$T=1$ states in ${}^{74}\mathrm{Rb}$ and their ${}^{74}\mathrm{Kr}$ analogs

Charge symmetry breaking effects that perturb analog symmetry between nuclei are usually small but are important in extracting reliable Fermi matrix elements from “superallowed” β decays and testing conserved vector current theory, especially for the heavier cases. We have used the ${}^{40}\mathrm{Ca}$(${}^{36}\mathrm{Ar}$, $\mathit{pn}$)${}^{74}\mathrm{Rb}$ and ${}^{40}\mathrm{Ca}$(${}^{40}\mathrm{Ca}$,$\alpha \mathit{pn}$)${}^{74}\mathrm{Rb}$ reactions at 108, 123 and 160 MeV, respectively, to populate ${}^{74}\mathrm{Rb}$ and determine the analog distortion through comparison of $T=1$ states in ${}^{74}\mathrm{Rb}$ with their corresponding ${}^{74}\mathrm{Kr}$ levels. We have traced the analogs of the ${}^{74}\mathrm{Kr}$ ground-state band in ${}^{74}\mathrm{Rb}$ to a candidate spin $J=8$ state and determined the Coulomb energy differences. They are small and positive and increase smoothly with spin. New $T=0$ states were found that better delineate the deformed band structure and clarify the steps in deexcitation from high spin. A new $T=0$ band was found. No evidence was found for γ decay to or from a low-lying $J^{\pi }=0^{+}$ state in ${}^{74}\mathrm{Rb}$ despite a careful search.

Figure 1

The composite decay scheme for ${}^{74}\mathrm{Rb}$ inferred from this work. The band numbering follows that of Ref. [15]. The γ-ray intensities, as indicated by the widths of the arrows, are taken primarily from the ${}^{40}\mathrm{Ca}$ + ${}^{40}\mathrm{Ca}$ study at 160 MeV. A portion of the ground-state band for the isobar ${}^{74}\mathrm{Kr}$ is shown at the left, with relative γ-ray intensities taken from the high-spin study of Rudolph et al. [28].

Figure 2

Sample γ-ray angular distributions obtained from the ${}^{40}\mathrm{Ca}$ + ${}^{40}\mathrm{Ca}$ 160-MeV data set. Due to the low production cross section for ${}^{74}\mathrm{Rb}$, the data from neighboring rings of Gammasphere detectors were added together to improve the statistics as a function of angle. The data in the panels on the left were obtained from γ-gated spectra correlated with the detection of one proton and one α in the Microball array. The data in the panels on the right were obtained from γ-ray “singles” spectra coincident with the detection of one proton, one α, and one neutron. The solid curves represent fits to the data assuming an alignment of $\sigma /J=0.35$, and with the spins as shown in Table .

Figure 3

Spectra showing γ rays in coincidence with the particular γ-ray transition indicated in each panel. The data are from the ${}^{40}\mathrm{Ca}$ + ${}^{40}\mathrm{Ca}$ study at 123 MeV. This set of spectra is provided to support the revised placement of the 303- and 519-keV transitions of ${}^{74}\mathrm{Rb}$.

Figure 4

Gamma-ray spectra obtained from the addition of selected $\gamma \gamma$ gates applied to the ${}^{40}\mathrm{Ca}$ + ${}^{40}\mathrm{Ca}$ at 160-MeV data set. The upper panel shows the sum of all double gates involving the 443-, 248-, 636-, 813- or 974-keV transitions of band 5. The lower panel focuses on band 3 and shows the sum of double gates in which one gate belongs to the set of (478-, 491-, and 694-keV) transitions and the second gate to the set of (884-, 1051-, and 1182-keV) transitions. The presence of the 730-keV γ ray indicates that it is also a member of band 3.

Figure 5

Gamma-ray spectrum resulting from a gate on the 783-keV transition from the $6^{+}$ state at 1836 keV in ${}^{74}\mathrm{Rb}$. The spectrum was obtained from the ${}^{40}\mathrm{Ca}$ + ${}^{40}\mathrm{Ca}$ at 123-MeV data set.