Alanine pellet dosimeter efficiency in a 40 kV x-ray beam relative to cobalt-60

The electron paramagnetic resonance (EPR) response of alanine pellet dosimeters irradiated in a 40kV x-ray beam with effective energy 9keV was investigated, which is a factor of three lower than for currently available literature data. The response was compared to the EPR response of alanine pellets irradiated in a reference cobalt-60 field to obtain the relative efficiency of the dosimeter – the ratio of detector response per dose-to-detector in the x-ray beam relative to the reference field. Due to the low filtration of the beam, and subsequent low half-value layer, a correction for the dose gradient within the pellet was necessary to implement for the EPR response of x-ray irradiated pellets. Calculation of the dose gradient in a pellet, together with dose-to-alanine to dose-to-water ratio, was carried out using the EGSnrc Monte Carlo code. The relative efficiency of the alanine pellet dosimeter in this x-ray field compared to a reference cobalt-60 field was found to be G_Q,Q0=0.91±0.04.