The quality dependence of LiF TLD in megavoltage photon beams: Monte Carlo simulation and experiments

The quality dependence of LiF TLD in megavoltage photon beams with qualities from 60Co gamma-rays to 25 MV x-rays has been studied experimentally against ion chamber measurements and theoretically by Monte Carlo simulation using the EGS4 Monte Carlo code system. The experimental findings are that the energy dependence of 1 mm thick TLD-100 (micro-rods and chips) on average decreases slowly from 1.0 for 60Co gamma-rays to 0.989 +/- 1.3% for 6 MV x-rays (TPR20/10 = 0.685) and to 0.974 +/- 1.3% for 25 MV x-rays (TPR20/10 = 0.800) relative to 60Co gamma-rays. The Monte Carlo results vary from 0.991 +/- 0.9% for 6 MV x-rays to 0.978 +/- 0.8% for 25 MV x-rays. Differences between chips and micro-rods were negligible and there was no difference in the energy dependence between TLDs irradiated in water or Perspex (PMMA). The Monte Carlo simulation shows that the contribution to the total absorbed dose from photon interactions in the 1 mm diameter and 6 mm long TLD material varies from 50% for 60Co gamma-rays to 10% for 25 MV x-rays. When the diameter of the TLD micro-rod was increased from 1 mm to 5 mm there was no significant change in response computed by Monte Carlo even though the dose contribution to the total dose scored in the TLD material from photon interactions in the cavity increased to 85% for 60Co gamma-rays and 30% for 25 MV x-rays.