U. Kneissl

Systematic study of low-lying dipole excitations in 156,158,160Gd by photon scattering

Abstract Nuclear resonance fluorescence experiments have been performed on the deformed nuclei 156,158,160Gd using 4.1 MeV bremsstrahlung and high-resolution Ge-γ-detectors. Precise excitation energies, reduced transition probabilities and decay branching ratios of numerous new spin-1 states have been extracted from the energies and angular distributions of the scattered photons. The results are compared with electron-scattering data and discussed with respect to the orbital M1 mode and predicted collective E1 excitations.

Photoexcitation of dipole modes in 160,162,164Dy

Abstract High resolution photon scattering experiments on 160,162,164 Dy have revealed considerable fragmentation of dipole strength into two groups around 2.5 MeV and around 3 MeV. In assuming that the new J =1 states near 3 MeV correspond to magnetic excitations, the B (M1)↑ sums are effectively twice the sd-IBA-2 SU (3) value for g -factors obtained from magnetic moments. The new data also show a sudden drop in B (M1)↑ strength from Dy to Er.

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.

Measurement of the total cross section for 235U and 238U photofission in the δ-resonance region

Abstract The total cross section for the photoemission of fission fragments σ(γ, F) has been measured for 235U and 238U using tagged bremsstrahlung photons in the energy range 120–460 MeV. The fission fragments were detected in parallel plate avalanche detectors. The results are compared with other information on the photonuclear absorption cross section.

Observation of orbital magnetic dipole strength in the actinide nuclei 232Th and 238U

Abstract Precise experimental information has been obtained on the distribution of orbital M1 strength using high resolution nuclear resonance fluorescence and inelastic electron scattering. We report on the first observation of those J π = 1 + states in actinide nuclei that are excited by the M1 scissors mode. The M1 strength induced through the convection current part of the M1 operator is closely centered around 2 MeV in both nuclei. The observed spreading of strength is again of the same magnitude that has been observed already in rare-earth nuclei.

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.

Total photonuclear absorption cross section of Pb measured with quasi-monochromatic photons between 25 and 106 MeV

Abstract The σ ( γ , xn ) cross sections of Pb have been measured above the giant dipole resonance region up to 106 MeV with a quasi-monochromatic photon beam obtained by the annihilation in flight of monochromatic positrons. The total cross section decreases linearly with energy from 20 mb at 35 MeV to 12 mb at 106 MeV. The integrated cross section up to 140 MeV amounts to (1.80 ± 0.2) classical dipole sums.

A survey ofΔK=0 dipole transitions from low lyingJ=1 states in rare earth nuclei

High resolution photon scattering experiments on various Nd-, Gd-, Dy-, Er- and Yb-isotopes yielded new information about the energy and absolute ΔK=0 groundstate transition strengths ofJ=1 levels with excitation energies between 1.5 and 4 MeV. The lowest of theseJπ=1− states are suggested to be the bandheads of aKπ=0− octupole vibrational band, whereas the origin of the higher lying states is unknown.

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.

Observation of low-lying collective dipole transitions in the rare-earth nuclei 172,174,176Yb

Abstract Precise experimental data have been obtained on the distribution of low lying collective spin-one states in the rare-earth nuclei 172,174,176 Yb using high-resolution nuclear resonance fluores cence. We report on excitation energies, spins and decay-widths of these states. The data show that the dipole strength below 4 MeV is strongly fragmented. We discuss the influence of the fragmentation on the so-called M1 “scissors mode”, and compare upper limits for M1 strengths to those measured for the neighbouring nuclei.