# Monte Carlo calculation and verification of the geometrical factors for the NPDGamma experiment

## TL;DR

This paper uses Monte Carlo simulations to accurately determine detector geometrical factors in the NPDGamma experiment, improving the measurement of parity-violating asymmetry in neutron capture on hydrogen.

## Contribution

It introduces a modified MCNPX model to calculate detector sensitivities accounting for asymmetries, validated by experimental measurements.

## Key findings

- Monte Carlo model accurately predicts detector sensitivities.
- Validation through gamma-ray asymmetry measurements from chlorine capture.
- Enhanced correction methods improve asymmetry extraction.

## Abstract

The NPDGamma experiment measures the parity-violating asymmetry in $\gamma$-ray emission in the capture of polarized neutrons on liquid parahydrogen. The sensitivity to the asymmetry for each detector in the array is used as a parameter in the extraction of the physics asymmetry from the measured data. The detector array is approximately cylindrically symmetric around the target and a step-wise sinusoidal function has been used for the sensitivity in the previous iteration of the NPDGamma experiment, but deviations from cylindrical symmetry necessitate the use of a Monte Carlo model to determine corrections to the geometrical factors. For the calculations, source code modifications to MCNPX were done in order to calculate the sensitivity of each cesium iodide detector to the physics asymmetry. We describe the MCNPX model and results from calculations and how the results are validated through measurement of the parity violating asymmetry of $\gamma$-rays from neutron capture on chlorine.