Coulomb shifts and shape changes in the mass 70 region

The technique of recoil β tagging has been developed which allows prompt γ decays in nuclei from excited states to be correlated with electrons from their subsequent short-lived β decay. This technique is ideal for studying nuclei very far from stability and improves in sensitivity for very short-lived decays and for high decay Q-values. The method has allowed excited states in ${}^{78}\mathrm{Y}$ to be observed for the first time, as well as an extension in the knowledge of $T=1$ states in ${}^{74}\mathrm{Rb}$. From this new information it has been possible to compare Coulomb energy differences (CED) between $T=1$ states in ${}^{70}\mathrm{Br}$/${}^{70}\mathrm{Se}$, ${}^{74}\mathrm{Rb}$/${}^{74}\mathrm{Kr}$, and ${}^{78}\mathrm{Y}$/${}^{78}\mathrm{Sr}$. The $A=70$ CED exhibit an anomalous behavior which is inconsistent with all other known CED. This behavior may be accounted for qualitatively in terms of small variations in the Coulomb energy arising from shape changes.

Figure 1

Recoil-β tagged γ ray spectra for (a) ${}^{78}\mathrm{Y}$ and (b) ${}^{74}\mathrm{Rb}$ [8]. The insets show a sum of gates on the 281, 506, and 615 keV, and a gate on the 784 keV transitions, in their respective recoil-β tagged $E_{\gamma }\text{−}{E}_{\gamma }$ matrices. Details of the time correlations and positron energy limits are given in the text.

Figure 2

CED between $T_z=(0,1)$ pairs as a function of spin for the cases: (a) $A=$22, 26, 42, 46, 50, and 54 and (b) $A=66$, 70, 74, and 78. Data for (a) and $A=66$ were taken from Refs. [9] and [21], respectively. Open symbols and dashed lines for (b) represent tentatively assigned levels in the $N=Z$ system considered.