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[en] A laser-induced fast-ionization wave (FIW) is the post laser-breakdown phenomenon observed when laser intensity is 1013 W m-2 in atmospheric pressure. Its propagation velocity exceeds 10 km s-1. The discharge structure has similarity with streamers and sprites. An earlier study demonstrated that the velocity of the FIW is proportional to the laser intensity. The electron temperature and density in the FIW were found using emission spectroscopy. In this study, a collisional-radiative model with 79 levels of atomic population was developed to investigate the dynamics of excitation population density and the neutral particle temperature in a FIW. Using the gas temperature-dependence of the depopulation frequency for energy level p = 5 (4s'[1/2]1), the reasonable neutral particle temperature at the front of FIW is obtained by comparing experimentally observed spectra in the fine structure of 4s-4p. (author)
[en] Strongly correlated many-body systems often display the emergence of simple patterns and regular behaviour of their global properties. Phenomena such as clusterization, collective motion and appearance of shell structures are commonly observed across different size, time, and energy scales in our universe. Although at the microscopic level their individual parts are described by complex interactions, the collective behaviour of these systems can exhibit strikingly regular patterns. This contribution provides an overview of the experimental signatures that are commonly used to identify the emergence of shell structures and collective phenomena in distinct physical systems. Examples in macroscopic systems are presented alongside features observed in atomic nuclei. The discussion is focused on the experimental trends observed for exotic nuclei in the vicinity of nuclear closed-shells, and the new challenges that recent experiments have posed in our understanding of emergent phenomena in nuclei.
[en] Deformation-energy surfaces of 54 even-even isotopes of Pt, Hg and Pb nuclei with neutron numbers up to 126 are investigated within a macroscopic-microscopic model based on the Lublin-Strasbourg-Drop macroscopic energy and shell plus pairing-energy corrections obtained from a Yukawa-folded mean-field potential at the desired deformation. A new, rapidly converging Fourier shape parametrization is used to describe nuclear shapes. The stability of shape isomeric states with respect to non-axial and higher-order deformations is investigated.
[en] E1 transition properties such as the reduced transition probabilities, excitation energies and photon–absorption cross-sections have been theoretically investigated for Ta nucleus within the framework of Translational and Galileo Invariant-Quasiparticle Phonon Nuclear Model (TGI-QPNM). The model Hamiltonian includes the single-particle and the isovector dipole–dipole interaction terms along with the restoration forces. The strength of the isovector dipole–dipole interaction has been chosen to be χ=500/A MeV⋅fm. Theoretical calculations show that in addition to the M1 excitations, there is considerable amount of E1 transitions especially between 2.6–3 MeV, which gives remarkable contribution to the fragmentation in the low-energy region of the dipole spectrum. Thus, the agreement between theory and experiment in terms of the fragmentation increases. Furthermore, the photon–absorption cross-sections in the Pigmy Dipole Resonance (PDR) region below the neutron separation energy (S) is compatible with experimental data.
[en] The exact solution of spherical mean-field plus multi-pair interaction model with two non-degenerate j-orbits, which is an extension of the widely used standard (two-body) pairing model, is derived based on the Bethe–Richardson–Gaudin approach. The Bethe–Richardson–Gaudin equations in determining eigenstates and the corresponding eigen-energies of the model are provided and exemplified with up to three-pair interactions. With a suitable parameterization of the overall multi-pair interaction strengths, the model with one adjustable parameter and valence nucleons confined in the 1d and 0g orbits is applied to fit binding energies of Sn. It is shown that the ground-state occupation probabilities of nucleon pairs calculated from this model and those from the standard pairing model are almost the same with perfect ground-state overlap of the two models. A noticeable feature of the multi-pair interactions is that the even-odd staggering of pairing interaction strength appearing in the standard pairing model due to the Pauli-blocking is suppressed. As the result, the pairing interaction strength of the model only depends on the number of valence nucleon pairs in the system.
