2,152 research outputs found
Hygroscopicity distribution concept for measurement data analysis and modeling of aerosol particle mixing state with regard to hygroscopic growth and CCN activation
This paper presents a general concept and mathematical framework of particle hygroscopicity distribution for the analysis and modeling of aerosol hygroscopic growth and cloud condensation nucleus (CCN) activity. The cumulative distribution function of particle hygroscopicity, H(κ, Dd) is defined as the number fraction of particles with a given dry diameter, Dd, and with an effective hygroscopicity parameter smaller than the parameter κ. From hygroscopicity tandem differential mobility analyzer (HTDMA) and size-resolved CCN measurement data, H(κ, Dd) can be derived by solving the κ-Köhler model equation. Alternatively, H(κ, Dd) can be predicted from measurement or model data resolving the chemical composition of single particles.
A range of model scenarios are used to explain and illustrate the concept, and exemplary practical applications are shown with HTDMA and CCN measurement data from polluted megacity and pristine rainforest air. Lognormal distribution functions are found to be suitable for approximately describing the hygroscopicity distributions of the investigated atmospheric aerosol samples.
For detailed characterization of aerosol hygroscopicity distributions, including externally mixed particles of low hygroscopicity such as freshly emitted soot, we suggest that size-resolved CCN measurements with a wide range and high resolution of water vapor supersaturation and dry particle diameter should be combined with comprehensive HTDMA measurements and size-resolved or single-particle measurements of aerosol chemical composition, including refractory components. In field and laboratory experiments, hygroscopicity distribution data from HTDMA and CCN measurements can complement mixing state information from optical, chemical and volatility-based techniques. Moreover, we propose and intend to use hygros
Time Dependence of Chemical Freeze-out in Relativistic Heavy Ion Collisions
We investigate chemical and thermal freeze-out time dependencies for strange
particle production for CERN SPS heavy ion collisions in the framework of a
dynamical hadronic transport code. We show that the Lambda yield changes
considerably after hadronization in the case of Pb+Pb collisions, whereas for
smaller system sizes (e.g. S+S) the direct particle production dominates over
production from inelastic rescattering. Chemical freeze-out times for strange
baryons in Pb+Pb are smaller than for non-strange baryons, but they are still
sufficiently long for hadronic rescattering to contribute significantly to the
final Lambda yield. Based on inelastic and elastic cross section estimates we
expect the trend of shorter freeze-out times (chemical and kinetic), and thus
less particle production after hadronization, to continue for multi-strange
baryons.Comment: 10 pages, 7 postscript figure
Effect of Statistical Fluctuation in Monte Carlo Based Photon Beam Dose Calculation on Gamma Index Evaluation
The gamma-index test has been commonly adopted to quantify the degree of
agreement between a reference dose distribution and an evaluation dose
distribution. Monte Carlo (MC) simulation has been widely used for the
radiotherapy dose calculation for both clinical and research purposes. The goal
of this work is to investigate both theoretically and experimentally the impact
of the MC statistical fluctuation on the gamma-index test when the fluctuation
exists in the reference, the evaluation, or both dose distributions. To the
first order approximation, we theoretically demonstrated in a simplified model
that the statistical fluctuation tends to overestimate gamma-index values when
existing in the reference dose distribution and underestimate gamma-index
values when existing in the evaluation dose distribution given the original
gamma-index is relatively large for the statistical fluctuation. Our numerical
experiments using clinical photon radiation therapy cases have shown that 1)
when performing a gamma-index test between an MC reference dose and a non-MC
evaluation dose, the average gamma-index is overestimated and the passing rate
decreases with the increase of the noise level in the reference dose; 2) when
performing a gamma-index test between a non-MC reference dose and an MC
evaluation dose, the average gamma-index is underestimated when they are within
the clinically relevant range and the passing rate increases with the increase
of the noise level in the evaluation dose; 3) when performing a gamma-index
test between an MC reference dose and an MC evaluation dose, the passing rate
is overestimated due to the noise in the evaluation dose and underestimated due
to the noise in the reference dose. We conclude that the gamma-index test
should be used with caution when comparing dose distributions computed with
Monte Carlo simulation
Towards an Achievable Performance for the Loop Nests
Numerous code optimization techniques, including loop nest optimizations,
have been developed over the last four decades. Loop optimization techniques
transform loop nests to improve the performance of the code on a target
architecture, including exposing parallelism. Finding and evaluating an
optimal, semantic-preserving sequence of transformations is a complex problem.
