Pile Neutron Cross Sections of the Heavier Plutonium Isotopes

The effective pile neutron capture cross sections of the heavier Pu isotopes have been measured by irradiations, in the Chalk River Reactor (NRX) and in the Materials Testing Reactor. The values in barns for Pu²⁴⁰, Pu²⁴¹, Pu²⁴², Pu²⁴³, Pu²⁴⁴, and Pu²⁴⁵ were found to be: 530 ± 50, 390 ± 80, 30 ± 10, 170 ± 90, 1.5 ± 0.3, and 260 ± 145, respectively. In addition, a thermal neutron absorption cross section of 1450 ± 250 barns was found for Pu²⁴¹ and a thermal neutron fission cross section of 1060 ± 210 barns for Pu²⁴¹. Using these cross sections and an assumed flux of 3 × 10¹⁴ cm^-2 sec^-1, curves were calculated and plotted showing the variation in composition of plutonium as a function of integrated flux.