Spatial and spectral measurement of laser-driven protons through radioactivation

The simultaneous measurement of the spatial profile and spectrum of laser-accelerated protons is important for further optimization of the beam qualities and applications. We report a detailed study regarding the underlying physics and regular procedure of such a measurement through the radioactivation of a stack composed of aluminum, copper, and CR-39 plates as well as radiochromic films (RCFs). After being radioactivated, the copper plates are placed on imaging plates (IPs) to detect the positrons emitted by the reaction products through contact imaging. The spectrum and energy-dependent spatial profile of the protons are then obtained from the IPs and confirmed by the measured ones from the RCFs and CR-39 plates. We also discuss the detection range, influence of electrons, radiation safety, and spatial resolution of this measurement. Finally, insights regarding the extension of the current method to online measurements and dynamic proton imaging are also provided.