Radiophotoluminescence response of color centers in lithium fluoride crystals, thin films and TLD-100 pellets for gamma-ray dosimetry

Passive solid-state detectors based on lithium fluoride (LiF) crystals and TLD-100 pellets were irradiated in the range from 0.65 to 1000 Gy and LiF thin films from 20 to 1000 Gy at different dose-rates within the two dose ranges, with the 60 Co source of the Calliope irradiation facility at ENEA Casaccia, Italy. The 1 mm-thick LiF crystals and pellets were commercially available, while the LiF films were grown, by thermal evaporation on bare and Al coated Si(100) substrates, with nominal thickness of 0.5, 1 and 2 μm, at ENEA Frascati, Italy. The principle of operation of these radiation detectors is the optical reading of visible radiophotoluminescence (RPL) emitted by radiation-induced F 2 and F 3 + color centers (CCs). The RPL spectra of the LiF crystals and of the TLD-100 pellets were measured under continuous wave laser excitation at 445 nm, while for the LiF films the spectrally integrated RPL signal was measured by a fluorescence microscope equipped with a blue LED as pumping source. In pellets, the RPL signal intensity showed a linear behavior as a function of the irradiation dose, which is independent from the dose rate. Despite of the close similarity of the RPL spectra, a slightly supra-linear behavior was found for the LiF crystals. Due to the LiF films limited thickness and thus the low RPL signal, a linear RPL response was clearly measured only at the highest doses, for both kind of substrates and all the investigated thicknesses. On the basis of these results, further investigations are under way to understand the observed behaviors in order to exploit the RPL of these LiF-based detectors for dosimetry of gamma rays in a very wide dose range up to MGy. • LiF crystals, thin films and TLD-100 pellets were gamma irradiated up to 1 kGy • F 2 , F 3 + CCs RPL spectra measured under cw laser excitation in crystals and pellets • RPL showed linear response with dose and dose-rate independence in pellets • F 2 , F 3 + CCs RPL measured by a fluorescence microscope in LiF thin films • RPL showed linear response at the highest doses in films on Si and Al substrates