[en] In multiple Compton scattering, the incident gamma photon is scattered more than once due to finite dimensions of the sample. It serves as a noise for the exact evaluation of electron momentum distribution in Compton profile, Compton cross section and non-destructive testing of in-assessable large samples. To overcome these difficulties, there is a need to characterise the contribution of multiple scattering as a function of atomic number, scattering angle and incident gamma photon energy. In the present work, measurements are carried out to investigate the multiple scattering of 662 keV photons with tin sample of different thickness in the forward and backward hemispheres. The scattered photon flux is recorded by a NaI(Tl) scintillation detector and with an HPGe solid-state detector to check for the trend of saturation thickness as a function of resolution of the detector. The number of multiply scattered photons is found to be increasing with the sample thickness up to a particular thickness value called saturation thickness beyond which it remains constant with increase in target thickness. The effect of detector collimator size on multiply scattered photons is also studied. Both the detectors show the same trend of saturation thickness as a function of thickness. Angular distribution of multiply scattered photons shows symmetry around 90° scattering angle. The low energy side of multiple scattered photon spectrums is also characterised as a function of sample thickness and order of scattering. Monte Carlo calculations support the present experimental results. (author)
[en] The constrained Hartree-Fock-Bogoliubov approximation, based on the recent parametrization D1M* of the Gogny energy density functional, is used to describe fission in 435 superheavy nuclei. The Gogny-D1M* parametrization is benchmarked against available experimental data on inner and second barrier heights, excitation energies of the fission isomers and half-lives in a selected set of Pu, Cm, Cf, Fm, No, Rf, Sg, Hs and Fl nuclei. Results are also compared with those obtained with the Gogny-D1M energy density functional. A detailed study of the minimal energy fission paths is carried out for isotopic chains with atomic numbers 100 ≤Z≤ 126 including very neutron-rich sectors up to around 4 MeV from the two-neutron driplines. Single-particle energies, ground state deformations, pairing correlations, two-nucleon separation energies and barrier heights are also discussed. In addition to fission paths, the constrained Hartree-Fock-Bogoliubov framework provides collective masses and zero-point quantum rotational and vibrational energies. Those quantities are building blocks within the Wentzel-Kramer-Brillouin formalism employed to evaluate the systematic of the spontaneous fission half-lives tSF. The competition between spontaneous fission and -decay is studied, through the computation of the -decay half-lives t using a parametrization of the Viola-Seaborg formula. From the comparison with the available experimental data and the results obtained with other theoretical approaches, it is concluded that D1M* represents a reasonable starting point to describe fission in heavy and superheavy nuclei.
[en] We present extensions and new developments of the in-medium no-core shell model (IMNCSM), which is a novel ab initio many-method that merges the multi-reference in-medium similarity renormalization group (MR-IM-SRG) with the no-core shell model (NCSM) - two complementary and successful ab initio many-body methods. Within the IM-NCSM framework, the MR-IM-SRG employs a correlated NCSM reference state and unitarily transforms observables such that the reference state is decoupled. Consequently, the model-space convergence of a subsequent NCSM calculation is substantially accelerated - demonstrating the power of the IM-SRG decoupling scheme - and the IM-NCSM can treat nuclei that are out of reach for traditional NCSM calculations. In earlier applications we already employed the IM-NCSM for addressing scalar observables w.r.t. ground and excited states in even open-shell nuclei, however, this initial formulation of the IM-NCSM had several restrictions that we eliminate in this work. Due to the spherical formulation of the IM-SRG equations - which is mandatory from a computational point of view - the total angular momentum of the reference state is required to vanish and, thus, the IM-NCSM was restricted to the treatment of even nuclei. The particle-attached/particle-removed extension overcomes this restriction and makes odd nuclei accessible. Furthermore, the spherical formulation of the IM-SRG equations did not account for non-scalar operators and, therefore, the consistent transformation of electromagnetic observables was not possible. By deriving and implementing the IM-SRG equations corresponding to non-scalar observables, we open up the possibility to calculate electromagnetic observables from the IM-NCSM. These observables are sensitive to different aspects of the wave functions and, therefore, ideal for validating theoretical models and new opportunities for fruitful collaborations with experimentalists are opened up. For the transformation of observables we implemented a Magnus-type transformation, which determines the generator for the IM-SRG transformation and greatly reduces the computational effort. Considering numerical applications, we employ the IM-NCSM for the calculation of groundstate energies, excitation energies, radii, magnetic dipole moments, electric quadrupole moments, B(M1) transitions, and B(E2) transitions, where we study various medium-mass nuclei up to calcium isotopes. These calculations are already converged at very small model-space sizes-showing the great advantage of the IM-NCSM - and the results are compatible with large-scale NCSM calculations. These applications demonstrate that the IM-NCSM is now capable of addressing the full range of nuclear structure observables - including spectroscopic and electromagnetic observables - in fully open-shell nuclei.