Visible photoluminescence of aggregate colour centres in lithium fluoride thin films for low-energy proton beam radiation detectors at high doses

Irradiation of thermally evaporated LiF thin films with a proton beam of 3 MeV nominal energy, produced by a linear accelerator, in the fluence range of 1011-1015 protons/cm2, induces the formation of stable CCs, mainly the primary F centre and the aggregate F2 and F3+ defects. A comparison with irradiations performed at 7 MeV shows that the spectrally integrated PL as a function of the absorbed dose is independent on the selected beam energy, at least as far as typical LiF film thicknesses are concerned. The PL behaviour vs. dose can be described by a linear growth which covers up to three order of magnitude, followed by saturation at high values (> ≈105 Gy). The spectral contributions of F2 and F3+ CCs to the detected PL, in the red and in the green respectively, under laser pumping were carefully analysed in order to investigate behaviour differences of these defects at high doses.