Quasiparticle-phonon Model Research Articles

Nuclear reactions as a tool to study the microscopic structure of pygmy and giant resonances

An advanced microscopic approach based on energy-density functional theory and the quasiparticle-phonon model has been used to study nuclear excitations up to giant dipole resonance energies. In addition, our nuclear structure model has been extended to include reaction theory. The theoretical spectral distributions compared with experimental data performed with different probes and techniques are able to provide us with information about the microscopic structure of the pygmy dipole resonance as well as its collectivity. Evaluations of electric dipole (E1) transition strengths and branching ratios at low energy reveal different properties of the nuclear excitations and the effect of quasi-continuum coupling. The present studies will support above-neutronthreshold γ experiments at ELI-NP targeting γ decays of giant and pygmy resonances, as well as studies on multi-step γ decays by low-lying states.

Low-lying dipole strength distribution inPb204

Dipole and quadrupole strength distribution of $^{204}\mathrm{Pb}$ was investigated via a nuclear resonance fluorescence experiment using bremsstrahlung produced using an electron beam at a kinetic energy of 10.5 MeV at the linear accelerator ELBE. We identified 136 states resonantly excited at energies from 3.6 to 8.4 MeV. Spins of the excited states were deduced by angular distribution ratios of $\ensuremath{\gamma}$ rays observed at scattering angles of ${90}^{\ensuremath{\circ}}$ and ${127}^{\ensuremath{\circ}}$ with respect to the incident $\ensuremath{\gamma}$ beam. The analysis of the measured $\ensuremath{\gamma}$-ray spectra includes the quasicontinuum of levels at high energy. Monte Carlo simulation of $\ensuremath{\gamma}$-ray cascades were performed to obtain the intensities of inelastic transitions and branching ratios of the ground-state transitions. The present experimental results were used to investigate the electric dipole $(E1)$ strengths by comparison with predictions from the quasiparticle-phonon model with the self-consistent energy density functional.

Electromagnetic transitions between low-lying nonrotational states of odd-neutron nuclei in α -decay chains starting from Hs265,267,269

Background: Calculations of the structure of the low-lying states of nuclei with $Z\ensuremath{\ge}100$ play an important role in understanding the properties of nuclei belonging to the new region of the nuclide chart, which is available now for experimental study.Purpose: We calculate quasiparticle-phonon structure and the reduced $\ensuremath{\gamma}$-transition probabilities for the excited states with excitation energies up to 1 MeV for nuclei with $Z\ensuremath{\ge}100$.Methods: The quasiparticle-phonon model, which takes into account the quasiparticle-phonon interaction of different multipolarities, is used as a basis for the calculations.Results: The quasiparticle-phonon structure and the $\ensuremath{\gamma}$-reduced transition probabilities of odd-neutron $^{265,267,269}\mathrm{Hs}$, $^{261,263,265}\mathrm{Sg}$, $^{257,259,261}\mathrm{Rf}$, $^{253,255,257}\mathrm{No}$, and $^{249,251,253}\mathrm{Fm}$ are calculated. The $\ensuremath{\alpha}$-decay chains starting from $^{265,267,269}\mathrm{Hs}$ are analyzed.Conclusion: It is shown that below 500 keV the structure of the nuclear states is mainly exhausted by the single-quasiparticle component. The quasiparticle-phonon interaction starts to play an important role at excitation energies above 500 keV. The nuclei before Fm in the $\ensuremath{\alpha}$-decay chains starting from $^{265,267,269}\mathrm{Hs}$ have at least two $\ensuremath{\alpha}$-decay lines.

Neutral-current supernova neutrino-nucleus scattering off I127 and Cs133

A large number of the presently running neutrino and dark-matter experiments use thallium-doped cesium-iodide CsI[Tl] crystals, sodium-doped cesium-iodide CsI[Na] crystals, or thallium-doped sodium-iodide NaI[Tl] crystals. In the present paper we calculate elastic and inelastic cross sections for neutral-current supernova-neutrino scattering off $^{127}\mathrm{I}$ and $^{133}\mathrm{Cs}$, relevant for experiments using CsI[Tl], CsI[Na], or NaI[Tl] crystals. We study also the cross sections folded with two-parameter Fermi-Dirac distributions of the supernova-neutrino spectrum. The adopted nuclear-theory framework is the microscopic quasiparticle-phonon model, able to operate in large single-particle valence bases and use G-matrix based two-nucleon interactions with a phenomenological renormalization.

