Reaction rate for two--neutron capture by 4^4He

Recent investigations suggest that the neutrino--heated hot bubble between the nascent neutron star and the overlying stellar mantle of a type--II supernova may be the site of the r--process. In the preceding $\alpha$--process building up the elements to $A \approx 100$, the $^4$He(2n,$\gamma$)$^6$He-- and $^6$He($\alpha$,n)$^9$Be--reactions bridging the instability gap at $A=5$ and $A=8$ could be of relevance. We suggest a mechanism for $^4$He(2n,$\gamma$)$^6$He and calculate the reaction rate within the $\alpha$+n+n approach. The value obtained is about a factor 1.6 smaller than the one obtained recently in the simpler direct--capture model, but is at least three order of magnitude enhanced compared to the previously adopted value. Our calculation confirms the result of the direct--capture calculation that under representative conditions in the $\alpha$--process the reaction path proceeding through $^6$He is negligible compared to $^4$He($\alpha$n,$\gamma$)$^9$Be.Comment: 13 pages, 4 postscript figures, to appear in "Zeitschrift f. Physik A", changed internet address and filename, the uuencoded postscript file including the figures is available at ftp://is1.kph.tuwien.ac.at/pub/ohu/twoneutron.u

T=1 states in Rb74 and their Kr74 analogs

Charge symmetry breaking effects that perturb analog symmetry between nuclei are usually small but are important in extracting reliable Fermi matrix elements from "superallowed" β decays and testing conserved vector current theory, especially for the heavier cases. We have used the Ca40(Ar36, pn)Rb74 and Ca40(Ca40,αpn)Rb74 reactions at 108, 123 and 160 MeV, respectively, to populate Rb74 and determine the analog distortion through comparison of T=1 states in Rb74 with their corresponding Kr74 levels. We have traced the analogs of the Kr74 ground-state band in Rb74 to a candidate spin J=8 state and determined the Coulomb energy differences. They are small and positive and increase smoothly with spin. New T=0 states were found that better delineate the deformed band structure and clarify the steps in deexcitation from high spin. A new T=0 band was found. No evidence was found for γ decay to or from a low-lying Jπ=0+ state in Rb74 despite a careful search

Terminating states in the positive-parity structures of As 67

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=12 nucleus As67 have been studied by using the Ca40(Ar36,2αp)As67 reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson-Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g92 particles and that both possess noncollective terminating states

Alignment delays in the N = Z nuclei 72Kr, 76Sr, and 80Zr

The ground state rotational bands of the N = Z nuclei 72Kr, 76Sr, and 80Zr have been extended into the angular momentum region where rotation alignment of particles is normally expected. By measuring the moments of inertia of these bands we have observed a consistent increase in the rotational frequency required to start pair breaking, when compared to neighboring nuclei. 72Kr shows the most marked effect. It has been widely suggested that these “delayed alignments” arise from np-pairing correlations. However, alignment frequencies are very sensitive to shape degrees of freedom and normal pairing, so the new experimental observations are still open to interpretation

Testing mean-field models near the N=Z line: γ-ray spectroscopy of the Tz=1/2 nucleus 73Kr

Excited states in the N=Z+1 nucleus 73Kr have been investigated using the 40Ca(36Ar, 2pn) and 40Ca(40Ca, α2pn) reactions at 145 and 160 MeV, respectively. γ rays were detected using the Gammasphere array and events were recorded in coincidence with charged-particle and neutron detectors. The three previously observed bands were extended to high spin, and a new unfavored positive-parity band has been observed. The alignment characteristics and decay properties of the bands are all consistent with large-deformation prolate rotation, with no clear evidence for oblate bands or shape coexistence. This is quite different from neighboring 72,74Kr, indicating a strong shape-stabilizing role for the valence neutron. The experimental results are compared to extended total Routhian surface, cranked Nilsson Strutinsky, and cranked relativistic mean-field calculations. The results suggest that the paired calculations lack some important physics. Neutron-proton correlations may be the missing ingredient. There is also evidence for an unusual band crossing in the negative-parity bands, which may indicate the presence of T=0 pairing correlations. At high spin all the models can reproduce the experimental data

High-resolution in-beam particle spectroscopy —New results on prompt proton emission from \chem{^{58}Cu}

Prompt proton decay lines in $^{58}$Cu have been studied by means of high-resolution in-beam particle-$\gamma$ coincidence spectroscopy using the gammasphere Ge-detector array in conjunction with a dedicated set of ancillary detectors including four $\Delta E$-$E$ silicon-strip telescopes. High-spin states in $^{58}$Cu have been populated via the heavy-ion fusion-evaporation reaction \chem{^{28}Si({}^{36}Ar,1\alpha 1p 1n)} at 148 MeV beam energy. The full-width at half maximum for the proton peak could be reduced significantly compared to earlier experiments. The results indicate that only one prompt proton decay branch exists in the decay-out of the well-deformed band of $^{58}$Cu