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The decay properties of the Pygmy Dipole Resonance (PDR) have been investigated in the semi-magic N=82 nucleus 140Ce using a novel combination of nuclear resonance fluorescence and γ–γ coincidence techniques. Branching ratios for transitions to low-lying excited states are determined in a direct and model-independent way both for individual excited states and for excitation energy intervals. Comparison of the experimental results to microscopic calculations in the quasi-particle phonon model exhibits an excellent agreement, supporting the observation that the Pygmy Dipole Resonance couples to the ground state as well as to low-lying excited states. A 10% mixing of the PDR and the [21+ x PDR] is extracted.
The technique of self absorption has been applied for the first time to study the decay pattern of low-lying dipole states of 140Ce. In particular, ground-state transition widths 0 and branching ratios 0/ to the ground state have been investigated in the energy domain of the pygmy dipole resonance. Relative self-absorption measurements allow for a model-independent determination of 0. Without the need to perform a full spectroscopy of all decay channels, also the branching ratio to the ground state can be determined. The experiment on 140Ce was conducted at the bremsstrahlung facility of the superconducting Darmstadt electron linear accelerator S-DALINAC. In total, the self-absorption and, thus, 0 were determined for 104 excited states of 140Ce. The obtained results are presented and discussed with respect to simulations of γ cascades using the DICEBOX code.
We present an extensive experimental study of the recently predicted pygmy quadrupole resonance (PQR) in Sn isotopes, where complementary probes were used. In this study, (α,α' γ ) and (γ , γ') experiments were performed on 124Sn. In both reactions, Jπ = 2+ states below an excitation energy of 5 MeV were populated. The E2 strength integrated over the full transition densities could be extracted from the (γ , γ') experiment, while the (α,α'γ ) experiment at the chosen kinematics strongly favors the excitation of surface modes because of the strong α-particle absorption in the nuclear interior. The excitation of such modes is in accordance with the quadrupole-type oscillation of the neutron skin predicted by a microscopic approach based on self-consistent density functional theory and the quasiparticle-phonon model (QPM). The newly determined γ -decay branching ratios hint at a non-statistical character of the E2 strength, as it has also been recently pointed out for the case of the pygmy dipole resonance (PDR). This allows us to distinguish between PQR-type and multiphonon excitations and, consequently, supports the recent first experimental indications of a PQR in 124Sn.
The ( J, T ) = (1, 1) parity doublet in 20Ne at 11.26 MeV is a good candidate to study parity violation in nuclei. However, its energy splitting is known with insufficient accuracy for quantitative estimates of parity violating effects. To improve on this unsatisfactory situation, nuclear resonance fluorescence experiments using linearly and circularly polarized γ -ray beams were used to determine the energy difference of the parity doublet E = E(1−) − E(1+) = −3.2(±0.7)stat( +0.6 −1.2)sys keV and the ratio of their integrated cross sections I (+) s,0 /I (−) s,0 = 29(±3)stat( +14 −7 )sys. Shell-model calculations predict a parityviolating matrix element having a value in the range 0.46–0.83 eV for the parity doublet. The small energy difference of the parity doublet makes 20Ne an excellent candidate to study parity violation in nuclear excitations.
The transitional nucleus 154Gd was investigated using a combination of a photon scattering experiment and a γγ-coincidence study following the β decay of 154Tb. A novel decay channel from the scissors mode to the band head of the β-band was observed. Its transition strength B(M1; 1sc+ → 0β+) was determined. An IBM-2 calculation reveals a correlation of this decay channel and the shape phase transition between spherical and deformed nuclei.
The method of relative self absorption is based on the technique of nuclear resonance fluorescence measurements. It allows for a model-independent determination of ground-state transition widths, natural level widths, and, consequently, of branching ratios to the ground state for individual excitations. Relative self–absorption experiments have been performed on the nuclei 6Li and 140Ce. In order to investigate the total level width for the 0+1, T = 1 level at 3563 keV in 6Li, a high-precision self-absorption measurement has been performed. In the case of 140Ce, self absorption has been applied for the first time to study decay widths of dipole-excited states in the energy regime of the pygmy dipole resonance.
We analysed our experimental recent findings of the dipole response of the odd-mass stable nucleus 205Tl within the quasi-particle phonon model. Using the phonon basis constructed for the neighbouring 204Hg and wave function configurations for 205Tl consisting of a mixture of quasiparticle ⊗ N-phonon configurations (N=0,1,2), only one group of fragmented dipole excited states has been reproduced at 5.5 MeV in comparison to the experimental distribution which shows a second group at about 5 MeV. The computed dipole transition strengths are mainly of E1 character which could be associated to the pygmy dipole resonance.
The electric dipole strength distribution in 130Te has been investigated using the method of Nuclear Resonance Fluorescence. The experiments were performed at the Darmstadt High Intensity Photon Setup using bremsstrahlung as photon source and at the High Intensity -Ray Source, where quasi-monochromatic and polarized photon beams are provided. Average decay properties of 130Te below the neutron separation energy are determined. Comparing the experimental data to the predictions of the statistical model indicate, that nuclear structure effects play an important role even at sufficiently high excitation energies. Preliminary results will be presented.
The decay behavior of low-lying dipole states in 140Ce was investigated exploiting the γ3-setup at the HIγS facility using quasi-monochromatic photon beams. Branching ratios of individual excited states as well as average branching ratios to low-lying states have been extracted using γ – γ coincidence measurements. The comparison of the average branching ratios to QPM calculations shows a remarkable agreement between experiment and theory in the energy range from 5.0 to 8.5 MeV.
A series of photon scattering experiments has been performed on the double-beta decay partners 76Ge and 76Se, in order to investigate their dipole response up to the neutron separation threshold. Gamma-ray beams from bremsstrahlung at the S-DALINAC and from Compton-backscattering at HIGS have been used to measure absolute cross sections and parities of dipole excited states, respectively. The HIGS data allows for indirect measurement of averaged branching ratios, which leads to significant corrections in the observed excitation cross sections. Results are compared to statistical calculations, to test photon strength functions and the Axel-Brink hypothesis.