Background: Mainly because of its long half-life and despite its scientific relevance, spectroscopic measurements of Lu176 forbidden β decays are very limited and lack formulation of shape factors. A direct precise measurement of its Q value is also presently unreported. In addition, the description of forbidden decays provides interesting challenges for nuclear theory. The comparison of precise experimental results with theoretical calculations for these decays can help to test underlying models and can aid the interpretation of data from other experiments. Purpose: Perform the first precision measurements of Lu176β-decay spectra and attempt the observation of its electron capture decays, as well as perform the first precision direct measurement of the Lu176β-decay Q value. Compare the shape of the precisely determined experimental β spectra to theoretical calculations, and compare the end point energy to that obtained from an independent Q value measurement. Method: The Lu176β-decay spectra measurements and the search for electron capture decays were performed with an experimental setup that employed lutetium-containing scintillator crystals and a NaI(Tl) spectrometer for coincidence counting. The β decay Q value was determined via high-precision Penning trap mass spectrometry (PTMS) with the LEBIT facility at the National Superconducting Cyclotron Laboratory. The β-spectrum calculations were performed within the Fermi theory formalism with nuclear structure effects calculated using a shell model approach. Results: Both β transitions of Lu176 were experimentally observed and corresponding shape factors formulated in their entire energy ranges. The search for electron capture decay branches led to an experimental upper limit of 6.3×10-6 relative to its β decays. The Lu176β-decay and electron capture Q values were measured using PTMS to be 1193.0(6) and 108.9(8) keV, respectively. This enabled precise β end point energies of 596.2(6) and 195.3(6) keV to be determined for the primary and secondary β decays, respectively. The conserved vector current hypothesis was applied to calculate the relativistic vector matrix elements. The β-spectrum shape was shown to significantly depend on the Coulomb displacement energy and on the value of the axial vector coupling constant gA, which was extracted according to different assumptions. Conclusion: The implemented self-scintillation method has provided unmatched observations of Lu176, independently validated by the first direct measurements of its β-decay Q value by Penning trap mass spectrometry. Theoretical study of the main β transition led to the extraction of very different effective gA and log10f values, showing that a high-precision description of this transition would require a realistic nuclear structure with nucleus deformation.

Background: The understanding and description of forbidden decays provides interesting challenges for nuclear theory. These calculations could help to test underlying nuclear models and interpret experimental data. Purpose: Compare a direct measurement of the La138β-decay Q value with the β-decay spectrum end-point energy measured by Quarati et al. using LaBr3 detectors [Appl. Radiat. Isot. 108, 30 (2016)ARISEF0969-804310.1016/j.apradiso.2015.11.080]. Use new precise measurements of the La138β-decay and electron capture (EC) Q values to improve theoretical calculations of the β-decay spectrum and EC probabilities. Method: High-precision Penning trap mass spectrometry was used to measure cyclotron frequency ratios of La138, Ce138, and Ba138 ions from which β-decay and EC Q values for La138 were obtained. Results: The La138β-decay and EC Q values were measured to be Qβ=1052.42(41) keV and QEC=1748.41(34) keV, improving the precision compared to the values obtained in the most recent atomic mass evaluation [Wang, Chin. Phys. C 41, 030003 (2017)1674-113710.1088/1674-1137/41/3/030003] by an order of magnitude. These results are used for improved calculations of the La138β-decay shape factor and EC probabilities. New determinations for the Ce138 2EC Q value and the atomic masses of La138, Ce138, and Ba138 are also reported. Conclusion: The La138β-decay Q value measured by Quarati et al. is in excellent agreement with our new result, which is an order of magnitude more precise. Uncertainties in the shape factor calculations for La138β decay using our new Q value are reduced by an order of magnitude. Uncertainties in the EC probability ratios are also reduced and show improved agreement with experimental data.