Development of a new tracking detector with fine-grained nuclear emulsion for sub-MeV neutron measurement

Abstract

Abstract We have developed a new sub-MeV neutron detector with high position resolution, energy resolution, and directional sensitivity, and low background. The detector is based on a super-fine-grained nuclear emulsion, called a nano-imaging tracker (NIT), and it is capable of detecting neutron-induced proton recoils as tracks through topological analysis with sub-micrometric accuracy. We used a type of NIT with AgBr:I crystals of ($98 \pm 10$) nm size dispersed in gelatin. First, we calibrated the performance of the NIT device for detecting monochromatic neutrons with sub-MeV energy generated by nuclear fusion reactions, and the detection efficiency for recoil proton tracks of more than 2 $\mu$m range was consistently 100% (the 1 $\sigma$ lower limit was 83%) in accordance with expectations from manual analysis. In addition, the recoil energy and angle distribution obtained good agreement with kinematical expectations. The primary neutron energy was reconstructed using these, and it was evaluated as 42% full width at half maximum at 540 keV. Furthermore, we demonstrated a newly developed automatic track recognition system dedicated to track ranges of more than a few micrometers. It achieved a recognition efficiency of ($74 \pm 4$)%, and the recoil energy and angle distribution obtained good agreement with manual analysis. Finally, it indicated a very high rejection power for $\gamma$-rays.