# Measurement of the scintillation and ionization response of liquid xenon at MeV energies in the EXO-200 experiment

## TL;DR

This study measures the scintillation and ionization responses of liquid xenon at MeV energies using EXO-200 data, providing key parameters for improving rare event detection in future experiments.

## Contribution

It presents the first detailed measurements of absolute charge and light yields at MeV energies in liquid xenon, including the W-value and recombination fluctuations.

## Key findings

- W-value measured as 11.5 eV with uncertainties
- Recombination fluctuations deviate from lower-energy extrapolations
- Developed a semi-empirical energy resolution model

## Abstract

Liquid xenon (LXe) is employed in a number of current and future detectors for rare event searches. We use the EXO-200 experimental data to measure the absolute scintillation and ionization yields generated by $\gamma$ interactions from $^{228}$Th (2615~keV), $^{226}$Ra (1764~keV) and $^{60}$Co (1332~keV and 1173~keV) calibration sources, over a range of electric fields. The $W$-value that defines the recombination-independent energy scale is measured to be $11.5~\pm~0.5$~(syst.)~$\pm~0.1$~(stat.) eV. These data are also used to measure the recombination fluctuations in the number of electrons and photons produced by the calibration sources at the MeV-scale, which deviate from extrapolations of lower-energy data. Additionally, a semi-empirical model for the energy resolution of the detector is developed, which is used to constrain the recombination efficiency, i.e., the fraction of recombined electrons that result in the emission of a detectable photon. Detailed measurements of the absolute charge and light yields for MeV-scale electron recoils are important for predicting the performance of future neutrinoless double beta decay detectors.