The sequence is guided using heuristics and/or analytical models and there is
no way of knowing how close it gets to optimal performance or if there is any
headroom for improvement. This paper makes two contributions. First, it uses a
comparative analysis of loop optimizations/transformations across multiple
compilers to determine how much headroom may exist for each compiler. And
second, it presents an approach to characterize the loop nests based on their
hardware performance counter values and a Machine Learning approach that
predicts which compiler will generate the fastest code for a loop nest. The
prediction is made for both auto-vectorized, serial compilation and for
auto-parallelization. The results show that the headroom for state-of-the-art
compilers ranges from 1.10x to 1.42x for the serial code and from 1.30x to
1.71x for the auto-parallelized code. These results are based on the Machine
Learning predictions.Comment: Accepted at the 31st International Workshop on Languages and
Compilers for Parallel Computing (LCPC 2018
Magnetism and superconductivity driven by identical 4 states in a heavy-fermion metal
The apparently inimical relationship between magnetism and superconductivity
has come under increasing scrutiny in a wide range of material classes, where
the free energy landscape conspires to bring them in close proximity to each
other. This is particularly the case when these phases microscopically
interpenetrate, though the manner in which this can be accomplished remains to
be fully comprehended. Here, we present combined measurements of elastic
neutron scattering, magnetotransport, and heat capacity on a prototypical heavy
fermion system, in which antiferromagnetism and superconductivity are observed.
Monitoring the response of these states to the presence of the other, as well
as to external thermal and magnetic perturbations, points to the possibility
that they emerge from different parts of the Fermi surface. This enables a
single 4 state to be both localized and itinerant, thus accounting for the
coexistence of magnetism and superconductivity.Comment: 4 pages, 4 figure
Spin-Boson Hamiltonian and Optical Absorption of Molecular Dimers
An analysis of the eigenstates of a symmetry-broken spin-boson Hamiltonian is
performed by computing Bloch and Husimi projections. The eigenstate analysis is
combined with the calculation of absorption bands of asymmetric dimer
configurations constituted by monomers with nonidentical excitation energies
and optical transition matrix elements. Absorption bands with regular and
irregular fine structures are obtained and related to the transition from the
coexistence to a mixing of adiabatic branches in the spectrum. It is shown that
correlations between spin states allow for an interpolation between absorption
bands for different optical asymmetries.Comment: 15 pages, revTeX, 8 figures, accepted for publication in Phys. Rev.
Pion-nucleus reactions in a microscopic transport model
We analyse pion-nucleus reactions in a microscopic transport model of the BUU
type, which propagates nucleons, pions, deltas and N(1440)-resonances
explicitly in space and time. In particular we examine pion absorption and
inelastic scattering cross sections for pion kinetic energies T(pi) =85-315MeV
and various target masses. In general, the mass-dependence of the experimental
data is well described for energies up to the delta-resonance (\approx 160 MeV)
while the absorption cross sections are somewhat overestimated for the higher
energies. In addition we study the possible dynamical effects of delta- and
pion-potentials in the medium on various observables as well as alternative
models for the in-medium delta-width.Comment: 31 pages, UGI-93-0
HBT: A (mostly) experimental overview
I will present a review of the field of Hanbury Brown-Twiss interferometry in
relativistic heavy-ion collisions. The "HBT puzzle" is explored in detail,
emphasizing recent theoretical attempts to understand the persisting puzzle. I
also present recent experimental results on azimuthally sensitive HBT, HBT of
direct photons, and some surprises in the comparison of HBT results from p+p
and Au+Au collisions at RHIC.Comment: 8 pages, 3 figures. Proceedings of the Quark Matter 2004 conference
(Oalkland, CA, USA, January 2004
Probing nuclear expansion dynamics with -spectra
We study the dynamics of charged pions in the nuclear medium via the ratio of
differential - and -spectra in a coupled-channel BUU (CBUU)
approach. The relative energy shift of the charged pions is found to correlate
with the pion freeze-out time in nucleus-nucleus collisions as well as with the
impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb
force provides a 'clock' for the expansion of the hot nuclear system. Detailed
comparisons with experimental data for at 1 GeV/A and at
2.0 GeV/A are presented.Comment: 21 pages, latex, figures include
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