R.D. Heil

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.

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.

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.

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.

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.

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 “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.