Emergent Collectivity in Nuclei and Enhanced Proton-Neutron Interactions

Enhanced proton-neutron interactions occur in heavy nuclei along a trajectory of approximately equal numbers of valence protons and neutrons. This is also closely aligned with the trajectory of the saturation of quadrupole deformation. The origin of these enhanced p-n interactions is discussed in terms of spatial overlaps of proton and neutron wave functions that are orbit-dependent. It is suggested for the first time that nuclear collectivity is driven by synchronized filling of protons and neutrons with orbitals having parallel spins, identical orbital and total angular momenta projections, belonging to adjacent major shells and differing by one quantum of excitation along the z-axis. These results may lead to a new approach to symmetry-based theoretical calculations for heavy nuclei.Comment: 6 pages, 4 figure

Enhanced low-energy γ\gamma-decay strength of 70^{70}Ni and its robustness within the shell model

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $\gamma$-emission probability at very low $\gamma$ energies. In this work, we present measurements of the $\gamma$-decay strength of $^{70}$Ni over the wide range $1.3 \leq E_{\gamma} \leq 8$ MeV. A significant enhancement is found in the $\gamma$-decay strength for transitions with $E_\gamma < 3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above $E_\gamma \simeq 5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\gamma$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich $^{72,74,76}\mathrm{Ni}$.Comment: 9 pages, 9 figure

Octupole deformation in light actinides within an analytic quadrupole octupole axially symmetric model with a Davidson potential

The analytic quadrupole octupole axially symmetric model, which had successfully predicted Ra-226 and Th-226 as lying at the border between the regions of octupole deformation and octupole vibrations in the light actinides using an infinite well potential (AQOA-IW), is made applicable to a wider region of nuclei exhibiting octupole deformation, through the use of a Davidson potential, beta(2) + beta(4)(0)/beta(2) (AQOA-D). Analytic expressions for energy spectra and B(E1), B(E2), B(E3) transition rates are derived. The spectra of Ra222-226 and Th-224,Th-226 are described in terms of the two parameters phi(0) (expressing the relative amount of octupole vs quadrupole deformation) and beta(0) (the position of the minimum of the Davidson potential), while the recently determined B(EL) transition rates of Ra-224, presenting stable octupole deformation, are successfully reproduced. A procedure for gradually determining the parameters appearing in the B(EL) transitions from a minimum set of data, thus increasing the predictive power of the model, is outlined

A mini-Wigner effect in p-n interactions in heavy nuclei and the 0[110] transformation in the Nilsson scheme

We show that δV_pn values in the rare earth region show peaks, reminiscent of the spikes at N = Z in light nuclei, but occurring at N_val ∼ Z_val. These peaks, evident for both even and odd Z values, are interpreted in terms of large spatial overlaps of respective proton and neutron wave functions whose Nilsson quantum numbers are related by δK[δN,δn_z,δΛ]  =  0[110[. That is, the wave functions differ only by a single oscillator quantum in the z-direction. The implications of this for the development of collectivity and deformation in heavy nuclei, and the locus of this development, are discussed