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The excitation functions of the reactions 238 Pu(α, 2n) and 241 Am(p, 2n), both leading to 240 Cm under fission competition, are measured between threshold and 8 MeV excess energy. The two identical excitation functions show a bump-like structure 5 MeV above threshold, which is impossible to explain by the use of statistical reaction theories with standard level density expressions. A compound reaction calculation based on shell dependent level densities for the neutron and fission channels, respectively, with the inclusion of pairing leads to a detailed understanding of the observed structure.

An experimental arrangement is described which enables the detection of fission isomers with half-lives in the ps to ns region. The arrangement is based on the recoil-distance method, and with special targets which are plane within ±1 μm the shortest measurable half-life becomes 5 ps if the fission isomers are produced in α-particle induced reactions. Calculations of the decay curves and the detection efficiency for various half-lives are presented. In order to demonstrate the applicability of the method, two examples are given of fission isomer decays with half-lives close to the upper and lower limits of the accessible half-life region.

Levels in 39K have been populated using the (p, p'γ) reaction and the decays of levels below 7.3 MeV have been established by a particle-gamma coincidence experiment. The mean lifetimes of 24 levels have been determined in a singles experiment using the Doppler shift attenuation method. Limits on the lifetimes of a further 10 levels in 39K have been obtained. Discussion of the properties of levels in 39K is given.

Delayed fission events due to isomeric fission have been produced using He3 and He4 beams from the MP Tandem Van de Graaff accelerator. The isomeric nuclei which recoil from the target and subsequently fission in flight were detected by a square cone array of polycarbonate foil detectors mounted along the beam direction. From the distribution of fission tracks along the detector foils, half-lives in the nanosecond region were deduced.

Targets of 233, 235, 236, 238 U were bombarded with douterons and protons in the energy range from 11 to 20 MeV. Excitation functions for the production of fission isomers were obtained and are interpreted in terms of direct reaction processes. The most probable reactions are (d, pχγ) and (d, pnχγ) leading to isomers assigned to 236 U and 238 U with half-lives of 70 nsec and 110 nsec respectively.

A semiempirical systematics of known fission isomer half-lives is proposed. For doubly odd and odd-mass nuclei there seems to be a pronounced correlation between the half-lives and the liquid-drop model fissility parameter with an additional modulation due to the variation of shell effects with neutron number. For even nuclei the observed isomeric half-lives are nearly constant irrespective of Z and A . A model based on a simplified double-humped fission barrier is discussed which may account for the general trend of fission isomeric half-lives of doubly odd and odd-mass nuclei. Parameters are given which reproduce the observed half-lives. Excitation energies and fission barrier heights are compared with the predictions of the model. Other explanations are considered for isomers in doubly even nuclei which do not follow the systematics.

A new extremely shortlived fission isomer presumably in245Bk has been detected following the bombardment of243Am with4He particles of 26 MeV. Proton bombardment of243Am gives some evidence for the excitation of two shortlived fission activities which are tentatively assigned to isomers in241,242Cm. When irradiating targets of237Np,241Am, and243Am with deuterons in the energy range from 12 to 20 MeV previously known isomers are observed. Excitation functions are given for the production of these fission isomers.

Strutinsky type calculations are extended to lighter nuclei. Secondary minima in the potential energy surface generally considered to account for the existence of fission isomers occur also in the lighter mass region. Results for 40 Ca are compared with experimental evidences.

Level-crossing technique applying parallel electric and magnetic fields has been used to investigate the influence of an electric fieldE z on the hyperfine structure of the 4p 2 P 3/2-state. The Stark shift of the level-crossing nearH z =155 Oe and the modification of the magnetic zero-field level-crossing signal due to the electric fieldE z was observed. Both experimental results can be explained with a Stark-constantβ=7.2(1.1)kc(kV/cm)−2.

Activity and γγ-coincidence spectra have been studied following 16O bombardment of 92Mo. Evidence for the population of a new β-decaying state in 106In with a half-life of 6.26±0.24 min is presented. A partial level scheme of 106Cd is deduced.