Competition between Allowed and First-Forbidden β Decay: The Case of Hg 208 → Tl 208

6 pags., 4 figs., 1 tab.The β decay of Hg208 into the one-proton hole, one neutron-particle Tl81208127 nucleus was investigated at CERN-ISOLDE. Shell-model calculations describe well the level scheme deduced, validating the proton-neutron interactions used, with implications for the whole of the N>126, Z<82 quadrant of neutron-rich nuclei. While both negative and positive parity states with spin 0 and 1 are expected within the Qβ window, only three negative parity states are populated directly in the β decay. The data provide a unique test of the competition between allowed Gamow-Teller and Fermi, and first-forbidden β decays, essential for the understanding of the nucleosynthesis of heavy nuclei in the rapid neutron capture process. Furthermore, the observation of the parity changing 0+→0-β decay where the daughter state is core excited is unique, and can provide information on mesonic corrections of effective operators.This work was supported by the European Union under Contracts No. 262010 (ENSAR) and No. 654002 (ENSAR2), the Science and Technology Facilities Council (UK), the German BMBF under Contract No. 05P18PKCIA and “Verbundprojekt 05P2018,” the MINECO Projects No. FPA2015-65035-P, No. RTI2018- 098868-B-I00, No. FPA2015-64969-P, and No. FPA2017- 87568-P (Spain), FWO-Vlaanderen (Belgium), GOA/ 2015/010 (BOF KU Leuven), the Excellence of Science programme (EOS-FWO), the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (BriX network P7/12), the Romanian IFA project CERN-RO/ISOLDE and the Polish National Science Centre under Contracts No. UMO-2015/18/M/ST2/00523 and No. UMO-2019/33/N/ST2/03023. P. H. R. and S. M. J. acknowledge support from the UK Department for Business, Energy and Industrial Strategy via the National Measurement Office. Zs. P. acknowledges support from the ExtreMe Matter Institute EMMI at the GSI Helmholtzzentrum fr Schwerionenforschung, Darmstadt, Germa

Investigation of the Δn = 0 selection rule in Gamow-Teller transitions : The β-decay of 207 Hg

Gamow-Teller β decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This Δn=0 requirement plays a major role in the β decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical r-process pathway below both Z=50 (for N>82) and Z=82 (for N>126). The level of forbiddenness of the Δn=1ν1g 9/2 →π0g 7/2 transition has been investigated from the β − decay of the ground state of 207 Hg into the single-proton-hole nucleus 207 Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible γ-ray transitions depopulating the π0g 7/2 −1 state in 207 Tl, an upper limit of 3.9×10 −3 % was obtained for the probability of this decay, corresponding to log⁡ft>8.8 within a 95% confidence limit. This is the most stringent test of the Δn=0 selection rule to date

Decay studies in the A∼225 Po-Fr region from the DESPEC campaign at GSI in 2021

International audienceThe HISPEC-DESPEC collaboration aims at investigating the structure of exotic nuclei formed in fragmentation reactions with decay spectroscopymeasurements, as part of the FAIR Phase-0 campaign at GSI. This paper reportson first results of an experiment performed in spring 2021, with a focus on β-decaystudies in the Po-Fr nuclei in the 220 <A< 230 island of octupole deformationexploiting the DESPEC setup. Ion-beta correlations and fast-timing techniques arebeing employed, giving an insight into this difficult-to-reach region