Reconstructing the direction of reactor antineutrinos via electron scattering in Gd-doped water Cherenkov detectors

TL;DR

This study explores how Gd-doped water Cherenkov detectors can determine the direction of reactor antineutrinos through elastic scattering, assessing background effects and experimental conditions for effective directional reconstruction.

Contribution

It demonstrates the feasibility of using water Cherenkov detectors with optimized conditions to reconstruct antineutrino directions at significant distances from reactors.

Findings

Directional reconstruction possible at >10 km with proper background control.

Low background PMTs and fiducialization are crucial for sensitivity.

Optimal conditions enable >3σ significance in large detectors.

Abstract

The potential of elastic antineutrino-electron scattering in a Gd-doped water Cherenkov detector to determine the direction of a nuclear reactor antineutrino flux was investigated using the recently proposed WATCHMAN antineutrino experiment as a baseline model. The expected scattering rate was determined assuming a 13-km standoff from a 3.758-GWt light water nuclear reactor and the detector response was modeled using a Geant4-based simulation package. Background was estimated via independent simulations and by scaling published measurements from similar detectors. Background contributions were estimated for solar neutrinos, misidentified reactor-based inverse beta decay interactions, cosmogenic radionuclides, water-borne radon, and gamma rays from the photomultiplier tubes (PMTs), detector walls, and surrounding rock. We show that with the use of low background PMTs and sufficient fiducialization, water-borne radon and cosmogenic radionuclides pose the largest threats to sensitivity. Directional sensitivity was then analyzed as a function of radon contamination, detector depth, and detector size. The results provide a list of experimental conditions that, if satisfied in practice, would enable antineutrino directional reconstruction at 3$\sigma$ significance in large Gd-doped water Cherenkov detectors with greater than 10-km standoff from a nuclear reactor.