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

TL;DR

FaNS-2 is a segmented fast neutron spectrometer designed to measure neutron flux and spectra from a few MeV to 1 GeV, utilizing capture-gated spectroscopy for background rejection and improved energy reconstruction.

Contribution

This paper introduces the design, construction, and characterization of FaNS-2, a novel segmented neutron spectrometer with enhanced sensitivity and energy resolution for low-flux neutron measurements.

Findings

Achieved 5% energy resolution at 14 MeV

Successfully characterized with calibrated sources and monoenergetic neutron fields

Demonstrated capability to measure cosmogenic neutron spectra

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.