Search

Results of a 208 Pb (γ,γ′) experiment are presented aiming at an identification of the 2 + member of the long-sought two-octupole phonon multiplet. Four E2 excitations have been observed below 6.5 MeV excitation energy, two of them for the first time. However, in contrast to new results of calculations within the quasiparticle–phonon nuclear model (QPM), no obvious candidate for the two-octupole phonon vibration could be found in the present study. We discuss the J π =2 + states detected in this as well as previous experiments with respect to their possible two-octupole phonon structure.

The low-energy dipole strength distributions in 165Ho and 169Tm have been studied in nuclear resonance fluorescence experiments at the S-DALINAC utilizing a Euroball Cluster detector. In the energy interval between 2.5 and 4.0 MeV, where the scissors mode is expected, 35 ground state transitions could be observed in 165Ho and 53 in 169Tm. Assuming M1 character for all these transitions corresponds to ΣB(M1) ↑= 1.54(23), μN2 and ΣB(M1) ↑= 2.15(63), μN2 for 165Ho and 169Tm, respectively. A statistical analysis is applied to the measured spectra which is capable of reconstructing the complete strength, including contributions of transitions below the observation limit, with the ratio between M1 and E1 excitations and their total strengths taken as an average over the even-mass neighbours. This results in summed M1 transition strengths of ΣB(M1) ↑= 3.54−0.95+0.75, μN2 (165Ho) and ΣB(M1) ↑= 3.36−0.57+1.00μN2 (169Tm). The large variations of the summed dipole strength and the fragmentation in other deformed odd-mass nuclei can be reproduced by the same statistical ansatz. The total low-lying M1 strength in heavy deformed odd-mass nuclei is in agreement with the findings in the neighbouring even-mass nuclides as well as with sum-rule predictions.

Results of a nat Cr(γ,γ′) experiment at the S-DALINAC are reported for energies up to 7 MeV utilizing a Euroball Cluster detector. The excitation of a J = 1 state at 5544 keV is observed which is believed to belong to 52 Cr because of the strength of the signal and the large natural abundance. By a variety of empirical arguments a quadrupole-octupole-coupled two-phonon 1 − character is suggested for this transition. Quasiparticle random-phase approximation (QRPA) calculations are able to reproduce the energy and B (E1) transition strength remarkably well. The underlying microscopic structure is suggested to be more complex than a pure two-phonon picture. Furthermore, the calculations indicate a pure neutron ( 2p 3 2 −1 2p 1 2 ) magnetic dipole structure for an excitation experimentally seen at 5098 keV which would provide a direct measure of ground-state correlations in 52 Cr.

High-spin states in 177 W have been populated using the 164 Dy( 18 O,5n) reaction at a beam energy of 83 MeV. Six new high- K bands based on 3-, 5- and 7-quasiparticle excitations have been observed. Nilsson configurations for these bands are discussed, and experimental and calculated g -factors are compared. Decays of the K π = 29 2 + 5-quasiparticle state to the members of the K π = 7 2 + band have been observed. The decay rates are compared with predictions calculated within a tunneling model where the nuclear shapes are considered to change along the γ direction in the ( ϵ 2 , γ) deformation plane.

The nucleus 69Ge has been studied in the reactions 66Zn(α,n)69Ge and 60Ni(12C,2pn)69Ge. Using γ-spectroscopic methods energies, intensities, excitation functions, γγ-coincidences, angular distributions, and the linear polarization of γ-rays as well as lifetimes were measured. From the α-induced reaction, states up to spin J = 212 and Ex = 4267 keV have been established in 69Ge including 178 new transitions and 62 new levels. Spin and parity were determined for 79 levels. High-spin states populated via the reaction 12C + 60Ni show 61 new transitions and 27 new levels up to spin J = 372 and Ex = 9182 keV. For 19 levels ps-lifetimes were determined. Eight complete multiplets originating from the coupling of the odd neutron to the core 68Ge have been identified and candidates for further multiplets are proposed. Significant deviations from the simple De-Shalit core-coupling model were found. Shell-model calculations, which were performed for 68Ge and 69Ge with the code RITSSCHIL on a parallel computer in a large configuration space, show that proton excitations from the 2p32 to higher shells play an important role for the detailed structure of the particle-core multiplets.

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.

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.

Neutron particle-hole states in the doubly magic nucleus 208 Pb were investigated using a Euroball Cluster detector and five usual HPGe detectors at the Osiris cube spectrometer in Cologne. 126 γ-ray transitions with energies up to 7.2 MeV could be detected 14 for those for the first time. The states were excited with the 208 Pb( p , p ' γ ) 208 Pb reaction via isobaric analogue resonances in 209 Bi at E p = 16.5, 17, 17.45, 17.5 and 18.5 MeV and though the subcoulomb 207 Pb( d , pγ ) 208 Pb reaction at E d = 10 MeV. For most of the excited states assignments for the particle and/or the hole component of the configuration could be made, 22 of these are new.

We report on a nuclear resonance fluorescence experiment on the semi-magic odd-mass fp -shell nucleus 51 V. The detected dipole strength distribution is discussed on the basis of modern shell-model calculations employing a model space allowing for excitations of protons and neutrons from the 1 f 7/2 to the (2 p 1/2 ,2 p 3/2 ,1 f 5/2 ) shells. The calculations indicate that the main body of transitions is of magnetic dipole type, and they are capable of reproducing the observed distribution well. Possible electric dipole and quadrupole contributions are also discussed.

We have performed recoil-distance and Doppler-shift-attenuation lifetime measurements in the reaction 54Fe(32S, 3p)83Y at 103–107 MeV beam energy. By analyzing the Doppler-broadened line shapes of the 964 keV 21 2 + → 17 2 + yrast transition in coincidence with feeding and depopulating transitions, the average side-feeding time and lifetime of this state have been deduced as τF = 0.6(2) ps and τ = 0.62(10) ps, respectively. A similar analysis of the 811 keV 17 2 + → 13 2 + transition gave values of τF = 0.65(25) ps and τ = 1.5(2) ps, respectively for the 17 2 + yrast state. Feeding times of the higher transitions in the positive-parity yrast band have been estimated from their Dopplerbroadened lineshapes measured in coincidence with depopulating transitions only. The feeding times are compared with the results of Monte Carlo calculations of the time development of the continuum γ-ray flux, and a damping width of 2–3 MeV has been deduced. The measured level lifetimes support the regular band structures with the deformation β2 = 0.33 previously inferred from the transition energies.