1,721,097 research outputs found
Electron distortion in quasifree (e,e′p) reactions
No full textEffects of Coulomb distortion of the electron wave functions in the quasifree (e, e′p) reactions are investigated in DWBA. Electron waves are described by a high energy approximation. The coincidence cross section is calculated in a nonrelativistic approach based on the impulse approximation. Results are presented and discussed for different nuclei in different kinematical conditions. Coulomb distortion gives two different effects. The first one is to change the momentum transfer into an effective momentum transfer. This effect is dominant for light nuclei and decreases by increasing incident electron energy. The second one is the focusing of the electron wave in the nuclear Coulomb field. This effect, almost energy independent, is sizeable in medium and heavy nuclei. It increases the cross section and in some cases becomes very large
Meson exchange currents in electromagnetic one-nucleon emission
No full textThe 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(g ,p)15N reactions. MEC effects are in general rather small in (e,e'p), while in (g ,p) they are always large and important to obtain a consistent description of (e,e'p) and (g ,p) data, with the same
spectroscopic factors. The calculated (g ,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
Electron-Nucleon Interaction in Quasi-Free Scattering. II Non-Relativistic Hamiltonian thorugh Fourth Order
No full textThe interaction of non-relativistic nucleons with relativistic electrons can be expressed by a Hamiltonian expanded in a power series of the recoil nucleon velocity. In this paper third- and fourth-order terms are calculated. Free electron-nucleon-scattering and quasi-free electron scattering on 12C are studied and results through various orders are compared with the relativistic formulae. A different off-shell behaviour of the relativistic approach is found, which is independent of the higher-order terms and seems responsible for the different results of knock-out reactions
Two-proton emission induced by electron scattering
No full textThe theoretical framework of the electron-induced reactions where two nucleons are emitted is developed, pointing out analogies and differences with respect to the case of single-nucleon knockout. The triple-coincidence cross section is written in terms of the structure functions, which represent the response of the nucleus to the various components of the electromagnetic interaction. The cross section of the (e, e′2p) reaction is calculated in an unfactorized form, by integrating over the coordinates of the two ejected protons. The role of realistic short-range correlations and of the distortion of the two outgoing particles is investigated in the case of a coplanar and symmetrical kinematics. In this situation, the choice of the optical potential is not crucial, even though distortion is important. On the contrary, the cross section is very sensitive to the choice of the correlation function
Self-consistent center-of-mass correction and the charge form factor of 4He
No full textThe Hartree-Fock approximation is used for 4He in the case of Lipkin's Hamiltonian with several nuclear forces. A comparison between experimental and theoretical charge form factor is done.
The discreancies cannot be interpreted as due to spurious c.m. effects
Polarization observables and asymmetry in electromagnetic quasi-free proton knockout
No full textPolarization observables and helicity asymmetry of the coincidence cross section for the electromagnetic knockout of polarized protons from unpolarized targets are investigated in the quasi-elastic region. Specific calculations, based on the theoretical framework of the one-photon exchange approximation, are shown for a 16O target nucleus as an example to study the sensitivity to final state interactions, mesonic effects and isobaric configurations. Peculiar properties of the helicity asymmetry are analyzed and discussed
Proton Recoil Polarization in (e,e'pp) Reactions
No full textThe general formalism of nucleon recoil polarization in the (e,e'N N) reaction is given. Numerical predictions are presented for the components of the outgoing proton polarization and of the polarization transfer coefficient in the specific case of the exclusive 16O(e,e'pp)14C knockout reaction leading to discrete states in the residual nucleus. Reaction calculations are performed in a direct knockout framework where final-state interactions and one-body and two-body currents are included. The two-nucleon overlap integrals are obtained
from a calculation of the two-proton spectral function of 16O where long-range and short-range correlations are
consistently included. The comparison of results obtained in different kinematics confirms that resolution of
different final states in the 16O(e,e'pp)14C reaction may act as a filter to disentangle and separately investigate
the reaction processes due to short-range correlations and two-body currents and indicates that measurements
of the components of the outgoing proton polarization may offer good opportunities to study short-range
correlations
Variational approachto 4He with separation of centre-of-mass motion
No full textThe ground state energy and wavefunction of 4He are calculated in a variational approach with separation of centre-of-mass motio
Complete determination of scattering amplitudes and nucleon polarization in electromagnetic knockout reactions
No full textThe electron-induced knockout reactions are investigated aiming at the complete determination of scattering amplitudes. This result can be achieved only by exploiting spin degrees of freedom and therefore through polarization experiments where the components of the nuclear response are separated. Particular situations are envisaged where at least a partial determination can be obtained by measurements compatible with the present experimental facilities. Numerical predictions are presented and discussed for polarization of an outgoing proton or neutron in coplanar kinematics. The role of final-state interactions and of Coulomb distortion of electron waves is investigated. In the case of an outgoing neutron, the two-step charge-exchange contribution, due to an isospin-dependent term in the optical potential, is also included
Realistic one-particle density matrix and single particle-description of nuclei
No full textStarting from the experimental data on elastic electron scattering, realistic one-particle density matrices for 16O and 40Ca are obtained with fractional occupation numbers
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