Detection of fluorescent low-energy proton tracks in lithium fluoride thin films on silicon substrates

Recently, Fluorescent Nuclear Track Detectors (FNTDs) have been demonstrated in lithium fluoride (LiF) crystals exploiting the visible radiophotoluminescence (RPL) of F₂ and F₃⁺ color centers (CCs). In this paper, solid-state radiation detectors based on LiF thin films, grown by thermal evaporation on Si(100) substrates, were tested as FNTDs with ∼1 MeV proton beams produced by the vertical low-energy extraction line of the TOP-IMPLART linear accelerator in operation at ENEA Frascati. They were irradiated with the film surface approximately parallel to the beam propagation direction (i.e. the cleaved film edge was directly exposed to the incident beam). For the first time, at fluences between 10⁷ and 10⁸ protons/cm², luminescent images of single proton tracks were visualized in LiF films using a fluorescence microscope at high magnification. RPL spectra under continuous-wave blue laser excitation were also measured to study the behavior of the F₂ and F₃⁺ emission bands, whose intensities show a different dependence on the excitation power. Increasing the proton beam fluence by two orders of magnitude, the superposition of a higher number of tracks allowed to fully detect the luminescent Bragg curve of the beam, even at this low proton energy. The energy spectrum of the proton beam was estimated by best fitting this luminescent Bragg curve, using FLUKA simulations of energy deposition in a LiF layer on Si. The layer was accurately modeled, and its density was derived from ellipsometric measurements of the refractive index.