Speaker
Description
Crucial questions remain unanswered in the heaviest accessible region of $N=Z$ nuclei, where both the neutron and proton Fermi levels are located well inside the $g_\frac{9}{2}$ region. The details of how collectivity varies for $N=Z$ nuclei between $^{56}$Ni and $^{100}$Sn, and the location, and extent, of the maximum collectivity presents a demanding test of our best nuclear-structure models - see e.g. [1]. The reduced transition probability $B(E2: 2_1^+\rightarrow 0_1^+$) remains one of the most sensitive probes of quadrupole collectivity and can provide an indication of nuclear deformation. To date the heaviest $N=Z$ systems, for which $B(E2: 2_1^+\rightarrow 0_1^+$) have been measured, are $^{78}$Y (odd-odd) and $^{80}$Zr (even-even) [2]. The results demonstrate the presence of rapidly changing nuclear collectivity with the addition of nucleons beyond mass 70. An issue of much contemporary significance in this region is whether $N=Z$ nuclei are likely to show clear evidence for isoscalar ($T=0, J>0$) np-pairing correlations [3]. A measurement of the $B(E2: 2_1^+\rightarrow 0_1^+$) can potentially shed light on these issues. For example, calculations [4] suggest that $T=0$ np pairing plays an important role in both the evolution of the moments of inertia in the $N=Z$ nucleus $^{88}$Ru and the absolute value of the predicted $B(E2: 2_1^+\rightarrow 0_1^+$). Indeed, the structure of the $^{88}$Ru yrast band exhibits a delayed rotational alignment [5] which has been interpreted in terms of the presence of such isoscalar np pairing. In this contribution, new results from FRIB on the lifetime of the $2^+$ state in $^{88}$Ru will be presented - the heaviest $N=Z$ nucleus for which such a measurement has been possible. We will also report on progress towards the lifetime measurement of the $T=1$ $2^+$ state in odd-odd $N=Z$ $^{86}$Tc.
The experiment was performed in April 2023 at FRIB. A 250 MeV/u $^{124}$Xe beam was used to produce fragmentation beams of $^{88}$Tc and $^{89}$Ru, separated using the new ARIS spectrometer at FRIB. Final fragments were identified using the S800 spectrometer, with $\gamma$ rays recorded using GRETINA. The TRIPLEX plunger device was utilised in order to determine the lifetimes, and hence $B(E2: 2_1^+\rightarrow 0_1^+$) values, for the $N=Z$ nuclei $^{88}$Ru and odd-odd $^{86}$Tc. The first results for the measured $B(E2: 2_1^+\rightarrow 0_1^+$) for $^{88}$Ru will be presented and compared with state-of-the-art shell-model and DFT calculations. For the shell model, two new approaches are available and have been applied to the new results for $^{88}$Ru. The first is the new Discrete Nonorthognal shell-model approach [6], which applies mean-field and beyond-mean-field techniques, and the second is the large-scale shell model (LSSM) using a new interaction, ZBM3 [7], operating in the $f_{5/2},p,g_{9/2},d_{5/2},s_{1/2}$ space. The new results for $^{88}$Ru present the first opportunity for $N=Z$ results to be evaluated in such a model space.
References
[1] K. Kaneko, Y. Sun and T. Mizusaki, Phys. Rev. C 97 (2021) 054326.
[2] R. D. O.Llewellyn, M. A. Bentley, R. Wadsworth et al, Phys. Rev. Lett. 124, (2020) 152501.
[3] S. Frauendorf and A.O. Macchiavelli, Prog. in Part. and Nucl. Phys. 78, (2014) 24.
[4] K. Kaneko et al, Nucl. Phys. A, 957, (2017) 144-153.
[5] B. Cederwall et al., Phys. Rev. Lett. 124, (2020) 062501.
[6] D. Dao and F. Nowacki, Phys. Rev. C 105, (2022) 054314.
[7] J. Ha et al, submitted to Nature Physics (2024)
