Purpose: To investigate the feasibility of using two optical storage phosphor dosimeters of different proton numbers (Zeff) for simultaneous proton dose and linear-energy-transfer (LET) measurements.
Methods: A high Zeff commercial BaFBrI:Eu2+ storage phosphor detector was investigated and compared with KCl:Eu2+, a low Zeff (Zeff=18) storage phosphor made in-house. BaFBrI:Eu2+’s proton stopping-power-ratio (SPR) was calculated using the Bethe-Bloch theory with elemental data obtained via energy dispersive x-ray spectroscopy (EDS) analysis. Then, its optical characteristics were investigated using excitation and emission spectral analysis. Finally, its dosimetric response under varying proton irradiation conditions was measured under a reference proton SOBP geometry (Range: 15.0 g/cm2, Modulation: 10.0 g/cm2) generated using a Mevion S250 proton therapy unit. The LET environment varied with depth in water and their values were calculated using analytical methods. The dosimetric responses of BaFBrI:Eu2+ under varying irradiation conditions including dose linearity, dose-rate dependence, dynamic range, temporal profiles, radiation hardness and LET dependence were measured via photostimulated luminescence (PSL) intensity measurements.
Results: EDS analysis revealed its exact elemental composition to be BaFBr0.85I0.15:Eu2+ (Zeff=49) and subsequent analytical calculations showed a good, monotonic signal separation by virtue of the proton energy dependence of its proton SPR. Its PSL excitation and emission spectrum overlapped with KCl:Eu2+’s showing good cross-compatibility. In addition, it had good proton dose linearity, dose-rate independence and high reusability. It showed a strong LET-dependent PSL under-response across an LET spectrum from 0.64 keV/μm to 11.1 keV/μm that could be sufficiently explained using stopping-power-ratio variations.
Conclusion: We have demonstrated that BaFBr0.85I0.15:Eu2+ and KCl:Eu2+ constitute a pair of compatible storage phosphor dosimeters having desirable optical and dosimetric properties and having the ability to measure absolute proton doses (using PSL intensities from the near LET-independent KCl:Eu2+ dosimeters) and LET values (via the relative PSL under-response of the high Zeff BaFBr0.85I0.15:Eu2+ dosimeters) simultaneously.