Prolate-oblate shape coexistence in 75Kr

Abstract

The collective bands of 75Kr were extended up to spin 45/2 using the compound reactions 5°Cr(28Si,2pn), 54Fe(24Mg,2pn) and 58Ni(2°Ne,2pn)YSKr. Lifetimes were measured by RDDS with nine OSIRIS detectors in coincidence. Mixing ratios were determined by measurements of internal conversion coefficients, angular distribution and correlations. Deviations between different measurements are explained by the short lifetimes leading to angle dependent intensity losses due to Doppler shift. Spins and parities were assigned from angular distributions, excitation functions and DCO ratios measured with the OSIRIS_12 spectrometer. The band head spin of the negative parity yrast band was established as 3/2 via DCO ratios and internal conversion coefficients. Further bands at low excitation energy were found. In accordance to Woods-Saxon cranking model calculations, these weaker populated sidebands are interpreted to be built on oblate deformed intrinsic states (/32 ~ -0.2), while the strongly populated bands are built on prolate deformed states (~2 ~ +0.4). The interpretation of the lqp g9/2 yrare band, to be generated from oblate deformation, is further supported by A1 = 0 transitions into the yrast band and a similar oblate deformed g9/z structure in the isotone 73Se. The experimental level energies, branching ratios, transition probabilities and mixing ratios are compared to rotor model calculations. The deviations between experiment and rotor model calculations are interpreted to be based on mixing between prolate and oblate states.

Shape evolution at high spin states in Kr and Br isotopes

“…The plot indicates some irregularities around spin I = 25/2h in the yrast positive parity bands of both nuclei. The increase in DMI is interpreted as an alignment of proton g 9/2 orbital on the basis of Cranking model for 75 Kr [5]. Similarly, in case of 75 Br this irregularity has been interpreted as alignment of neutron g 9/2 orbital in DMI vs. spin plots at the above mentioned spin values.…”

“…[4,5] were identified in our experiment also and used as a reference for lifetime The lifetimes of high spin states above 15/2 were measured using DSAM technique in the present work. Doppler broadened line shapes were analyzed by using LINESHAPE program developed by J. C. Wells [14].…”

“…The intensities of side feeding transitions were obtained from experimentally determined relative intensities quoted in Ref. [5,4] from the intensity balance of each state. The uncertainty involved in the lifetimes and consequently in the transition quadrupole moments, (Q t ) were determined by the statistical method using subroutine MINOS [16].…”

“…The occupation of unique parity g 9/2 orbital plays a dominating role for the shape competition and quasi particle alignment with small changes in nucleon number in this mass region. Due to competing collectivity of prolate oblate shapes, neutron deficient Se [3] Br [4], Kr [5,6] and with N ∼ 40 are well known to exhibit shape coexistence at low, excitation energy. During recent years, development of state of art, Indian National Gamma Array (INGA) [7], which is a multi detector array provides an excellent opportunity to study these exotic phenomena in unstable nuclei, lying far from stability.…”

“…The yrast positive and negative parity bands of these nuclei are based on the [422]5/2 + and [301]3/2 − Nilsson orbitals, which is also verified by theoretical calculations. 75 Kr nucleus, which lies between two well deformed 74 Kr and 76 Kr nuclei, was studied only up to low spin states through recoil distance measurement [5] and found to have large quadrupole deformation β 2 ≈ 0.40. However, Lighter odd A Bromine isotopes, 75 Br and 77 Br were studied by Solomon et al [4] and Sylvan et al [8] which predicted two distinct rotational bands in positiveparity and excited band with high K values.…”