Sub-keV energy calibration of CONUS+ via 71Ge M-shell neutron activation

TL;DR

The paper reports a sub-keV energy calibration of the CONUS+ germanium detector using 71Ge M-shell X-ray lines from neutron activation, reducing energy scale uncertainty for CEvNS measurements.

Contribution

It demonstrates the first clear resolution of the 71Ge M-shell X-ray line at sub-keV energies, validating energy calibration techniques for CONUS+ detector.

Findings

Achieved below 4% uncertainty in signal prediction from energy calibration.

Resolved the 71Ge M-shell X-ray line at (158.7+-1.4) eVee.

Validated energy reconstruction down to the detection threshold.

Abstract

The CONUS+ experiment has recently reported the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) of reactor antineutrinos on germanium nuclei and is now entering a precision phase. The dominant uncertainty in the first measurement was the energy scale, which contributed 14% to the uncertainty of the prediction of the combined signal. We present a dedicated neutron activation campaign in which one of the new 2.4 kg CONUS+ germanium detectors was irradiated with a strong 241AmBe source, demonstrating that a contribution below 4% to the uncertainty of signal prediction is achievable. For the first time, the 71Ge M-shell X-ray line was clearly resolved at (158.7+-1.4) eVee, validating the CONUS+ energy reconstruction down to the detection threshold. This validation includes the understanding of the energy scale, the energy resolution, the trigger efficiency, and the correct separation of physical from noise events. These results establish the foundation for a future activation campaign at the Kernkraftwerk Leibstadt reactor site, strengthening the CONUS+ energy calibration and extending its sensitivity to precision CEvNS and beyond Standard Model physics measurements.