Results from high resolution inelastic electron scattering and nuclear resonance fluorescence experiments on 156Gd are combined to yield information on the distribution of orbital magnetic dipole strength. The magnetic dipole strength connected with the new M1 mode is closely centered in five Jπ = 1+ states around 3 MeV excitation energy.

A compact time-of-flight spectrometer for measurements of fission fragment distributions is described. The start detector consists of a channel plate arrangement with an electrostatic mirror. The time resolution of PPACs, serving as stop detectors, has been optimized.

Time distributions of fission fragments from subbarrier photofission of 238U have been measured using a pulsed bremsstrahlung beam (EBS = 3.9–4.3 MeV). The observed half-life of 238mU (T1/2 = 155±33 ns) is in good agreement with previous results obtained at higher photon energies. The measured yield ratio of isomeric to prompt fission (Yiso/Ypr ≳1) directly demonstrates for the first time the overwhelming contribution of the isomeric fission in the energy range of the so-called “isomeric shelf”. The results are compared with the most recent calculations.

The construction and performance of several arrangements of parallel plate avalanche detectors are described. The set ups can be used for the measurement of: fragment time of flight, photofission cross sections, out of plane angular distributions, inclusive and exclusive electrofission cross sections. First results are discussed, demonstrating the wide field of applicabilities of these detectors.

Absolute electrofission cross sections for 182,184,186W, natPt, and 209Bi have been measured using solid-state track detectors and parallel-plate avalanche counters for the detection of the fission fragments. The measured cross sections — together with others known from literature — have been analyzed within an extended statistical model to deduce fission barriers and compared with theoretical calculations. From the derived barrier height for platinum, it is concluded that the pairing strength G most likely does not change with deformation (G = const). All experimental fission barriers agree very well with theoretical predictions for G = const.

The comparison of fission fragment angular distributions in electron-and positron-induced fission is proposed as a new method to test virtual photon spectra. The sensitivity of this technique is shown by first experiments on232Th (bombarding energy: 9 and 16 MeV).

Strong low-energy collective transitions in the deformed nuclei 156,158,160Gd have been observed with nuclear resonance fluorescence techniques utilizing 3.5 MeV bremsstrahlung and a high-resolution Ge(Li) detector. It is most probable that a new collective M1 mode, predicted by several theoretical models of deformed nuclei and discovered recently in 156,158Gd(e, e′) experiments, has been observed. The Gd(γ, γ′) measurements reveal a strong coupling of the levels to the first rotational 2+ state as expected in the IBA-2 model. If all the transitions reported are due to M1 excitations, a fragmentation of strength is present in 158,160Gd and the sums of B(M1)↑ values amount to 1.5 ± 0.3, 2.3 ± 0.5, and 2.3 ± 0.6 μ02 for 156,158,160Gd, respectively.

Strong low-energy collective transitions in the deformed nuclei 156,158,160 Gd have been observed with nuclear resonance fluorescence techniques utilizing 3.5 MeV bremsstrahlung and a high-resolution Ge(Li) detector. It is most probable that a new collective M1 mode, predicted by several theoretical models of deformed nuclei and discovered recently in 156,158 Gd(e, e′) experiments, has been observed. The Gd(γ, γ′) measurements reveal a strong coupling of the levels to the first rotational 2 + state as expected in the IBA-2 model. If all the transitions reported are due to M1 excitations, a fragmentation of strength is present in 158,160 Gd and the sums of B (M1)↑ values amount to 1.5 ± 0.3, 2.3 ± 0.5, and 2.3 ± 0.6 μ 0 2 for 156,158,160 Gd, respectively.

Absolute photofission cross sections of /sup 235/U and /sup 238/U have been measured with quasimonoenergetic photons from e/sup +/ annihilation and direct fragment detection between 11.5 and 30 MeV. The results obtained in the energy range of the giant dipole resonance (up to 18 MeV) are compared with those from previous experiments.

The half-life of the 238U shape isomer and its yield ratio in a (γ, γ') reaction have been measured by pulsed beam techniques at a bremsstrahlung endpoint energy of 12 MeV. From the results (T12 = 146 ± 22 ns, Yiso/Ypr = (6.6 ± 1.0) × 10−6) the isomeric fission cross section has been deduced. Combining this information with the results of a previous 238U(γ, xnγ) study, an upper limit for the branching ratio Γγ/Γf|II < 13 for the decay of 238mU can be obtained. The decay properties of the 236U and 238U shape isomers are discussed.

Absolute cross sections for electron- and positron-induced fission of 238U have been measured in the energy range 10 – 35 MeV. The cross sections were compared with the corresponding photo-fission cross sections using DWBA virtual photon spectra. The reliability of the virtual photon formalism to extract E2 strength from inclusive electrodis-integration experiments is critically discussed.

Absolute cross sections for electron- and positron-induced fission of 238U have been measured in the energy range 10–35 MeV. The cross sections were compared with the corresponding photo-fission cross sections using DWBA virtual-photon spectra. The reliability of the virtual-photon formalism to extract E2 strength from inclusive electrodisintegration experiments is critically discussed.

The formalism for fragment angular distributions in photofission with unpolarized and linearly polarized photons is summarized. First experiments on232Th using linearly polarized bremsstrahlung are described. The results for the analyzing power show for the first time in a model independent way that the low energy photofission can be explained byelectric dipole excitation. The possibilities of this new method are discussed, in particular when using monoenergetic tagged photons, which are available soon at new high duty cycle electron accelerators.

The controversial results for the fission decay of the isoscalar giant quadrupole resonance in $^{238}\mathrm{U}$ have been investigated by electron- and positron-induced fission experiments (${E}_{e}=10\ensuremath{-}35$ MeV). The measured cross-section ratio $\frac{{\ensuremath{\sigma}}^{\ensuremath{-}}}{{\ensuremath{\sigma}}^{+}}$ and absolute cross sections were analyzed with use of available distorted-wave Born-approzimation virtual-photon spectra. Within this analysis no fission decay of the giant quadropole resonance could be detected, in contrast to a recent inclusive electrofission work.

The preparation of large area uranium layers (diameter 65 mm) with a thickness of 2–3 mg/cm2 by electrolysis on Al foils is described. The layers are used in a multiple parallel plate avalanche detector with 30 counter gaps for photofission experiments. The performances and applications of this detector set-up are discussed.