Coriolis mixing of the K = 1 and K = 0 mixed symmetry states in the well deformed even–even nuclei

The Coriolis matrix elements responsible for mixing of the $$1^+ K=1$$ and $$1^+ K=0$$ states are calculated in the framework of the Quasiparticle Phonon Model for several Gd and Dy isotopes. In many considered cases these matrix elements are equal to several tens of keV and are comparable with energy distances between the mixed levels. The results obtained indicates that Gd isotopes could be more suitable for finding deviations from Alaga rules in M1 transitions from $$1^+$$ state to the states of the ground band.

Mixed Symmetry States in 92Zr and 94Mo Nuclei

Mixed-symmetry states of 92Zr and 94Mo isotopes are investigated with the use of the collective models, Interacting Boson Model-2 (IBM-2) and Quasiparticle Phonon Model (QPM). The energy spectra and electromagnetic transition rates for these isotopes are calculated. The results of IBM-2 and QPM are compared with available experimental data. We have obtained a satisfactory agreement, and IBM-2 gives a better description. In these isotopes, we observe a few states having a mixed symmetry such as 2+2, 2+3, 3+1, and 1+s. It is found that these isotopes have a vibrational shape corresponding to the U(5) symmetry.

Microscopic Structure of the Low-Energy Electric Dipole Response of ^{120}Sn.

The microscopic structure of the low-energy electric dipole response, commonly denoted as pygmy dipole resonance (PDR), was studied for ^{120}Sn in a ^{119}Sn(d,pγ)^{120}Sn experiment. Unprecedented access to the single-particle structure of excited 1^{-} states below and around the neutron-separation threshold was obtained by comparing experimental data to predictions from a novel theoretical approach. The novel approach combines detailed structure input from energy-density functional plus quasiparticle-phonon model theory with reaction theory to obtain a consistent description of both the structure and reaction aspects of the process. The presented results show that the understanding of one-particle-one-hole structures of the 1^{-} states in the PDR region is crucial to reliably predict properties of the PDR and its contribution to nucleosynthesis processes.

Structure of high-lying levels populated in the 96Y →96Zr β decay

The nature of the high-lying final levels of the 96Ygsβ decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum, has been investigated in high-resolution γ-ray spectroscopy following the β decay as well as in a campaign of inelastic photon scattering experiments. The comprehensive approach establishes 1− levels associated with the Pygmy Dipole Resonance as high-lying final levels in the β decay. Branching ratios extracted from β decay complement photon scattering and allow the absolute E1 excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of the 1− levels below the Qβ value, which are investigated in the Quasiparticle Phonon Model. The calculations reveal that the components populated in β decay contribute only with small amplitudes to the complex wavefunction of these 1− levels. A comparison of the β decay results to data from total absorption γ-ray spectroscopy demonstrates a good agreement between both measurements.

Dipole response in Te128,130 below the neutron threshold

Background: Numerous studies of the ground-state decay of the pygmy dipole resonance (PDR) have been carried out in the past. However, data on the decay of the PDR to low-lying excited states is still very scarce due to limitations of the sensitivity to weak branching transitions of experimental setups.Purpose: We present a detailed examination of the low-energy dipole response of $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$ below their neutron separation thresholds of 8.8 and 8.5 MeV, respectively.Methods: Photonuclear reactions with the subsequent $\ensuremath{\gamma}$-ray spectroscopy of the decay channel with continuous-energy bremsstrahlung at varying endpoint energies and linearly polarized quasimonochromatic $\ensuremath{\gamma}$-ray beams with energies ranging from 2.7 to 8.9 MeV in steps of roughly 250 keV were used for probing the decay behavior of the low-energy dipole response in $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$. In addition, $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}}{\ensuremath{\gamma}}^{\ensuremath{'}\ensuremath{'}})$ reactions were used to study the population of low-lying states of $^{128}\mathrm{Te}$.Results: Spin-parity quantum numbers and reduced transition probabilities are determined for individual photo-excited states. The analysis of average decay properties for nuclear levels in narrow excitation-energy bins enable the extraction of photoabsorption cross sections, average branching ratios to the ${2}_{1}^{+}$ state, and the distinction between $E1$ and $M1$ transitions to the ground state and to the ${2}_{1}^{+}$ state accounting for resolved and unresolved transitions.Conclusions: Above 5 MeV, the experimental data are in reasonable agreement to calculations within the quasiparticle phonon model. The major fraction of the ground-state decay channel is due to $E1$ transitions, while less than $5--10$% stem from $M1$ transitions. Furthermore, first direct experimental evidence is provided that the population of the ${2}_{1}^{+}$ state of $^{128}\mathrm{Te}$ via primary $\ensuremath{\gamma}$-ray transitions from excited states in the PDR region from 5 to 9 MeV is dominated by $E1$ transitions of ${1}^{\ensuremath{-}}$ states.

