Shell-model interpretation of theβ−decay ofBig

Abstract

The P decay of the Bi ground state (Tip~= 61 min) aud the isomers Bi (Tig2 --27 min), aud Bi (T, g2 --8 min) are considered in the spherical shell model. The wave functions and energy spectra of all even and odd states in the Kuo-Herling model space were calculated for both Bi and Po. Possible candidates for the two Bi isomers were located from these results and the allowed and first-forbidden P decays rates were formed using a recently developed parametrization of first-forbidden decays in the lead region. It was concluded that Bi is the yrast 8 state and Bi the yrast 18 state of Bi. The decay rates calculated for these two P emitters are in excellent agreement with experiment. The predictions for the decay of the ground state of Bi are not in good agreement with experiment, illustrating the diKculty of calculating weak first-forbidden decay modes reliably.

Direct Observation of Long-Lived Isomers inBi212

“…As discussed by Warburton [5], the second isomer can be associated with the calculated I π = 18 − state, now at 1456 keV, which is in agreement with the present measurement of 1478 (30) keV, but inconsistent with the previous value of >1910 keV [4]. According to the calculations, the highest-spin state at lower energy than the isomer is an I π = 15 − state 35 keV lower (see Fig.…”

“…With only four nucleons (one proton and three neutrons) outside the doubly magic core of 208 Pb, shell-model calculations should be reliable for 212 Bi. Surprisingly, however, the estimate of E * >1910 from its β-decay rate [4] is substantially different from the calculated energy of 1496 keV for the best isomer candidate, with I π = 18 − [5]. Notwithstanding these contrary indications, if the differences could be properly understood, then the possibility of exploiting the isomer for energy-release studies could be addressed.…”

“…Nevertheless, it remains poorly characterised, with, for example, unmeasured excitation energy. This quantity is needed foremost to test the predictive power of nuclear shell-model calculations [5], which themselves are required for modeling elemental synthesis in explosive rapid-neutron-capture (r-process) astrophysical environments [6,7]. With only four nucleons (one proton and three neutrons) outside the doubly magic core of 208 Pb, shell-model calculations should be reliable for 212 Bi.…”

“…Shell-model calculations of the excited states of 212 Bi were carried out with the OXBASH code [33] by Warburton [5], who considered in detail the possible isomer spin and parity assignments. The ground state and first isomer were interpreted as having I π = 1 − , and 8 − or 9 − , respectively.…”

EnhancedE1transitions andα+208Pb(3−<

“…The nucleus 212 Po serves as a testing ground to answer this question as it contains two neutrons and two protons outside the doubly magic nucleus 208 Pb. Though the yrast band structure in 212 Po is certainly reproduced well in a shell-model calculation [3], understanding the α-decay width of the ground state and the electric quadrupole (E2) transition rates among the band members definitely calls for a model that includes explicitly the degree of α clustering. The large α-decay width can successfully be accounted for in a microscopic model that combines the shell model and the α + 208 Pb(0 + ) cluster model [4].…”