Monte Carlo counting efficiencies for measuring Rn in a charcoal canister

The level of 222Rn adsorbed in an activated charcoal canister may be determined by direct measurement of the canister, using a NaI detector. The counting efficiency of this arrangement depends on the Rn distribution in the charcoal, and this varies as a function of sampling period. Provided the Rn profile can be predicted, a Monte Carlo program can be used to simulate the counting arrangement. The Rn distribution in a canister used for surface emanation measurements can be described accurately by use of an empirical expression. Three time dependent coefficients were used in the expression to determine the shape of the distribution as a function of sampling period. Counting efficiencies were estimated by use of a simple Monte Carlo program and these were all found to be in close agreement with measured values for various sampling periods. (orig.)