Observation of Resonances in 239Pu (n,alpha) Reactions

A measurement performed at the Argonne National Laboratory (ANL) Intense Pulsed Neutron Source (IPNS) facility studied the incident neutron energy-dependent charged particle and fission fragment yields from neutron-induced reactions on 239Pu. This initial measurement in a planned series of measurements tested the use of coincidence measurement techniques to determine absolute neutron capture and neutron-induced-fission cross sections for the minor actinides. The analysis of the data showed a set of previously unknown resonances identified as the result of (n,a) reaction channel. These resonances differ from the known fission and radiative capture resonances in 239Pu and do not correspond to any known capture resonances from structural material in and around the 239Pu target assembly. The new resonances decay uniquely via a-emission, and this presents new states not explained by current models. The yield of the observed (n,a) reaction channel is 30 ± 2 times as strong as the fission channel, as determined by comparing the high-energy a-particle peak area and the fission fragment peak area in the Si detectors.