Search Results (18 total)

Photon scattering experiments on the deformed even-even nuclei ^{164,166,168,170}Er provided detailed information on the excitation energies, spins, transition probabilities, and K quantum numbers of low-lying dipole excitations. In the case of ¹⁶⁸Er, in addition, parities of the excited states were determined model independently by measuring the linear polarization of the scattered photons using a sectored Ge-Compton polarimeter with a BGO–anti-Compton shield. The results for M1 excitations are compared with the systematics of low-lying orbital M1 strength (‘‘scissors mode’’) in the neighboring rare-earth nuclei. The observed enhanced E1 transitions are discussed in terms of collective excitations (K=0 octupole bandheads or candidates for 2⁺⊗3^- two-phonon excitations) and in the framework of recent microscopic quasiparticle phonon model calculations. © 1996 The American Physical Society.

A photon scattering experiment on ¹⁴³Nd yielded detailed information about the energies, cross sections, and absolute strengths of dipole transitions between 1.4 and 4.1 MeV. Besides some well-known levels at energies around 2 MeV we observed a number of very strong dipole excitations between 2.8 and 3.8 MeV. This is the energy region where one expects the two-phonon-(2⁺⊗3^-)⊗ particle multiplet which arises due to the coupling of the unpaired neutron with (2⁺⊗3^-) coupled states in the even-even core nucleus ¹⁴²Nd. The summed dipole strength between 2.8 and 3.6 MeV in ¹⁴³Nd, according to the sum rule, is comparable to the E1 strength of the 1^- member of the multiplet in the core nucleus ¹⁴²Nd. Model calculations in a harmonic approximation with a quadrupole-quadrupole interaction show very good agreement with the experiment and indicate the dominant two-phonon-particle structure (2⁺⊗3^-⊗f_7/2) of the excitations in ¹⁴³Nd.

Systematic nuclear resonance fluorescence experiments, including model independent parity determinations, provided evidence for enhanced E1 excitations near 2.5 MeV excitation energy in the deformed nuclei ¹⁵⁰Nd, ¹⁶⁰Gd, and ^162,164Dy. The corresponding J^π=1^- states are interpreted as two-phonon excitations due to the coupling of octupole (J=3^-,K=1) and quadrupole γ vibrations (J=2⁺,K=2). The results can be explained in the framework of the sdf interacting-boson-approximation model and the dynamic collective model describing octupole vibrations and their coupling to quadrupole vibrations in deformed nuclei.

Nuclear resonance fluorescence experiments are reported to search for enhanced M1 scissors mode states in the deformed odd-mass nucleus ¹⁶³Dy. A concentration of dipole strengths near 3 MeV excitation energy is found, which fits nicely into the systematics observed for M1 excitations in the neighboring even-even Dy isotopes. The observed strength distribution and the decay branching ratios are discussed in the context of the interacting boson-fermion model.

A photon scattering experiment has been performed on the odd-A nucleus ¹⁴³Nd. Strong dipole excitations have been detected in the energy region around 3.2 MeV. This is near the expected position of the two-phonon (2⁺⊗3^-) multiplet in the even-even neighboring nucleus ¹⁴²Nd. The summed dipole strength between 2.8 and 3.6 MeV in ¹⁴³Nd is in agreement with the sum rule for weak particle coupling to the core nucleus ¹⁴²Nd. Model calculations in a harmonic approximation are in a very good agreement with the experiment and they suggest a two-phonon (2⁺⊗3^-) particle structure of the excitations in ¹⁴³Nd.

The ¹⁶⁵Ho (α,t) and (³He,d) reactions have been measured at beam energies of 40 and 25 MeV to look for the K^π=1⁺{9/2^-[514]-7/2^-[523]} two-proton band structure in ¹⁶⁶Er. Reaction products were analyzed using a quadrupole-three-dipole spectrometer. Distorted-wave Born approximation predictions were used to extract candidate states with angular momentum transfer l=5. The measured strengths of all the candidates for l=5 transitions were compared with the Nilsson fingerprint pattern of the K^π=1⁺{9/2^-[514]-7/2^-[523]} band to identify the structure. No strong population of this configuration was found. At best two tentative candidates for the I^π=3⁺ and 4⁺ members of this band could be identified, which would represent at most 38% of the expected strength for this configuration. The bandhead for this tentative K^π=1⁺ excitation does not correspond to any I^π=1⁺ state observed in the ¹⁶⁶Er(γ,γ’) reaction.

Measurements of the linear polarization of resonantly scattered photons have been used for model independent parity assignments in nuclear resonance fluorescence experiments (NRF). Previous NRF investigations of the even-even Nd isotopes were completed by polarization measurements on the transitional nucleus ¹⁴⁶Nd. With these new parity assignments the systematics of low lying M1 strengths can be studied along the transition from spherical to deformed nuclear shapes. The total M1 strength in the excitation energy range of 2–4 MeV in the investigated even-even Nd isotopes increases proportionally to the square of the deformation parameter. These findings represent a reliable confirmation of the results recently reported for the Sm isotopes by the Darmstadt group. This deformation dependence can be explained by various recent theoretical approaches.

