96 research outputs found
Radiative pion capture in nuclei: a continuum shell-model approach
The radiative pion capture process in nuclei is approached by using a
continuum shell-model description of the nucleus, together with a
phenomenological treatment of the two particle-two hole effects. It is found
that these effects play an important role to reproduce the observed
experimental photon energy distribution. This distribution as well as the
integrated one depends significantly on the details of the mean field
potential. This makes this process interesting to investigate the nuclear
structure dynamics.Comment: 21 pages, LateX file + 5 figures, epsf.st
A model of short-range correlations in the charge response
The validity of a model treating the short-range correlations up to the first
order is studied by calculating the charge response of an infinite system and
comparing the obtained results with those of a Fermi Hypernetted Chain
calculation.Comment: 6 pages, 3 Postscript figures, to be published on Phys. Rev.
A Method to Determine the In-Air Spatial Spread of Clinical Electron Beams
We propose and analyze in detail a method to measure the in-air spatial
spread parameter of clinical electron beams. Measurements are performed at the
center of the beam and below the adjustable collimators sited in asymmetrical
configuration in order to avoid the distortions due to the presence of the
applicator. The main advantage of our procedure lies in the fact that the dose
profiles are fitted by means of a function which includes, additionally to the
Gaussian step usually considered, a background which takes care of the dose
produced by different mechanisms that the Gaussian model does not account for.
As a result, the spatial spread is obtained directly from the fitting procedure
and the accuracy permits a good determination of the angular spread. The way
the analysis is done is alternative to that followed by the usual methods based
on the evaluation of the penumbra width. Besides, the spatial spread found
shows the quadratic-cubic dependence with the distance to the source predicted
by the Fermi-Eyges theory. However, the corresponding values obtained for the
scattering power are differing from those quoted by ICRU nr. 35 by a factor ~2
or larger, what requires of a more detailed investigation.Comment: 11 pages, 5 Postscript figures, to be published in Medical Physic
Superscaling in electroweak excitation of nuclei
Superscaling properties of 12C, 16O and 40Ca nuclear responses, induced by
electron and neutrino scattering, are studied for momentum transfer values
between 300 and 700 MeV/c. We have defined two indexes to have quantitative
estimates of the scaling quality. We have analyzed experimental responses to
get the empirical values of the two indexes. We have then investigated the
effects of finite dimensions, collective excitations, meson exchange currents,
short-range correlations and final state interactions. These effects strongly
modify the relativistic Fermi gas scaling functions, but they conserve the
scaling properties. We used the scaling functions to predict electron and
neutrino cross sections and we tested their validity by comparing them with the
cross sections obtained with a full calculation. For electron scattering we
also made a comparison with data. We have calculated the total charge-exchange
neutrino cross sections for neutrino energies up to 300 MeV.Comment: 19 pages, 12 figures, 1 table; to be published in Physical Review
Magnetic excitations in nuclei with neutron excess
The excitation of the , and modes in O, O,
O, O, Ca, Ca, Ca and Ca nuclei is
studied with self-consistent random phase approximation calculations.
Finite-range interactions of Gogny type, containing also tensor-isospin terms,
are used. We analyze the evolution of the magnetic resonances with the
increasing number of neutrons, the relevance of collective effects, the need of
a correct treatment of the continuum and the role of the tensor force.Comment: 18 pages, 12 figures, 2 tables, accepted for publication in Physical
Review
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