Development of a time-of-flight spectrometer for characterizing the quasi-monoenergetic neutron beam driven by a 30-MeV proton cyclotron

By developing a time-of-flight (TOF) spectrometer, this study experimentally examines the spectrum of a quasi-monoenergetic neutron (QMN) source generated by irradiating 30-MeV protons from a cyclotron onto a beryllium target. The repetitive irradiation of proton bunches on the target at a radio frequency results in periodic distributions of coincidence events between gamma-ray- and neutron-related signals, necessitating a specific method to extract the neutron spectrum from the TOF measurements. An effective time window of 13.67 ns in the TOF spectrum, corresponding to the period between proton bunches, was identified for analyzing neutrons with energies ranging from 16.19 to 30 MeV. The retrieved neutron spectrum distinctly exhibits a peak at 26 MeV, verifying the spectrum simulated by a Monte Carlo model and supporting the QMN beam’s use in downstream applications. Additionally, a fast-timing scintillator module and its associated algorithm were developed to measure the proton bunch duration on a sub-nanosecond scale, enabling the estimation of the energy resolution in the spectrum, approximately 8% at 30 MeV. These techniques, developed for characterizing the neutron spectrum and proton bunch duration, significantly enhance the quality assurance processes for operating a proton-cyclotron-based neutron source.