Muon capture and fission phenomena in actinide muonic atoms

The interactions of stopped muons with 232Th, 235238U, 237Np, and 239242Pu nuclei were studied with the muon beam of the Los Alamos Meson Physics Facility. Time distributions were obtained of electrons produced by bound-muon decay and of neutrons and fission fragments resulting from nuclear μ capture. In addition, time distributions of neutrons attributed to μ capture by fission fragments were obtained; these neutrons were identifed by the prior observation of a fission caused by a radiationless transition in the muon cascade, resulting in μ capture by one of the fission fragments thus produced. The experimental results are applied to questions concerning the phenomenology of μ capture in heavy elements. The previously observed apparent dependence of observed capture rate on experimental method is shown to be caused by misidentification of events resulting from the disappearance of muons bound to fission fragments, rather than by population of shape-isomeric states in the excited nucleus following a radiationless transition. The true capture rates characteristic of the nuclei studied are compared with theoretical predictions. Capture rates for muons bound to fission fragments are inferred and found to be consistent with capture rates expected to be exhibited by muons bound to heavy fragments produced in the asymmetric mass division, rather than to light fragments. This finding is shown to be consistent with an adiabatic treatment of the muon motion at the time of fission. Alterations in the GAMMA/sub n/GAMMA/sub f/ ratios for nuclei to which muons are bound are taken as evidence for a change in the height of the fission barrier owing to a lessening of the Coulomb repulsion in the presence of the muon