N. Nicolay

In-band M1 and E2 transition rates and collective structures in 128Ba

Abstract Subpicosecond mean lifetimes of eight excited states in 128Ba populated via the 96Zr(36S,4n) reaction were measured by the Doppler-shift attenuation (DSA) technique using a line-shape analysis. The differential decay-curve method (DDCM) was applied for the lifetime determination. The B(E2) values in the yrast band indicate that the first band-crossing is with a proton S-band. The configuration πh 11 2 d 5 2 of the negative-parity semi-decoupled bands is confirmed by the measured B(E2, I → I − 2) and B(M1, I → I − 1) transition strengths. The higher-lying “dipole” band in 128Ba can be described as a high-K four-quasiparticle band built on the prolate configuration (πh 11 2 d 5 2 ) ⊗ (νh 11 2 g 7 2 ) .

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.

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.

Complete scissors mode strength in heavy deformed odd-mass nuclei: a case study of 165Ho and 169Tm

Abstract 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.

Photon scattering off 52Cr: two-phonon E1 strength at the N = 28 shell closure?

Abstract 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.

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.

Particle excitations and collectivity in the N = 48 nuclei 83Br and 85Rb

Abstract Excited states of the nuclei 83Br and 85Rb were populated in the reactions 82Se(7Liα2n) and 82Se(7Li,4n), respectively, using the 7Li beams of the FN tandem accelerator in Cologne (E = 32 MeV) and of the 120 cm cyclotron in Rossendorf (E = 35 MeV). Gamma-gamma-particle coincidences were measured with six Ge and fourteen Si detectors installed at the detector array Cologne OSIRIS CUBE. This technique enabled a reaction channel selection. Multipole orders of the γ-rays were derived from directional correlations of coincident γ-rays and from angular distributions. Mean lifetimes were deduced for seven levels in 83Br and two levels in 85Rb using the Doppler-shift-attenuation method. The level scheme of 83Br has been extended up to ( 21 2 + ) and 15 2 − states. In 85Rb a level sequence built on the ground state has been established up to 15 2 ( − ) and a new cascade of fast M1 transitions starting with a 17 2 ( − ) state at 3198.2 keV has been found. Furthermore, several medium-spin states have been newly introduced. The level sequences in both nuclei have been interpreted in terms of the shell model. The calculations performed in the model space π(0 g 9 2 , 1 p 1 2 , 1 p 3 2 , 0 f 5 2 ) ν(0 g 9 2 , 1 p 1 2 ) reproduce single-particle characteristics as well as collective properties of the level sequences.

Multiparticle-hole states of high spin in N < 50, A ≈ 90 nuclei: 3. The odd-odd nucleus 88Nb

Abstract High-spin states of the neutron deficient nucleus 88 Nb have been studied following the reaction 56 Fe ( 35 Cl, 2pn) 88 Nb at 120 and 123 MeV bombarding energy. Neutron gated and pure γγ-coincidences were measured with a neutron-γγ-coincidence setup; in addition γγ-coincidences were detected with the OSIRIS-cube at the Cologne tandem accelerator. Via the analysis of γγ-coincidence spectra the level scheme of 88 Nb has been extended up to 9.7 MeV excitation energy. About 88 transitions and 40 new levels have been placed in the level scheme. The experimental level energies and branching ratios are compared to predictions of shell model calculations in the restricted ( p 1/2 , g 9/2 ) configuration space.

Shell-model states and collectivity in 83Br48 and 85Rb48

Excited states of the nuclei 83Br and 85Rb were populated in the reactions 82Se(7Li,α2n) and 82Se(7Li,4n), respectively, using the 7Li beams of the FN tandem accelerator in Cologne (E = 32 MeV) and of the 120 cm cyclotron in Rossendorf (E = 35 MeV). Gamma-gamma-particle coincidences were measured with six Ge and fourteen Si detectors installed at the detector array Cologne OSIRIS CUBE. This technique enabled a reaction channel selection. Multipole orders of the γ rays were derived from directional correlations of coincident γ rays and from angular distributions. Mean lifetimes were deduced for seven levels in 83Br and two levels in 85Rb using the Doppler-shift-attenuation method. The level scheme of 83Br has been extended up to (21/2+) or 15/2− states. In 85Rb a level sequence built on the ground state has been established up to 15/2(−) and a new cascade of fast M1 transitions starting with a 17/2(−) state at 3198.2keV has been found. Furthermore, several medium-spin states have been newly introduced. The level sequences in both nuclei have been interpreted in terms of the shell model. The calculations performed in the model space π(0g9/2, 1p½, 1p3/2, 0f5/2)v(0g9/2, 1p½) reproduce single-particle characteristics as well as collective properties of the level sequences.

Excited States in the Transitional N=45 Nucleus 85Zr

Excited states in the transitional nucleus85Zr were studied via the fusion evaporation reaction56Fe(35Cl,αpn)85Zr at 120 MeV. A level scheme reaching up to probable spins of (33/2+) at 6.2 MeV and (27/2−) at 4.9 MeV was established and compared to neighbouring nuclei.