Fast γ-ray coincidence timing using a BGO detector array in a nuclear reaction experiment

Abstract The ( 7 Li, 7 Be) nuclear charge-exchange reaction has been investigated at E( 7 Li ) = 350 MeV . Coincidences were measured between the charged 7 Be reaction products in the focal plane of a magnetic spectrometer and 430-keV Doppler-shifted γ-rays from the decay in flight of excited 7 Be ejectiles. The latter were detected in a compact BGO detector array. Fast timing was used to optimize the separation between prompt and randoms events. The BGO output signals display the statistical distribution of the arrival times of individual photoelectrons from the photocathode in the photomultiplier tubes. A timing resolution of 6.5 ns relative to the accelerator RF was obtained. A simple model was successfully developed to simulate the BGO output signals with a statistical distribution of a predetermined number of photoelectrons modulated by the light output response of the scintillator and the single-photoelectron 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.