K. Wienhard

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.

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.

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.

Detection of prompt deexcitation γ-rays following bremsstrahlung-induced reactions with the aid of large volume Ge(Li) detectors

Abstract An experimental arrangement at the bremsstrahlung beam of a 65 MeV electron linac has been optimized with regard to the detection of prompt deexcitation γ-rays from small target quantities by Ge(Li) detectors. The 100 cm 3 Ge(Li) diodes have been calibrated by means of radioactive sources and radiative proton capture reactions. The influence of counting rate effects and background radiation during the bremsstrahlung burst are discussed. Deexcitation photon spectra from 28 Si(γ,γ′), 25 Mg(γ,χγ′) and 31 P(γ,χγ′) reactions are presented.

Phase shift analysis of elastic proton scattering by12C

Angular distributions of the polarization and cross section in12C (p,p)12C were measured at 24 energies between 9.95 and 10.90 MeV. A phase shift analysis of the data was carried out. The results show four resonances in13N atEx=11.40, 11.64, 11.67 and 11.82 MeV with spins 5/2+, 5/2−, 3/2− and 3/2+.

Nuclear resonance flourescence in 22Ne

Abstract States at 5.328, 6.851, 8.553, 9.165 and 10.203 MeV in 22 Ne have been observed in a nuclear resonance fluorescence experiment using bremsstrahlung and high resolution Ge(Li) diodes. Photon scattering cross sections, γ-ray decay widths to the ground and first excited states, and lifetimes are derived. The results are compared with inelastic electron scattering data and shell model calculations.

Charged particles from the giant dipole resonance of 26Mg

Abstract The 26 Mg(γ, p) and 26 Mg(γ, α) 90° differential cross sections and the 26 Mg(γ, p) total cross section have been deduced from particle spectra in the excitation energy region from 16.0 to 23.0 MeV. The (γ, p) cross section is compared with neutron data and with results from a 26 Mg( γ , xγ ′) experiment to clarify the role of isospin effects in the formation of the giant dipole resonance of 26 Mg.