The microdosimetric one-hit detector model for calculating the relative efficiency of the alanine pellet dosimeter in low energy x-ray beams

The alanine pellet dosimeter is a widely used reference dosimeter in both medical and industrial dosimetry across a wide range of beam qualities. A challenge when using alanine in low energy x-ray beams is its strong energy dependence; a significant decline is observed in the alanine response per dose-to-water relative to irradiations in a cobalt-60 reference field. This decrease is caused by the physical difference in alanine to water dose ratios combined with a radiochemical decrease in the intrinsic detector efficiency but is difficult to characterize mainly due to experimental uncertainties.

Here we have applied a microdosimetric one-hit detector model to characterize the intrinsic detector efficiency of the alanine pellet dosimeter in low energy x-ray beams. Microdosimetric distributions were estimated from track structure calculations using the Geant4-DNA Monte Carlo software, where literature data was used to determine free model parameters.

The model was applied to two sets of x-ray spectra with low (40 kV to 170 kV) and medium (100 kV to 300 kV) tube potential, covering a wide range of beam qualities. A relative detector efficiency of 0.937 was obtained for the low energy set with variations between -1.0% and 1.5%, whereas the efficiency varied between approximately 0.925 and 0.985 for the medium energy set, with a strong correlation to the half-value layer of the beam.

It is concluded, that the tube potential and half-value layer of an x-ray beam is not sufficient characterization to uniquely determine the relative efficiency of an alanine pellet dosimeter. However, the variation in relative efficiency with respect to the half-value layer is small.