Measurements and computational analysis of the natural decay of Lu176

Abstract

Background: Mainly because of its long half-life and despite its scientific relevance, spectroscopic measurements of 176 Lu 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 176 Lu β-decay spectra and attempt the observation of its electron capture decays, as well as perform the first precision direct measurement of the 176 Lu β-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 176 Lu β-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 176 Lu 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 176 Lu β-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 g A , which was extracted according to different assumptions. Conclusion:The implemented self-scintillation method has provided unmatched observations of 176 Lu, 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 g A and log 10 f values, showing that a high-precision description of this transition would require a realistic nuclear structure with nucleus deformation.

Systematics of Log Ft Values for Β-, and Ec/Β+ Transitions

“…Below these bars, both the centroid and the width of each distribution are indicated by an additional data point and its error bars, respectively. In case of first forbidden non-unique transitions with ∆J=1 and ∆π=yes with Z<80, there are only two cases with a comparatively large log f t of approximately 20, that occur in 176 Lu decay, well known to be affected by nucleus deformation [53]. Otherwise, the maximum is 14.334.…”

Status of CHIP-TRAP: The Central Michigan University High-Precision Penning Trap

“…Measurements at Penning trap facilities with ions from a LAS have spanned the nuclear chart from, for example, 6,7 Li [30] to 249 Cf [28]. Ions from a LAS serve as reference masses for on-line measurements with short-lived isotopes, calibrants for tuning a system and investigating systematics, and as the ions of interest for nuclear and neutrino physics studies, e.g., [31][32][33][34][35][36][37].…”