Quasiparticle-phonon Model Research Articles

Dipole γ-ray transition rates in 238U

M1 and E1 transition rates from the ground to excited states and between excited states in 238U are calculated within the quasiparticle-phonon nuclear model with the wave functions consisting of one- and two-phonon terms. We show that there are relatively large M1 transitions between 2+ states in the low-energy region. The fragmentation of the one-phonon states strongly affects M1 and E1 strength distributions. The correlation takes place between E1 and E3 transition strengths. We show that there are fast E1 and M1 transitions between large components of the wave functions differing by an octupole or quadrupole phonon.

Nuclear orientation study of the decay of 173Lu

A decay study of 173Lu oriented at low temperatures in a gadolinium host is carried out. Directional distributions of 12 γ-rays in 173Yb are measured using the high-resolution Ge(Li) detectors. The E2 M1 and M2 E1 multipole mixing ratios of mixed γ-transitions are deduced. The energies and structure of the rotational band levels in 173Yb, reduced E2 and M1 transition probabilities and E2 M1 mixing ratios are calculated in the framework of the semi-microscopic quasiparticle-phonon model with a Coriolis interaction included. In general, good agreement is obtained between the experimental results and theory.

Dipole strength distribution and specific properties of excitations in the second minimum in actinide nuclei

Vibrational states in the first minimum in 238U and 240Pu and in the second minimum in 240Pu have been calculated within the quasiparticle-phonon nuclear model. The results of calculation of vibrational states in both minima are compared. Specific properties of the vibrational states in the second minimum in actinide nuclei are discussed.

New experimental access to the two-phonon octupole vibration in 208Pb

The feasibility of a new experimental method to identify two-phonon octupole vibrations is investigated for the example of 208Pb. The 2 + member of the 3 I − ⊗ 3 I − multiplet can be excited with the (γ, γ′) reaction and a sizable E1 decay to the 3 I − level serves as a signature of the two-phonon character. Numerical estimates within the quasiparticle-phonon model indicate that such an experiment should be possible with state-of-the-art large volume Ge detectors like the EUROBALL cluster module.

Excitation and decay properties of the |3 1− ⊗ 3 1−; 2 +〉 state in 208Pb

A nuclear resonance fluorescence experiment using Euroball Cluster detectors is proposed with the aim to identify the long-sought two-octupole-phonon vibration in 208Pb. Based on quasiparticle-phonon model calculations and countrate estimates from previous measurements the experiment seems feasible.

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

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.

A EUROBALL module at the S-DALINAC — Investigation of elementary and magnetic excitation modes

Results from the first application of a EUROBALL Cluster module in (γ, γ′) experiments at the S-DALINAC are reported. The scissors mode was observed for the first time in a γ-soft nucleus, 196Pt. A combination of detailed spectroscopy of 165Ho and 169Tm with a fluctuation analysis technique demonstrates that the total scissors mode strength in odd-mass nuclei is comparable to that in the even neighbours, but fragmentation is so strong that large parts are below detection thresholds. The search for the spin M1 resonance in the heavy deformed nucleus 154Sm gives a hint on rather large cancellations of spin and orbital contributions. The fine structure of the E1 response in the semimagic 140Ce below particle threshold is fully resolved and successfully interpreted in the microscopic quasiparticle-phonon model.

Separation energy dependence of hole form factors

Abstract Form factors of fragmented hole states are studied within the quasiparticle-phonon model, using the inhomogeneous equation method. The validity of this method is successfully checked by comparison with coupled equation solutions in schematic vibrational model cases. A systematic investigation of form factors is performed for neutron and proton hole states in the valence and first inner shells of 208 Pb. Large fluctuations of form factor radii are observed for individual levels superimposed on a general increase with separation energy. Average characteristics are introduced for groups of levels, namely the mean form factors, summed source terms and correction potentials, and their behaviour is presented. The role of the relative values of the interaction radius parameter and binding well radius is discussed in detail.

Systematics of low-lying dipole excitations in the deformed even-even nuclei 164,166,168,170Er.

Photon scattering experiments on the deformed even-even nuclei $^{164,166,168,170}\mathrm{Er}$ provided detailed information on the excitation energies, spins, transition probabilities, and K quantum numbers of low-lying dipole excitations. In the case of $^{168}\mathrm{Er}$, 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 ${2}^{+}$\ensuremath{\bigotimes}${3}^{\mathrm{\ensuremath{-}}}$ two-phonon excitations) and in the framework of recent microscopic quasiparticle phonon model calculations. \textcopyright{} 1996 The American Physical Society.

Particle-hole and particle-particle correlations in neodymium isotopes

Abstract Excited states in 140, 142, 144, 146 Nd nuclei, up to an excitation energy of about 5 MeV, were investigated by (p,t) experiments performed with a good energy resolution. These data, together with proton and deuteron scattering data from a previous experiment, are compared with Quasi-Particle Phonon Model evaluations, in which the competition between particle-hole and particle-particle residual interactions is considered. The B (Eλ) distributions are satisfactorily reproduced. The 146, 148 Nd(p,t) reaction data are well accounted for, while difficulties are found in reproducing those for 142, 144 Nd(p,t). Limitations and improvements of the model are discussed.

