A method for picosecond lifetime measurements for neutron-rich nuclei: (2) Timing study with scintillation counters

Abstract

A delayed coincidence timing system consisting of a thin NE111A plastic scintillator and a BaF 2 crystal coupled to XP2020 and XP2020Q photomultipliers, respectively, and operated in a fast dynode mode was developed. Its performance was optimized over a dynamic range from 0.3 to 1.6 MeV for γ-rays. For this range a walk of ±22 ps and a time resolution of 96 ps (FWHM) were measured for β-rays with a mean energy of about 600 keV coincident with 1.37 MeV γ-rays from a 24Na source. The high sensitivity of this detector system has been demonstrated by using a β-γ delayed coincidence mode to measure known picosecond lifetimes of nuclear states in neutron-rich nuclei at the TRISTAN fission product mass separator. Mean-lives as short as 51±3 ps have been measured by the slope method and 10±7 ps by the centroid shift method. Lifetimes of several nuclear states in 142Ba, 142,144Ce and 146Nd measured by the slope and by the centroid shift method are reported.