Student Research: Lyle A. Rathbun
This thesis report will demonstrate that the pancake GM (a small, gas-filled radiation detector employing the Geiger-Mueller principle of gas multiplication) can accurately measure skin dose rates from quantities of radioactive material typically found in skin contamination incidents. Accurate skin dose rate measurements are achievable if the response of the pancake GM can be shown to mimic fluence-to-dose conversion factors for all spectra with average beta energies above 100 keV. A thin window, a shallow chamber, a flat anode with sharp edges, and a quench gas with a high affinity for electrons allow the pancake GM to measure dose rates rather than simply counts per minute. However, the pancake GM retains the large signal pulses, simplicity of operation, and low cost of other Geiger-Mueller detectors. The pancake GM detector is superior to the conventional GM, the ion chamber, and the plastic scintillator for measuring skin dose rates and approximately equal in accuracy to a small gas flow proportional detector for such measurements. This thesis is intended to consider the practical health physics problem of measuring dose to the skin from a very thin layer of material emitting beta radiation on or near the surface of the skin.
Despite efforts in recent years to substitute more sensitive gas-flow detectors, the pancake GM Detector remains the detector of choice for direct contamination surveys in United States nuclear facilities. Nearly every location where workers exit a contaminated area has a pancake GM. It is usually the first detector used to measure skin contamination. The advantages and disadvantages of this detector were studied and the results are presented in this thesis.