Photoluminescence of Colour Centres in Thermally-Evaporated LiF Films for Radiation Imaging Detectors

Abstract

Lithium fluoride (LiF) is a well-known dosimeter material in pure (McLaughlin et al. Nucl Instrum Methods 175:17–18, 1980) and doped (Lakshmanan et al. Phys Status Solidi (a) 153:265–273, 1996) form. Radiation detectors based on microcrystalline dispersion of LiF in a polymeric matrix have been introduced for gamma and electron high-dose dosimetry (Kovacs et al. Radiat Phys Chem 57:691–695, 2000). In recent years the area of growth and characterisation of LiF thin films has seen a considerable expansion. Polycrystalline LiF films grown by thermal evaporation were proposed and tested as nuclear sensors for neutrons (Cosset et al. Thin Solid Films 303:191–195, 1997) and for gamma dosimetry (Montecchi et al. Point defects in lithium fluoride films induced by gamma irradiation, ch. 116. In: Proceedings of the 7th international conference on advanced technology and particle physics, Como, pp 819–825, 2002). In the last years many efforts have been devoted to the development of novel imaging detectors for extreme-ultraviolet radiation and soft X-rays (Baldacchini et al. J Nanosci Nanotechnol 3:483–486, 2003), hard X-rays (Almaviva et al. Appl Phys Lett 89(5):054102, 2006), as well as low (Baldacchini et al. J Phys Condens Matter 10:857–867, 1998) and high energy electrons. Such solid-state detectors are based on the optical reading of photoluminescence (PL) from stable, visible-emitting colour centres (CCs), produced by irradiation with ionising radiations. These aggregate electronic defects are F₂ and F₃ ⁺ CCs (two electrons bound to two and three close anion vacancies, respectively), which possess almost overlapping absorption bands, peaked at about 450 nm, called M band (Nahum, Phys Rev 157:817–830, 1967). By optical pumping in this spectral region, F₂ and F₃ ⁺ CCs emit broad PL bands peaked at 678 and 541 nm, respectively. LiF films of different thickness were grown by thermal evaporation on different substrates, such as glass and silica, as well as plastic ones (Di Lazzaro et al. Extreme ultraviolet marking system for anti-counterfeiting tags with adjustable security level.