1,832 research outputs found
Bremsstrahlung Pair Production In Relativistic Heavy Ion Collision
We calculate production of electron- and muon-pairs by the bremsstrahlung
process in hadron collisions and compare it with the dominant two-photon
process. Results for the total cross section are given for proton-proton and
heavy-ion collisions at energies of the Relativistic Heavy Ion Collider (RHIC)
and the Large Hadron Collider (LHC).Comment: 6 pages, Latex, 9 figures using epsf-style. Accepted for publication
in Z. Phys.
Electron-positron pair production in the external electromagnetic field of colliding relativistic heavy ions
The results concerning the production in peripheral highly
relativistic heavy-ion collisions presented in a recent paper by Baltz {\em{et
al.}} are rederived in a very straightforward manner. It is shown that the
solution of the Dirac equation directly leads to the multiplicity, i.e. to the
total number of electron-positron pairs produced by the electromagnetic field
of the ions, whereas the calculation of the single pair production probability
is much more involved. A critical observation concerns the unsolved problem of
seemingly absent Coulomb corrections (Bethe-Maximon corrections) in pair
production cross sections. It is shown that neither the inclusion of the
vacuum-vacuum amplitude nor the correct interpretation of the solution of the
Dirac equation concerning the pair multiplicity is able the explain (from a
fundamental point of view) the absence of Coulomb corrections. Therefore the
contradiction has to be accounted to the treatment of the high energy limit.Comment: 6 pages, 4 Postscript figures, uses svjour.cls/svepj.cl
Towards a model-independent constraint of the high-density dependence of the symmetry energy
Neutron-proton elliptic flow difference and ratio have been shown to be
promising observables in the attempt to constrain the density dependence of the
symmetry energy above the saturation point from heavy-ion collision data. Their
dependence on model parameters like microscopic nucleon-nucleon cross-sections,
compressibility of nuclear matter, optical potential, and symmetry energy
parametrization is thoroughly studied. By using a parametrization of the
symmetry energy derived from the momentum dependent Gogny force in conjunction
with the T\"{u}bingen QMD model and comparing with the experimental FOPI/LAND
data for 197Au+197Au collisions at 400 MeV/nucleon, a moderately stiff, x=-1.35
+/- 1.25, symmetry energy is extracted, a result that agrees with that of a
similar study that employed the UrQMD transport model and a momentum
independent power-law parametrization of the symmetry energy. This contrasts
with diverging results extracted from the FOPI ratio
available in the literature.Comment: 18 pages, 3 figures, accepted for publication in Phys. Rev.
Mechanisms for Direct Breakup Reactions
We review some simple mechanisms of breakup in nuclear reactions. We mention
the spectator breakup, which is described in the post-form DWBA. The relation
to other formulations is also indicated. An especially important mechanism is
Coulomb dissociation. It is a distinct advantage that the perturbation due to
the electric field of the nucleus is exactly known. Therefore firm conclusions
can be drawn from such measurements. Some new applications of Coulomb
dissociation for nuclear astrophysics are discussed.Comment: 17 pages, 5 figures, to appear in the proceedings of the RCNP-TMU
Symposium on Spins in Nuclear and Hadronic Reactions, October 16-18 199
Focusing of high-energy particles in the electrostatic field of a homogeneously charged sphere and the effective momentum approximation
Abstract.: The impact of the strongly attractive electromagnetic field of heavy nuclei on electrons in quasi-elastic (e, e') scattering is often accounted for by the effective momentum approximation. This method is a plane wave Born approximation which takes the twofold effect of the attractive nucleus on initial- and final-state electrons into account, namely the modification of the electron momentum in the vicinity of the nucleus, and the focusing of electrons towards the nuclear region leading to an enhancement of the corresponding wave function amplitudes. The focusing effect due to the attractive Coulomb field of a homogeneously charged sphere on a classical ensemble of charged particles incident on the field is calculated in the highly relativistic limit and compared to results obtained from exact solutions of the Dirac equation. The result is relevant for the theoretical foundation of the effective momentum approximation and describes the high-energy behavior of the amplitude of continuum Dirac waves in the potential of a homogeneously charged sphere. Our findings indicate that the effective momentum approximation is a useful approximation for the calculation of Coulomb corrections in (e, e') scattering off heavy nuclei for sufficiently high electron energies and momentum transfe
Modification of surface energy in nuclear multifragmentation
Within the statistical multifragmentation model we study modifications of the
surface and symmetry energy of primary fragments in the freeze-out volume. The
ALADIN experimental data on multifragmentation obtained in reactions induced by
high-energy projectiles with different neutron richness are analyzed. We have
extracted the isospin dependence of the surface energy coefficient at different
degrees of fragmentation. We conclude that the surface energy of hot fragments
produced in multifragmentation reactions differs from the values extracted for
isolated nuclei at low excitation. At high fragment multiplicity, it becomes
nearly independent of the neutron content of the fragments.Comment: 11 pages with 13 figure
Local formation of nitrogen-vacancy centers in diamond by swift heavy ions
We exposed nitrogen-implanted diamonds to beams of swift uranium and gold
ions (~1 GeV) and find that these irradiations lead directly to the formation
of nitrogen vacancy (NV) centers, without thermal annealing. We compare the
photoluminescence intensities of swift heavy ion activated NV- centers to those
formed by irradiation with low-energy electrons and by thermal annealing. NV-
yields from irradiations with swift heavy ions are 0.1 of yields from low
energy electrons and 0.02 of yields from thermal annealing. We discuss possible
mechanisms of NV-center formation by swift heavy ions such as electronic
excitations and thermal spikes. While forming NV centers with low efficiency,
swift heavy ions enable the formation of three dimensional NV- assemblies over
relatively large distances of tens of micrometers. Further, our results show
that NV-center formation is a local probe of (partial) lattice damage
relaxation induced by electronic excitations from swift heavy ions in diamond.Comment: to be published in Journal of Applied Physic
Coulomb distortion of relativistic electrons in the nuclear electrostatic field
Abstract.: Continuum states of the Dirac equation are calculated numerically for the electrostatic field generated by the charge distribution of an atomic nucleus. The behavior of the wave functions of an incoming electron with given asymptotic momentum in the nuclear region is discussed in detail and the results are compared to different approximations used in the data analysis for quasielastic electron scattering off medium and highly charged nuclei. It is found that most of the approximations provide an accurate description of the electron wave functions in the range of electron energies above 100 MeV typically used in experiments for quasielastic electron scattering off nuclei only near the center of the nucleus. It is therefore necessary that the properties of exact wave functions are investigated in detail in order to obtain reliable results in the data analysis of quasielastic (e, e'p) knockout reactions or inclusive quasielastic (e, e') scattering. Detailed arguments are given that the effective momentum approximation with a fitted potential parameter is a viable method for a simplified treatment of Coulomb corrections for certain kinematical regions used in experiments. Numerical calculations performed within the framework of the single-particle shell model for nucleons lead to the conclusion that our results are incompatible with calculations performed about a decade ago, where exact electron wave functions were used in order to calculate Coulomb corrections in distorted-wave Born approximation. A discussion of the exact solutions of the Dirac equation for free electrons in a Coulomb field generated by a point-like charge and some details relevant for the numerical calculations are given in the appendi
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