Lifetime measurements of short-lived excited states, and shape changes in 69As and 66Ge nuclei

Abstract

Background: The nuclear shape is a macroscopic feature of an atomic nucleus that is sensitive to the underlying nuclear structure in terms of collectivity and the interaction between nucleons. Therefore, the evolution of nuclear shapes has attracted many theoretical and experimental nuclear structure studies. The structure of the A≈ 70 , N≈ Z nuclei, lying far from the stability line, is interesting because a particularly strong proton-neutron correlation may occur here due to the occupation of the same orbits by nucleons of both types. In this region, different particle configurations drive a nucleus towards various deformed shapes: prolate, oblate, octupole, or nonaxial. These nuclear shapes change rapidly with nucleon number and also with angular momentum. This is reflected by a presence of different structures (bands) of excited states which exhibit a broad range of lifetimes. Purpose: The aim of this paper is to determine lifetimes of some high-spin excited states in 69 As and 66 Ge nuclei to examine the shape evolution in these neutron-deficient nuclei. Methods: Lifetimes of high-spin states in 69 As and 66 Ge have been measured by using the Doppler-shift attenuation technique with the GASP and recoil filter detector setup at the Laboratori Nazionali di Legnaro. The nuclei of interest were produced in the 32 S ( 95 MeV ) + 0.8 mg / cm 2 40 Ca fusion-evaporation reaction. The strongest reaction channels 3p and α 2 p led to the 69 As and 66 Ge final nuclei, respectively. Using γ − γ -recoil coincidences we were able to determine very short lifetimes (in the femtosecond range) in the residual nuclei of interest. Results: In 69 As , the extracted lifetimes are τ = 72 ( − 32 , +45 ) fs for the 33 / 2 + state at 7897 keV and τ < 85 fs for the 37 / 2 + state at 9820 keV. For the 66 Ge case, the lifetime of the 11 − state at 7130 keV is τ = 122 ( ± 41 ) fs. Lifetimes in 69 As and 66 Ge reported in this paper have been measured for the first time in the present experiment. Conclusions: The results are discussed in the terms of deformation and shape evolution in 69 As and 66 Ge . The quadrupole moments deduced from the measured lifetimes were compared with the cranked Woods-Saxon-Strutinsky calculations by means of the total Routhian surface method. It turns out that Band 3 in 69 As shows an oblate-prolate shape transition, and above spin 33 / 2 + it corresponds to a prolate collective structure with β 2 ≈ 0.27 and γ ≈ 20 ∘ . In turn, in 66 Ge the negative-parity band built on the 7− state at 4205 keV corresponds to a triaxial shape with β 2 = 0.33 and γ = 31 ∘ . Analysis of the transitional quadrupole moments derived from the experimental and theoretical ones points to a significant change of deformation in the 69 As and 66 Ge nuclei with increasing rotational frequency.