Low-lying electric dipole γ-continuum for the unstable 62,64Fe nuclei: Strength evolution with neutron number

The γ-ray emission from the nuclei 62,64Fe following Coulomb excitation at bombarding energy of 400-440 AMeV was measured with special focus on E1 transitions in the energy region 4-8 MeV. The unstable neutron-rich nuclei 62,64Fe were produced at the FAIR-GSI laboratories and selected with the FRS spectrometer. The γ decay was detected with AGATA. From the measured γ-ray spectra the summed E1 strength is extracted and compared to microscopic quasi-particle phonon model calculations. The trend of the E1 strength with increasing neutron number is found to be fairly well reproduced with calculations that assume a rather complex structure of the 1− states (three-phonon states) inducing a strong fragmentation of the E1 nuclear response below the neutron binding energy.

Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in ^{208}Pb.

New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in ^{208}Pb are presented. They were obtained from (d,p) and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for ^{208}Pb and establish (d,p) as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the strength distributions are discussed and compared to large-scale shell model (LSSM) and energy-density functional plus quasiparticle-phonon model theoretical approaches to elucidate the microscopic structure of the PDR in ^{208}Pb.

Supernova neutrino scattering off Gadolinium odd isotopes in water Cherenkov detectors

In this work the supernova neutrino (SN) charged-current interactions with Gd odd isotopes (A=155 and 157) are studied. We use measured spectra and the quasiparticle-phonon model (MQPM) to calculate the charged current response of odd Gd isotopes to supernova neutrinos. Flux-averaged cross sections are obtained considering quasi-thermal neutrino spectra.

On the excitation of the 2^+_1 state in ^{12}C in the (e,e'gamma ) reaction

The excitation of the carbon 2^+ state at 4.439 MeV by 70{-}150 MeV electron impact and its subsequent decay to the ground state by photon emission is described within the distorted-wave Born approximation. The transition densities are obtained from the nuclear quasiparticle phonon model. The photon angular distributions are compared with earlier results and with experiment, including the influence of bremsstrahlung. Predictions for spin asymmetries in the case of polarized electron impact are also made.

Neutrino scattering off the 95,97Mo isotopes

In this work we perform calculations of the cross sections for neutral-current neutrino scattering off the 95,97Mo isotopes. Both the incoherent and coherent con- tributions to the cross sections are considered. The wave functions of the initial and final nuclear states are constructed in the context of the quasiparticle phonon model (MQPM). The response of the aformentioned nuclei to supernova neutrinos are computed by folding the obtained cross sections with a two-parameter Fermi-Dirac distribution.

Excitation of the electric pygmy dipole resonance by inelastic electron scattering⋆

To complete earlier studies of the properties of the electric pygmy dipole resonance (PDR) obtained in various nuclear reactions, the excitation of the 1$^-$ states in $^{140}$Ce by $(e,e')$ scattering for momentum transfers $q=0.1-1.2$~fm$^{-1}$ is calculated within the plane-wave and distorted-wave Born approximations. The excited states of the nucleus are described within the Quasiparticle Random Phase Approximation (QRPA), but also within the Quasiparticle-Phonon Model (QPM) by accounting for the coupling to complex configurations. It is demonstrated that the excitation mechanism of the PDR states in $(e,e')$ reactions is predominantly of transversal nature for scattering angles $\theta_e \approx 90^o-180^o$. Being thus mediated by the convection and spin nuclear currents, the $(e,e')$ like the $(\gamma,\gamma')$ reaction, may provide additional information to the one obtained from Coulomb- and hadronic excitations of the PDR in $(p,p')$, $(\alpha,\alpha')$, and heavy-ion scattering reactions. The calculations predict that the $(e,e')$ cross sections for the strongest individual PDR states are in general about three orders of magnitude smaller as compared to the one of the lowest $2^+_1$ state for the studied kinematics, but that they may become dominant at extreme backward angles.

