902 research outputs found
Quasi-particle spectra, absorption spectra, and excitonic properties of sodium iodide and strontium iodide from many-body perturbation theory
We investigate the basic quantum mechanical processes behind non-proportional
response of scintillators to incident radiation responsible for reduced
resolution. For this purpose, we conduct a comparative first principles study
of quasiparticle spectra on the basis of the approximation as well as
absorption spectra and excitonic properties by solving the Bethe-Salpeter
equation for two important systems, NaI and SrI. The former is a standard
scintillator material with well-documented non-proportionality while the latter
has recently been found to exhibit a very proportional response. We predict
band gaps for NaI and SrI of 5.5 and 5.2 eV, respectively, in good
agreement with experiment. Furthermore, we obtain binding energies for the
groundstate excitons of 216 meV for NaI and 19525 meV for SrI. We
analyze the degree of exciton anisotropy and spatial extent by means of a
coarse-grained electron-hole pair-correlation function. Thereby, it is shown
that the excitons in NaI differ strongly from those in SrI in terms of
structure and symmetry, even if their binding energies are similar.
Furthermore, we show that quite unexpectedly the spatial extents of the highly
anisotropic low-energy excitons in SrI in fact exceed those in NaI by a
factor of two to three in terms of the full width at half maxima of the
electron-hole pair-correlation function.Comment: 10 pages, 9 figure
The Cranked Nilsson-Strutinsky versus the Spherical Shell Model: A Comparative Study of pf-Shell Nuclei
A comparative study is performed of a deformed mean field theory, represented
by the cranked Nilsson-Strutinsky (CNS) model, and the spherical shell model.
Energy spectra, occupation numbers, B(E2)-values, and spectroscopic quadrupole
moments in the light pf shell nuclei are calculated in the two models and
compared. The result is also compared to available experimental data which are
generally well described by the shell model. Although the Nilsson-Strutinsky
calculation does not include pairing, both the subshell occupation numbers and
quadrupole properties are found to be rather similar in the two models. It is
also shown that ``unpaired'' shell model calculations produce very similar
energies as the CNS at all spins. The role of the pairing energy in the
description of backbending and signature splitting in odd-mass nuclei is also
discussed.Comment: 14 pages, 20 figures, submitted to Phys.Rev.
Spreading widths of giant resonances in spherical nuclei: damped transient response
We propose the universal approach to describe spreading widths of monopole,
dipole and quadrupole giant resonances in heavy and superheavy spherical
nuclei. Our approach is based on the ideas of the random matrix distribution of
the coupling between one-phonon and two-phonon states generated in the random
phase approximation. We use the Skyrme interaction SLy4 as our model
Hamiltonian to create a single-particle spectrum and to analyze excited states
of the doubly magic nuclei Sn, Pb and 126. Our results
demonstrate that the universal approach enables to describe gross structure of
the spreading widths of the considered giant resonances.Comment: 6 pages, 2 figure
Nuclear pairing and Coriolis effects in proton emitters
We introduce a Hartree-Fock-Bogoliubov mean-field approach to treat the
problem of proton emission from a deformed nucleus. By substituting a rigid
rotor in a particle-rotor-model with a mean-field we obtain a better
description of experimental data in Ho. The approach also elucidates
the softening of kinematic coupling between particle and collective rotation,
the Coriolis attenuation problem.Comment: 2 pages, 1 figur
Adiabatic geometric phases in hydrogenlike atoms
We examine the effect of spin-orbit coupling on geometric phases in
hydrogenlike atoms exposed to a slowly varying magnetic field. The marginal
geometric phases associated with the orbital angular momentum and the intrinsic
spin fulfill a sum rule that explicitly relates them to the corresponding
geometric phase of the whole system. The marginal geometric phases in the
Zeeman and Paschen-Back limit are analyzed. We point out the existence of nodal
points in the marginal phases that may be detected by topological means.Comment: Clarifying material added, one figure removed, journal reference
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Two-phonon structures for beta-decay theory
The -decay rates of Ca have been studied within a microscopic
model, which is based on the Skyrme interaction T45 to construct
single-particle and phonon spaces. We observe a redistribution of the
Gamow-Teller strength due to the phonon-phonon coupling, considered in the
model. For Sc, the spin-parity of the ground state is found to be .
We predict that the half-life of Ca is 0.3 ms, while the total
probability of the emission is 6.1%. Additionally, the random
matrix theory has been applied to analyse the statistical properties of the
spectrum populated in the -decay to elucidate the obtained
results.Comment: 4 pages, 1 figure, proceedings of International Conference on Nuclear
Structure and Related Topics (NSRT18), June 3-9 2018, Burgas, Bulgari
Origin of resolution enhancement by co-doping of scintillators: Insight from electronic structure calculations
It was recently shown that the energy resolution of Ce-doped LaBr
scintillator radiation detectors can be crucially improved by co-doping with
Sr, Ca, or Ba. Here we outline a mechanism for this enhancement on the basis of
electronic structure calculations. We show that (i) Br vacancies are the
primary electron traps during the initial stage of thermalization of hot
carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are
associated with deep levels; (iii) Sr doping increases the Br vacancy
concentration by several orders of magnitude; (iv) binds
to resulting in a stable neutral complex; and (v) association
with Sr causes the deep vacancy level to move toward the conduction band edge.
The latter is essential for reducing the effective carrier density available
for Auger quenching during thermalization of hot carriers. Subsequent
de-trapping of electrons from complexes then
can activate Ce dopants that have previously captured a hole leading to
luminescence. This mechanism implies an overall reduction of Auger quenching of
free carriers, which is expected to improve the linearity of the photon light
yield with respect to the energy of incident electron or photon
Microscopic Origin of Quantum Chaos in Rotational Damping
The rotational spectrum of Yb is calculated diagonalizing different
effective interactions within the basis of unperturbed rotational bands
provided by the cranked shell model. A transition between order and chaos
taking place in the energy region between 1 and 2 MeV above the yrast line is
observed, associated with the onset of rotational damping. It can be related to
the higher multipole components of the force acting among the unperturbed
rotational bands.Comment: 7 pages, plain TEX, YITP/K-99
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