712 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
Correlations in Nuclear Masses
It was recently suggested that the error with respect to experimental data in
nuclear mass calculations is due to the presence of chaotic motion. The theory
was tested by analyzing the typical error size. A more sensitive quantity, the
correlations of the mass error between neighboring nuclei, is studied here. The
results provide further support to this physical interpretation.Comment: 4 pages, 2 figure
The Coherent Crooks Equality
This chapter reviews an information theoretic approach to deriving quantum
fluctuation theorems. When a thermal system is driven from equilibrium, random
quantities of work are required or produced: the Crooks equality is a classical
fluctuation theorem that quantifies the probabilities of these work
fluctuations. The framework summarised here generalises the Crooks equality to
the quantum regime by modeling not only the driven system but also the control
system and energy supply that enables the system to be driven. As is reasonably
common within the information theoretic approach but high unusual for
fluctuation theorems, this framework explicitly accounts for the energy
conservation using only time independent Hamiltonians. We focus on explicating
a key result derived by Johan {\AA}berg: a Crooks-like equality for when the
energy supply is allowed to exist in a superposition of energy eigenstates
states.Comment: 11 pages, 3 figures; Chapter for the book "Thermodynamics in the
Quantum Regime - Recent Progress and Outlook", eds. F. Binder, L. A. Correa,
C. Gogolin, J. Anders and G. Adess
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
Correlation studies of fission fragment neutron multiplicities
We calculate neutron multiplicities from fission fragments with specified
mass numbers for events having a specified total fragment kinetic energy. The
shape evolution from the initial compound nucleus to the scission
configurations is obtained with the Metropolis walk method on the
five-dimensional potential-energy landscape, calculated with the
macroscopic-microscopic method for the three-quadratic-surface shape family.
Shape-dependent microscopic level densities are used to guide the random walk,
to partition the intrinsic excitation energy between the two proto-fragments at
scission, and to determine the spectrum of the neutrons evaporated from the
fragments. The contributions to the total excitation energy of the resulting
fragments from statistical excitation and shape distortion at scission is
studied. Good agreement is obtained with available experimental data on neutron
multiplicities in correlation with fission fragments from U(n,f). At higher neutron energies a superlong fission mode appears which
affects the dependence of the observables on the total fragment kinetic energy.Comment: 12 pages, 10 figure
Semiclassical Theory of Bardeen-Cooper-Schrieffer Pairing-Gap Fluctuations
Superfluidity and superconductivity are genuine many-body manifestations of
quantum coherence. For finite-size systems the associated pairing gap
fluctuates as a function of size or shape. We provide a parameter free
theoretical description of pairing fluctuations in mesoscopic systems
characterized by order/chaos dynamics. The theory accurately describes
experimental observations of nuclear superfluidity (regular system), predicts
universal fluctuations of superconductivity in small chaotic metallic grains,
and provides a global analysis in ultracold Fermi gases.Comment: 4 pages, 2 figure
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
- …