed on the resulting count rate bands and a mathematical relationship between the known β dose rates and the filtered count rate measurements was formulated.