High precision measurement of the $^{99}$Tc $\beta$ spectrum

TL;DR

This study achieved highly precise measurements of the $^{99}$Tc beta spectrum using multiple detection methods and theoretical calculations, providing detailed spectral shape data and fundamental nuclear parameters.

Contribution

It presents the first high-precision measurement of the $^{99}$Tc beta spectrum with multiple independent methods and comprehensive theoretical analysis, improving nuclear data accuracy.

Findings

Measured the $^{99}$Tc beta spectrum down to less than 1 keV.

Determined the $Q_{eta}$ value as 295.82(16) keV.

Estimated the effective axial-vector coupling constant $g_A^{ ext{eff}}$ as 1.530(83).

Abstract

Highly precise measurements of the $^{99}$Tc beta spectrum were performed in two laboratories using metallic magnetic calorimeters. Independent sample preparations, evaluation methods and analyses yield consistent results and the spectrum could be measured down to less than 1 keV. Consistent beta spectra were also obtained via cross-evaluations of the experimental data sets. An additional independent measurement with silicon detectors in a $4\pi$ configuration confirms the spectrum shape above 25 keV. Detailed theoretical calculations were performed including nuclear structure and atomic effects. The spectrum shape was found to be sensitive to the effective value of the axial-vector coupling constant. Combining measurements and predictions, we extracted $Q_{\beta}=$295.82(16) keV and $g_A^{\text{eff}} = 1.530 (83)$. Furthermore, we derived the mean energy of the beta spectrum $\overline{E}_{\beta}$=98.45(20) keV, $\log f = -0.47660 (22)$ and $\log ft = 12.3478 (23)$.