Cause of the ≈3 eV discrepancy in K-GS/L-GS peak centroids

Determine whether the approximately 3 eV discrepancy between the measured centroid of the K-capture to the 7Li ground-state (K-GS) peak in the 7Be electron-capture recoil spectrum and the value expected from the sum of the 7Li nuclear recoil energy and the Li 1s binding/relaxation energy arises from an unrecognized systematic error in the energy calibration or from quenching of nuclear-recoil signals in tantalum-based superconducting tunnel junction (STJ) detectors; quantify any such calibration bias or quenching effect and assess its impact on the energy scale for both K- and L-capture peaks.

Background

In the BeEST Phase-III data, the centroid of the primary K-GS peak is measured at 108.50(11) eV, which is several eV lower than the value expected from adding the well-determined 7Li recoil energy to the Li 1s relaxation energy. The authors discuss two possible explanations: an unidentified systematic in the energy calibration or a small quenching of nuclear signals in STJ detectors.

They note that the L-GS centroid is consistent with the same recoil energy plus an approximately 3 eV contribution from the Li 2s hole, implying both hypotheses are plausible within existing measurements. The question is left open pending further investigation, but its resolution is important for absolute energy calibration and interpretation of potential beyond-standard-model signatures.