Letteratura scientifica selezionata sul tema "Quasiparticle Random Phase Approximation (QRPA)"

The ground state of a many body Hamiltonian considered in the quasiparticle representation is redefined by accounting for the quasiparticle quadrupole pairing interaction. The residual interaction of the newly defined quasiparticles is treated by the quasiparticle random phase approximation (QRPA). Solutions of the resulting equations exhibit specific features. In particular, there is no interaction strength where the first root is vanishing. A comparison with other renormalization methods is presented. Application to a single [Formula: see text]-shell allows for the results interpretation by comparing them with those obtained by exact calculations.

RPA and its quasiparticle generalization (QRPA) have been widely used to study electromagnetic transitions and beta decays in medium and heavy nuclei, being the pn-QRPA charge exchange mode extensively employed in the description of single and double beta decays in vibrational nuclei. However develops a collapse, i.e. it presents imaginary eigen-values for strengths beyond a critical value of the force. Extensions called renormalized QRPA (RQRPA) do not develop any collapse going beyond the simplest quasiboson approximation, however they present several drawbacks which will be analyzed.

One of the main methods used to microscopically describe collective states in atomic nuclei is the quasiparticle random-phase approximation (QRPA). However, due to its high computational cost, systematic studies covering the full nuclear chart are rare. In this work we show the first results of our systematic large-scale QRPA calculations. We do this by means of the quasiparticle finite-amplitude method (QFAM), which significantly reduces computation times. We use two kinds of interactions, the covariant DD-PC1 and a novel chiral interaction.

The nuclear structure physics of double beta decay transitions is reviewed starting from the consideration of fundamental symmetries of the nuclear many body problem. The problems found in the use of the Quasiparticle Random Phase Approximation (QRPA) and related approximations, in dealing with the calculation of nuclear double beta decay observables, are understood in terms of the mixing between isospin collective and intrinsic variables.

Low-frequency modes of excitation in deformed neutron-rich nuclei are studied by the use of the deformed quasiparticle-random-phase approximation (QRPA) based on the coordinate-space Hartree–Fock–Bogoliubov formalism. We investigate the microscopic structure of the low-lying Kπ = 0+ modes in neutron-rich Mg , Cr and Fe isotopes. It is found that the spatially extended structure of neutron quasiparticle wave functions around the Fermi level brings about a striking enhancement of the transition strengths. It is also found that the fluctuation of the pairing field plays an important role in generating coherence among two-quasiparticle excitations of neutron. Recent progress towards the self-consistent deformed QRPA calculation using the Skyrme density functional is also presented.

Abstract Isoscalar monopole (IS0) and dipole (IS1) states located below and around the alpha-particle threshold Sα in prolate 24Mg are investigated within fully self-consistent Skyrme Quasiparticle Random-Phase Approximation (QRPA) approach. It is shown that these states demonstrate strong mean-field (MF) effects: dipole vorticity, deformation-induced monopole-quadrupole (IS0/IS2) and dipole-octupole (IS1/IS3) coupling. At the same time, the density distributions show precursors of a cluster structure. So we get a cluster/MF duality with a strong dominance of MF impact. IS0 QRPA results are compared with iThemba Lab and RCNP (α, α′) data.

Abstract The excitation of pygmy dipole resonance (PDR) and giant dipole resonance (GDR) in even–even 154–164Dy isotopes is examined through quasiparticle random-phase approximation (QRPA) with the effective interactions that restore the broken translational and Galilean invariances. In each isotope, an electric response emerges by showing ample distribution at energies below and above 10 MeV. We, therefore, study the transition cross-sections and probabilities, photon strength functions, transition strengths, isospin character, and collectivity of the predicted E1 responses.

We investigate tensor effects in pygmy dipole excitations for the case of neutron-rich nuclei 68 Ni and 124 Sn using effective nucleon–nucleon Skyrme interaction. We use the Hartree–Fock–Bogoliubov (HFB) theory and employ the quasiparticle random phase approximation (QRPA). We calculate and compare the PDR and also GDR strength in the PDR–GDR energy region for QRPA calculations with and without tensor correlations. The most obvious results for the dipole excitations calculations are strongly dependent on the tensor terms. We see that the tensor correlations are more active at around 14–20 MeV , especially for the neutron-rich nuclei 68 Ni . We also compare the PDR calculations with their experimental results for the different proton–neutron tensor coupling constants.

