Bragg peak imaging in lithium fluoride films by photoluminescent color centers induced by proton beams

LiF films grown on glass and Si(100) substrates by thermal evaporation were used as radiation imaging detectors to evaluate the Bragg peak position of a 3 MeV proton beam produced by a linear accelerator. The proton irradiation of LiF films induces the formation of stable color centers, mainly the primary F centers and the aggregate F₂ and F³⁺ defects, whose local concentrations are proportional to the deposited energy. Under optical pumping in the blue spectral region, F₂ and F³⁺ color centers emit broad photoluminescence bands located at 678 and 541 nm, respectively, which can be detected by conventional fluorescence microscopy. Photoluminescence images of the top surface of the exposed LiF films allowed obtaining the Bragg peak position of the proton beam, whose evaluation is in satisfactory agreement with software simulations.