Prompt gamma rays emitted from individual fragments in neutron-induced fission

Prompt gamma rays associated with moving fission fragments of specific masses have been observed in thermal neutron-induced fission of 235U. The geometrical layout of the apparatus prevents detection of photons emitted from stopped fragments and determines the effective time resolution to about 1 ns after fission. Spectra are measured by means of a high-resolution Ge(Li) detector with 3. 5 % photopeak efficiency for 60Co. An 8 inch diameter x 9 inch Nal(Tl) anti-Compton shield suppresses the Compton distribution in the spectra and the recording of events produced by fast fission neutrons. The coincident fission-fragment masses are deduced from their correlated kinetic energies as measured by two Si solid-state detectors. The observed Doppler shift in gamma-ray energy allows the assignment of lines to single members of fragment pairs. Data are processed in a 256 x 256 x 2048 channel matrix. The main objective of these experiments is a study of the properties of individual neutron-rich nuclei far off the stability line. (author)