Unfolding Neutron Spectrum with Markov Chain Monte Carlo at MIT Research Reactor with He-3 Neutral Current Detectors

TL;DR

This paper develops a method using Markov Chain Monte Carlo to unfold neutron spectra from detector data at MIT research reactor, aiding neutrino experiments by characterizing neutron backgrounds and exploring shielding options.

Contribution

It introduces an MCMC-based unfolding technique for neutron spectra at a research reactor, improving background understanding for neutrino detection experiments.

Findings

Neutron spectrum characterized across multiple energy ranges.

Shielding effectiveness evaluated for background reduction.

Implications for deploying neutrino detectors at MITR discussed.

Abstract

The Ricochet experiment seeks to measure Coherent (neutral-current) Elastic Neutrino-Nucleus Scattering using dark-matter-style detectors with sub-keV thresholds placed near a neutrino source, such as the MIT (research) Reactor (MITR), which operates at 5.5 MW generating approximately 2.2e18 neutrinos/second in its core. Currently, Ricochet is characterizing the backgrounds at MITR, the main component of which comes in the form of neutrons emitted from the core simultaneous with the neutrino signal. To characterize this background, we wrapped Bonner cylinders around a He-3 thermal neutron detector, whose data was then unfolded via a Markov Chain Monte Carlo (MCMC) to produce a neutron energy spectrum across several orders of magnitude. We discuss the resulting spectrum and its implications for deploying Ricochet at the MITR site as well as the feasibility of reducing this background level via the addition of polyethylene shielding around the detector setup.