FAST GAMMA-RAY COINCIDENCE TIMING USING A BGO DETECTOR ARRAY IN A NUCLEAR-REACTION EXPERIMENT

The (Li-7, Be-7) nuclear charge-exchange reaction has been investigate d at E(Li-7)=350 MeV. Coincidences were measured between the charged B e-7 reaction products in the focal plane of a magnetic spectrometer an d 430-keV Doppler-shifted gamma-rays from the decay in flight of excit ed Be-7 ejectiles. The latter were detected in a compact EGO detector array. Fast timing was used to optimize the separation between prompt and randoms events. The EGO output signals display the statistical dis tribution of the arrival times of individual photoelectrons from the p hotocathode in the photomultiplier tubes. A timing resolution of 6.5 n s relative to the accelerator RF was obtained. A simple model was succ essfully developed to simulate the EGO output signals with a statistic al distribution of a predetermined number of photoelectrons modulated by the light output response of the scintillator,and the single-photoe lectron gain distribution and transit time through the photomultiplier tube. It is concluded that the timing resolution is primarily limited by the small number of photoelectrons.