Second- and Third-Order Anharmonic Effects within the Quantum Many-Body Theory

The paper presents a brief review of the results in the self-consistent theory of the anharmonic effects of the second and third orders in the phonon production amplitude, based on the quantum many-body theory. Numerical (for second-order effects) and theoretical analyses of new, i.e., three- and four-quasiparticle correlations in the ground state (backward-going graphs) in magic nuclei and in nuclei with pairing are given. A formula for the probability of transition between one- and two-phonon states is obtained and compared to the solution of a similar problem within the quasiparticle-phonon model. It is shown that the approach under consideration contains a number of new effects.

Fine structure of the pygmy quadrupole resonance in 112,114Sn

The electric quadrupole response in 112,114Sn isotopes is investigated by energy-density functional (EDF) and three-phonon quasiparticle-phonon model (QPM) theory with special emphasis on electric quadrupole excitations located above the first collective 2+ state and below 5 MeV. Additional quadrupole strength clustering as a sequence of states similar to the recently observed pygmy quadrupole resonance in 124Sn is found. The spectral distributions and transition densities of these 2+ states show special features being compatible with oscillations of a neutron skin against the isospin-symmetric nuclear core. Furthermore, two (p, p′γ) Doppler-shift attenuation (DSA) coincidence experiments were performed at the SONIC@HORUS setup. 2+ states with excitation energies up to 4.2 MeV were populated in 112,114Sn. Lifetimes and branching ratios were measured allowing for the determination of E2 transition strengths to the ground state. A stringent comparison of the new data to EDF+QPM theory in 112Sn and 114Sn isotopes hints at the occurrence of a low-energy quadrupole mode of unique character which could be interpreted as pygmy quadrupole resonance.

Charged-current neutrino-nucleus scattering off Xe isotopes

Xenon detectors are used in the search for dark matter and neutrinoless double-beta decay ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta})$. As the next-generation detectors reach masses in the ton scale, neutrinos from astrophysical sources are soon predicted to become background in such detectors. Theoretical predictions of neutrino scattering cross sections and information of nuclear structure effects therein are crucial in accounting for the background. We perform calculations for differential and total cross sections of charged-current neutrino scattering off the most abundant xenon isotopes. The nuclear-structure calculations are made in the proton-neutron quasiparticle random-phase approximation and the microscopic quasiparticle phonon model for even-mass and odd-mass nuclei, respectively. We compute total cross sections as a function of the neutrino energy, and also give estimates of total averaged cross sections for $^{8}\mathrm{B}$ solar neutrinos and supernova neutrinos using realistic energy distributions.

Low-lying octupole isovector excitation in Nd144

The nature of low-lying 3(-) levels in Nd-144 was investigated in the Nd-143(n, gamma gamma) cold neutron-capture reaction. The combination of the high neutron flux from the research reactor at the Institut Laue-Langevin and the high gamma-ray detection efficiency of the EXILL setup allowed the recording of gamma gamma coincidences. From the coincidence data precise branching ratios were extracted. Furthermore, the octagonal symmetry of the setup allowed angular-distribution measurements to determine multipole-mixing ratios. Additionally, in a second measurement the ultra-high resolution spectrometer GAMS6 was employed to conduct lifetime measurements using the gamma-ray induced Doppler-shift technique (GRID). The confirmed strong M1 component in the 3(3)(-) -> 3(1)(-) decay strongly supports the assignment of the 3(3)(-) level at 2779 keV as low-lying isovector octupole excitation. Microscopic calculations within the quasiparticle phonon model confirm an isovector component in the wave function of the 3(3)(-) level, firmly establishing this fundamental mode of nuclear excitation in near-spherical nuclei.

Third Order Anharmonic Effects in Many-Body Nuclear Quantum Theory

The quantum theory of third order anharmonic effects in the photon production amplitude developed by V.A. Khodel has been generalized for nuclei with pairing and for a more accurate description of low-energy collective phonons. An expression for the transition amplitude between two- and one-phonon states has been obtained. This expression has been compared to the solution of a similar problem considered within the quasiparticle–phonon model. It has been shown that this approach describes a number of new effects, including three- and four-quasiparticle correlations in the ground state (backward-going graphs).