J. Drexler

Distribution of orbital magnetic dipole strength in 156Gd

Abstract Results from high resolution inelastic electron scattering and nuclear resonance fluorescence experiments on 156 Gd 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 J π = 1 + states around 3 MeV excitation energy.

Arrangements of parallel plate avalanche detectors for electrofission and photofission experiments

Abstract 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.

Photofission with linearly polarized photons

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.

Absolute cross sections for electron- and positron-induced fission of 238U and tests of DWGA virtual-photon spectra

Abstract Absolute cross sections for electron- and positron-induced fission of 238 U 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 preparation of large area uranium target layers for a multiple parallel plate fission detector

Abstract The preparation of large area uranium layers (diameter 65 mm) with a thickness of 2–3 mg/cm 2 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.

The “isomeric shelf” in the deep subbarrier photofission of 238U

Abstract 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.

A new method to test virtual photon spectra

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).

Fission barriers of preactinide nuclei from electrofission experiments

Abstract Absolute electrofission cross sections for 182,184,186 W, nat Pt, and 209 Bi 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.