# Improved STEREO simulation with a new gamma ray spectrum of excited gadolinium isotopes using FIFRELIN

## TL;DR

This paper improves the simulation of gamma ray spectra from gadolinium isotopes after neutron capture, enhancing the accuracy of the STEREO experiment's response modeling using FIFRELIN-generated cascades.

## Contribution

Introduces a new gamma ray spectrum model for gadolinium isotopes using FIFRELIN, significantly improving simulation-data agreement in the STEREO experiment.

## Key findings

- Enhanced simulation accuracy for neutron capture gamma spectra.
- Generated a comprehensive database of deexcitation cascades.
- Improved detection efficiency modeling in the STEREO experiment.

## Abstract

The STEREO experiment measures the electron antineutrino spectrum emitted in a research reactor using the inverse beta decay reaction on H nuclei in a gadolinium loaded liquid scintillator. The detection is based on a signal coincidence of a prompt positron and a delayed neutron capture event. The simulated response of the neutron capture on gadolinium is crucial for the comparison with data, in particular in the case of the detection efficiency. Among all stable isotopes, $^{155}$Gd and $^{157}$Gd have the highest cross sections for thermal neutron capture. The excited nuclei after the neutron capture emit gamma rays with a total energy of about 8 MeV. The complex level schemes of $^{156}$Gd and $^{158}$Gd are a challenge for the modeling and prediction of the deexcitation spectrum, especially for compact detectors where gamma rays can escape the active volume. With a new description of the Gd(n,${\gamma}$) cascades obtained using the FIFRELIN code, the agreement between simulation and measurements with a neutron calibration source was significantly improved in the STEREO experiment. A database of ten millions of deexcitation cascades for each isotope has been generated and is now available for the user.