(Digital Presentation) Color Centers Photoluminescence in Lithium Fluoride Thin-Film-on-Silicon Detectors for Proton Bragg Curves Imaging
Authors: Montereali Rosa Maria , Nichelatti Enrico , Nigro Valentina , Picardi Luigi , Piccinini Massimo , Ronsivalle Concetta , Vincenti Maria Aurora 
Published: November 2022

The photoluminescence (PL) properties of radiation-induced color centers (CCs) in lithium fluoride (LiF) crystals and thin films find applications in optically-pumped solid-state lasers and photonic light-emitting microdevices operating at room temperature (RT) in the visible and near-infrared spectral range [1]. Among their peculiarities, the broad tunability and high emission quantum efficiency, combined with the wide optical transparency of the hosting LiF matrix.

On the other hand, LiF dosimeters based on thermoluminescence (TL) of point defects in LiF crystals and pellets have been the most widely used family of phosphors in TL dosimetry, mainly for their high radiation sensitivity at low doses and the LiF tissue-equivalence, which is essential for meaningful medical applications.

The excellent thermal and optical stabilities of the laser-active F2 and F3
+ electronic defects, consisting of two electrons bound to two and three adjacent anion vacancies, respectively, whose efficient visible photoluminescence is located in the green-red spectral range under simultaneous excitation with blue-light pumping, make radiation detectors based on LiF crystals and thin films attractive for X-ray imaging [2] at nanoscale, related to the atomic-scale dimensions of such point defects.

These passive radiation imaging sensors are based on the optical reading of visible radiophotoluminescence (RPL) of aggregate CCs locally created and stored in LiF, by using conventional and advanced fluorescence microscopy techniques for latent images acquisition.

In the last years they were successfully tested for proton beam advanced diagnostics and dosimetry at increasing energies from 1.5 to 35 MeV, showing a linear RPL response as a function of absorbed doses in a wide dynamic range [4], long-term stability against fading and non-destructive reading capability. With suitable irradiation geometries of LiF crystals, it is possible to record the energy that protons deposit in the material (Bragg curve) as a bi-dimensional spatial distribution of luminescent CCs, even at dose values below 50 Gy, typically utilized in protontheraphy [5].

We have been investigating the feasibility to extend this approach to optically transparent LiF thin films thermally evaporated on Si(100) substrates. Despite of their low thickness, we take advantage of the enhanced PL response of F2 and F3
+ CCs, related to the presence of the flat and smooth Si substrate [6], which is optically reflective at the excitation and emission wavelengths utilized in the fluorescence microscope. The irradiations were performed with proton beams produced by the linear accelerator TOP-IMPLART (Oncological Therapy with Protons - Intensity Modulated Proton Linear Accelerator for RadioTherapy), under development at ENEA C.R. Frascati, with the cut edge perpendicular to the proton beam direction. The latent two-dimensional fluorescence images of the CC distributions generated in the polycrystalline LiF layers show a systematic increase in depth of the Bragg peak with respect to LiF crystals.

The results obtained in LiF films at increasing proton energies are presented and discussed, also in comparison with those in LiF crystals, in order to explain the observed behaviors and highlight advantages and limits of versatile LiF film radiation detectors.

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