2025/06/21 by Song, J. W., Ramalho, M., Lee, M. K. +9
#FOS: Physical sciences #Nuclear Experiment (nucl-ex)
paper · doi:10.48550/arxiv.2506.17544
Measurements of β-spectral shapes is an important way to examine the effective value of the weak axial coupling g\rm A. These stu dies focus specifically on forbidden non-unique β- transitions, as only in these cases is the spectral shape directly sensitive to th e ratio g\rm A/g\rm V. Here, the value of the weak vector coupling constant, g\rm V, is fixed at 1.0 according to the Conserve d Vector Current (CVC) hypothesis. In previous studies for the fourth-forbidden non-unique β- decays of 113Cd [J.~Kostensalo et al., Phys. Lett. B 822, 136652 (2021)] and 115In [A.~F. Leder et al., Phys. Rev. Lett. 129, 232502 (2022) and L. Pagnanini et al., Phys. Rev. Lett. 133, 122501 (2024)] a quenched value was determined for the ratio g\rm A/g\rm V using g\rm V=1.0. A notable exception is the recent measurement and analysis of the second-forbidden non-unique β-decay transition in 99Tc, performed by M. Paulsen et al., Phys. Rev. C 110, 05503(2024). Where an enhanced ratio g rm A/g\rm V=1.526(92) was suggested. To resolve this apparently contradictory situation with the effective value of g\rm A, we hav e performed calculations based on the nuclear shell model (NSM) Hamiltonians glekpn, jj45pnb, and the MQPM approach with a careful considera tion of the small relativistic vector nuclear matrix element (sNME). The theoretical spectra were compared to the 99Tc β-decay sp ectrum by using the 4π gold absorber with a Metallic Magnetic Calorimeter (MMC). In all cases, we found that the data matches well with reduced g\rm A/g\rm V values of 1.0--1.2. Our result contradicts the previously reported measurement for 99Tc and instead sup ports a quenched axial coupling as reported for other isotopes.