First measurement of beta decay half-lives in neutron-rich Tl and Bi isotopes

Abstract

Neutron-rich isotopes around lead, beyond N=126, have been studied exploiting the fragmentation of an uranium primary beam at the FRS-RISING setup at GSI. For the first time β-decay half-lives of 219 Bi and 211,212,213 Tl isotopes have been derived. The half-lives have been extracted using a numerical simulation developed for experiments in high-background conditions. Comparison with state of the art models used in r-process calculations is given, showing a systematic underestimation of the experimental values, at variance from close-lying nuclei.

β-decay studies of neutron-rich Tl, Pb, and Bi isotopes

“…As in other β-decay studies with the RISING setup [7,8,32,33], implantations were accepted when one pixel of the DSSSD detectors registered a high-energy event in coincidence with a signal under particle threshold in the veto scintillator. The electrons emitted in their decay deposit typically several hundreds of keV in the active stopper.…”

“…The ion-β correlations were employed to study the decay of the fragmentation residues 211-213 Tl, 215 Pb, and 218,219 Bi. Such nuclei are characterized by long β-decay half-lives [8] that induce correlation intervals up to several hundred times longer than the beam cycle, thus increasing the number of δ or accidental β electrons in the spectra. To resolve the specific γ transitions from the random contribution of other radioactive species, the β-like events recorded during the beam-on periods were excluded from the data analysis.…”

“…Whereas standard microscopic-macroscopic models emphasize the importance of the allowed Gamow-Teller β-decay strength [3], self-consistent microscopic treatments stress the role of firstforbidden transitions in the reduction of predicted lifetimes [4,5]. From an experimental viewpoint, this issue is also a source of debate: Recent β-decay studies for N < 126 nuclei hint at a substantial contribution of forbidden transitions to the total β strength [6,7], while half-life measurements beyond 208 Pb [8][9][10] point towards a more complex landscape where the interplay between allowed and first-forbidden decays depends on the differing valence configurations. Detailed information on β-decay strength functions and Q β values is required in order to address the present discrepancies between theory and experiment.…”

“…This wealthy dataset has already provided some important results, such as the identification of seniority isomers along the neutron-rich Pb chain [26], the measurement of β half-lives in neutron-rich Tl and Bi isotopes [8], and the observation of new μs isomers in the neutron-rich 210 Hg nucleus [27].…”

New μs isomers in the neutron-rich 210 Hg nucleus

“…Further experimental details can be found in Ref. [4,14]. Figure 1, upper panel, shows the γ-ray spectrum following the isomeric decays of 210 Hg.…”