N. Huxel

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.

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.

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.

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 .

Resonant photon scattering on the semi-magic nucleus 89Y up to 7 MeV

Abstract The semi-magic nucleus 89Y has been investigated in a (γ,γ′) experiment at an endpoint energy of the bremsstrahlung of E0 = 7 MeV. The scattered photons have been detected with a EUROBALL Cluster detector. The observed excitations are discussed in the framework of the shell model and a coupling of the unpaired proton to multi-phonon structures in the doubly even neighbours. Transitions above 4.7 MeV are considered to have E1 character. Around 6.3 MeV an unusually large concentration of E1 strength is found. Its origin is likely to correspond to similar structures in the 88Sr and 90Zr isotones which can be interpreted to result from the constructive interference of strong two-phonon amplitudes with weak admixtures from the low-energy tail of the giant dipole resonance.

Absolute measurement of spatial and spectral characteristics of bremsstrahlung using the photoexcitation of nuclear isomers

A method of obtaining absolute, direct measurements of the spatial and spectral characteristics of bremsstrahlung is discussed. This technique, called x‐ray activation of nuclei (XAN) is based on the use of well‐known photonuclear reactions which populate long‐lived nuclear isomers. These populations sample incident photon continua at discrete excitation energies and effectively store this information for convenient retrieval following the irradiation of gram‐sized targets. Recently a series of experiments has been conducted which has significantly expanded the available data for the photoexcitation of a wide range of isomers at higher energies. Thus it has become feasible to extend the use of XAN to energies approaching 4 MeV. The utility of this technique is demonstrated by the characterization of bremsstrahlung from the newly installed research linac at the University of Texas at Dallas.