Visible photoluminescence of color centers in lithium fluoride detectors for low-energy proton beam Bragg curve imaging and dose mapping

Abstract

In the last few years, the visible photoluminescence (PL) of radiation-induced laser-active F2 and F3+ aggregate color centers (CCs) in lithium fluoride, LiF, has been successfully used for advanced diagnostics of low-energy proton beams. A systematic study of the optical properties of LiF crystals and thin films exposed in different irradiation geometries to a proton beam of nominal energy 7 MeV is under way. The ionization induces the stable formation of primary and aggregate CCs in the crystalline lattice, among which the F2 and F3+ defects possess broad emission bands in the red and green spectral ranges, respectively, when optically pumped at wavelengths close to 450 nm. Their PL intensity has been found to be linearly proportional to the dose absorbed by the host material over at least three orders of magnitude, so that LiF solid-state dosimeters based on spectrally-integrated PL reading can be envisaged. A simple defect formation model that takes into account the energy released in the material, crystals and films, together with the saturation of CC concentration at high doses, allows obtaining both the full Bragg curve reconstruction with its dose distribution with depth and the accumulated bi-dimensional transversal dose-mapping from the visible fluorescence images latently stored in LiF crystals and thin films, respectively. Results obtained by using a fluorescence microscope are presented and discussed with the aim of highlighting some quantitative aspects of the PL behavior of CCs both in LiF crystals and films, which are relevant for low-energy proton-beam Bragg curve imaging and dose mapping in a wide interval of doses up to the ones at which strong CC saturation occurs.