Development and Characterization of a High Sensitivity Segmented Fast Neutron Spectrometer (FaNS-2)

Abstract

We present the development of a segmented fast neutron spectrometer (FaNS-2) based upon plastic scintillator and ${}^3$He proportional counters. It was designed to measure both the flux and spectrum of fast neutrons in the energy range of few MeV to 1 GeV. FaNS-2 utilizes capture-gated spectroscopy to identify neutron events and reject backgrounds. Neutrons deposit energy in the plastic scintillator before capturing on a ${}^3$He nucleus in the proportional counters. Segmentation improves neutron energy reconstruction while the large volume of scintillator increases sensitivity to low neutron fluxes. A main goal of its design is to study comparatively low neutron fluxes, such as cosmogenic neutrons at the Earth's surface, in an underground environment, or from low-activity neutron sources. In this paper, we present details of its design and construction as well as its characterization with a calibrated ${}^{252}$Cf source and monoenergetic neutron fields of 2.5 MeV and 14 MeV. Detected monoenergetic neutron spectra are unfolded using a Singular Value Decomposition method, demonstrating a 5% energy resolution at 14 MeV. Finally, we discuss plans for measuring the surface and underground cosmogenic neutron spectra with FaNS-2.

20 pages, 13 figures, Submitted to JINST

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• Verification of He-3 proportional counters fast neutron sensitivity through a comparison with He-4 detectors
• Design of a mobile neutron spectrometer for the Laboratori Nazionali del Gran Sasso (LNGS)