Vibrational states in odd deformed nuclei

The energies and wave functions of the nonrotational states in odd deformed nuclei are calculated in the quasiparticle-phonon model of the nucleus. It is shown that the number of vibrational states in odd nuclei is many times larger than the number of vibrational states in even-even nuclei. The wave functions of the overwhelming majority of these states with excitation energies in the range 1.5–2.5 MeV possess a dominant term of the type quasiparticle⊗phonon.

Particle decay of high-lying states in odd nuclei

A method for calculating non-statistical particle decay of excited states in odd nuclei is presented. Using the quasiparticle-phonon model partial cross sections and branching ratios for the neutron decay of the high angular momentum states in 209Pb and 91Zr excited by means of the (α, 3He) reaction have been evaluated. The calculated branching ratios are compared with existing experimental data.

Neutron decay of high angular momentum states excited in transfer reactions.

A method for calculating nonstatistical particle decay of excited states in odd nuclei is presented. Using the quasiparticle-phonon model partial cross sections and branching ratios for the neutron decay of the high angular momentum states in $^{209}\mathrm{Pb}$ and $^{91}\mathrm{Zr}$ excited by means of the (\ensuremath{\alpha}${,}^{3}$He) reaction have been evaluated. The calculated branching ratios are compared with existing experimental data. A general agreement is found except for the ${5}^{\mathrm{\ensuremath{-}}}$ channel in $^{90}\mathrm{Zr}$ where practically all the observed decay is attributed to statistical process. \textcopyright{} 1996 The American Physical Society.

The115In(γ, γ′) reaction as a test of the quasi-particle phonon model with complex configurations in odd-mass nuclei

The115In(γ, γ′) reaction was measured at the S-DALINAC using bremsstrahlung with endpoint energiesE 0=3.1, 4.6 and 5.2 MeV. In the excitation energy range of 1–5 MeV in total 32 transitions were observed, 18 of which were hithertoo unknown. The results are compared to two quasiparticle-phonon model calculations, one with a model space of ‘quasiparticle⊗phonon’ and one extended by ‘quasiparticle⊗two-phonon’ configurations. A substantial improvement is achieved in the description of low-lying115In states by the consideration of the more complex configurations. The calculations quantitatively account for the summed115In(γ, γ′) integrated cross sections and show a comparable amount of fragmentation of the individual transition strengths. The model interpretation of the microscopic structure of the experimentally observed transitions is discussed.

Structural characteristics of 142Ce through inelastic neutron scattering.

The excited levels of $^{142}\mathrm{Ce}$ have been studied using the (n,${\mathit{n}}^{\ensuremath{'}}$\ensuremath{\gamma}) reaction. Excitation functions, angular distributions, and Doppler shifts were measured for \ensuremath{\gamma} rays from levels up to an excitation energy of 3.3 MeV; multipole-mixing and branching ratios and transition rates were deduced. Theoretical predictions of electromagnetic transition rates were compared for the interacting boson model, the quasiparticle phonon model, and the particle-core coupling model with experimental values. Evidence is found supporting the fragmentation of mixed-symmetry strength among neighboring $^{142}\mathrm{Ce}$ excited levels. This fragmentation is evinced in the ${2}^{+}$ excitations through strong M1 transitions; however, the ${3}^{+}$ mixed-symmetry strength is not manifested directly through strong M1 transitions, but rather is implied through model calculations. Candidates for multiphonon excitations are identified for the three-quadrupole and quadrupole-octupole phonon quintets.

Microscopic quasiparticle-phonon description of odd-A Xe isotopes

A microscopic quasiparticle-phonon model (MQPM), based on QRPA phonons, is developed. Both the phonons and the quasiparticles are constructed using the same realistic two-body interaction which yields a consistent framework for the study of the properties of the model. The MQPM is used to calculate energy spectra and beta-decay properties of 127-133Xe. The resulting energies and log ft values are compared with experimental results.

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.

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

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 |2 1 + ⊗ 2 1 + ⊗ 3 1 −; 2 −〉 configuration of the upper intermediate level at 3.101 MeV which exclusively decays to a close-lying two-phonon |2 1 + ⊗ 3 1 f-; 1 f-f> state at 2.747 MeV. The (γ,γf') reaction strongly excites the latter and also indicates the 1 f- member of the three-phonon 2 1 + ⊗ 2 1 + ⊗ 3 1 f- septuplet at 3.091 MeV.

162Dy studied with (n, γ), (n, n′γ), (d, p) and (d, t) reactions

Abstract The γ-ray spectra of 162 Dy were measured by (n, γ), (n, γγ) and (n, n′γ) reactions. In the 161 Dy(d, p) and 163 Dy(d, t) reactions the levels up to 2600 keV and 3200 keV, respectively, were identified. The level scheme was extended by additional rotational states up to 2190 keV. The vibrational and two-quasiparticle states were described within the quasiparticle-phonon nuclear model.

Semimicroscopic description of the odd-A Te isotopes.

The structure of low-lying levels in odd-mass Te isotopes (115[le][ital A][le]131) is discussed in the framework of the quasiparticle-phonon model. In addition, the properties of [sup 131]Te are also investigated by means of three-particle cluster-phonon coupling, where the pairing force is taken as the residual particle-particle interaction. Excitation energies and spectroscopic factors are calculated and compared with the experimental data.