16 research outputs found
Meson exchange currents in electromagnetic one-nucleon emission
The role of meson exchange currents (MEC) in electron- and photon-induced
one-nucleon emission processes is studied in a nonrelativistic model including
correlations and final state interactions. The nuclear current is the sum of a
one-body and of a two-body part. The two-body current includes pion seagull,
pion-in-flight and the isobar current contributions. Numerical results are
presented for the exclusive 16O(e,e'p)15N and 16O(\gamma,p)15N reactions. MEC
effects are in general rather small in (e,e'p), while in (\gamma,p) they are
always large and important to obtain a consistent description of (e,e'p) and
(\gamma,p) data, with the same spectroscopic factors. The calculated (\gamma,p)
cross sections are sensitive to short-range correlations at high values of the
recoil momentum, where MEC effects are larger and overwhelm the contribution of
correlations.Comment: 9 pages, 6 figure
Overlap functions in correlation methods and quasifree nucleon knockout from O
The cross sections of the () and () reactions on O
are calculated, for the transitions to the ground state and the first
excited state of the residual nucleus, using single-particle overlap
functions obtained on the basis of one-body density matrices within different
correlation methods. The electron-induced one-nucleon knockout reaction is
treated within a nonrelativistic DWIA framework. The theoretical treatment of
the () reaction includes both contributions of the direct knockout
mechanism and of meson-exchange currents. The results are sensitive to details
of the different overlap functions. The consistent analysis of the reaction
cross sections and the comparison with the experimental data make it possible
to study the nucleon--nucleon correlation effects.Comment: 26 pages, LaTeX, 5 Postscript figures, submitted to PR
Relativistic corrections in (gamma,N) knockout reactions
We develop a fully relativistic DWIA model for photonuclear reactions using
the relativistic mean field theory for the bound state and the Pauli reduction
of the scattering state which is calculated from a relativistic optical
potential. Results for the 12C(gamma,p) and 16O(gamma,p) differential cross
sections and photon asymmetries are displayed in a photon energy range between
60 and 257 MeV, and compared with nonrelativistic DWIA calculations. The
effects of the spinor distortion and of the effective momentum approximation
for the scattering state are discussed. The sensitivity of the model to
different prescriptions for the one-body current operator is investigated. The
off-shell ambiguities are large in (gamma,p) calculations, and even larger in
(gamma,n) knockout.Comment: LaTeX2e, 18 pages, and 6 figure
Surto de intoxicação por narasina em suínos An outbreak of narasin poisoning in swine
Descreve-se um surto de intoxicação por narasina em suínos, cujo diagnóstico baseou-se no histórico, quadro clínico-patológico, na reprodução da enfermidade pela experimentação com a ração comercial originalmente oferecida aos suínos, bem como pelo exame químico posterior que detectou narasina em quantidades suficientemente elevadas para causar intoxicação. Apesar de os quadros clínico-patológicos das intoxicações natural e experimental serem da mesma natureza, houve algumas diferenças. Nos casos naturais o índice de letalidade foi elevado, enquanto que na intoxicação experimental nenhum animal morreu. Os exames histopatológicos revelaram, em parte dos casos naturais, além das lesões dos músculos estriados, também lesões nas fibras musculares cardíacas, últimas não verificadas nos casos experimentais. Sugere-se que essas diferenças poderiam ser explicadas pelo estresse ao qual os animais eram submetidos na granja pelo grande número de animais nas baias, ao contrário dos animais experimentais que eram mantidos em compartimentos individuais.<br>An outbreak of narasin poisoning in swine is described. The diagnosis was based on the history, clinical-pathological findings, the reproduction of the disease by the administration of the feed originally given to the animals and by chemical analysis which showed doses sufficiently high to cause poisoning. Inspite that the clinical-pathological picture of the natural and experimental poisoning was of the same nature, there were some differences. In the natural outbreak lethality was high, but in the experiments none of the animals died. In the natural cases besides the lesions in the muscles, also lesions of the heart muscle fibres were seen, not detected in the esperimental animals. It is suggested that these differences could be due to the stress to which the animals were submitted in the piggery by the great number of animals per box, whereas the experimental animals were kept individually