[Paper Review] Total Neutron Cross-section Measurement on CH with a Novel 3D-projection Scintillator Detector

Agarwal, A., H. Budd+60 more|arXiv (Cornell University)|Jun 28, 2022

Neutrino Physics Research4 citations

TL;DR

This study presents a novel 3D-projection scintillator detector to measure total neutron cross-sections on hydrocarbon (CH) across 98–688 MeV neutron kinetic energy. Using precise vertex reconstruction and systematic uncertainty modeling, the experiment achieves a total cross-section of 0.36 ± 0.05 barns in the 98–688 MeV range, showing good agreement with the Geant4 Bertini model and providing critical data to reduce systematic uncertainties in long-baseline neutrino oscillation experiments.

ABSTRACT

In order to extract neutrino oscillation parameters, precision long-baseline neutrino oscillation experiments rely on detailed models of neutrino interactions with nuclei. These models constitute an important source of systematic uncertainty, partially because detectors to date have been blind to final state neutrons. Three-dimensional projection scintillator trackers comprise components of the near detectors of the next generation long-baseline neutrino experiments. Due to the good timing resolution and fine granularity, this technology is capable of measuring neutron kinetic energy in neutrino interactions on an event-by-event basis and will provide valuable data for refining neutrino interaction models and ways to reconstruct neutrino energy. Two prototypes have been exposed to the neutron beamline at Los Alamos National Laboratory (LANL) in both 2019 and 2020, with neutron energies between 0 and 800 MeV. In order to demonstrate the capability of neutron detection, the total neutron-scintillator cross section is measured and compared to external measurements. The measured total neutron cross section in scintillator between 98 and 688 MeV is 0.36 $\pm$ 0.05 barn.

Motivation & Objective

• To measure the total neutron cross-section on hydrocarbon (CH) across a broad energy range (98–688 MeV) to improve neutrino interaction models in long-baseline oscillation experiments.
• To address the significant systematic uncertainty in neutrino cross-section models arising from undetected final-state neutrons in previous experiments.
• To validate the performance of a novel 3D-projection scintillator tracker in reconstructing neutron vertex positions with high precision.
• To quantify and minimize systematic uncertainties from detector alignment, light yield, time resolution, and reconstruction effects.
• To provide a benchmark measurement consistent with existing data while reducing reliance on simulation-based assumptions for neutron detection.

Proposed method

• Employed a 3D-projection scintillator tracker with fine granularity and excellent timing resolution to reconstruct neutron interaction vertices along the z-axis.
• Used exponential fitting to the reconstructed z-vertex distribution across energy bins to extract the total cross-section, with systematic uncertainties varied independently and summed in quadrature.
• Performed extensive validation checks: consistency between data and Monte Carlo simulations, day-by-day data consistency, fitting range variation, and inclusion of constant background terms.
• Reconstructed cross-sections were validated against true MC cross-sections, showing bias within uncertainty bounds.
• Applied geometric acceptance corrections and accounted for invisible scattering, collimator interactions, and detection inefficiencies in the uncertainty budget.
• Used a multi-layered detector configuration with 38 z-layers to ensure statistical and spatial resolution necessary for energy-dependent cross-section extraction.

Experimental results

Research questions

• RQ1 How accurately can a 3D-projection scintillator detector measure the total neutron-CH cross-section across 98–688 MeV?
• RQ2 To what extent do systematic uncertainties—especially from detector alignment, light yield, and reconstruction—affect the final cross-section measurement?
• RQ3 How does the measured total neutron-CH cross-section compare to the Geant4 Bertini model in the 98–688 MeV range?
• RQ4 What is the impact of neutron-H interactions on the overall CH cross-section, and how does the measurement constrain this contribution?
• RQ5 Can the detector’s reconstruction performance be validated against Monte Carlo simulations with minimal bias?

Key findings

• The total neutron-CH cross-section was measured as 0.36 ± 0.05 barns in the energy-integrated range of 98–688 MeV, with a χ²/d.o.f. of 22.03/38.
• For neutron energies below 200 MeV, the measured cross-section is slightly higher than the Geant4 Bertini model, with a χ²/d.o.f. of 16.1/18.
• In the 200–688 MeV range, the measurement shows good agreement with the Geant4 Bertini model, indicating reliable simulation performance in this region.
• The total uncertainty is dominated by detection-related systematic effects, including invisible scattering, light yield, and reconstruction uncertainties.
• Validation checks—including day-by-day consistency, fitting range variation, and background term inclusion—confirmed robustness and consistency of the analysis.
• The measured cross-section is consistent with existing neutron–carbon total cross-section measurements, indicating a minor contribution from neutron–hydrogen interactions in CH.