Hindrance of heavy-ion fusion at extreme sub-barrier energies in open-shell colliding systems

The excitation function for the fusion-evaporation reaction 64Ni + 100Mo has been measured down to a cross section of ∼5 nb. Extensive coupled-channels calculations have been performed, which cannot reproduce the steep falloff of the excitation function at extreme sub-barrier energies. Thus, this system exhibits a hindrance for fusion, a phenomenon that has been discovered only recently. In the S-factor representation introduced to quantify the hindrance, a maximum is observed at Es = 120.6 MeV, which corresponds to 90% of the reference energy Esref, a value expected from systematics of closed-shell systems. A systematic analysis of Ni-induced fusion reactions leading to compound nuclei with mass A = 100-200 is presented in order to explore a possible dependence of fusion hindrance on nuclear structure

The 44Ti(α, p) reaction and its implication on the 44Ti yield in supernovae

Cross sections for the 44Ti(α, p)47V reaction which significantly affects the yield of 44Ti in supernovae were measured in the energy range 5.7MeV ≤ Ec.m. ≤ 9 MeV, using a beam of radioactive 44Ti. The cross sections and the deduced astrophysical reaction rates are larger than the results from theoretical calculations by about a factor of 2. The implications of this increase in the reaction rate for the search of supernovae using space-based gamma detectors are discussed

Study of the 56Ni(d, p)57 Reaction and the Astrophysical 56Ni(p, γ)57Cu Reaction Rate

The single-particle character of states outside the doubly magic (radioactive) nucleus 56Ni has been determined through a measurement of the (d, p) neutron transfer reaction using inverse kinematics. From the spectroscopic factors of the low-lying states in 57Ni, the astrophysically interesting yield for the 56Ni(p, γ) reaction to the mirror nucleus 57Cu has been calculated, utilizing charge symmetry. The rate for this reaction in the temperature range typical of novae, supernovae, and x-ray bursts is found to be more than 10 times higher than previously assumed

81Kr in the Great Artesian Basin, Australia: a new method for dating very old groundwater

The measurement of cosmogenic 81Kr (t1/2=(2.29±0.11)×105 yr) has been proposed for many years as a reliable tool for groundwater dating in the range from 105 to 106 yr. In this paper, we report on the first use of 81Kr to determine the age of groundwater from four wells in the Great Artesian Basin in Australia. As the concentration of 81Kr in old groundwater is only a few hundred atoms per liter, krypton was extracted from large (16 000 l) groundwater samples and was analyzed for the isotopic abundance of 81Kr by accelerator mass spectrometry (AMS) with a cyclotron. 81Kr/Kr isotope ratios of (1.54±0.22)×10−13, (1.78±0.26)×10−13, (2.19±0.28)×10−13 and (2.63±0.32)×10−13, respectively, were measured for these samples. It is reasonable to assume that krypton dissolved in surface water in contact with the atmosphere has the known atmospheric 81Kr/Kr ratio of (5.20±0.40)×10−13. The observed reduction of isotope ratios in the groundwater samples can then be interpreted as being due to radioactive decay since recharge. This results in respective groundwater ages of: (4.02±0.51)×105 yr, (3.54±0.50)×105 yr, (2.87±0.38)×105 yr and (2.25±0.42)×105 yr. The main emphasis of this paper lies on the description of the analytic procedure to extract a reliable 81Kr signal from large groundwater samples. Although the uncertainties are still relatively large (primarily due to counting statistics caused by the low cyclotron AMS efficiency), the new technique enabled for the first time a definite determination of residence times for old groundwater with 81Kr. It thus confirms the hope that this radionuclide may become a very valuable tool for groundwater dating