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Weak interactions

Electron captures sample a larger fraction of the Gamow-Teller strength function than beta-decays, due to the high electron Fermi energies in late phases of stellar evolution, stellar collapse, and in explosive events like type Ia and II supernovae. Such environments contain a number of unstable fp-shell nuclei, among them 55-60Co, 56-61Ni, 54-58Mn, and 54-59Fe. They can be studied via charge-exchange reactions using radioactive nuclear beams. Astrophysical tabulations based on shell model matrix elements are only available for light nuclei in the sd-shell. For heavier nuclei, more simplified approaches based on average positions of the Gammon-Teller giant resonance and average matrix elements have been utilised until now. A new Monte-Carlo shell model technique allows calculations in the fp-shell (and at finite temperatures) and reproduces the measured GT+-distribution very well. As a next step this method should be applied to those nuclei which cannot be measured with current techniques [34]. Neutrino induced transmutation of nuclei play an important role in type II supernovae. The intense neutrino flux leads to heating via neutrino and anti-neutrino captures on neutrons and protons and inelastic neutrino scattering on nuclei. The latter could affect the outcome of the r-process by neutrino spallation of neutron-rich nuclei and produce also a significant amount of rare isotopes like 7Li, 11B, 19F, or 180Ta (neutrino-nucleosynthesis). These processes require neutrino-nucleus cross sections. Since 1990 accelerator-based measurements of neutrino induced reactions on nuclei have become feasible (KARMEN, LSND) [35] (see the working group on neutrino physics and fundamental interactions). Neutrino sources in these experiments provide (as one component) monoenergetic $\nu_\mu$ (29.8 MeV) neutrinos and $\nu_e$ and $\overline{\nu}_\mu$ with continuous energy spectra up to 52.8 MeV, similar to the neutrino energies in supernovae. Theory can be tested with such measured cross sections, complementary data from other weak interactions in nuclei like beta-decay and muon capture rates, and even electron scattering data, as the weak and electromagnetic interactions are related. The Continuum Random Phase Approximation (CRPA), which combines the usual RPA treatment with a correct description of the particle states in the continuum [36], has passed these tests, provided a good description of the giant (dipole and spin-dipole) resonances in 12C and 16O, and has been found to reproduce well the total muon capture rates in nuclei like 12C, 16O, and 40Ca. Future neutrino experiments at the European Spallation Source (ESS) would offer unique possibilities to further explore neutrino properties and interactions with nuclei. 
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