The Measurement of Short-Lived Delayed Photoneutrons from Fission-Fragment Gamma Rays of ²³⁵U in Heavy Water

The kinetic equations for a heavy-water-moderated reactor have terms describing the production of neutrons in the photodisintegration of deuterons. A technique has been developed for the measurement of these short-lived delayed-photoneutron groups. A ²³⁵U target was placed in the center of a sphere of D₂O and a chopped neutron beam from the VPI reactor was allowed to impinge on the target. The time for a revolution of the chopper varied from 86.4 to 0.463 sec in six steps and there are two pulses per revolution. The ratio of “beam-on” target time to “beam-off” target time is 1 to 16.5. During the beam-off time interval, the production rate of delayed neutron and delayed photoneutrons as a function of time was stored in a time base multichannel analyzer. The half-lives of two new delayed-photoneutron groups were determined to be 7.39 × 10⁻² and 4.02 × 10⁻³ sec with relative abundances of 0.0292 and 0.170, respectively. The relative abundances represent the fractions of the delayed-neutron and delayed-photoneutron groups.