Spin-aligned neutron-proton pair mode in atomic nuclei

Abstract

Shell model calculations using realistic interactions reveal that the ground and low-lying yrast states of the $N=Z$ nucleus $^{92}_{46}$Pd are mainly built upon isoscalar neutron-proton pairs each carrying the maximum angular momentum J=9 allowed by the shell $0g_{9/2}$ which is dominant in this nuclear region. This structure is different from the ones found in the ground and low-lying yrast states of all other even-even nuclei studied so far. The low-lying spectrum of excited states generated by such correlated neutron-proton pairs has two distinctive features: i) the levels are almost equidistant at low energies and ii) the transition probability $I\rightarrow I-2$ is approximately constant and strongly selective. This unique mode is shown to replace normal isovector pairing as the dominating coupling scheme in $N=Z$ nuclei approaching the doubly-magic nucleus $^{100}$Sn.Comment: 11 pages, 5 figures, version to appear in Phys. Rev. C (Rapid communication

Symmetries in theg9/2shell

“…With regards to the works of Refs. [12][13][14][15], this suggests the J = J max pairing is a better approximation in the upper half of a j shell rather than the lower half.…”

“…One obvious distinction of our work relative to that of Refs. [12][13][14][15] is that we are dealing with higher spins than the ones that they consider. In the Nature article [12], they measure J = 0, 2, 4, and 6 and also discuss J = 8, whereas the PDS that we consider occur for J = 11 + and beyond.…”

“…[12][13][14][15]). In the Nature article [12], an experiment is presented in which an almost equal-spaced spectrum is found in N = Z 92 Pd for J = 0 + , 2 + , 4 + , and 6 + .…”

N=Znuclei: a laboratory for neutron–proton collective mode

“…It was suggested that in this nucleus, as well as in neighboring nuclei like 96 Cd, the properties of the low-lying states can be classified by a spin-aligned np pair coupling scheme [62,63]. That is, the ground state wave functions do not consist mainly of pairs of neutrons (νν) and protons (ππ) coupled to zero angular momenta, but rather of isoscalar np pairs (νπ) coupled to the maximum angular momentum J, which in the shell 0g 9/2 is J = 9 [62,63]. A detailed shell-model analysis of the spinaligned np pair coupling was performed in Refs.…”

“…A detailed shell-model analysis of the spinaligned np pair coupling was performed in Refs. [63,64] based on coefficients of fractional parentage and multistep shell model calculations.…”