Measurement of cosmogenic $^9$Li and $^8$He production rates at RENO

TL;DR

This study measures the production rates of cosmogenic $^9$Li and $^8$He isotopes at RENO, providing insights into background sources for neutrino experiments and establishing a power-law relationship with muon energy.

Contribution

It presents the first detailed measurement of $^9$Li and $^8$He production rates at RENO and confirms a power-law dependence of yields on muon energy.

Findings

Measured $^9$Li and $^8$He production rates at near and far detectors.

Established a power-law relationship between isotope yield and muon energy.

Quantified the yields and their dependence on muon energy with high precision.

Abstract

We report the measured production rates of unstable isotopes $^9$Li and $^8$He produced by cosmic muon spallation on $^{12}$C using two identical detectors of the RENO experiment. Their beta-decays accompanied by a neutron make a significant contribution to backgrounds of reactor antineutrino events in precise determination of the smallest neutrino mixing angle. The mean muon energy of its near (far) detector with an overburden of 120 (450) m.w.e. is estimated as 33.1 +- 2.3 (73.6 +- 4.4) GeV. Based on roughly 3100 days of data, the cosmogenic production rate of $^9$Li ($^8$He) isotope is measured to be 44.2 +- 3.1 (10.6 +- 7.4) per day at near detector and 10.0 +- 1.1 (2.1 +- 1.5) per day at far detector. This corresponds to yields of $^9$Li ($^8$He), 4.80 +- 0.36 (1.15 +- 0.81) and 9.9 +- 1.1 (2.1 +- 1.5) at near and far detectors, respectively, in a unit of 10$^{-8}$ $\mu^{-1}$ g${^-1}$ cm${^2}$. Combining the measured $^9$Li yields with other available underground measurements, an excellent power-law relationship of the yield with respect to the mean muon energy is found to have an exponent of $\alpha$ = 0.75 +- 0.05.