Low-energy protons shallow spread-out Bragg peak imaging with a lithium fluoride crystal

The 35 MeV proton beam produced by a modular linear accelerator was used to irradiate at grazing incidence a LiF crystal through a spinning Mylar multi-sector range modulation wheel and a Pyrex range shifter. Starting from a measured pristine depth dose curve, these energy-modulation devices were designed to plan a test shallow spread-out Bragg peak in water. The irradiation of the LiF crystal produced in its lattice a volume distribution of color centers, which could be visualized in a fluorescence microscope as a fluorescent image under blue-light illumination. Since the intensity of this fluorescent image was proportional point-by-point to the absorbed energy, it was elaborated to obtain an experimental depth dose curve in the LiF crystal, from which the proton beam energy distribution was estimated. This latter was finally used to evaluate the corresponding spread-out Bragg peak that would be obtained in water and compare it to the designed one.