No abstract available

No abstract available

[en] Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work that introduced a strong spin-orbit interaction to the nuclear shell model potential. However, 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), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron-proton pairing, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N=Z=46 nucleus ⁹²Pd. Gamma rays emitted following the ⁵⁸Ni(³⁶Ar,2n)⁹²Pd fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N=Z nuclei. Such strong, isoscalar neutron-proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis. (authors)

[en] Gamma-ray spectroscopy is intensively used at GANIL to measure low lying states in exotic nuclei on the neutron-rich as well as on the neutron-deficient side of the nuclear chart. On the neutron deficient border, gamma-rays have been observed for the first time in ⁹²Pd. The level scheme which could be established points to the role of isoscalar pairing. On the neutron rich side, the lifetime of excited states in nuclei around ⁶⁸Ni have been been measured using the plunger technique. This allows us to study the evolution of collectivity in a broad range of nuclei. In 2014 GANIL will host the AGATA array for a campaign of at least 2 years. This array is based on the gamma-ray tracking technique, which allows an impressive gain in resolving power

[en] Shell-model calculations using realistic interactions reveal that the ground and low-lying yrast states of the N=Z nucleus ₄₆⁹²Pd 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 that 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→I-2 is approximately constant and strongly selective. This unique mode is shown to replace normal isovector pairing as the dominant coupling scheme in N=Z nuclei approaching the doubly magic nucleus ¹⁰⁰Sn.

[en] Shell model calculations reveal that the ground and low-lying yrast states of the N = Z nuclei ⁹²₄₆Pd and ⁹⁶Cd are mainly built upon isoscalar spin-aligned 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 mode of excitation is unique in nuclei and indicates that the spin-aligned pair has to be considered as an essential building block in nuclear structure calculations. In this contribution we will discuss this neutron-proton pair coupling scheme in detail. In particular, we will explore the competition between the normal monopole pair coupling and the spin-aligned coupling schemes. Such a coupling may be useful in elucidating the structure properties of N = Z and neighboring nuclei.

[en] A wide range of research topics in different fields of physics can be addressed by study of the self-conjugate N = Z nuclei, such as the np pairing, isospin symmetry, the rp-process and the properties of the electroweak interaction. This contribution focuses on the spectroscopy of N∼Z nuclei towards ¹⁰⁰Sn. The latest results on the isomeric decay spectroscopy of N∼Z nuclei below ¹⁰⁰Sn, such as the N = Z + 2 nuclides ⁹⁴Pd and ⁹⁶Ag, the N = Z nuclide ⁹⁶Cd and so on are highlighted. New opportunities in in-beam γ spectrscopy of N∼Z nuclei towards ¹⁰⁰Sn, like ⁹⁰Rh and ⁹²Pd, with radioactive ion beams are discussed. (author)

[en] A study is carried out on the role of the aligned neutron-proton pair with angular momentum J=9 and isospin T=0 in the low-energy spectroscopy of the N=Z nuclei ⁹⁶Cd, ⁹⁴Ag, and ⁹²Pd. Shell-model wave functions resulting from realistic interactions are analyzed in terms of a variety of two-nucleon pairs corresponding to different choices of their coupled angular momentum J and isospin T. The analysis is performed exactly for four holes (⁹⁶Cd) and carried further for six and eight holes (⁹⁴Ag and ⁹²Pd) by means of a mapping to an appropriate version of the interacting boson model. The study allows the identification of the strengths and deficiencies of the aligned-pair approximation.

[en] Background: Nucleon-pair correlations play an important role in low-lying states of atomic nuclei. Purpose: The aim is to study empirical proton-neutron interactions and investigate the contribution from spin-aligned proton-neutron pairs (with spin 9 and isospin 0) in low-lying states of ⁹⁶Cd and ⁹²Pd. Methods: The empirical proton-neutron interaction is obtained by using nuclear binding energies of a few neighboring nuclei and the nuclear shell model. The contribution from spin-aligned proton-neutron pairs in the shell model wave functions is evaluated by using the nucleon-pair approximation with isospin symmetry. Results: The empirical proton-neutron interaction is reasonably consistent with isoscalar interactions calculated by using the shell model. There exists an additional binding energy in both even-even and odd-odd nuclei originated from proton-neutron interactions. Spin-aligned isoscalar pairs play an important role in the 0₁⁺, 2₁⁺, 4₁⁺, 6₁⁺, 12₁⁺, 14₁⁺and 16₁⁺ states of ⁹⁶Cd and the 0₁⁺, 2₁⁺ states of ⁹²Pd. For the 0₁⁺ and 2₁⁺ states, the conventional isovector SD pairs are more relevant. Conclusion: Both isoscalar and isovector nucleon pairs play an important role in low-lying states of N = Z nuclei. This is a consequence of nonorthogonality feature of nucleon-pair basis. (authors)

[en] In the simple and common picture of nuclear structure, when moving several nucleons or more away from 'magic' neutron and proton numbers (which reflect major gaps in the energy level spectrum), an emergence of 'collectivity', is expected. For even-even nuclei this is normally associated with a lowering of the first excited 2+ state energy accompanied by an increase in the 2⁺ → 0⁺ transition strength, signaling that the wave function spreads to multiple coherent particle-hole components and opening the vibrational or rotational degrees of freedom. These quantities are hence important benchmarks for nuclear models. In particular, the reduced electric quadrupole transition probability, B(E2), connecting the lowest-lying excited states and the ground state directly probes the corresponding wave functions. Recent advancements of experimental techniques now enable us to study not only excited state energies but also electromagnetic transition strengths across long isotopic chains providing more stringent tests of theory. The results reveal some striking differences and deficiencies in the predictive power of current nuclear structure models. This document is composed of an abstract and the slides of the presentation. (author)