140 research outputs found
Quantum states and specific heat of low-density He gas adsorbed within the carbon nanotube interstitial channels: Band structure effects and potential dependence
We calculate the energy-band structure of a He atom trapped within the
interstitial channel between close-packed nanotubes within a bundle and its
influence on the specific heat of the adsorbed gas. A robust prediction of our
calculations is that the contribution of the low-density adsorbed gas to the
specific heat of the nanotube material shows pronounced nonmonotonic variations
with temperature. These variations are shown to be closely related to the band
gaps in the adsorbate density of states
Systematic model behavior of adsorption on flat surfaces
A low density film on a flat surface is described by an expansion involving
the first four virial coefficients. The first coefficient (alone) yields the
Henry's law regime, while the next three correct for the effects of
interactions. The results permit exploration of the idea of universal
adsorption behavior, which is compared with experimental data for a number of
systems
Quantum virial expansion approach to thermodynamics of He adsorbates in carbon nanotube materials: Interacting Bose gas in one dimension
I demonstrate that He adsorbates in carbon nanotube materials can be
treated as one-dimensional interacting gas of spinless bosons for temperatures
below 8 K and for coverages such that all the adsorbates are in the groove
positions of the carbon nanotube bundles. The effects of adsorbate-adsorbate
interactions are studied within the scheme of virial expansion approach. The
theoretical predictions for the specific heat of the interacting adsorbed gas
are given.Comment: 5 PS figure
Implementation and analysis of list mode algorithm using tubes of response on a dedicated brain and breast PET
In this work we present an innovative algorithm for the reconstruction of PET images based on the List-Mode (LM) technique which improves their spatial resolution compared to results obtained with current MLEM algorithms. This study appears as a part of a large project with the aim of improving diagnosis in early Alzheimer disease stages by means of a newly developed hybrid PET-MR insert. At the present, Alzheimer is the most relevant neurodegenerative disease and the best way to apply an effective treatment is its early diagnosis. The PET device will consist of several monolithic LYSO crystals coupled to SiPM detectors. Monolithic crystals can reduce scanner costs with the advantage to enable implementation of very small virtual pixels in their geometry. This is especially useful for LM reconstruction algorithms, since they do not need a pre-calculated system matrix. We have developed an LM algorithm which has been initially tested with a large aperture (186 mm) breast PET system. Such an algorithm instead of using the common lines of response, incorporates a novel calculation of tubes of response. The new approach improves the volumetric spatial resolution about a factor 2 at the border of the field of view when compared with traditionally used MLEM algorithm. Moreover, it has also shown to decrease the image noise, thus increasing the image quality. © 2012 Elsevier B.V. All rights reserved.This work was supported by the Centre for Industrial Technological Development co-funded by FEDER through the Technology Fund (DREAM Project, IDI-20110718), by the Spanish Plan Nacional de Investigacion Cientifica, Desarrollo e Innovacion Tecnologica (I+D+I) under Grant. No. FIS2010-21216-CO2-01TEO 2008/114.Moliner Martínez, L.; Correcher, C.; González Martínez, AJ.; Conde Castellanos, PE.; Hernández Hernández, L.; Orero Palomares, A.; Rodríguez Álvarez, MJ.... (2013). Implementation and analysis of list mode algorithm using tubes of response on a dedicated brain and breast PET. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 702:129-132. https://doi.org/10.1016/j.nima.2012.08.029S12913270
TomograPy: A Fast, Instrument-Independent, Solar Tomography Software
Solar tomography has progressed rapidly in recent years thanks to the
development of robust algorithms and the availability of more powerful
computers. It can today provide crucial insights in solving issues related to
the line-of-sight integration present in the data of solar imagers and
coronagraphs. However, there remain challenges such as the increase of the
available volume of data, the handling of the temporal evolution of the
observed structures, and the heterogeneity of the data in multi-spacecraft
studies.
We present a generic software package that can perform fast tomographic
inversions that scales linearly with the number of measurements, linearly with
the length of the reconstruction cube (and not the number of voxels) and
linearly with the number of cores and can use data from different sources and
with a variety of physical models: TomograPy
(http://nbarbey.github.com/TomograPy/), an open-source software freely
available on the Python Package Index. For performance, TomograPy uses a
parallelized-projection algorithm. It relies on the World Coordinate System
standard to manage various data sources. A variety of inversion algorithms are
provided to perform the tomographic-map estimation. A test suite is provided
along with the code to ensure software quality. Since it makes use of the
Siddon algorithm it is restricted to rectangular parallelepiped voxels but the
spherical geometry of the corona can be handled through proper use of priors.
