THE NEUTRON CAPTURE CROSS SECTIONS OF Pu²³⁸, Pu²⁴², AND Am²⁴³ IN THE THERMAL AND EPICADMIUM REGIONS

Thermal neutron capture cross sections of 403 ± 8 b. for Pu 238 , 18.6 ± 0.8 b. for Pu 242 , and 73.6 ± 1.8 b. for Am 243 have been measured by the activation method assuming 36.4 b. for the thermal neutron cross section of Co 59 . Samples of plutonium (90% Pu 242 – 10% Pu 238 ) were irradiated for 1 to 5 months in the NRX reactor. The growth of Pu 239 , Am 243 (the daughter of the 5 hr. Pu 243 ), and Cm 244 (the daughter of the 25 min. Am 244 ) was determined by mass analysis and by alpha spectrum analysis and total alpha counting. Resonance capture integrals (from 0.5 ev. to ∞) of 3260 ± 280 b. for Pu 238 , 1275 ± 30 b. for Pu 242 , and 2290 ± 50 b. for Am 243 were found using cobalt and U 238 as standards. Values of 48.6 b. and 282 b. were taken as the resonance integrals of Co 59 and U 238 respectively.The fission cross section of Pu 242 was zero within experimental error.