Geant4 Simulation of Neutron Capture in Pure and Gadolinium-Doped Water for Neutrino Detectors

Neutron capture is a key process for detecting and characterizing neutrino interactions in large water-Cherenkov and gadolinium-doped detectors. Its timing and spatial signatures determine how efficiently delayed neutron signals can be separated from prompt backgrounds and therefore set fundamental limits on event reconstruction and background rejection. We present a detailed Geant4 (11.2.2) simulation of neutron capture in pure water and in water doped with 0.1 %, 0.2 %, and 0.5 % gadolinium (Gd) by mass. Primary neutrons were generated isotropically with three initial energy spectra—thermal (0.01–0.1 eV), intermediate (0.1 eV–100 keV), and fast (0.1–10 MeV)—each sampled with a uniform probability distribution. For every material–spectrum combination we recorded four observables: (i) mean neutron-capture time, (ii) capture-time distributions and their cumulative probabilities, (iii) total track length prior to capture, and (iv) capture radius relative to the primary neutron production vertex. The results show that at 0.5 % Gd, the mean capture time falls from roughly 200 µs in pure water to 5–9 µs across all neutron spectra, while the mean capture radius decreases from about 73 mm (thermal)–180 mm (fast) to 11 mm (thermal)–158 mm (fast). These findings quantify the dependence of capture dynamics on both absorber content and neutron energy and provide geometry-independent benchmarks—made possible by the use of a simulation volume large enough to ensure essentially complete neutron capture—for optimizing the time-window selection, fiducial-volume definition, and background suppression of next-generation neutrino detectors.