V. Van der Sluys

Electroinduced two-nucleon knockout and correlations in nuclei

Abstract A model to calculate cross sections for electroinduced two-nucleon emission from finite nuclei is presented. Short-range correlations in the wave functions and meson-exchange contributions to the photoabsorption process are implemented. Effects of the short-range correlations are studied with the aid of a perturbation expansion method with various choices of the Jastrow correlation function. The model is used to investigate the relative importance of the different reaction mechanisms contributing to the A(e,e′pn) and A(e,e′pp) process. Representative examples for the target nuclei 12C and 16O and for kinematical conditions accessible with contemporary high-duty cycle electron accelerators are presented. A procedure is outlined to calculate the two-nucleon knockout contribution to the semi-exclusive (e,e′p) cross section. Using this technique we investigate to what extent far semi-exclusive (e,e′p) reactions can be used to detect high-momentum components in the nuclear spectral function.

MULTINUCLEON MECHANISMS IN (GAMMA,N) AND (GAMMA-NN) REACTIONS.

The similarities in the experimental indications for multinucleon mechanisms in ([gamma],[ital p]) and ([ital e],[ital e][prime][ital p]) processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive ([gamma],[ital N]) reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behavior of the [sup 12]C([gamma],[ital p]) and [sup 12]C([gamma],[ital n]) data at high excitation energies in the residual nucleus. In the calculations, effects from nonresonant and resonant pion exchange currents are included. Photoabsorption on these currents is predicted to produce the major contributions to the exclusive [sup 16]O([gamma],[ital n][sub 0])[sup 15]O process at photoenergies above the pion threshold. Double differential cross sections for photon induced [ital pp] and [ital pn] emission from [sup 16]O are calculated and compared with the data.

Fragmentation of single-particle strength in spherical open-shell nuclei: Application to the spectral functions in 142Nd

Abstract A method is presented for the calculation of single-particle strength distributions in spherical semi-magic open-shell nuclei. The model is based upon the equation-of-motion method and differs from the standard quasiparticle-phonon model in that also backward-going terms are included in the self-energy of the nucleons. In the limit to closed shells the model reduces to the microscopic particle-vibration method with RPA-like polarization propagators. The model is applied to the proton particle and hole spectral functions in the odd nuclei adjacent to 142Nd.

Two-body currents in inclusive electron scattering

The longitudinal and transverse structure functions are calculated for inclusive electron scattering from {sup 12}C and {sup 40}Ca in the quasielastic and dip region. The microscopic model presented here incorporates two-body currents derived from one-pion exchange and intermediate {Delta} excitation. The reaction mechanism involves both one-nucleon and two-nucleon knockout processes. It is demonstrated that, even for quasielastic kinematics, two-body currents give a substantial contribution to the transverse structure function. Furthermore, the observed excess of transverse strength in the dip region between the quasielastic and delta peak can be partially ascribed to direct two-nucleon knockout following photoabsorption on a two-body current.

Multinucleon mechanisms in ([gamma],[ital N]) and ([gamma],[ital NN]) reactions

The similarities in the experimental indications for multinucleon mechanisms in ([gamma],[ital p]) and ([ital e],[ital e][prime][ital p]) processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive ([gamma],[ital N]) reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behavior of the [sup 12]C([gamma],[ital p]) and [sup 12]C([gamma],[ital n]) data at high excitation energies in the residual nucleus. In the calculations, effects from nonresonant and resonant pion exchange currents are included. Photoabsorption on these currents is predicted to produce the major contributions to the exclusive [sup 16]O([gamma],[ital n][sub 0])[sup 15]O process at photoenergies above the pion threshold. Double differential cross sections for photon induced [ital pp] and [ital pn] emission from [sup 16]O are calculated and compared with the data.

