Measurement of the energy spectrum of laser-accelerated protons using FNTD: Development of an easy and quick method for energy spectrometry

Abstract

This paper reports the derivation of energies of proton beams accelerated by an intense laser using a fluorescent nuclear track detector (FNTD), which is Al2O3 single crystal doped with C and Mg. Using the FNTD, we acquire 3D images of incoming tracks easily, consecutively and rapidly and we can follow the fluorescence intensity of the proton tracks towards their terminals. By counting the number of proton tracks that disappeared at a certain depth, we determine the range of the tracks. Then, we evaluate energies of incoming protons from the evaluated range. The maximum energy of accelerated proton is 3.3 MeV. The results obtained from FNTD measurements are in good agreement with those obtained from the relationship between the incident energy and the distance calculated from the spatial distribution of the magnetic field. The FNTD provides the energy spectrum quickly after the laser shot within few hours, much faster than conventional nuclear track detectors such as CR-39 plastic and nuclear emulsion detectors, which require several days due to the chemical treatments needed. The FNTD is a promising detector for the diagnosis of beams accelerated by an intense laser, which generates a complex mixed radiation field.