220 research outputs found
Thorotrast and in vivo thorium dioxide: numerical simulation of 30 years of alpha radiation absorption by the tissues near a large compact source
Background: The epidemiology of the slightly radioactive contrast agent named
Thorotrast presents a very long latency period between the injection and the
development of the related pathologies. It is an example of the more general
problem posed by a radioactive internal contaminant whose effects are not
noteworthy in the short term but become dramatic in the long period. A point
that is still to be explored is fluctuations (in space and time) in the
localized absorption of radiation by the tissues. Methods: A Monte Carlo
simulation code has been developed to study over a 30 year period the daily
absorption of alpha radiation by micrometer sized portions of tissue placed at
a distance of 0-100 micrometers from a model source, that approximates a
compact thorium dioxide source in liver or spleen whose size is larger or equal
to 20 micrometers. The biological depletion of the daughter nuclei of the
thorium series is taken into account. The initial condition assumes chemically
purified natural thorium. Results: Most of the absorbed dose is concentrated in
a 25 micrometer thick layer of tissue, adjacent to the source boundary.
Fluctuations where a target region with a volume of 1 cube micrometer is hit by
3-5 alpha particles in a day or in a shorter period of time are relevant in a
1-10 micrometer thick layer of tissue adjacent to the source boundary, where
their frequency is larger than the Poisson law prediction.Comment: In press on Physica Medica, available online at the journal site
since february 21th, 201
A study on the feasibility of a precise measurement of the -dependence of the cross sections for Drell-Yan experiments at moderate energies
Recently, a reconsideration of Drell-Yan cross sections at moderate energies
and masses has suggested the possibility of relevant enhancements of the cross
sections in some kinematical regions. If confirmed, these predictions could
largely affect the planning of Drell-Yan experiments aimed at transverse spin
measurements after 2010. More in general, the problem is present of a precision
measurement of the dependence of Drell-Yan cross sections. Here we
discuss the feasibility of such a measurement within short time at the COMPASS
apparatus, and its relevance for the PANDA experiment.Comment: 4 pages, 4 figures, in print on Physical Review
Periodic interference structures in the time-like proton form factor
An intriguing and elusive feature of the timelike hadron form factor is the
possible presence of an imaginary part associated to rescattering processes. We
find evidence of that in the recent and precise data on the proton timelike
form factor measured by the BABAR collaboration. By plotting these data as a
function of the 3-momentum of the relative motion of the final proton and
antiproton, a systematic sinusoidal modulation is highlighted in the
near-threshold region. Our analysis attributes this pattern to rescattering
processes at a relative distance of 0.7-1.5 fm between the centers of the
forming hadrons. This distance implies a large fraction of inelastic processes
in interactions, and a large imaginary part in the related reaction because of unitarity.Comment: 5 pages 3 figures - Discussion modified. To appear in Phys Rev
Letter
Azimuthal asymmetries in exclusive four-particle "Drell-Yan" events, with lepton-antilepton plus proton-antiproton detection, in proton-antiproton scattering
I study hard collisions between unpolarized protons and antiprotons where a
lepton-antilepton pair is detected in coincidence with a final
proton-antiproton pair, and no more particles are produced, in the regime 10
GeV^2 4 GeV, q_T < 3 GeV/c. The present work is
centered on azimuthal asymmetries. Because of momentum conservation, a
Boer-Mulders term in the momentum distribution of a quark implies a balancing
effect in the momentum distribution of some spectators. This produces azimuthal
asymmetries of the final hadrons. To analyze this, I have organized a
parton-level MonteCarlo generator where a standard cos(2phi)-asymmetry of the
dilepton distribution is produced, thanks to a soft rescattering process
between an active quark coming from a hadron and a spectator anti-diquark
coming from the other hadron. This produces cos(2phi)-asymmetries of the final
hadron pair. Hadron and lepton asymmetries have the same size.Comment: final versio
T-odd distributions, breaking of long range correlations, and sudden entropy changes, in Drell-Yan high-energy processes
T-odd parton distribution functions in a Drell-Yan process are here studied
by examining the evolution of the internal statistical properties of the
interacting hadrons. T-odd functions are shown to be a signature of the
irreversible process in which a hadronic state characterized by long range
correlation properties (hadronic phase) decays to produce a cloud of
independent partons (partonic phase) because of initial/final state
interactions. The relevant considered variable is the rate of increase of the
entropy of the hadronic system. This quantity is shown to be roughly equal to
the decay rate of the hadronic state. Conditions for getting a leading twist
T-odd effect are established on this basis. Last, the relevant case of a large
entropy increase associated with transverse-dominated initial/final state
interactions is analyzed.Comment: 18 printed pages, no figures. Revised version, with added subsections
to sections 4 and 5, and added references to the introduction. In print on
Phys.Rev.
Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes
The doping dependent isotope effect on the critical temperature (Tc) is
calculated for multi-band multi-condensate superconductivity near a 2.5
Lifshitz transition. We focus on multi-band effects that arises in
nano-structures and in density wave metals (like spin density wave or charge
density wave) as a result of the band folding. We consider a superlattice of
quantum stripes with finite hopping between stripes near a 2.5 Lifshitz
transition for appearing of a new sub-band making a circular electron-like
Fermi surface pocket. We describe a particular type of BEC (Bose-Einstein
Condensate) to BCS (Bardeen-Cooper-Schrieffer condensate) crossover in
multi-band / multi-condensate superconductivity at a metal-to-metal transition
that is quite different from the standard BEC-BCS crossover at an
insulator-to-metal transition. The electron wave-functions are obtained by
solving the Schr\"odinger equation for a one-dimensional modulated potential
barrier. The k-dependent and energy dependent superconducting gaps are
calculated using the k-dependent anisotropic Bardeen-Cooper-Schrieffer (BCS)
multi-gap equations solved joint with the density equation, according with the
Leggett approach currently used now in ultracold fermionic gases. The results
show that the isotope coefficient strongly deviates from the standard BCS value
0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for
the metal-to-metal transition. The critical temperature Tc shows a minimum due
to the Fano antiresonance in the superconducting gaps and the isotope
coefficient diverges at the point where a BEC coexists with a BCS condensate.
On the contrary Tc reaches its maximum and the isotope coefficient vanishes at
the crossover from a polaronic condensate to a BCS condensate in the new
appearing sub-band.Comment: 8 pages, 4 ps figure
Nucleon Spin Structure with hadronic collisions at COMPASS
In order to illustrate the capabilities of COMPASS using a hadronic beam, I
review some of the azimuthal asymmetries in hadronic collisions, that allow for
the extraction of transversity, Sivers and Boer-Mulders functions, necessary to
explore the partonic spin structure of the nucleon. I also report on some Monte
Carlo simulations of such asymmetries for the production of Drell-Yan lepton
pairs from the collision of high-energy pions on a transversely polarized
proton target.Comment: talk delivered to the "International Workshop on Structure and
Spectroscopy", Freiburg, March 19-21, 2007; 18 pages, RevTeX4 style, 8
figures with 10 .eps file
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