$B\left(\text{GT}\right)$ strength from $\beta$-decay measurements and inferred shape mixing in ${}^{74}\mathrm{Kr}$

A total absorption spectrometer, dedicated to the study of very short-lived atomic species, has been built and installed at the CERN∕ISOLDE mass separator. The $\beta$ decay of the neutron-deficient ${}^{74}\mathrm{Kr}$ nucleus has been studied using this new device. The Gamow-Teller strength distribution has been observed over most of the $Q_{EC}$ window, and a total strength of $0.69\left(3\right)$ $g_A^2∕4\pi$ has been measured for states with excitation energy below $3\text{MeV}$. Shape mixing in the ${}^{74}\mathrm{Kr}$ ground state is inferred from a comparison of the experimental strength distribution with self-consistent, deformed, quasiparticle-random-phase-approximation calculations.

Figure 1

Schematic diagram of the experimental setup. Heavy ions $\left(\text{HI}\right)$ are implanted into an aluminized mylar tape in the center of the $\text{NaI}$ crystal. All sensitive detectors are placed inside a four layer shielding that reduces the counting rate due to neutron and $\gamma$-background activities. Collimators, placed upstream from the collection point, limit the beam spot size on the tape.

Figure 2

X-ray spectrum obtained with the germanium planar detector and showing the ${\mathrm{K}}_{\alpha }$ and ${\mathrm{K}}_{\beta }$ transitions in ${}^{74}\mathrm{Br}$. The expected positions of the ${\mathrm{K}}_{\alpha }$ and ${\mathrm{K}}_{\beta }$ transitions in ${}^{74}\mathrm{Se}$ are also shown. The shaded areas show the energy gates used for background subtraction.

Figure 3

(a) Direct TAgS spectrum. (b) TAgS spectrum in coincidence with the $\text{Br}$ ${\mathrm{K}}_{\alpha }$ and ${\mathrm{K}}_{\beta }$ transitions in the planar detector. (c) Final background subtracted TAgS spectrum corresponding to the EC component of the ${}^{74}\mathrm{Kr}$ decay. Arrows indicate the ${}^{74}\mathrm{Kr}$ and ${}^{74}\mathrm{Br}$ $Q_{EC}$ values.

Figure 4

Experimental Gamow-Teller strength distribution measured for ${}^{74}\mathrm{Kr}$ (filled circles) compared to self-consistent HF calculations [39, 41] assuming for the ${}^{74}\mathrm{Kr}$ ground state (a) an oblate $\left({\beta }_0=-0.15\right)$ shape, and (b) a prolate $\left({\beta }_0=+0.39\right)$ shape. The calculations were performed using the $\text{SG2}$ Skyrme interaction.

Figure 5

Accumulated GT strength in ${}^{74}\mathrm{Kr}$ as a function of the excitation energy of the daughter nucleus. The dotted and dashed lines correspond to the oblate and prolate solutions of HF calculations using the SG2 interaction, respectively.