The beta decay of Tz=-2 64Se and its descendants: the T=2 isobaric multiplet

TL;DR

The paper investigates the beta decay of the heaviest $T_z=-2$ nucleus with a stable mirror partner, $^{64}$Se, to illuminate the $A=64$, $T=2$ isobaric multiplet. Using high-intensity fragmentation of $^{78}$Kr at the RI Beam Factory and a combination of WAS3ABi implantation, EURICA gamma spectroscopy, and BigRIPS identification, the authors reconstruct decay schemes for $^{64}$Se, $^{64}$As, and $^{63}$Ge. They report the first beta-delayed proton emission from $^{64}$Se and $^{64}$As, determine half-lives and proton branching ratios, and deduce the ground-state mass of $^{64}$As with good precision, together with an IMME-based estimate for $^{64}$Se. Comparisons with charge-exchange data on the mirror $^{64}$Zn→$^{64}$Ga system reveal substantial isospin symmetry for the $T=2$ IAS up to the IAS and provide insights into the Anti Analogue State and multiplet structure, with new information on excited states in the $N=Z$ nucleus $^{64}$Ge.

Abstract

In this paper we present our results on the decay of 64Se. It is the heaviest Tz=-2 nucleus that both beta decays and has a stable mirror partner Tz=+2, thus allowing comparison with charge exchange reaction studies. The beta decays of 64Se and its descendants were studied at the RIKEN Nishina Center (Tokyo, Japan) following their production in the fragmentation of 78Kr on a beryllium target. Beta-delayed gamma-ray and particle radiation was identified for each of the nuclei in the decay chain allowing us to obtain decay schemes for 64Se, 64As, and 63Ge. Thus new excited states could be found for the descendant nuclei, including the interesting case of the N=Z nucleus 64Ge. Furthermore we observed for the first time the beta-delayed proton emission of 64Se and 64As. Based on these results we obtained proton branching ratios of 48.0(9)% in 64Se decay and 4.4(1)% in 64As decay. We obtained a half-life value of 22.5(6) ms for 64Se decay and half-lives slightly more precise than those in the literature for each nucleus involved in the decay chain. Using our results on the excited levels of 64As and the mass excess in the literature for 63Ge we obtained -39588(50) keV for the mass excess of 64As. Then based on the IMME we obtained the mass excess of -27429(88) keV for 64Se by extrapolation. The mirror process of 64Se beta decay, the charge exchange reaction 64Zn(3He,t)64Ga, has already been measured allowing us to study the mirror symmetry through the comparison of the weak force (beta decay) and strong force (charge exchange reaction). An interpretation of the decay schemes based on the idea of the Anti Analogue State is proposed.

Figures (26)

• Figure 1: Section of the chart of the nuclides. Blue circles indicate the even-even $T_{z}\!=\!-2$ and -1 nuclei where the beta decay has been studied in previous campaigns at GSI and GANIL (see text) using the fragmentation of [58]Ni beams, marked in purple, as well as in the present campaign at RIKEN using [78]Kr beams, also in purple. Yellow circles indicate the corresponding stable mirror nuclei where CE reactions can be carried out. The main focus of the present article is the study of the beta decay of the [64]Se isotope and its descendants.
• Figure 2: The proton number $Z$ versus the mass over charge ratio $A/Q$ for the fragments produced in the fragmentation of 345 MeV/u [78]Kr ions on a natural Be target. The nuclei studied in this work are labelled.
• Figure 3: The radioactive decay chain starting with [64]Se. The decays with half-lives shorter than $200\mathrm{~ms}$ are highlighted with a circle. Known half-lives are shown in black, while the half-lives determined in this work are shown in color.
• Figure 4: The figure shows the $\gamma$-ray spectrum in time correlation with implants of [64]Se nuclei. The $\gamma$-rays assigned to [64]Se decay in [64]As are shown in green and marked with $\oplus$, the $\gamma$-ray assigned to [64]Se $\beta$-delayed p decay in [63]Ge is shown in blue and marked with $\spadesuit$, the $\gamma$-rays assigned to [64]As decay in [64]Ge are shown in pink and marked with $\clubsuit$ and the $\gamma$-ray assigned to [63]Ge decay in [63]Ga is shown in red with $\blacklozenge$. The annihilation radiation and x-rays are shown in black. The $511\mathrm{~keV}$ annihilation peak shows a low energy shoulder corresponding to a $\gamma$-ray in the decay of [64]Se, see text.
• Figure 5: The figure shows the $\gamma$-ray spectrum in time correlation with implants of [64]As nuclei within a $440\mathrm{~ms}$ implant-decay correlation window. Unplaced $\gamma$-rays are shown in black and marked with $\Box$, together with the Lead x-rays and the positron annihilation peak. The $\gamma$-rays assigned to [64]As $\beta$ decay are shown in pink and marked with $\clubsuit$.