The measurement of picosecond lifetimes in neutron rich nuclei

The FEST (Fast Electronic Scintillation Timing) technique for measuring nuclear level lifetimes in far from stability neutron rich nuclei is presented. The method, developed at the TRISTAN on-line mass separator at BNL, uses fast plastic and BaF2 scintillation detectors and high resolution Ge detectors in a fast-slow β-γ-γ coincidence counting system. The detectors can be calibrated over a wide range of γ ray energies due to the use of a ΔE β detector with a timing response that is essentially independent of the incident β energy. By using a well defined set of calibration and correction procedures, it is possible to measure lifetimes 40 ≥ τ ≥ 3 ps in nuclei that are far from stability via the centroid shift method. For τ > 40 ps, the lifetime is extracted by determining the slope of the delayed portion of the time spectrum. The technique is discussed along with comparisons to levels of known lifetimes and results from recent measurements