We describe the main features of the code and show three practical examples
of multi-spacecraft tomographic inversions using STEREO/EUVI and STEREO/COR1
data. Static and smoothly varying temporal evolution models are presented.Comment: 21 pages, 6 figures, 5 table
Levinson's theorem and scattering phase shift contributions to the partition function of interacting gases in two dimensions
We consider scattering state contributions to the partition function of a
two-dimensional (2D) plasma in addition to the bound-state sum. A partition
function continuity requirement is used to provide a statistical mechanical
heuristic proof of Levinson's theorem in two dimensions. We show that a proper
account of scattering eliminates singularities in thermodynamic properties of
the nonideal 2D gas caused by the emergence of additional bound states as the
strength of an attractive potential is increased. The bound-state contribution
to the partition function of the 2D gas, with a weak short-range attraction
between its particles, is found to vanish logarithmically as the binding energy
decreases. A consistent treatment of bound and scattering states in a screened
Coulomb potential allowed us to calculate the quantum-mechanical second virial
coefficient of the dilute 2D electron-hole plasma and to establish the
difference between the nearly ideal electron-hole gas in GaAs and the strongly
correlated exciton/free-carrier plasma in wide-gap semiconductors such as ZnSe
or GaN.Comment: 10 pages, 3 figures; new version corrects some minor typo
Noise Characteristics of a Four-Jet Impingement Device Inside a Broadband Engine Noise Simulator
The noise generation mechanisms for four directly impinging supersonic jets are investigated employing implicit large eddy simulations with a higher-order accurate weighted essentially non-oscillatory shock-capturing scheme. Impinging jet devices are often used as an experimental apparatus to emulate a broadband noise source. Although such devices have been used in many experiments, a detailed investigation of the noise generation mechanisms has not been conducted before. Thus, the underlying physical mechanisms that are responsible for the generation of sound waves are not well understood. The flow field is highly complex and contains a wide range of temporal and spatial scales relevant for noise generation. Proper orthogonal decomposition of the flow field is utilized to characterize the unsteady nature of the flow field involving unsteady shock oscillations, large coherent turbulent flow structures, and the sporadic appearance of vortex tubes in the center of the impingement region. The causality method based on Lighthill's acoustic analogy is applied to link fluctuations of flow quantities inside the source region to the acoustic pressure in the far field. It will be demonstrated that the entropy fluctuation term in the Lighthill's stress tensor plays a vital role in the noise generation process. Consequently, the understanding of the noise generation mechanisms is employed to develop a reduced-order linear acoustic model of the four-jet impingement device. Finally, three linear acoustic FJID models are used as broadband noise sources inside an engine nacelle and the acoustic scattering results are validated against far-field acoustic experimental data
Ionization degree of the electron-hole plasma in semiconductor quantum wells
The degree of ionization of a nondegenerate two-dimensional electron-hole
plasma is calculated using the modified law of mass action, which takes into
account all bound and unbound states in a screened Coulomb potential.
Application of the variable phase method to this potential allows us to treat
scattering and bound states on the same footing. Inclusion of the scattering
states leads to a strong deviation from the standard law of mass action. A
qualitative difference between mid- and wide-gap semiconductors is
demonstrated. For wide-gap semiconductors at room temperature, when the bare
exciton binding energy is of the order of T, the equilibrium consists of an
almost equal mixture of correlated electron-hole pairs and uncorrelated free
carriers.Comment: 22 pages, 6 figure
Assessment of dosimetric errors induced by deformable image registration methods in 4D pencil beam scanned proton treatment planning for liver tumours
PURPOSE: Respiratory impacts in pencil beam scanned proton therapy (PBS-PT) are accounted by extensive 4D dose calculations, where deformable image registration (DIR) is necessary for estimating deformation vector fields (DVFs). We aim here to evaluate the dosimetric errors induced by different DIR algorithms in their resulting 4D dose calculations by using ground truth(GT)-DVFs from 4DMRI. MATERIALS AND METHODS: Six DIR methods: ANACONDA, Morfeus, B-splines, Demons, CT Deformable, and Total Variation, were respectively applied to nine 4DCT-MRI liver data sets. The derived DVFs were then used as input for 4D dose calculation. The DIR induced dosimetric error was assessed by individually comparing the resultant 4D dose distributions to those obtained with GT-DVFs. Both single-/three-field plans and single/rescanned strategies were investigated. RESULTS: Differences in 4D dose distributions among different DIR algorithms, and compared to the results using GT-DVFs, were pronounced. Up to 40 % of clinically relevant dose calculation points showed dose differences of 10 % or more between the GT. Differences in V95(CTV) reached up to 11.34 ± 12.57 %. The dosimetric errors became in general less substantial when applying multiple-field plans or using rescanning. CONCLUSION: Intrinsic geometric errors by DIR can influence the clinical evaluation of liver 4D PBS-PT plans. We recommend the use of an error bar for correctly interpreting individual 4D dose distributions
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