Yield of Photo-Neutrons fromU235Fission Products in Heavy Water

The photo-disintegration of the deuteron has been used to study the hard $\gamma$-rays emitted by fission products of ${\mathrm{U}}^{235}$1. The neutrons created in the process were used as the indicator of the presence of hard $\gamma$-rays. The fission products were placed at the center of a 10″ radius sphere of heavy water. Conclusions about the periods and yields of the hard $\gamma$-rays were made from the total number of photo-neutrons being captured in a large tank of oil surrounding the sphere of heavy water. Eight half-lives were found: 2.5 sec., 41 sec., 2.4 min., 7.7 min., 27 min., 1.6 hr., 4.4 hr., and 53 hr. The shortest one and longest one of these are least reliable. Eighty-five percent of the photo-neutrons appear in the two shortest half-lives, the 2.5-sec. component being three times as intense as the 41 sec. component. The total saturated activity of the photo-neutrons for an infinite amount of heavy water was calculated from the 10″ radius sphere measurements to be about 16.5 percent of the saturated delayed neutron activity. It is calculated that there must be of the order of one to two photons of energy above 2.2 Mev emitted per fission by fission products with half-lives greater than one second.