High-resolution diagnostics of the TOP-IMPLART 71 MeV proton beam using photoluminescent color centers in LiF crystals

Optically active point defects, known as color centers (CCs), are created in the crystal lattice of lithium fluoride (LiF) by irradiation with various types of ionizing radiation. Some of these CCs emit light in the red and green regions of the visible spectrum when optically excited with blue light. When a proton beam irradiates a LiF crystal, a volume distribution of CCs is formed, with defect concentration point-by-point proportional to the absorbed dose for values up to approximately 10⁵ Gy. By illuminating the irradiated crystal with blue light in a fluorescence microscope, a luminescent image produced by the CCs can be observed and recorded. A high-resolution diagnostics both for spot imaging and energy in a proton accelerator has been developed based on this technique. Regarding energy, a luminescent replica of the Bragg curve in LiF is extracted and analyzed using a theoretical model of Bragg curve applied to dose deposition, taking into account the crystal dimensions. We report an application of this method to the 71 MeV TOP-IMPLART linac at ENEA Frascati, where it was used to evaluate the beam energy spectrum at both the crystal position and the accelerator exit.