γ-induced reactions

High energy γ-rays – allowing the excitation of a probe nucleus above its particle separation thresholds – are produced as secondary beams with different mechanisms and characteristics. If a high energy electron beam is decelerated or stopped in a solid radiator bremsstrahlung is emitted in a continuous spectrum with energies up to the beam energy. If a laser photon is scattered off a relativistic electron its energy is boosted into the MeV-range. The spectrum of these Laser Compton Backscattered (LCB) photons depends on the selected solid angle due to the double-differential character of the underlying Compton effect.

Bremsstrahlung is usually used to perform experiments with the activation technique. If the reaction is investigated with spectra of different maximum energies the energy dependence of the reaction cross section and the (ground-state) reaction rate at stellar temperatures can be extracted from the measured yields. To perform an in-beam experiment, LCB photons or tagged photons are used to provide a reasonable energy resolution. At an up-to-date LCB-photon facility like, e.g., HIγS at Duke University the photon intensities are for a wide energy range high enough to study γ-induced reactions with both activation and in-beam techniques.

The reaction cross sections of unstable isotopes can be investigated in inverse kinematics using Coulomb dissociation. The relativistic radioactive ion beam impinges on a high-Z probe and is excited in the virtual photon field. Knowing the complete kinematics of the reaction the excitation energy can be extracted and, thus, the reaction cross section.



Experiments

LAND/R3B at GSI
HIγS at Duke University