W. Wilke

Absolute photofission cross sections for 232Th and 235,238U measured with monochromatic tagged photons (20 Mev < Eγ < 110 Mev)

Abstract The absolute photofission cross sections for 232 Th and 235, 238 U has been studied from 20 to 110 MeV using the photon tagging technique at the Saclay linear electron accelerator (ALS). The fission fragments were directly detected using a parallel plate avalanche detector (PPAD). The basic experimental results are a set of data giving σ ( E γ , fission). These results are compared with the few other measurements performed with monochromatic photons. From a comparison of the present fission data with the total absorption cross section, the total fission probability was deduced. The behaviour of this probability was discussed in terms of a cascade model for fission-evaporation competition. Quite good agreement is found with a calculation based on the hybrid model of pre-equilibrium reactions.

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.

Systematics of photonuclear yields and cross sections for plutonium and uranium fission isomers

Abstract Half-lives and yields in photonuclear reactions have been measured for shape isomers in U and Pu isotopes by pulsed-beam techniques bombarding 235,238 U and 239,240,242 Pu targets with brems-strahlung. Isomeric fission cross sections have been deduced from the measured isomeric to prompt yield ratios within an evaporation model using absolute prompt fission data. The results are compared with data from particle-induced reactions.

A 4π double obelisk of position sensitive PPACs for new vistas in photofission experiments

An arrangement of two-dimensional position sensitive PPACs (parallel plate avalanche counters) covering a large solid angle (> 50% of 4π) for use in a new type of photofission experiments is described. For the first time simultaneous measurements of both the photofission fragment angular and mass distributions are enabled by this device. The mass information is obtained by the double time of flight technique (with modest accuracy) whereas an excellent angular resolution of < 1° is achieved. The detector performances are outlined. First applications in high intensity bremsstrahlung and tagged photon experiments are 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.

Angular distributions of photofission fragments of particular mass

Abstract Fragment angular and mass distributions have been measured simultaneously for photon energies near the fission barrier of 236 U. Different angular distributions have been observed for particular fragment mass regions. This establishes a clear relationship between the quantum numbers of the transition states and the fragment mass division. The results are discussed within the transition channel concept of the double humped barrier for different fission paths and the more recent model of multi-ecit channels.

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.

Yield ratio for the two 241Pu fission isomers in the 242(γ, n) reaction☆

Abstract The half-lives and yield ratios for the two 241 Pu fission isomers have been measured in the 242 Pu(γ, n) reaction. The observed half-life for the long-lived isomer (T 1 2 = 20.5 ± 2.2 μs) is in agreement with previous data, and the existence of a short-lived 34 ± 7 ns isomer in 241 Pu could be confirmed. The measured yield ratios are Y iso / Y pr = (9.2 ± 0.8) × 10 −5 and Y iso / Y pr = (3.7 ± 0.7) × 10 −5 , respectively. From a statistical model analysis of the isomeric fission yield ratio, Y long iso / Y short iso = 2.5 ± 0.6, a spin assignment for the two isomers is attempted. Possible spin combinations are compared with single-particle shell-model calculations and with available spectroscopic data for the other even-odd Pu isotopes.