K. Huber

Critical consideration of the statistical model analysis of photonuclear isomeric cross-section ratios

Abstract We have measured isomeric cross-section ratios for the photonuclear reactions 181 Ta(γ, 3n) 178m,g Ta, 142 Nd(γ, n) 141m,g Nd, 110 Pd(γ, n) 109m 1 ,m 2 ,g Pd, 108 Pd(γ, n) 107m 1 ,m 2 Pd, 100 Mo(γ, n) 99m,g Mo, 92 Mo(γ, n) 91m,g Mo and 89 Y(γ, n) 88m 1 ,m 2 Y. The experimental results were compared with statistical model calculations according to the method of Huizenga and Vandenbosch. The applicability and limits of this method, especially the simple decision model for the population of the isomeric state and the ground state of the residual nucleus, were tested and discussed. Additionally, it is shown that for photonuclear reactions on magic nuclei, contributions of a direct reaction mechanism are not negligible.

Population of the 236U shape isomer in a photonuclear reaction

Abstract A fission isomer with a half-life of 115 ± 5 ns and a yield ratio Y iso / Y prompt = (2.02 ± 0.16) × 10 −5 was observed in bremsstrahlung-induced fission of natural uranium. The isomer is ascribed to 236 U populated via a 238 U(γ, 2n) reaction. The integrated cross section for isomeric fission is determined to be σ int = 32 ± 6 μb MeV. Comparing this value with a calculated total isomer production cross section, a branching ratio of the isomer decay of Γ γ II / Γ f II ≈ 6 can be deduced.

Applicability of a simple parallel plate avalanche detector to photofission experiments

A simple parallel plate avalanche detector (PPAD) has been tested for its applicability to photofission experiments at intense bremsstrahlung beams. Performance tests with a 252Cf spontaneous fission source showed a good time resolution (≈ 500 ps) and discrimination between α-particles and fission fragments. An investigation of the 236U fission isomer via the 238U(γ, 2n) reaction at the bremsstrahlung beam of the 65 MeV Giessen linac demonstrates the advantages of the PPAD. The results [T12 = 118 ± 7 ns] excellently agree with previous data obtained with semiconductor detectord. The time of data collection can be reduced by one order of magnitude.

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.

Symmetric and asymmetric yields in the photofission of232Th,235U,238U and239Pu

Measurements are presented for the mass and energy distributions of fragments from the photofission of232Th,235U,238U and239Pu. The experiments were performed for bremsstrahlung endpoint energies between 15 and 55 MeV, using silicon surface barrier detectors. The results are discussed with respect to the competition between the symmetric and asymmetric fission modes.

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.

Search for a γ-decay of the 236U shape isomer☆

A pulsed beam experiment has been carried out to search for a γ-decay of the 236U shape isomer. In the 238U(γ, 2nγ) reaction no shape isomeric γ-transition has been detected. From the experiments an upper limit (Γγ/Γf)II, max < 1.5 for the branching ratio for a single high energy γ-transition (Eγ = 2−2.5 MeV) has been derived. It is concluded that an important part of the shape isomeric γ-decay strength of 236U has to be attributed to low energy γ-transitions with Eγ ≦ 1 MeV.

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.

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.