P. von Neumann-Cosel

Fine structure of the E1 response in 140Ce below the particle threshold

Abstract The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (γ,γ′) experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.

Low-energy dipole-strength distributions in 148,150,152,154Sm

Abstract We present the results of nuclear-resonance fluorescence measurements at the S-DALINAC for a chain of evenvl isotopes 148–154 Sm, covering the excitation-energy region of 2–4 MeV. The orbital M1 strength distributions are compared with recent microscopic calculations. The summed E1 strengths are discussed in terms of macroscopic clustering and octupole deformation prescriptions of Iachello.

Complete level scheme of 124Te up to 3 MeV

Excited states of 124Te were measured with the transfer reactions 123Te(d,p)124Te, 125Te(d,t)124Te and 123Sb(3He,d)124Te and with the inelastic scattering reaction ZUTe(p,p)lUTe at the Tandem Accelerator in Munich. Levels were obtained up to 3.5 MeV with energy resolutions of 3-6 keV. The lUTe(3,,3/)124Te reaction was studied at the S-DALINAC accelerator in Darmstadt for bremsstrahlung endpoint energies Eo = 3.5 and 5 MeV. The 123Te(n,T)lUTe reaction with thermal neutrons was investigated in Riga and in Dubna including ~,-~, coincidences. In Dubna the special method of sum coincidences was applied, analyzing the Y-3 cascades to the ground state and the first 2 + level. Based on these experiments a level scheme was constructed with states up to 4.5 MeV which is assumed to be complete up to 3 MeV for angular momenta J< 4h. A quasiparticle-phonon model calculation with a configuration space of up to three-phonon coupling permits a good description for many of the low-energy states. Transition probabilities, E2/M1 mixing ratios and branching ratios between these levels are generally well described. The quantitative reproduction indicates a two-phonon quadrupole-octupole coupled 1 - structure of the states most strongly excited in the (~,, T) reaction. The 2- member of the 2 + ®2 + ®3 i- septuplet is identified in the unusual decay cascade following thermal neutron capture. A candidate for a 2 + state with mixed symmetry structure is proposed.

Quasi-elastic nucleon transfer and single charge exchange in 48Ti + 42Ca collisions

Abstract Single nucleon transfer and single charge exchange have been studied for quasi-elastic collisions of 48 Ti and 42 Ca at E lab = 240, 300 and 385 MeV. Specific features below 12 MeV excitation energy are displayed in all of the measured particle energy spectra for each channel. These have been accounted for in the case of nucleon transfer by a one-step direct transfer mechanism in which all of the available hole and bound particle states in the donor and acceptor nucleus, respectively, have been included. This core excitation model was extended to single charge exchange (SCX) two-step transfers in which a proton and a neutron are exchanged. The absolute magnitude and distribution of strength in the SCX channel was obtained. The contributions from direct charge exchange via the isovector parts of the NN interaction has also been investigated in a microscopic model and is found to contribute of the order of 1–5% to the charge exchange cross section. The distribution of Fermi and Gamow-Teller strength is discussed.

Two-phonon J = 1 states in even-mass Te isotopes with A = 122–130☆

Abstract Excited states of the nuclei 122,126,130 Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E γ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1 − state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1 + state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

Unusual neutron-capture gamma-ray cascade in 124Te: A fingerprint of octupole-coupled multiphonon states☆

Abstract The properties of the special levels in 124Te that are strongly populated by thermal neutron-capture γ-rays have been determined with 123Te(n,γ) and 124Te(γ,γ′) experiments. The unusual decay pattern can be consistently explained within the quasiparticle-phonon model assuming a three-phonon |21+ ⊗ 21+ ⊗ 31−; 2−〉 configuration of the upper intermediate level at 3.101 MeV which exclusively decays to a close-lying two-phonon |21+ ⊗ 31f-; 1f-f> state at 2.747 MeV. The (γ,γf) reaction strongly excites the latter and also indicates the 1f- member of the three-phonon 21+ ⊗ 21+ ⊗ 31f- septuplet at 3.091 MeV.

Absolute calibration of low energy, thick target bremsstrahlung☆

Abstract The applicability of the Monte Carlo program GEANT3 for the calculation of low energy, thick target bremssstrahlung is investigated. The results of GEANT3, version 15, show very good correspondence to those of the well established EGS4 code. In contrast it is shown that GEANT3, version 11, leads to erroneous angular distributions, total intensities and even spectral shapes for angles far from the critical angle. The use of the 115In(γ, γ′)115Inm reaction for an absolute calibration of bremsstrahlung spectra is also demonstrated. This method relies on the reconstruction of the isomer yield excitation function from a set of 115In levels with well characterized properties. It can be utilized for continuous photon spectra up to about 3 MeV.

Structure of intermediate states in the photoexcitation of the 89Y isomer

Abstract Resonant photon scattering off 89Y was investigated in a measurement of the 89Ym excitation function for bremsstrahlung endpoint energies E0 = 2−5 MeV and in a nuclear-resonance-fluorescence experiment with E0 = 5 MeV. The results are compared to a quasiparticle-phonon model calculation. Besides a well-known single-particle M1 transition at low energies, the photoexcitation spectrum is governed by transitions to states built by coupling of the dominant p 1 2 hole ground-state configuration to collective quadrupole phonons in the neighbouring 90Zr. The detailed decay cascade to the isomer reproduces the experimental finding of only two intermediate states with about equal strength and explains the suppression of other possible transitions due to the nature of the particular E1 matrix element. The theoretical isomer branching ratios are small compared to the experiment, but depend critically on details of the model.

Resonant photoexcitation of isomers. 115Inm as a test case

Abstract Large integrated cross sections for the photoexcitation of 115 In m have been detected around E x =3MeV. Complementary nuclear resonance fluorescence studies prove that the g.s. transition strength is largely confined to the same energy region and the intermediate excited states are identified. Unified-model calculations indicate magnetic dipole lg 9 2 → lg 7 2 spin-flip transitions as the major excitation mechanism.

Search for low-lying magnetic dipole strength in the heavy odd-mass nucleus 165Ho

Abstract The excitation energy region up to 4 MeV in 165 Ho was studied by high-resolution nuclear resonance fluorescence (NRF) and inelastic electron scattering. Magnetic dipole transition strength is observed into a few levels below 2.4 MeV, which can be explained as intrinsic single-particle transitions. The search for the “scissors mode” in the 3 MeV region revealed no transition with B (M1)↑⩾0.1 μ N 2 .