Characterizing neutrino and neutron fluxes from the Spallation Neutron Source for the COHERENT experiment

Coherent elastic neutrino-nucleus scattering (CEvNS), a process by which a neutrino scatters from the whole nucleus, has been observed by the COHERENT collaboration in multiple detectors using pulsed neutrinos produced by the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. This work contributes to the improved understanding of neutron and neutrino fluxes, which all COHERENT subsystems use to interpret their observations, as COHERENT transitions from discovery to precision measurements of CEvNS.

In the first half of this work, we walk through the prediction of the neutrino flux and spectra at the SNS using Geant4 simulation, and establish that there is an associated 10% uncertainty on these calculations due to a lack of pion-production data for SNS operating conditions. This large systematic is now the dominant uncertainty limiting COHERENT’s precision physics goals. To contribute to the reduction of this uncertainty, we also perform design studies for a new deployed D₂O detector to experimentally normalize our neutrino flux.

The only observable signature of CEvNS is a low-energy nuclear recoil, which means neutrons cause a similar signal to neutrinos in our sensitive detectors. In the second half of this work, we transition to the characterization of the beam-related neutron flux from the SNS. We monitor the neutron flux near COHERENT detectors using a dedicated subsystem: the Multiplicity and Recoil Spectrometer (MARS). In this work, we simulate this mobile, Gd-doped plastic scintillator and begin to develop a neutron response matrix that can be used to unfold the incident neutron spectrum. We also perform a rate analysis to study the incoming flux at multiple locations near our CEvNS detectors.