Application of the time-of-flight technique for lifetime measurements with relativistic beams of heavy nuclei

A novel method for picosecond lifetime measurements of excited γ -ray emitting nuclear states has been developed for fast beams from fragmentation reactions. A test measurement was carried out with a beam of 124Xe at an energy of ∼ 55 MeV / u . The beam ions were Coulomb excited to the 2 1 + state on a movable target. Excited nuclei emerged from the target and decayed in flight after a distance related to the lifetime. A stationary degrader positioned downstream with respect to the target was used to further reduce the velocity of the excited nuclei. As a consequence, the γ -ray decays from the 2 1 + excited state that occurred before or after traversing the degrader were measured at a different Doppler shift. The γ -ray spectra were analyzed from the forward ring of the Segmented Germanium Array; this ring positioned at 37 ∘ simultaneously provides the largest sensitivity to changes in β and the best-energy resolution. The ratio of intensities in the peaks at different Doppler shifts gives information about the lifetime if the velocity β is measured. The results and range of the application of the method are discussed.