Evidence for a spin-aligned neutron–proton paired phase from the level structure of 92Pd

Abstract

3The general phenomenon of shell structure in atomic nuclei has been understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and Suess [1].They realized that the experimental evidence for nuclear magic numbers could be explained by introducing a strong spin-orbit interaction in the nuclear shell model potential.However, our detailed knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have been predicted to favor a new type of nuclear superfluidity; isoscalar neutron-proton pairing [2][3][4][5][6], in addition to normal isovector pairing (see Fig. 1). Despite many experimental efforts these predictions have not been confirmed. Here, we report on the first observation of excited states in N = Z = 46 nucleus 92 Pd. Gamma rays emitted following the 58 Ni( 36 Ar,2n) 92 Pd fusionevaporation reaction were identified using a combination of state-of-the-art highresolution -ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutronproton coupling scheme, different from the previous prediction [2][3][4][5][6]. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling [7,8]) in the ground and low-lying excited states of the heaviest N = Z nuclei. The strong isoscalar neutron-proton correlations in these N = Z nuclei are predicted to have a considerable impact on their level structures, and to influence the dynamics of the stellar rapid proton capture nucleosynthesis process.For all known nuclei, including those residing along the N = Z line up to around mass 80, a detailed analysis of their properties such as binding energies [9] and the spectroscopy of the excited states [10] strongly suggests that normal isovector (T = 1) pairing is dominant at low excitation energies. On the other hand, there are long standing predictions for a change in the heavier N = Z nuclei from a nuclear superfluid dominated by isovector pairing to a structure where isoscalar (T = 0) neutron-proton (np) pairing has a major influence as the mass number increases towards the exotic doubly magic nucleus 100 Sn [2-6], the heaviest N = Z nucleus to be bound. N = Z nuclei with mass number > 90 can only be produced in the laboratory with very low The two-neutron (2n) evaporation reaction channel following formation of the 94 Pd compound nucleus, leading to 92 Pd, was very weakly populated with a relative yield of less than 10 −5 of the total fusion cross section. Gamma rays from decays of excited states in 92 Pd were identified by comparing γ-ray spectra in coincidence with two emitted neutrons and no charged particles with γ-ray spectra in coincidence with oth...

Superallowed Gamow–Teller decay of the doubly magic nucleus 100Sn

“…The knowledge of the underlying shell structure of nuclei in the vicinity of 100 Sn has to be determined with the highest possible accuracy to answer the burning questions in the nuclear structure, like the possibility of a new coupling scheme which might develop in the N=Z nuclei in the vicinity of 100 Sn [25]. The present measurement consists in a stringent test for LSSM calculations, in which the realistic character of such a coupling scheme still needs to be probed.…”

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

“…The low-lying yrast states in 92 46 Pd were recently reported [62]. This is the heaviest N = Z nucleus with measured spectrum so far.…”

“…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.…”

“…[64] we applied the method to study the spinaligned np pair coupling scheme [62,63]. We restricted restrict our calculations to the single 0g 9/2 shell with the interaction matrix elements taken from Ref.…”

Level structure above the 17+ isomeric state in Tm8369152

“…The similarities between the doubly magic nuclei 146 Gd and 208 Pb [1] and the expected influence of the proton-neutron interaction on structural features as observed in the 100 Sn region [3,[8][9][10]] also fueled further interest. As a result, a wealth of nuclear structure data has become available.…”