Collective nature of low-lying excitations in70,72,74Zn from lifetime measurements using the AGATA spectrometer demonstrator

Abstract

Background: Neutron-rich nuclei with protons in the fp shell show an onset of collectivity around N = 40. Spectroscopic information is required to understand the underlying mechanism and to determine the relevant terms of the nucleon-nucleon interaction that are responsible for the evolution of the shell structure in this mass region. Methods: We report on the lifetime measurement of the first 2 + and 4 + states in 70,72,74 Zn and the first 6 + state in 72 Zn using the recoil distance Doppler shift method. The experiment was carried out at the INFN Laboratory of Legnaro with the AGATA demonstrator, first phase of the Advanced Gamma Tracking Array of highly segmented, high-purity germanium detectors coupled to the PRISMA magnetic spectrometer. The excited states of the nuclei of interest were populated in the deep inelastic scattering of a 76 Ge beam impinging on a 238 U target. Results: The maximum of collectivity along the chain of Zn isotopes is observed for 72 Zn at N = 42. An unexpectedly long lifetime of 20 +1.8 −5.2 ps was measured for the 4 + state in 74 Zn. Conclusions: Our results lead to small values of the B(E2; 4 + 1 → 2 + 1 )/B(E2; 2 + 1 → 0 + 1 ) ratio for 72,74 Zn, suggesting a significant noncollective contribution to these excitations. These experimental results are not reproduced by state-of-the-art microscopic models and call for lifetime measurements beyond the first 2 + state in heavy zinc and nickel isotopes.

Collective features of Cr and Fe isotopes

“…This allows not only to describe the collective structure in nuclei below 68 Ni but also heavier nuclei such as the recently studied Cu and Zn isotopes [33][34][35][36]. Such a description implies to consider a wide shell model space that involves more than one main shell.…”

Investigating the large deformation of the 5/2+ isomeric state in Zn73 : An indicator for triaxiality

“…High-precision mass measurements revealed evidence for only a very weak subshell closure at 68 Ni, which disappears quickly when moving away from Z = 28 [3,4]. Collectivity effects appear with only two neutrons/protons above N = 40, Z = 28, and a maximum is found at N = 42 in the even-even systems (Zn, Ge, and Se) as indicated by B(E2; 0 + → 2 + ) and E(2 + ) measurements [5][6][7][8]. Various nuclear shapes are also found in this region, in particular for Ge and Se isotopes, such as the well-known shape coexistence and triaxial shapes [9][10][11].…”

First in-beamγ-ray study of the level structure of neutron-richS39

“…The coupling of large solid-angle magnetic spectrometers to arrays of escape-suppressed Ge detectors in studies of this type (see, e.g., Refs. [9][10][11]) has represented a very significant experimental advance in relation to earlier techniques that exploited large arrays of Ge detectors but provided no particle identification (see, e.g., Broda et al [12], Fornal et al [13], and Lee et al [14]). …”