No abstract

The transition rates for the 2(1)+ states in (62,64,66)Fe were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N = 40. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N = 40 as governed by the number of valence protons with respect to Z ≈ 30. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

β-decay properties of neutron-rich Ca isotopes have been obtained. Half-life values were determined for the first time for 54 Ca (86 ± 7 ms), 55 Ca (22 ± 2 ms), and 56 Ca (11 ± 2 ms). The half-life of 230 ± 60 ms deduced for 53 Ca is significantly longer than reported previously, where the decay chain 53 K → 53 Ca → 53 Sc was considered. A delayed γ ray with energy 247 keV was identified following β decay of 54 Ca, and is proposed to depopulate the 1 + 1 level in 54 Sc. The β-decay properties compare favorably with the results of shell model calculations completed in the full pf -space with the GXPF1 interaction. The half-lives of the neutron-rich Ca isotopes are also compared with gross β-decay theory. The systematic trend of the neutron-rich Ca half-lives is consistent with the presence of a subshell gap at N = 32.

The beta-decay properties of the N=Z nuclei 96Cd, 98In, and 100Sn have been studied. These nuclei were produced at the National Superconducting Cyclotron Laboratory by fragmenting a 120 MeV/nucleon 112Sn primary beam on a Be target. The resulting radioactive beam was filtered in the A1900 and the newly commissioned Radio Frequency Fragment Separator to achieve a purity level suitable for decay studies. The observed production cross sections of these nuclei are lower than predicted by factors of 10-30. The half-life of 96Cd, which was the last experimentally unknown waiting point half-life of the astrophysical rp process, is 1.03_{-0.21};{+0.24} s. The implications of the experimental T_{1/2} value of 96Cd on the abundances predicted by rp process calculations and the origin of A=96 isobars such as 96Ru are explored.

The slow β-decay of the 68 Se waiting point in the astrophysical rp-process can in principle be bypassed by a sequential two proton capture. The rate of this reaction sequence depends exponentially on the 69 Br proton separation energy. We studied β-delayed proton emission of 69 Kr and extracted a proton separation energy of −641(42) keV. In addition, we determined a 69 Kr β-decay half-life of T 1/2 = 28(1) ms and an excitation energy of 3153(55) keV of the 69 Kr isobaric analog state in 69 Br. X-ray burst model calculations show that regardless of the values of other uncertain masses in the region, the new S p ( 69 Br) allows for a reaction flow via 68 Se(2p, γ ) of at most 20%. Uncertainties are sufficiently reduced to conclude that 68 Se(2p, γ ) has at best a very small effect on burst light curve and composition, and that 68 Se is a strong rp-process waiting point. Our results also exclude the possibility of a suggested longer lived, so far unobserved, 69 Br ground state.(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP 3 . Astrophysical x-ray bursts are frequently observed and are thought to occur on the surface of accreting neutron stars [1][2][3][4]. The bursts are powered by a thermonuclear runaway in the accreted material, triggered by the triple-α reaction and the breakout from the hot CNO cycle through the αp-process. During the burst, temperatures quickly rise and become high enough for hydrogen to burn via the rp-process (rapid proton capture process)[5,6], a sequence of fast (p, γ ) reactions and slower β + -decays reaching Te in some bursts [7].