Speaker
Description
Spectroscopic properties of exotic nuclei are powerful tools to obtain a better insight on the evolution of nuclear structure far from the stability. Mid-shell nuclei are expected to exhibit collective behaviour which is typically reflected in the observation of low excitation energies of the first excited states and high transition probabilities. Moreover, the collectivity is expected to increase with the spin, causing both the $R_{4/2}=E_X(4^+)/E_X(2^+)$ to be higher than 2 and the $B_{4/2} = B(E2;4^+\to 2^+)/B(E2;2^+\to 0^+)$ to be higher than the unit. However, an increasing number of mid-shell nuclei had been found to present a $B_{4/2}<1$. This has already been observed in two neutron-deficient regions, one located close to the $Z=50$ shell closure and one in the rare-earth region, as in the case of Os and W isotopes. In particular, the osmium isotopic chain presents cases both in even-even nuclei, such as $^{168,170}$Os, and one in an even-odd nucleus, $^{169}$Os, where the $B_{4/2}$ ratio has been redefined as $B(E2; 21/2^+ \to 17/2^+) / B(E2; 17/2^+ \to 13/2^+)$. According to theory, this anomaly could only be explained by a change from collective to seniority-like regimes or by phenomena such as shape coexistence. However, none of these explanations can justify the anomalous $B_{4/2}$ ratio in Os isotopes. In this context, lifetime measurements of the excited states of these nuclei can provide a meaningful insight on the structure of the low-lying bands. An experiment aimed at measuring lifetimes in the neutron-deficient rare-earth region was performed at the Accelerator Laboratory of Jyv\"askyl\"a (Finland), using the Jurogam3 array coupled to the RITU spectrometer and the plunger device. The lifetimes were extracted using the Recoil Distance Doppler Shift method. From the measured lifetimes, it was possible to extract the reduced transition probabilities of the low-lying states and the $B_{4/2}$ ratio. The experimental results were then compared to potential energy surface calculations in order to shed light on the influence of the unpaired neutron. In this contribution, a summary of the performed experiment, the new results and the comparison with theory are presented.
