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 ∼55MeV/u. The beam ions were Coulomb excited to the 21+ 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 21+ 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-bar 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