Astrophysics

Reactions which are of particular interest for astrophysics are: Gamow-Teller transitions measured by charge-exchange reactions, spallation reactions, and Coulomb dissociation [1]. The nuclei that will be accessible at FAIR will allow exploring the reactions inverse to those relevant for the astrophysical rp- and rprocesses by utilizing the electromagnetic-excitation process at high beam energy. The cross sections for the direct capture reactions in the stellar environment are very small due to the stellar temperature (~20 keV) and the Coulomb barrier, while the cross sections for the inverse process, Coulomb dissociation, are much larger. Of special interest are the (γ,p) and (γ,n) cross section at very low fragment-nucleon relative energy. The proton tracking through the magnetic field of the large-acceptance dipole will result in a momentum resolution of ∆p\p~10^-3. For the neutrons, a similar resolution is achievable by making use of a long time-of-flight path of ~35 m (ToF resolution <100 ps), resulting in an invariant-mass resolution of better than 20 keV (at 100 keV relative energy), thus making cross-section measurement feasible at energies relevant for astrophysics, down to about 10 keV. Here, the lead target thickness is limited to about 100 mg/cm² due to multiple scattering and energy loss. Still, with beam intensities of ~104 ions/s differential cross sections of 100 µb/(10 keV bin) can be measured within a few days.

[1] C.A. Bertulani and G. Baur, Phys. Rep. 163 (1988) 299.