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Register a new userQuadrupole collectivity and shell closure in neutron-rich nuclei around $N=126$
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We present a comprehensive study on the low-lying states of neutron-rich Er, Yb, Hf, and W isotopes across the $N=126$ shell with a multi-reference covariant density functional theory. Beyond mean-field effects from shape mixing and symmetry restoration on the observables that are relevant for understanding quadrupole collectivity and underlying shell structure are investigated. The general features of low-lying states in closed-shell nuclei are retained in these four isotopes around $N=126$, even though the shell gap is overall quenched by about 30% with the beyond mean-field effects. These effects are consistent with the previous generator-coordinate calculations based on Gogny forces, but much smaller than that predicted by the collective Hamiltonian calculation. It implies that the beyond mean-field effects on the $r$-process abundances before the third peak at $Asim195$ might be more moderate than that found in A. Arcones and G. F. Bertsch, Phys. Rev. Lett. 108, 151101 (2012).
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The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.
Mass distributions of the fragments in the fission of $^{206}$Po and the N=126 neutron shell closed nucleus $^{210}$Po have been measured. No significant deviation of mass distributions has been found between $^{206}$Po and $^{210}$Po, indicating the absence of shell correction at the saddle point in both the nuclei, contrary to the reported angular anisotropy and pre-scission neutron multiplicity results. This new result provides benchmark data to test the new fission dynamical models to study the effect of shell correction on the potential energy surface at saddle point.
Characteristic quantities such as the penetration and preformation probabilities, assault frequency and tunneling times in the tunneling description of alpha decay of heavy nuclei are explored to reveal their sensitivity to neutron numbers in the vicinity of the magic neutron number $N$ = 126. Using realistic nuclear potentials, the sensitivity of these quantities to the parameters of the theoretical approach is also tested. An investigation of the region from $N=116$ to $N=132$ in Po nuclei reveals that the tunneling $alpha$ particle spends the least amount of time with an $N=126$ magic daughter nucleus. The shell closure at $N=126$ seems to affect the behaviour of the dwell times of the tunneling alpha particles and this occurs through the influence of the $Q$-values involved.
Neutron-rich nuclei in the vicinity of the $N=40$ island of inversion are characterized by shell evolution and exhibit deformed ground states. In several nuclei isomeric states have been observed and attributed to excitations to the intruder neutron $1g_{9/2}$ orbital. In the present study we searched for isomeric states in nuclei around $N=40$, $Z=22$ produced by projectile fragmentation at RIBF. Delayed $gamma$ rays were detected by the EURICA germanium detector array. High statistics data allowed for an updated decay scheme of $^{60}$V. The lifetime of an isomeric state in $^{64}$V was measured for the first time in the present experiment. A previously unobserved isomeric state was discovered in $^{58}$Sc. The measured lifetime suggests a parity changing transition, originating from an odd number of neutrons in the $1g_{9/2}$ orbital. The nature of the isomeric state in $^{58}$Sc is thus different from isomers in the less exotic V and Sc nuclei.
We discuss the present status of the description of the structure of the very neutron rich nuclei, in the framework of modern large scale shell model calculations. Particular attention is paid to the interaction related issues, as well as to the problems of the shell model approach at the neutron drip line. We present detailed results for nuclei around N=20 and, more briefly, we discuss some salient features of the regions close to N=8, 28 and 40. We show that most experimental features can be understood in a shell model context.
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