Two-nucleon knockout contributions to the 12C(e, e′p) reaction in the dip and Δ (1232) regions

Abstract The contributions from 12 C( e , e ′ pn ) and 12 C( e , e ′ pp ) to the semi-exclusive 12 C( e , e ′ p ) cross section have been calculated in an unfactorized model for two-nucleon emission. We assume direct two-nucleon knockout after virtual photon coupling with the two-body pion-exchange currents in the target nucleus. Results are presented at several kinematical conditions in the dip and Δ (1232) regions. The calculated two-nucleon knockout strength is observed to account for a large fraction of the measured ( e , e ′ p ) strength ahe two-nucleon emission threshold.

Effects of meson-exchange currents on the (e→,e’p) structure functions

The response functions for the unpolarized ([ital e],[ital e][prime][ital p]) and polarized ([ital [rvec e]],[ital e][prime][ital p]) reaction are calculated for medium-heavy nuclei under quasifree conditions. The formalism presented here incorporates both electron distortion effects and two-body currents related to meson-exchange and the [Delta](1232) excitation. The final-state interaction of the outgoing nucleon with the residual nucleus is handled in a self-consistent Hartree-Fock random phase approximation formalism. The effect of Coulomb distortion of the incoming and outgoing electron is included within a fully distorted wave calculation. The sensitivity of the results to the two-body currents is discussed for the five structure functions in quasielastic ([ital [rvec e]],[ital e][prime][ital p]) scattering off the target nuclei [sup 16]O and [sup 40]Ca. A selective sensitivity to the two-body currents is obtained in the longitudinal-transverse interference term [ital W][sub [ital L][ital T]] for which two-body currents can explain part of the discrepancy between the impulse-approximation calculations and the data.

Exclusive (e,e'p) reaction at high missing momenta.

The reduced ({ital e},{ital e}{sup {prime}}{ital p}) cross section is calculated for kinematics that probe high missing momenta. The final-state interaction is handled within a nonrelativistic many-body framework. One- and two-body nuclear currents are included. Electron distortion effects are treated in an exact distorted-wave calculation. It is shown that at high missing momenta the calculated ({ital e},{ital e}{sup {prime}}{ital p}) cross sections exhibit a pronounced sensitivity to ground-state correlations of the random-phase approximation type and two-body currents. The role of these mechanisms is found to be relatively small at low missing momenta. {copyright} {ital 1996 The American Physical Society.}

Systematic study of Coulomb distortion effects in exclusive (e,e'p) reactions.

A technique to deal with Coulomb electron distortions in the analysis of (e,e{sup {prime}}p) reactions is presented. Thereby, no approximations are made. The suggested technique relies on a partial-wave expansion of the electron wave functions and a multipole decomposition of the electron and nuclear current in momentum space. In that way, we succeed in keeping the computational times within reasonable limits. This theoretical framework is used to calculate the quasielastic (e,e{sup {prime}}p) reduced cross sections for proton knockout from the valence shells in {sup 16}O, {sup 40}Ca, {sup 90}Zr, and {sup 208}Pb. The final-state interaction of the ejected proton with the residual nucleus is treated within an optical potential model. The role of electron distortion on the extracted spectroscopic factors is discussed. {copyright} {ital 1997} {ital The American Physical Society}

Photon-induced proton knockout from 208Pb and 12C

Abstract Cross sections have been measured for the reactions 208Pb(γ,p) and 12C(γ,p) leading to the low-lying and continuum states in 207Tl and 11B using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the (e,e′p) reaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e′p) and (γ,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e′p) and (γ,p) results, i.e. no scaling is observed. Distorted-wave impulse-approximation (DWIA) calculations, of which the input is constrained by the (e,e′p) data, and random-phase approximation calculations (RPA) underestimate the 12C(γ,p) data by typically a factor five depending on angle and missing energy. For 208Pb the difference between the calculations and the data for the low-lying states is much less, but an increasing discrepancy between the data and the calculations is found in the continuum. The inclusion of meson-exchange currents (MEC) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less important for 208Pb as compared to 12C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on 208Pb despite the difference in the electromagnetic couplings involved.