β-decay half-lives and β-delayed neutron emission probabilities for several isotopes of Au, Hg, Tl, Pb, and Bi, beyond N=126

Abstract

Background: There have been measurements on roughly 230 nuclei that are &beta;-delayed neutron emitters. They range from ⁸He up to ¹⁵⁰La. Apart from ²¹⁰Tl, with a branching ratio of only 0.007%, no other neutron emitter has been measured beyond A=150. Therefore, new data are needed, particularly in the region of heavy nuclei around N=126, in order to guide theoretical models and help understand the formation of the third r-process peak at A&sim;195.</p> <p>Purpose: To measure both &beta;-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl, Pb, and Bi isotopes beyond N=126.</p> <p>Method: Ions of interest were produced by fragmentation of a ²³⁸U beam, selected and identified via the GSI-FRS fragment separator. A stack of segmented silicon detectors (SIMBA) was used to measure ion implants and &beta; decays. An array of 30 ³He tubes embedded in a polyethylene matrix (BELEN) was used to detect neutrons with high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time correlations. The latter were analyzed with an analytical model and results for the half-lives and neutron-branching ratios were derived by using the binned maximum-likelihood method.</p> <p>Results: Twenty new &beta;-decay half-lives are reported for ^{204&minus;206}Au, ^{208&ndash;211}Hg,^{211&ndash;216}Tl,^{215&ndash;218}Pb, and ^{218&ndash;220}Bi, nine of them for the first time. Neutron emission probabilities are reported for ^210,211Hg and ^{211&ndash;216}Tl.</p> <p>Conclusions: The new &beta;-decay half-lives are in good agreement with previous measurements on nuclei in this region. The measured neutron emission probabilities are comparable to or smaller than values predicted by global models such as relativistic Hartree Bogoliubov plus the relativistic quasi-particle random phase approximation (RHB + RQRPA).