F. Nunes (Universidade Fernando Pessoa, Porto)
Transcript of F. Nunes (Universidade Fernando Pessoa, Porto)
Nuclear Astrophysics
Gamow window
Reaction rate at a temperature T
Astrophysical S-factor
E0 and ΔE0
Nuclear Astrophysics: resonant vs direct
If lR=lmin
Reaction cross section:
Resonant part -> R-matrix
otherwise
Direct part -> next
Nuclear Astrophysics
Shift factor and penetration factor
At low energies
•For negative energies, the penetration factor has no meaning, but shift factor can still be defined•For neutron induces (no charge) the penetration factor goes as k2l+1.
Nuclear Astrophysics: radiative processes
Multipole decomposition of the E.M. field
The photon emission hamiltonian is weak compared to the nuclear, thus we use first order perturbation:
Long wave-length approximation
Nuclear Astrophysics: radiative processes
Radiative capture cross section in terms of the multipoles
Selection rules:
Nuclear Astrophysics: low energy S-factorElements that determine the low energy dependence
• κ – bound state pole• Li Initial centrifugal barrier• ηi Remnant of the initial Coulomb barrier• Lf Final centrifugal barrier• ηf Final Coulomb barrier
Nuclear Astrophysics: low energy S-factorTaylor expansions: of S(E) for charged particle
of (σ v) for neutrons
Nuclear Astrophysics: indirect methodsSub-threshold state
Above threshold the width relates to the ANC