Pile Neutron Capture Cross Sections of Np²³⁹

An activation method was used to measure the neutron capture cross sections of Np²³⁹. Two isomers of Np²⁴⁰ have been reported: a 7-min daughter of U²⁴⁰ and a 1-hr isomer produced by alpha bombardment of U²³⁸. Examination with a gamma scintillation spectrometer of the activity produced by neutron activation of Np²³⁹ gave positive evidence for the 7-min isomer as a product of neutron capture in Np²³⁹. The 1-hr isomer is also produced, although the evidence for it was complicated by fission product activity. Genetic linkage between the isomers was not detected and is less than 5 per cent. The measured cross sections for pile neutrons are 31 ± 6 barns and barns for the 7-min and 1-hr isomers. These values are based upon a 2.8 per cent abundance of the 1.5-Mev gamma of the 7-min isomer and a 38 per cent abundance for the 0.97-Mev gamma, of the 1-hr isomer. These results are relative to a gold thermal cross section of 99 barns.