COMPUTATIONS TO FACILITATE TEPC CALIBRATION WITH AR-37

Monte Carlo computations are performed to facilitate the calibration o f tissue equivalent proportional counters with Ar-37. As shown earlier , this calibration method is essential for small counters and for mult i-element detectors; besides providing the calibration factor it provi des also a quantitative assessment of the resolution of the counter. T o extend the use of this method to small simulated sites, geometrical effects are studied for spherical and cylindrical counters. As the sim ulated diameter decreases, the K-capture peak broadens and shifts towa rds lower energies. Frequency distributions of ionizations produced by Ar-37 decay particles are computed, and the most probable numbers of ion pairs due to K-capture events are given for different counter type s and for simulated diameters from 10 mu m down to 200 nm. The smalles t simulated diameter at which counters can be calibrated in terms of t he L-capture peak is found to be about 50 nm. By multiplication with t he W-value (the average energy required to produce an ion pair), the r esults are related to the distributions of energy imparted due to Ar-3 7 decays. The results can serve as reference values in the calibration of tissue equivalent proportional counters with Ar-37.