Delayed Neutron Yields in Pu, U-233, U-238, and Th Relative to Yield in U-235

Delayed neutron studies have been made in the Experimental Breeder Reactor (EBR), using a conventional sample transfer system and a neutron counter comprised of BF₃ tubes in a graphite geometry. Samples of Th, U²³³, U²³⁵, U²³⁸, and Pu were irradiated in a fast flux; samples of U²³³, U²³⁵, and Pu in a thermal flux. The ratio of the delayed neutron yield per fission (based on the longest four periods) to the delayed neutron yield per fast fission of U²³⁵ was determined as: for fast fission of U²³³, 0.414 ± 7.5%; for fast fission of Pu, 0.405± 7.5%; for fast fission of Th, 3.09 ± 17%; for fast fission of U²³⁸, 2.23 ± 7.5%. The ratio of fast fission to thermal fission delayed neutron yields was not significantly different from unity for all samples except Pu, where the ratio of thermal to fast fission yields was 0.888 ± 6%. This latter is believed to be primarily attributable to the 5% fraction of Pu²⁴⁰ in the sample.