1,214 research outputs found
The Cauchy problem for metric-affine f(R)-gravity in presence of a Klein-Gordon scalar field
We study the initial value formulation of metric-affine f(R)-gravity in
presence of a Klein-Gordon scalar field acting as source of the field
equations. Sufficient conditions for the well-posedness of the Cauchy problem
are formulated. This result completes the analysis of the same problem already
considered for other sources.Comment: 6 page
Motion of Isolated bodies
It is shown that sufficiently smooth initial data for the Einstein-dust or
the Einstein-Maxwell-dust equations with non-negative density of compact
support develop into solutions representing isolated bodies in the sense that
the matter field has spatially compact support and is embedded in an exterior
vacuum solution
Improved modelling of helium and tritium production for spallation targets
Reliable predictions of light charged particle production in spallation
reactions are important to correctly assess gas production in spallation
targets. In particular, the helium production yield is important for assessing
damage in the window separating the accelerator vacuum from a spallation
target, and tritium is a major contributor to the target radioactivity. Up to
now, the models available in the MCNPX transport code, including the widely
used default option Bertini-Dresner and the INCL4.2-ABLA combination of models,
were not able to correctly predict light charged particle yields. The work done
recently on both the intranuclear cascade model INCL4, in which cluster
emission through a coalescence process has been introduced, and on the
de-excitation model ABLA allows correcting these deficiencies. This paper shows
that the coalescence emission plays an important role in the tritium and
production and that the combination of the newly developed versions of the
codes, INCL4.5-ABLA07, now lead to good predictions of both helium and tritium
cross sections over a wide incident energy range. Comparisons with other
available models are also presented.Comment: 6 pages, 9 figure
New potentialities of the Liège intranuclear cascade (INCL) model for reactions induced by nucleons and light charged particles
The new version (INCL4.6) of the Li`ege intranuclear cascade (INC) model for
the description of spallation reactions is presented in detail. Compared to the
standard version (INCL4.2), it incorporates several new features, the most
important of which are: (i) the inclusion of cluster production through a
dynamical phase space coalescence model, (ii) the Coulomb deflection for
entering and outgoing charged particles, (iii) the improvement of the treatment
of Pauli blocking and of soft collisions, (iv) the introduction of experimental
threshold values for the emission of particles, (v) the improvement of pion
dynamics, (vi) a detailed procedure for the treatment of light-cluster induced
reactions taking care of the effects of binding energy of the nucleons inside
the incident cluster and of the possible fusion reaction at low energy.
Performances of the new model concerning nucleon-induced reactions are
illustrated. Whenever necessary, the INCL4.6 model is coupled to the ABLA07
deexcitation model and the respective merits of the two models are then
tentatively disentangled. Good agreement is generally obtained in the 200 MeV-2
GeV range. Below 200 MeV and down to a few tens of MeV, the total reaction
cross section is well reproduced and differential cross sections are reasonably
well described. The model is also tested for light-ion induced reactions at low
energy, below 100 MeV incident energy per nucleon. Beyond presenting the update
of the INCL4.2 model, attention has been paid to applications of the new model
to three topics for which some particular aspects are discussed for the first
time: production of clusters heavier than alpha particles, longitudinal residue
recoil velocity and its fluctuations, total reaction cross section and the
residue production cross sections for low energy incident light ions.Comment: 29 pages, 26 figure
The Gabor wave front set of compactly supported distributions
We show that the Gabor wave front set of a compactly supported distribution
equals zero times the projection on the second variable of the classical wave
front set
Good covers are algorithmically unrecognizable
A good cover in R^d is a collection of open contractible sets in R^d such
that the intersection of any subcollection is either contractible or empty.
Motivated by an analogy with convex sets, intersection patterns of good covers
were studied intensively. Our main result is that intersection patterns of good
covers are algorithmically unrecognizable.
More precisely, the intersection pattern of a good cover can be stored in a
simplicial complex called nerve which records which subfamilies of the good
cover intersect. A simplicial complex is topologically d-representable if it is
isomorphic to the nerve of a good cover in R^d. We prove that it is
algorithmically undecidable whether a given simplicial complex is topologically
d-representable for any fixed d \geq 5. The result remains also valid if we
replace good covers with acyclic covers or with covers by open d-balls.
As an auxiliary result we prove that if a simplicial complex is PL embeddable
into R^d, then it is topologically d-representable. We also supply this result
with showing that if a "sufficiently fine" subdivision of a k-dimensional
complex is d-representable and k \leq (2d-3)/3, then the complex is PL
embeddable into R^d.Comment: 22 pages, 5 figures; result extended also to acyclic covers in
version
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