A Multicrystal Gamma-Ray Spectrometer with Time-of-Flight Rejection of Neutron-Induced Background

To improve the rejection of neutron-induced background in a multicrystal gamma-ray spectrometer, nanosecond circuitry was designed that exploits the difference in flight times of neutrons and gamma rays produced in a target bombarded with a pulsed beam of monoenergetic protons. This circuitry allows the spectrometer to be gated on for a time (nominally 7 nsec) equal to the pulse width of the proton beam. Only those gamma rays produced during the proton pulse are accepted for energy analysis. These gamma rays arrive at the detector in a time short compared with the flight time of any other secondary radiation produced during that same beam pulse. The system. is used as a pair spectrometer for energies from 1.5 to 11 Mev, and as a total absorption spectrometer for energies below 2.5 Mev. The energies of the gamma rays produced by 160 Mev protons from the Harvard University synchrocyclotron in targets of Be, C, H2O, Al, Co, and Bi were measured. Spectral data were obtained and the results include determination of essentially all the gamma rays reviously described plus several more at higher energies. The neutron rejection ratio of the spectrometer varied with targets and ranged from 59% for C to 77% for H2O.