C. Wesselborg

Low-energy photon scattering off 142,146,148,150Nd: An investigation in the mass region of a nuclear shape transition☆

Abstract Nuclear resonance fluorescence experiments have been performed on the nuclei 142,146,148,150 Nd, which are located in the mass region of a nuclear shape transition from a spherical ( 142 Nd) to a well-deformed shape ( 150 Nd). Bremsstrahlung of 4.1 MeV endpoint energy served as the photon source. The scattered photons were detected by high-resolution Ge-spectrometers. Precise excitation energies, decay branching ratios, and ground-state decay widths of numerous new spin-1 states have been extracted.

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.

A sectored Ge-Compton polarimeter for parity assignments in photon scattering experiments

Abstract Parities of nuclear levels excited in photon scattering experiments can be determined by measurements of the linear polarization of the scattered photons. The needed theoretical formalism is summarized. A fourfold sectored single crystal Ge-Compton polarimeter, its operation and performances are described. The compact polarimeter has an outstanding coincidence efficiency ecoinc of about 25% and a good energy resolution of 2.2 keV at 1.333 MeV. The polarization sensitivity of the device as determined in (γ, γ ) polarization correlation measurements and several ( p , p ′ γ ) reaction studies amounts to about 20% at photon energies of 0.5 MeV and is about 10% at 4 MeV photon energies. The polarimeter has been successfully operated in systematic photon scattering experiments at the bremsstrahlung beam at the Stuttgart Dynamitron facility to investigate enhanced magnetic and electric dipole excitations in heavy deformed nuclei. Typical results are shown and discussed.

Low-lying E1 and M1 strengths in the deformed nucleus 160Gd☆

Abstract Measurements of the linear polarization of resonantly scattered photons have been used for model-independent parity determinations in nuclear resonance fluorescence experiments. The experiments have been performed at the bremsstrahlung facility of the Stuttgart Dynamitron accelerator. The use of two different Compton polarimeters, a five Ge-detector arrangement and a sectored single-crystal Ge-polarimeter enabled the parity assignments. Positive parities were established for a cluster of states near 3.3 MeV (M1 transitions) to be ascribed to the so-called scissors mode. In addition an enhanced electric dipole excitation near 2.5 MeV (2471 keV, B (E1) ↑ = 3.0 ± 0.4 × 10 −3 e 2 fm 2 ) has been observed as in the neighbouring isotopes 162 Dy and 150 Nd. These 1 − states systematically exhibit an uncommon decay branching hinting at K -mixing.

Measurements and simulations of low energy, thick target bremsstrahlung spectra

Abstract Nuclear resonance fluorescence (NRF) experiments on the isotopes 13 C, 23 Na, 27 Al, 31 P, 35 Cl, and 48 Ti with transitions of known strengths have been performed to determine the relative spectral shape of low energy, thick target bremsstrahlung spectra. The experimental results (endpoint energy E 0 = 4.1 MeV) are compared with Monte Carlo simulations. It is concluded that a linear function represents a rather precise approximation of the shape of this bremsstrahlung spectrum near the endpoint energy. Therefore, a linear interpolation of the photon flux between well known data points enables a fast and sufficiently precise analysis of NRF experiments using photon sources of this kind.

Parity assignments in nuclear resonance fluorescence experiments using compton polarimeters

Abstract Measurements of the linear polarization of resonantly scattered photons have been used for model-independent parity determinations in nuclear resonance fluorescence experiments. Two different Compton polarimeters, a five Ge detector arrangement and a sectored Ge(Li) detector, have been installed at the bremsstrahlung facility of the Stuttgart Dynamitron Accelerator. The following spin and parity assignments have been achieved to excited states (excitation energy E x ): 142 Nd: E x = 3.425 MeV, J π = 1 − ; 150 Nd: E x = 2.894 MeV, J π = 1 + ; E x = 2.994 MeV, J π = 1 + ; E x = 3.058 MeV, J π = 1 + ; 232 Th: E x = 2.043 MeV, J π = 1 + .

Electric and magnetic dipole excitations to bound states in 70,72,74,76Ge

Abstract The nuclei 70,72,74,76 Ge were studied by nuclear resonance fluorescence (NRF) experiments. Partially linearly polarized and unpolarized bremsstrahlung of 9 to 14 MeV endpoint energy was used at the Giessen 65 MeV electron linear accelerator; unpolarized bremsstrahlung of 4 MeV endpoint energy was used at the Stuttgart Dynamitron. The scattered photons were detected by Ge γ-ray spectrometers with high energy resolution. Multipolarities were determined by measuring the angular correlations between the beam and the scattered γ rays at different scattering angles. Precise excitation energies and ground-state decay widths of numerous (> 120) previously unknown spin-1 states were extracted. For 65 ground-state transitions (20 M1 transitions, 45 E1 transitions) parities were assigned, in a model-independent way, by polarized bremsstrahlung. A detailed distribution of electric and magnetic dipole strength in the even Ge isotopes was established.

Low-lying dipole excitations in the isotopes 113,114Cd

Abstract High-resolution nuclear resonance flourescence (NRF) experiments have been performed on the nuclei 113,114 Cd using bremsstrahlung and Ge-γ-spectrometers. Precise excitation energies, decay widths and decay branching ratios for numerous new levels (dipole excitations) in the energy range 2–4 MeV have been extracted from the measured energy spectra and angular distributions of the scattered photons. The NRF results are compared with IBA calculations and spectroscopic data from particle-induced reactions.