The probabilities of muon attachment to fission fragments in the prompt fission of a37Np have been determined as a function of fragment mass and total kinetic energy release in a (/~ -, fl f2 e-)-coincidence measurement. A strong dependence of the attachment probability on the fragment mass is observed. The experimental results are compared with theoretical predictions.

A study of muonic 238U has been performed in a combined (p-, 7f) and (#-, 7t') coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier of AEb = (0.6___ 0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p~ Is) and (3d-+ l s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p-~ls), (3 d ---, l s), and (3p--*ls) are (1.5• (5.7 • 1.7)%, and (5.3 • 1.9)%, respectively.

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Fission fragments from the reaction 237Np(p ,y f) have been measured in coincidence with muonic X-rays. The efficiency of the fission fragment detector is determined from (p , 7f)-data of the same experiment. The total fission probability per p-stop Pt has been measured as well as the fission probabilities Pf of the non-radiative muonic (3d~ ls)-and (2p~ls)-transitions; the latter has been divided into two parts leading to different mean excitation energies E: Pt=(54_+17)%, Pf(3d--*ls)=(41__21)%, Pf(2p--~ls, E= 6.218 MeV) = (61 _+ 19)%, andPf(2p~ls, E = 6.525 MeV) = (57 _+ 18)%. The influence of the muon on the fission barrier is discussed. The fission probability after muon capture is compared with a calculated value using a distribution of nuclear excitation energies following muon capture and the fission probability as measured in a a38U (3He, ~f)-reaction.

The probabilities for non-radiative (n.r.) excitations Pn.r. in the muonic nuclides 2~ a32Th, and a38U have been determined from (/z-, 77)-measurements by comparing the intensities of muonic X-ray transitions in single and coincidence spectra. The value of Pn.r. (3p~ 1 S), measured for the first time, is about 90% for the actinides 232Th and 238U, but only about 8 % for a~The value of Pn.r (3d~ls) is found to be 10% for 233Th, 13% for 238U, and about 4 % for 2~For 2~ a vanishing strength of the n.r. decay of the 2p-level is found, while for 232Th and for 238U n.r. strengths of about 20% and 26%, respectively, are observed. By regarding two subcomplexes of the 2p-* 1 s transitions leading to different mean excitation energies the n.r. transition probabilities were found to be different for 238U only, 21.6% and 31.1.%, respectively.