19F(α,n)22Na, 22Ne(p,n)22Na, and the Role of their Inverses in the Destruction of 22Na

The inverses of the 19F(α,n)22Na and 22Ne(p,n)22Na reactions may be important destruction mechanisms for 22Na in neutron-rich, high-temperature or explosive nucleosynthesis. I have measured the cross sections for the 19F(α,n)22Na and 22Ne(p,n)22Na reactions from threshold to 3.1 and 5.4 MeV, respectively. The absolute efficiency of the 4π neutron detector was determined by Monte Carlo calculations and calibrated using two standard sources and two nuclear reactions. Cross sections for the inverse reactions have been calculated using the principle of detailed balance, and reaction rates for both the reactions and their inverses determined for temperatures between 0.01 and 10 GK for 19F(α,n)22Na and between 0.1 and 10 GK for 22Ne(p,n)22Na

Evolution and Nucleosynthesis of Massive Stars and Related Nuclear Uncertainties

Properties of atomic nuclei important for the prediction of astrophysical reaction rates are reviewed. In the first part, a recent simulation of evolution and nucleosynthesis of stars between 15 and 25 solar masses is presented. This study is used to illustrate the required nuclear input as well as to give examples of the sensitivity to certain rates. The second part focusses on the prediction of nuclear rates in the statistical model (Hauser-Feshbach) and direct capture (DWBA). Some of the important ingredients are addressed. Discussed in more detail are approaches to predict level densities, parity distributions, and optical alpha+nucleus potentials.Comment: Invited talk at 17th Int. Nucl. Phys. Conf. of the EPS "Nuclear Physics in Astrophysics", Debrecen, Hungary, 2002 (new version: fixed typo in alpha potential parameters; note: the parameters are incorrect in the NPA paper

β-delayed α spectrum of 16N and the 12C(α,γ)16O cross section at low energies

The α spectrum following the β decay of 16N from the isotope separator TISOL has been measured by detecting 106 α particles in coincidence with 12C nuclei. These data, which show a low-energy interference anomaly accompanying the main α peak, permit a more precise determination of the p-wave amplitude of the astrophysically important reaction 12C(α,γ)16O. The α spectrum and previous γ-ray data have been fitted simultaneously by a K-matrix parametrization; a value of S(E=0.3 MeV)=57±13 keV b has been obtained for the E1 part of the 12C(α,γ)16O reaction

Constraints on the low-energy E1 cross section of 12C(α,γ)16O from the β-delayed α spectrum of 16N

The shape of the low-energy part of the β-delayed α-particle spectrum of 16N is very sensitive to the α+12C reduced width of the 7.117 MeV subthreshold state of 16O. This state, in turn, dominates the low-energy p-wave capture amplitude of the astrophysically important 12C(α,γ)16O reaction. The α spectrum following the decay of 16N has been measured by producing a low-energy 16N14N+ beam with the TRIUMF isotope separator TISOL, stopping the molecular ions in a foil, and counting the α particles and 12C recoil nuclei in coincidence, in thin surface-barrier detectors. In addition to obtaining the α spectrum, this procedure determines the complete detector response including the low-energy tail. The spectrum, which contains more than 106 events, has been fitted by R- and K-matrix parametrizations which include the measured 12C(α,γ)16O cross section and the measured α+12C elastic scattering phase shifts. The model space appropriate for these parametrizations has been investigated. For SE1(300), the E1 part of the astrophysical S factor for the 12C(α,γ)16O reaction at Ec.m.=300 keV, values of 79±21 and 82±26 keV b have been derived from the R- and K-matrix fits, respectively

Neutron production in (α,n) reactions

Neutrons can induce background events in underground experiments looking for rare processes. Neutrons in a MeV range are produced in radioactive decays via spontaneous fission and () reactions, and by cosmic rays. Neutron fluxes from radioactivity dominate at large depths ( km w. e.). A number of computer codes are available to calculate cross-sections of () reactions, excitation functions and neutron yields. We have used EMPIRE2.19/3.2.3 and TALYS1.9 to calculate neutron production cross-sections and branching ratios for transitions to the ground and excited states, and modified SOURCES4A to evaluate neutron yields and spectra in different materials relevant to high-sensitivity underground experiments. We report here a comparison of different models and codes with experimental data, to estimate the accuracy of these calculations