The experimental method consists of producing high-energy radioactive beams (with typically a few hundred MeV/nucleon kinetic energy) and of a kinematically complete measurement of breakup reactions in secondary targets. The measurement is exclusive or kinematically complete in the sense that all reaction products with velocities close to the beam velocity and gamma-rays are detected. A schematic drawing of the detection setup is shown below.
Secondary beams at the FRS target are selected by the FRS according to their magnetic rigidity only, thus mixed secondary beams containing isotopes with similar mass-over-charge ratio are transported to the experimental area. The incident projectiles are uniquely identifed on an event-by-event basis by utilizing energy loss and time-of-flight measurements.
In a similar manner, the fragments produced through interactions with the reaction target are identifed. Here, the magnetic rigidity is determined from three position measurements defning the trajectories of the charged projectile residues in the magnetic field of a large-gap dipole magnet placed behind the target. Additional energy loss and time-of-flight measurements allow unique identifcation of the outgoing fragments and determination of their momenta.
Neutrons emitted from the excited projectile or excited-projectile-like fragments are kinematically focussed in the forward direction and detected with high effciency (~ 90%) in the LAND neutron detector. The momenta of the neutrons are determined from the time-of-flight and position information. The angular range for fragments and neutrons covered by the detectors corresponds to a 4π measurement of the breakup in the rest frame of the projectile for fragment-neutron relative energies up to 5.5 MeV (at 500 MeV/nucleon beam energy).
At the high beam energies used, the gamma-rays need to be detected with good angular resolution in order to minimize Doppler-broadening effects. Two detectors are used alternatively: the Crystal Ball spectrometer, which consists of 160 NaI detectors covering almost the full solid angle, or a CsI array consisting of 144 submodules. The latter covers only the forward hemisphere, but with better angular resolution. Still, the resolution is limited by the Doppler broadening.
The excitation energy prior to decay is obtained by reconstructing the invariant mass combining the measurements. The resolution in excitation energy depends on the relative fragment-neutron kinetic energy and the resolution for measuring the gamma sum energy (in the projectile rest frame) in case of the population of excited states. It changes from about 200 keV close to the threshold to a few MeV in the region of the giant dipole resonance (at excitation energies around 15 MeV).