Difference in the relative response of the alanine dosimeter to megavoltage x-ray and electron beams

In order to increase the usefulness of the alanine dosimeter as a tool for quality assurance measurements in radiotherapy using MV x-rays, the response with respect to the dose to water needs to be known accurately. This quantity is determined experimentally relative to (60)Co for 4, 6, 8, 10, 15 and 25 MV x-rays from two clinical accelerators. For the calibration, kQ factors for ionization chambers with an uncertainty of 0.31% obtained from calorimetric measurements were used. The results, although not inconsistent with a constant difference in response for all MV x-ray qualities compared to (60)Co, suggest a slow decrease from approximately 0.996 at low energies (4-6 MV) to 0.989 at the highest energy, 25 MV. The relative uncertainty achieved for the relative response varies between 0.35% and 0.41%. The results are confirmed by revised experimental data from the NRC as well as by Monte Carlo simulations using a density correction for crystalline alanine. By comparison with simulated and measured data, also for MeV electrons, it is demonstrated that the weak energy dependence can be explained by a transition of the alanine dosimeter (with increasing MV values) from a photon detector to an electron detector. An in-depth description of the calculation of the results and the corresponding uncertainty components is presented in an appendix for the interested reader. With respect to previous publications, the uncertainty budget had to be modified due to new evidence and to changes of the measurement and analysis method used at PTB for alanine/ESR.