U.E.P. Berg

Photoexcitation of low-lying collective states in 156,158,160Gd

Abstract Strong low-energy collective transitions in the deformed nuclei 156,158,160 Gd have been observed with nuclear resonance fluorescence techniques utilizing 3.5 MeV bremsstrahlung and a high-resolution Ge(Li) detector. It is most probable that a new collective M1 mode, predicted by several theoretical models of deformed nuclei and discovered recently in 156,158 Gd(e, e′) experiments, has been observed. The Gd(γ, γ′) measurements reveal a strong coupling of the levels to the first rotational 2 + state as expected in the IBA-2 model. If all the transitions reported are due to M1 excitations, a fragmentation of strength is present in 158,160 Gd and the sums of B (M1)↑ values amount to 1.5 ± 0.3, 2.3 ± 0.5, and 2.3 ± 0.6 μ 0 2 for 156,158,160 Gd, respectively.

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.

Bound state M1 transitions in sd-shell nuclei☆

Abstract Nuclear resonance fluorescence experiments with unpolarized and linearly polarized bremsstrahlung and high-resolution Ge(Li) detectors have yielded model-independent values of magnetic dipole excitation strengths to highly excited states in 20,22 Ne, 24,26 Mg, 28,30 Si, and 32,34 S. The measured B ( M 1) values are quenched relative to the j - j limit of the shell model, but are in general agreement with predictions of recent shell-model calculations which treat intra-sd-shell configuration mixing completely.

Ground state dipole transitions in 58Ni

Abstract Nuclear resonance fluorescence measurements on 58 Ni with bremsstrahlung and Ge(Li) detectors were performed to search for bound state dipole excitations. Ten levels with ground state decay widths larger than 0.3 eV have been observed in the energy region between 6 and 10 MeV for which precise excitation energies, spins and lifetimes are reported. The measured transition probabilities are compared with theoretical estimates of E1 and M1 strength in 58 Ni. Since the 58 Ni (γ, γ) cross sections were determined relative to strong transitions in 208 Pb, the results of a separate 208 Pb(γ, γ) measurement are also presented.

A comparative study of the electric giant dipole resonance of 24−26Mg☆

Abstract The electric giant dipole resonance of 24−26 Mg has been explored up to 30 MeV excitation energy with bremsstrahlung. ΔE , E spectra of charged photo-particles and spectra of prompt deexcitation γ-rays from excited residual nuclear states were obtained at various bremsstrahlung endpoint energies. The 25 Mg(γ, p 0 ), (γ, d), 24,25 Mg(γ, α) differential cross sections as well as 24−26 Mg ( γ , xγ ′) integrated cross sections are presented. The results are discussed in terms of one-particle, one-hole excitations and isospin composition of giant resonance states. A comparison with calculations for 24 Mg gives poor agreement. Excitations from deeper shells were found in the giant dipole resonance of 24 Mg, but do not seem to be concentrated at higher energies. In 25 Mg, only weak excitations of this kind were found, and they are completely absent in 26 Mg.

Parities of dipole ground-state transitions in 52Cr☆

Abstract Nuclear resonance fluorescence experiments with polarized bremsstrahlung were performed to search for magnetic dipole strength. Previous interpretations that states at 7730, 7896 and 9787 keV in 52 Cr are due to M1 excitations can not be confirmed. These transitions are unexpectedly strong E1 excitations, exceeding 1 m Weisskopf unit in strength. The remaining identified M1 strength of ΣB (M1)↑ = 2.5 ± 0.7 μ k 2 is in agreement with the observation of missing M1 strength in heavy nuclei.

The photodisintegration of6Li and7Li

The photodisintegration of the stable Lithium isotopes has been investigated by spectroscopy of the emitted charged particles using the Giessen bremsstrahlung facility. Protons, deuterons, tritons,3He and alpha particles were detected and identified up to 20MeV particle energy. Angular distributions were measured using γ-ray energies up toEγ =50 MeV. They are compared with theoretical predictions and with other experiments. A remarkable low (γ, t)-cross section was found for both isotopes in disagreement to previous measurements using virtual photons. Coincidence measurements between the emitted particles were performed in order to study the manybody-breakups of7Li.