4,735 research outputs found
A method to correct differential nonlinearities in subranging analog-to-digital converters used for digital gamma-ray spectroscopy
The influence on -ray spectra of differential nonlinearities (DNL) in
subranging, pipelined analog-to-digital converts (ADCs) used for digital
-ray spectroscopy was investigated. The influence of the DNL error on
the -ray spectra, depending on the input count-rate and the dynamic
range has been investigated systematically. It turned out, that the DNL becomes
more significant in -ray spectra with larger dynamic range of the
spectroscopy system. An event-by-event offline correction algorithm was
developed and tested extensively. This correction algorithm works especially
well for high dynamic ranges
Low cost digital electronics for isotope analysis with microcalorimeters - final report
The overall goal of the Phase I research was to demonstrate that the digital readout electronics and filter algorithms developed by XIA for use with HPGe detectors can be adapted to high precision, cryogenic gamma detectors (microcalorimeters) and not only match the current state of the art in terms of energy resolution, but do so at a significantly reduced cost. This would make it economically feasible to instrument large arrays of microcalorimeters and would also allow automation of the setup, calibration and operation of large numbers of channels through software. We expected, and have demonstrated, that this approach would further allow much higher count rates than the optimum filter algorithms currently used. In particular, in measurements with a microcalorimeter at LLNL, the adapted Pixie-16 spectrometer achieved an energy resolution of 0.062%, significantly better than the targeted resolution of 0.1% in the Phase I proposal and easily matching resolutions obtained with LLNL readout electronics and optimum filtering (0.066%). The theoretical maximum output count rate for the filter settings used to achieve this resolution is about 120cps. If the filter is adjusted for maximum throughput with an energy resolution of 0.1% or better, rates of 260cps are possible. This is 20-50 times higher than the maximum count rates of about 5cps with optimum filters for this detector. While microcalorimeter measurements were limited to count rates of ~1.3cps due to the strength of available sources, pulser measurements demonstrated that measured energy resolutions were independent of counting rate to output counting rates well in excess of 200cps or more.. We also developed a preliminary hardware design of a spectrometer module, consisting of a digital processing core and several input options that can be implemented on daughter boards. Depending upon the daughter board, the total parts cost per channel ranged between 27, resulting in projected product prices of 160 per channel. This demonstrates that a price of $100 per channel is economically very feasible for large microcalorimeter arrays
Magnetic Phase Diagrams of Manganites-like Local-Moment Systems with Jahn-Teller distortions
We use an extended two-band Kondo lattice model (KLM) to investigate the
occurrence of different (anti-)ferromagnetic phases or phase separation
depending on several model parameters. With regard to CMR-materials like the
manganites we have added a Jahn-Teller term, direct antiferromagnetic coupling
and Coulomb interaction to the KLM. The electronic properties are
self-consistently calculated in an interpolating self-energy approach with no
restriction to classical spins and going beyond mean-field treatments. Further
on we do not have to limit the Hund's coupling to low or infinite values.
Zero-temperature phase diagrams are presented for large parameter intervals.
There are strong influences of the type of Coulomb interaction (intraband,
interband) and of the important parameters (Hund's coupling, direct
antiferromagnetic exchange, Jahn-Teller distortion), especially at intermediate
couplings.Comment: 11 pages, 9 figures. Accepted for publication in Phys. Rev.
Questioning the existence of a unique ground state structure for Si clusters
Density functional and quantum Monte Carlo calculations challenge the
existence of a unique ground state structure for certain Si clusters. For Si
clusters with more than a dozen atoms the lowest ten isomers are close in
energy and for some clusters entropic effects can change the energetic ordering
of the configurations. Isotope pure configurations with rotational symmetry and
symmetric configurations containing one additional isotope are disfavored by
these effects. Comparisons with experiment are thus difficult since a mixture
of configurations is to be expected at thermal equilibrium
Collisions of rigidly rotating disks of dust in General Relativity
We discuss inelastic collisions of two rotating disks by using the
conservation laws for baryonic mass and angular momentum. In particular, we
formulate conditions for the formation of a new disk after the collision and
calculate the total energy loss to obtain upper limits for the emitted
gravitational energy.Comment: 30 pages, 9 figure
Importance of high-angular-momentum channels in pseudopotentials for quantum Monte Carlo
Quantum Monte Carlo methods provide in principle a highly accurate treatment of the many-body problem of calculating the ground and excited states of condensed systems. In practice, however, uncontrolled errors, such as those arising from the fixed-node and pseudopotential approximations can be problematic. We show that the accuracy of some quantum Monte Carlo calculations is limited by the properties of currently available pseudopotentials. The use of pseudopotentials involves several approximations, and we will focus on one that is relatively simple to correct during the pseudopotential design phase. It is necessary to include angular-momentum channels in the pseudopotential for excited angular-momentum states and to choose the local channel appropriately to obtain accurate results. Variational and diffusion Monte Carlo calculations for Zn, O, and Si atoms and ions demonstrate these issues. Adding higher-angular-momentum channels into the pseudopotential description reduces such errors without a significant increase in computational cost
First phylogenetic analyses of galaxy evolution
The Hubble tuning fork diagram, based on morphology, has always been the
preferred scheme for classification of galaxies and is still the only one
originally built from historical/evolutionary relationships. At the opposite,
biologists have long taken into account the parenthood links of living entities
for classification purposes. Assuming branching evolution of galaxies as a
"descent with modification", we show that the concepts and tools of
phylogenetic systematics widely used in biology can be heuristically transposed
to the case of galaxies. This approach that we call "astrocladistics" has been
first applied to Dwarf Galaxies of the Local Group and provides the first
evolutionary galaxy tree. The cladogram is sufficiently solid to support the
existence of a hierarchical organization in the diversity of galaxies, making
it possible to track ancestral types of galaxies. We also find that morphology
is a summary of more fundamental properties. Astrocladistics applied to
cosmology simulated galaxies can, unsurprisingly, reconstruct the correct
"genealogy". It reveals evolutionary lineages, divergences from common
ancestors, character evolution behaviours and shows how mergers organize galaxy
diversity. Application to real normal galaxies is in progress. Astrocladistics
opens a new way to analyse galaxy evolution and a path towards a new
systematics of galaxies
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