Computations to facilitate TEPC calibration with 37Ar

Monte Carlo computations are performed to facilitate the calibration of tissue equivalent proportional counters with 37Ar. As shown earlier, this calibration method is essential for small counters and for multi-element detectors; besides providing the calibration factor it provides also a quantitative assessment of the resolution of the counter. To extend the use of this method to small simulated sites, geometrical effects are studied for spherical and cylindrical counters. As the simulated diameter decreases, the K-capture peak broadens and shifts towards lower energies. Frequency distributions of ionizations produced by 37Ar decay particles are computed, and the most probable numbers of ion pairs due to K-capture events are given for different counter types and for simulated diameters from 10 μm down to 200 nm. The smallest simulated diameter at which counters can be calibrated in terms of the L-capture peak is found to be about 50 nm. By multiplication with the W-value (the average energy required to produce an ion pair), the results are related to the distributions of energy imparted due to 37Ar decays. The results can serve as reference values in the calibration of tissue equivalent proportional counters with 37Ar.