Negative-parity structures and lifetime measurements inAs71

Abstract

High-spin states up to ( -+) and ( -) in As have been studied via the Ni(' F,n2p) reaction at 62 MeV using in-beam p-ray spectroscopic techniques. Based on a particle-p coincidence measurement a new decay sequence of negative parity has been assigned to As. Lifetimes for some E2 transitions were extracted from Doppler-shifted line shapes observed in coincidence mode, and experimental transition quadrupole moments were determined. They indicate a moderate to high collectivity in both the positive-parity yrast band and the new negative-parity decay sequence at high spins. Total Routhian surfaces calculated for states of both parities reveal strong triaxial shapes at high spins. A noncollective structure of '~and~states is interpreted as a three-quasiparticle excitation.PACS number(s): 21.10. Tg, 23.20.Lv, 27.50.+e

Effect of and alignments in the shape of 75Br from lifetime measurement

“…The change in Q t observed after the neutron alignment in the positive parity band (proton crossing is blocked) is in accordance with the decrease in triaxiality γ predicted by the TRS calculations. This is similar to the effect observed in 79 Rb (Z = 37) [17] while the effect reverses in 71 As (Z = 31) [16]. A change towards increasing γ is observed due to the proton alignments with corresponding decrease in Q t in the negative parity bands where the g 9/2 proton crossing is not blocked.…”

“…In light mass 71 As [16], the theoretical total energy surface calculations show a change in shape towards +ve γ after the neutron alignment whereas in heavier mass 79 Rb [17] the neutron alignment causes a shape change towards −ve γ . This suggests that the different orbitals in the f -p-g shell have different behaviour as a function of triaxiality parameter γ .…”

Competing single-particle and collective behavior in71Se

“…A similar pattern is observed in other nuclei with 5 2 − ground states such as 69 Se [11], 69 Ge [25], and 71 As [26][27][28], where the most strongly populated 13 2 − state shows two competing E2 decays to two relatively closely spaced 9 2 − states. The 581.6-and 621.8-keV decay branches of the 1378.7-keV state lead to respective states at 797.1 and 756.9 keV, which likely correspond to the 796.4-and 756.7-keV states observed in Ref.…”

“…Of course, a GFA fit to the 945.5-keV line shape would eliminate the need for this approximation, but this was not possible due to limited statistics. The difficulty in performing such a GFA fit was exacerbated by the fact that a gate on the transition just above the 945.5-keV decay in band 3 (1077.7 keV) includes to some extent the intense 1075.4-keV line in 71 As [26][27][28], which shows strong coincidences with the 943.5-keV overlapping contaminant peak.…”

“…5), it was clear that a long-lived feeding component was contributing to the observed line shape, probably due at least in part to feeding from the newly observed 21 Line shapes that were determined by gating from below (GFB) and gating from above (GFA) the given transition are indicated. 71 As [26] with a mean lifetime of 540(100) ps [26,38]. Thus we approximated the effect of direct feeding from the 21 2 (−) state in our GFB fits to the 945.5-keV transition by assuming a feeding lifetime of 540 ps, which resulted in a better fit (smaller χ 2 ) to the experimental line shape than if no long-lived feeding component was included.…”