Large liquid xenon detectors aiming for dark matter direct detection will soon become viable tools also for investigating neutrino physics. Information on the effects of nuclear structure in neutrino-nucleus scattering can be important in distinguishing neutrino backgrounds in such detectors. We perform calculations for differential and total cross sections of neutral-current neutrino scattering off the most abundant xenon isotopes. The nuclear-structure calculations are made in the nuclear shell model for elastic scattering and also in the quasiparticle random-phase approximation (QRPA) and microscopic quasiparticle-phonon model (MQPM) for both elastic and inelastic scattering. Using suitable neutrino energy distributions, we compute estimates of total averaged cross sections for ​8B solar neutrinos and supernova neutrinos.

The neutrinoless muon-to-electron conversion in nuclei is studied by using the renormalized quasiparticle random-phase approximation (RQRPA). This generalization of RPA is more reliable for the extremely small (μ-,e-) transition matrix elements than the ordinary QRPA because it restores the Pauli principle to a large extent. We apply the method to a set of nuclei throughout the periodic table, but we specifically investigate the 48Ti and 208Pb nuclei which are currently used as stopping targets at the PSI μ-e conversion experiments with the SINDRUM II spectrometer.

Cette thèse présente trois aspects centrés autour de la QRPA (Quasiparticle Random Phase Approximation).Le premier consiste en l'utilisation d'un code à symétrie axiale pour confronter des données calculéesà des résultats expérimentaux, et pour alimenter un code microscopique de réactions. Cette étapeest l'occasion d'analyser la spectroscopie du noyau à basse énergie (quelques dizaines de MeV), etplus spécifiquement (mais pas uniquement) la chaîne isotopique de l'étain (Z=50). Le second facetterepose sur l'amélioration d'un formalisme de calcul des opérateurs de transitions éléctromagnétiquesmultipolaires, et d'une méthode de généralisation du calcul de ces opérateurs permettant de faciliterla programmation en uniformisant le code pour les différentes multipolarités. Finalement, afin dedépasser la contrainte de la symétrie axiale, un nouveau code de calcul en symétrie triaxiale a étédéveloppé. Ses caractéristiques et son développement sont présentés, suivis des premiers résultatsdus à son exploitation
This thesis presents three aspects centered around the QRPA (Quasiparticle Random Phase Approximation).The first consists in the use of an axial code to confront computed data with experimental results andto feed a microscopic reaction code. This step is a chance to analyse low-energy spectroscopy (fewtens of MeV) of some nuclei, and more precisely (but not exclusively) the tin isotopic chain (Z=50).The second one relies on the improvement of the formalism to calculate multipolar electromagnetictransition operators, and a method to consolidate the computation of these operators, allowing toease the programming by unifying the code for different multipolarities. Finally, in order to overcomethe axial symmetry constraint, a new triaxial code has been developed. Its assets and developmentare presented, followed by the first batch of results