Measurements of the linear polarization of resonantly scattered photons have been used for model independent parity assignments in nuclear resonance fluorescence experiments. The novel result of the present study was the first observation of enhanced electric dipole excitations in the deformed nuclei ¹⁵⁰Nd and ¹⁶²Dy at excitation energies of 2.414 and 2.520 MeV, respectively. The transition energies and the enhanced B(E1)↑ strengths of 3 and 5×10^-3 e² fm² support the interpretation in terms of the predicted new type of collective electric dipole excitations in deformed nuclei due to reflection asymmetric shapes like octupole deformations and/or cluster configurations.

The saturation of the B(E2,0₁⁺→2₁⁺) strengths in the actinide nuclei is described very well by an analytical formula using shell closures at Z=82,126 and N=126,184. The transition towards saturation is much more gradual than in the rare earth nuclei and reaches a saturation value of 325±10 W.u. By assuming a similar correlation between B(E2) and summed B(M1) strengths as observed in the rare-earth region we predict for actinide nuclei a saturation of the summed B(M1,0₁⁺→1⁺) strengths below about 3 MeV of 3.7±0.4 μ_N². We compare our estimates for (g_π-g_ν)² with values obtained from extrapolations of averaged regional boson g factors.

Experiments designed to provide more information on the spectroscopy of ²¹⁴Bi, and on ²¹⁴Pb(β^-)²¹⁴Bi in particular, were undertaken because of interest in first-forbidden β decay in the lead region. The experiments consisted of γ-γ coincidences and angular correlations, conversion electron measurements, level lifetime determinations, and precision γ-ray energy measurements. The 352-keV level of ²¹⁴Bi was found to be a strong candidate (and the only candidate) for the 0₁^- state. Recent additions to the ²¹⁴Pb decay scheme are confirmed by γ-γ coincidence measurements. A careful evaluation of the ²¹⁴Bi level scheme is made with emphasis on separating experimentally based conclusions from speculations based on systematics and other ‘‘weak’’ arguments. Shell-model calculations of the spectroscopy of ^210,212Bi and ^210,212Pb(β^-)^210,212Bi were performed using a modification of the Kuo-Herling realistic interaction. These calculations and a generalized seniority model provide a basis for an examination of the systematics of the A=210, 212, 214 spectroscopy and β decay. The generalized seniority model is found to be a quite good approximation which provides a quantitative understanding of the ²¹⁴Pb decay rates.

Gamma rays from a mixed source of ⁵⁶Co+⁴⁶Sc were measured in a Ge(Li) detector. The ⁵⁶Co/⁴⁶Sc peak intensity ratios I₈₄₇/I₈₈₉ were determined by careful analysis of line shapes and underlying backgrounds. Based on the known ⁴⁶Sc adopted half-life value of 83.810(10) d, the fit to the intensity ratios corrected for ⁴⁶Sc decay from 107 runs over an elapsed time of 172 days gave a half-life value of 77.29(3) d for ⁵⁶Co.

In nuclear resonance fluorescence (NRF) measurements of well deformed rare-earth nuclei we observe for most of the excited J=1 levels of both parities the ground-state transition and the corresponding transition to the 2₁⁺ state. The experimental branching ratios R_exp=B(1→2)/B(1→0) are compared with the predictions of the Alaga rules. The examination of 171 levels found in NRF experiments yields that most values of R_exp are in good agreement with the Alaga rules. Nevertheless, some branching ratios suggest a possible mixing of K values.

Nuclear resonance fluorescence experiments have been performed on the deformed actinide nucleus ²³⁶U. Bremsstrahlung of 3.9 MeV endpoint energy has been used as the photon source. The scattered photons were detected by three high resolution Ge-γ-spectrometers installed at scattering angles of 92°, 128°, and 150°, respectively. Precise excitation energies, decay branching ratios, and ground state decay widths of numerous previously unknown spin 1 states in the excitation energy range 1.8–3.2 MeV have been extracted. The dipole strength has been found to be concentrated in the energy range 2.1–2.5 MeV. The systematics of the so-called scissors mode observed as a result of the previous (γ,γ’) and (e,e’) experiments on ²³²Th and ²³⁸U and, in particular, their combined analysis suggests likewise to attribute these new dipole excitations in ²³⁶U to the orbital M1 scissors mode.

Compton polarimeters have been used to measure the linear polarization of resonantly scattered photons. In the deformed nucleus ¹⁵⁰Nd the first direct and model-independent evidence has been obtained that in a cluster of enhanced dipole excitations near 3 MeV the three strongest transitions are of the same, positive parity.

Data from complementary experiments on the members of the (3^-νf_7/2) multiplets in _60¹⁴³Nd₈₃, _62¹⁴⁵Sm₈₃, and _64¹⁴⁷Gd₈₃ are summarized and the particle-vibration coupling is investigated. It is shown that for odd N=83 nuclei the quadrupole-quadrupole interaction gives the leading contribution in the energy displacements of the multiplet members and that it can essentially explain the spin order and the absolute values for the splitting of the observed particle-octupole multiplets.

A new method of fast-timing measurements using β-γ-γ coincidences with BaF₂ and plastic detectors has been used to measure a number of lifetimes for low-lying states in ¹¹⁶,118,120Cd in the range of 10–100 ps. The results are discussed in terms of the vibrational and intruder structure of these nuclei.