Solid state detectors based on point defects in lithium fluoride for advanced proton beam diagnostics

Lithium fluoride (LiF) is a well-known dosimeter material and it is sensitive to any kind of ionising radiation. In the last years, LiF crystals and thin films have been proposed and tested as solid-state x-ray [1,2] and neutron [3] imaging detectors, based on the optical reading of the photoluminescence (PL) of radiation-induced visible-emitting colour centres (CCs). Ion beams of different energies are widely investigated for applications ranging from material modifications to radiobiology and radiotherapy. Recently, proton beams of 3 MeV and 7 MeV energy, produced by a linear accelerator working as the injector of a protontherapy linac under development at ENEA Frascati [4], were used to irradiate at room temperature (RT) LiF crystals and thin films in the fluence range from 1010 to 1015 protons/cm2. The irradiation of LiF induces the formation of primary and aggregate CCs, which are stable at RT. The aggregate visible-emitting F2 and F3+ CCs (two electrons bound to two and three close anion vacancies, respectively) possess almost overlapping absorption bands around 450 nm. Under optical pumping in this spectral region they simultaneously emit broad PL bands peaked at 678 nm and 541 nm for F2 and F3+ CCs, respectively. Their PL intensity was carefully measured both in LiF crystals and in LiF films only 1 m thick grown by thermal evaporation on glass and silicon substrates [5]. They show different optical response as a function of the irradiation fluence and the results are encouraging for using LiF-based detectors for proton dosimetry over several orders of magnitude of dose range. By optical fluorescence microscopy, it was possible to record the transversal proton beam intensity profile by acquiring the PL image of irradiated LiF. It showed to be a versatile radiation imaging detector, as it could store information about the proton beam intensity on a large area with high spatial resolution and revealing even subtle intensity differences. Moreover, using cleaved LiF crystals and LiF films grown on Si(100) substrates irradiated in a particular geometry allowed one to measure the CC distribution with depth and imaging the Bragg peak, which gives an estimation of the proton beam energy. The sensitivity of the optical reading techniques and the high emission efficiency of the F2 and F3+ centres, combined with the good optical quality of the thermally evaporated LiF films provided encouraging results, which are under study in order to optimize the LiF film characteristics related to their PL response on selected fluence intervals, for dosimetry and imaging applications in protontherapy.