La quête d'une description microscopique complète du noyau atomique reste un problème ouvert après près d'un siècle de recherche. Les divers phénomènes présents dans le noyau, qui découlent principalement de sa nature quantique à plusieurs corps, ont conduit à la prolifération de modèles, chacun se spécialisant dans la description d'un ensemble donné de caractéristiques nucléaires. Pour comprendre pleinement le comportement collectif d'un point de vue microscopique, il est essentiel d'aller au-delà de l'approche statique du champ moyen. Toutefois, en raison principalement du coût de calcul élevé nécessaire, il existe relativement peu de méthodes de ce type qui permettent de réaliser des études systématiques sur l'ensemble de la carte nucléaire. Une exception notable est la méthode QRPA (Quasiparticle Random Phase Approximation), qui permet de décrire sur un pied d'égalité les excitations nucléaires individuelles et collectives tout en incorporant les effets d'appariement. Des études antérieures ont été réalisées avec la force Gogny D1M pour produire des fonctions de force gamma pour tous les noyaux. Néanmoins, pour une compréhension plus complète, l'utilisation d'autres interactions et approches au sein de la QRPA est primordiale. Dans cette thèse, de nouvelles études systématiques QRPA sont présentées, ainsi que de nouveaux développements numériques et formels autour de la QRPA. Tout d'abord, deux études systématiques des transitions gamma E1 sont discutées. Ici, le problème QRPA est abordé en utilisant la méthode des amplitudes finies (FAM), qui permet l'évaluation rapide des fonctions de force lissées. La première étude utilise le lagrangien effectif covariant DD-PC1 pour effectuer des calculs approfondis sur l'ensemble de la carte nucléaire. En outre, une autre étude examine les mêmes transitions dans des noyaux légers et de masse moyenne en utilisant des interactions chirales, qui fournissent une caractérisation réaliste de la force internucléon fondée sur la chromodynamique quantique à travers de la théorie effective des champs chirale. En particulier, nous présentons les tout premiers résultats de la QRPA chirale utilisant un champ moyen déformé triaxialement, explorant l'impact de cette déformation sur la réponse de la QRPA dans ²⁴Mg et ³²S. Au-delà des études systématiques de la QRPA, deux autres développements sont présentés. Tout d'abord, nous nous attaquons au problème de l'obtention d'états excités QRPA exacts à l'aide de l'approche FAM. Dans sa formulation originale, FAM était utilisé pour calculer les fonctions de force, tandis que l'obtention des états propres QRPA n'était possible que par une procédure de post-traitement. Dans cette thèse, nous introduisons une nouvelle méthode basée sur l'algorithme de Jacobi-Davidson, qui permet le calcul efficace de plusieurs états propres QRPA ciblés avec un temps de calcul significativement réduit par rapport à l'approche QRPA matricielle. Enfin, nous proposons une nouvelle formule simple pour corriger la violation du principe de Pauli dans la QRPA, qui est appliquée pour calculer les énergies de corrélation
The quest for a comprehensive microscopic description of the atomic nucleus remains an open problem after almost a century of research. The diverse phenomena present within the nucleus, primarily arising from its many-body quantum nature, have led to the proliferation models, each specializing in describing a given set of nuclear features. To fully understand collective behaviour from a microscopic perspective, it is essential to move beyond a static mean-field approach. However, due mainly to the high computational cost required to do so, relatively few of such methods exist that provide systematic studies across the entire nuclear chart. One notable exception is the Quasiparticle Random Phase Approximation (QRPA) method, which allows for the description of both single-particle and collective nuclear excitations in the same footing while incorporating pairing effects. Previous studies have been carried out with the Gogny D1M force to produce gamma-strength functions for all nuclei. Nevertheless, for a more complete understanding, the use of other effective interactions and approaches within QRPA is paramount. In this thesis, new systematic QRPA studies are presented, alongside some new numerical and formal developments around QRPA.First, two systematic studies of gamma E1 transitions are discussed. Here, the QRPA problem is addressed using the Finite Amplitude Method (FAM), which enables the rapid evaluation of smoothed strength functions. The first study employs the covariant effective Lagrangian DD-PC1 to conduct extensive calculations across the nuclear chart. Additionally, another study investigates the same transitions in light and mid-mass nuclei using chiral interactions, which provide a realistic characterization of the internucleon force grounded in Quantum Chromodynamics through effective field theory. Notably, we present the first-ever chiral-QRPA results using a triaxially deformed mean-field, exploring the impact of this deformation on the QRPA response in ²⁴Mg and ³²S. Beyond the systematic QRPA studies, two further developments are presented. Firstly, we tacklethe problem of obtaining exact QRPA excited states using the FAM approach. In its original formulation, FAM was used to calculate strength functions, while obtainig the QRPA eigenstates was only possible via a post-procesing procedure. In this thesis, we introduce a new method based on the Jacobi-Davidson algorithm, which enables the efficient calculation of several targeted QRPA eigenstates with significantly reduced computational time compared to the matrix QRPA approach. Lastly, we propose a new straightforward formula to correct for the violation of the Pauli principle in QRPA, which is applied to compute correlation energies