Shell model analysis of the $B(E2,2^{+}\to 0^{+})$ values in the $A=70,$ $T=1 \mathrm{triplet} {}^{70}\mathrm{Kr},$ ${}^{70}\mathrm{Br}, \mathrm{and} {}^{70}\mathrm{Se}$

The $B(E2,2^{+}\to 0^{+})$ transition strengths of the $T=1$ isobaric triplet ${}^{70}\mathrm{Kr},$ ${}^{70}\mathrm{Br},$ ${}^{70}\mathrm{Se},$ recently measured at the RIKEN Radioactive Isotope Beam Factory (RIBF), are discussed in terms of state-of-the-art large scale shell model calculations using the JUN45 and $\mathrm{JUN}45+\mathrm{LNPS}$ plus Coulomb interactions. In this Letter we argue that, depending on the effective charges used, the calculations are either in line with the experimental data within statistical uncertainties, or the anomaly happens in ${}^{70}\mathrm{Br},$ rather than ${}^{70}\mathrm{Kr}$. In the latter case, we suggest that it can be due to the presence of a hitherto undetected $1^{+}$ $T=0$ state below the yrast $2^{+}$ $T=1$ state. Our results do not support a shape change of ${}^{70}\mathrm{Kr}$ with respect to the other members of the isobaric multiplet.

Figure 1

$M_p\left(E2\right)$ in the $A=70$ isospin triplet (in $e{\mathrm{fm}}^2$). The experimental data are taken from Ref. [9] (see text for a discussion of the error bars). Results of the JUN45 (black squares) and $\mathrm{JUN}45+\mathrm{LNPS}$ (red squares) calculations. (a) ST effective charges; (b) DZ effective charges.

Figure 2

(a) Systematic of the low-lying $T=0$ and $T=1$ states in odd-odd nuclei relevant to the $A=70$ region. The $1^{+}$ in ${}^{70}\mathrm{Br}$, indicated by a question mark, is not known. (b) $\mathrm{JUN}45+\mathrm{LNPS}$ shell model results.

Figure 3

$B\left(M1\right)$ strength from the $2^{+}$ to a hypothetical $1^{+}$ required to explain a missing intensity of the $2^{+}\gamma$ ray (red dashed line and shaded area) and the excluded regions imposed by the experimental conditions of the setup in Ref. [9] (green shaded area; see text for details). The intersection of the shell model results (blue shaded area) with the empirically required strength determine a possible solution (magenta shaded area).