[Paper Review] A Geant4 simulation of particle production at the Spallation Neutron Source

COHERENT Collaboration, D. Akimov+81 more|arXiv (Cornell University)|Sep 22, 2021

Particle physics theoretical and experimental studies60 references4 citations

TL;DR

This paper presents a Geant4 simulation of neutrino production at the Spallation Neutron Source (SNS) using the QGSP_BERT physics list, modeling 1 GeV protons incident on a mercury target. It predicts a neutrino flux of 4.7 × 10⁷ ν cm⁻² s⁻¹ at 20 m from the target, with 99% originating from stopped π⁺ decay, and identifies the pion-production model uncertainty (±10%) as the dominant systematic in COHERENT's coherent elastic neutrino-nucleus scattering measurements.

ABSTRACT

Release of COHERENT collaboration software associated with the neutrino production studies of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory described in arXiv:2109.11049. This simulation investigates the particle production at the SNS using Geant4, and we include both the base simulation and our analysis scripts in this package.

Motivation & Objective

• To simulate the neutrino flux produced at the Spallation Neutron Source (SNS) using Geant4 for accurate prediction of particle yields.
• To quantify the uncertainty in the simulated neutrino flux arising from the underlying pion-production model, which affects COHERENT's coherent elastic neutrino-nucleus scattering measurements.
• To support the development of a direct measurement strategy for the SNS neutrino flux using a D₂O detector to normalize simulations.
• To enable future model validation for particles like π⁰, π⁻, and η relevant to dark matter searches at the SNS.
• To improve the accuracy of low-energy neutrino flux predictions for accelerator-based neutrino experiments using the SNS as a reference source.

Proposed method

• Utilizes Geant4.10.06 with the QGSP_BERT physics list to simulate proton-induced particle production in the SNS mercury target.
• Treats the SNS target as a point source to simplify flux calculations at 20 m distance.
• Models neutrino production primarily through the decay chain of stopped π⁺ mesons, which dominate the flux at low energies.
• Applies a 10% uncertainty estimate to the pion-production model, derived from experimental and theoretical inputs.
• Uses the well-understood νₑ + d → p + p + e⁻ cross section to design a future D₂O detector for direct flux normalization.
• Proposes extending the simulation framework to model β⁺/β⁻ decays from activated materials during proton spills.

Experimental results

Research questions

• RQ1 What is the predicted neutrino flux at 20 m from the SNS target under 1 GeV proton beam conditions?
• RQ2 What fraction of the total neutrino flux originates from stopped π⁺ decay compared to other sources?
• RQ3 How does the uncertainty in the pion-production model affect the overall systematic uncertainty in COHERENT's CEvNS measurements?
• RQ4 Can a D₂O detector deployed at the SNS achieve sufficient statistical precision to directly measure the νₑ flux and normalize simulations?
• RQ5 To what extent can the simulation framework be extended to model low-energy contributions from β decays of activated nuclei during proton spills?

Key findings

• The simulation predicts a total neutrino flux of 4.7 × 10⁷ ν cm⁻² s⁻¹ at 20 m from the SNS target for 1 GeV incident protons at 1.4 MW.
• Approximately 99% of the total neutrino flux is produced by the decay chain of stopped π⁺ mesons, confirming its dominance in low-energy neutrino production.
• The pion-production model uncertainty contributes a 10% systematic uncertainty to the simulated flux, which is now the dominant systematic in COHERENT's CEvNS measurements.
• The simulation framework is validated as a critical tool for estimating particle fluxes and beamline geometry effects at the SNS.
• A future D₂O detector with a mass of ~680 kg could achieve statistical precision comparable to four SNS beam-years, enabling direct normalization of the simulated flux.
• The simulation is being extended to model low-energy contributions from β⁺/β⁻ decays of activated materials during proton spills, which may affect future low-energy neutrino experiments.