Physical origin of the secondary low-energy component of the K-GS peak

Identify the physical mechanism responsible for the additional lower-energy component observed in the K-capture to 7Li ground-state (K-GS) peak of the 7Be electron-capture recoil spectrum measured with tantalum-based superconducting tunnel junction detectors, and determine whether it arises from energy loss by secondary radiation or from defect formation (e.g., lattice damage) near the implantation sites.

Background

The K-GS peak in both Phase-II and Phase-III data requires multiple components for accurate fitting. In particular, a smaller component appears at lower energy relative to the main K-GS peak. The authors note this feature is persistent but not yet explained.

They suggest potential causes including energy loss due to secondary radiation generated during the Auger relaxation or formation of lattice defects (e.g., Frenkel pairs) near the 7Be implantation site. Determining the mechanism is necessary to model the intrinsic lineshape and eliminate biases in searches for heavy neutrino admixtures.