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The ratios of prompt to delayed fission yields for the isotopes U-233, U-234, U-235, U-236, U-238, Np-237, Pu-242, and Pu-244 and the fission probabilities relative to each other have been investigated experimentally. Using the value of the total fission probability for Np-237 the absolute probabilities for prompt and delayed fission have been determined. The fission probabilities per muon capture P(fc) have been derived for all the isotopes and compared with an evaluation based on excitation functions from theory.

The excitation function of the fission probability PEEx) for238U has been measured in the reaction238U(α, α′f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowBnf, the threshold for second chance fission, and aboveBnf up toEx=37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityPf at excitation energies well aboveBnE. The role of the quasi-elastic knock-out process in this reaction is discussed.

A measurement of the α-spectrum in the region of the isoscalar giant resonances from 238 U at E α = 120 MeV in coincidence with out-of-plane fission fragments shows the validity of the hypothesis of axial symmetry with respect to the recoil axis. Similar to what was observed in previous in-plane measurements the fission probability in the giant resonance region is lower than that measured in photofission indicating a decreased fission probability for the isoscalar giant resonances.

The fission probabilities and angular distributions of the fission fragments for the (α, α'f) reaction on 232 Th and 238 U at a bombarding energy of 120 MeV have been measured from about 4 to 14 MeV excitation energy. Evidence for sub-barrier resonances has been found, the negativeparity ones occurring at the same excitation energy where photofission resonances have been observed. The data are analyzed with the two-humped barrier model. For 238 U the data are reasonably well fitted with barriers similar to those known from the literature. For 232 Th though, the outer barrier parameters are quite different: the height E B = 6.6 MeV and the width ( khω ) B = 1.2 MeV. Also for 232 Th, introducing an additional mass symmetric and axially asymmetric outer barrier, as was previously found necessary for 238 U, does not result in a good fit to the data at higher excitation energies.

The fission decay channel of 232 Th and 238 U has been investigated, using the (α, α'f) reaction at 120 MeV bombarding energy. The angular distributions of the fission fragments and the fission probabilities up to around 15 MeV excitation have been measured. No evidence for the fission decay of the giant quadrupole resonance has been found, although for 238 U, a weakly excited structure is seen in the (α, α'f) spectrum at about 9.5 MeV excitation at backward angles with respect to the recoil axis. This effect is similar to what has been found in a ( 6 Li, 6 Li'f) experiment reported recently. The over-all feature of the fission probability for excitation energies above the fission barrier are well reproduced by statistical calculations.

The total kinetic energy release and mass distributions in prompt and delayed muon induced fission of 238 U were measured. A yield of prompt symmetric fission (116 ⩽ A ⩽ 122) of (0.26 ± 0.10)% with a peak to valley ratio of 25 in the mass spectrum was observed. A ratio of prompt to delayed fission yields of (8.8 ± 0.3)% is deduced.

The fission decay of 238 U has been investigated using inelastic scattering of 120 MeV ga-particles to excite the 238 U nucleus. Angular correlations of the fission fragments have been measured for excitation energies between 5.7 and 15.7 MeV in coincidence with inelastically scattered α-particles between 0 and 3°. The difference in yield for fission in coincidence with inelastically scattered α-particles between 0–1.35° and 1.35–3° was used to deduce the fission decay of the giant monopole resonance. It was found that in the fission decay channel (22 ± 5)% of the E0 EWSR strength is located between 8 and 15 MeV excitation energy. The distribution of the deduced monopole strength is in agreement with recent theoretical calculations predicting splitting of the giant monopole resonance in deformed nuclei.

Spectra of 172 MeV α scattering coincident with fission decay indicate in addition to fission threshold effects pronounced structures at X x ∼ 14 and 21 MeV which are interpretedas decay of the giant monopole resonance and of new giant resonances ( L = 1 and 3) observed in singles (α, α′) experiments.

The mean life times of negative muons bound to actinide nuclei have been measured by detecting the time difference between a stopped muon and the arrival of fragments from delayed fission after muon capture. The deduced capture rates c are 1.392(4)·107/s for237Np, 1.290(7)·107/s for242Pu and 1.240(7)·107/s for244Pu. The results are compared with published data for the fission and the neutron decay channels and for the electron decay of the bound muon. Including a former measurement of c for239Pu, an isotopic dependence of the muon capture rates in the Pu isotopes is clearly observed.

Mass yield and total kinetic energy release (TKE) distributions of fragments from prompt and delayed muon induced fission, separately, have been measured for the isotopes235U,238U,237Np and242Pu. The distributions from prompt muon induced fission are compared with the corresponding distributions from hadronic reactions and from spontaneous fission (s.f.). The distributions from the delayed muonic fission processes are compared to the distributions for neutron and proton-induced fission. No mass distributions measured in the prompt muonic fission process show any signature, which can be attributed to the presence of the muon. Differences observed between the TKE distributions of prompt muon induced and hadron induced fission can be explained by the screening effect of the negative charge of the muon bound in the orbit of one of the fission fragments. The observed yield of symmetric muon induced fission was found to be defined merely by the value of the excitation energy.

Fission fragments in coincidence with protons inelastically scattered from the region of the fission barrier in 232Th have been measured for several angles. With the high resolution of ≅ 17 keV at Ep = 30 MeV a detailed excitation function of fission probability was obtained. The data were analyzed in the two-humped barrier model, giving barrier parameters for the outer barrier EB = 6.75 MeV, ℏωB =1,20 MeV which are remarkably higher than those known from the literature. The possibility of a triple-humped fission barrier is discussed.