Resolution of discrete final states in the16 O(e,e ′ pp) 14 C reaction may provide an interesting tool to discriminate between contributions from one-and two-body currents in this reaction. This is based on the observation that the 0 + ground state and first 2 + state of 14 C are reached predominantly by the removal of a 1 S0 pair from 16 O in this reaction, whereas other states mostly arise by the removal of a 3 P pair. This theoretical prediction has been supported recently by an analysis of the pair momentum distribution of the experimental data . In this paper we present results of reaction calculations performed in a direct knock-out framework where final-state interaction and one-and two-body currents are included. The two-nucleon overlap integrals are obtained from a calculation of the two-proton spectral function of 16 O and include both long-range and short-range correlations. The kinematics chosen in the calculations is relevant for recent experiments at NIKHEF and Mainz. We find that the knock-out of a 3 P proton pair is largely due to the (two-body) ∆-current. The 1 S0 pair knock-out, on the other hand, is dominated by contributions from the one-body current and therefore sensitive to two-body short-range correlations. This opens up good perspectives for the study of these correlations in the 16 O(e,e ′ pp) reaction involving the lowest few states in 14 C. In particular the longitudinal structure function f00, which might be separated with super-parallel kinematics, turns out to be quite sensitive to the NN potential that is adopted in the calculations.
Spectroscopic factors for one-nucleon knockout from 16 O are calculated for states with low excitation energy in 15 N with the Bonn-C potential. A method is proposed to deal with both short-and long-range correlations consistently. For this purpose a Green's function formalism is used and the self-energy in the Dyson equation is approximated as the sum of an energy-dependent Hartree-Fock ͑HF͒ term and dispersion and correlation terms of higher order in the G-matrix interaction. This G matrix is obtained by solving the Bethe-Goldstone equation with a Pauli operator which excludes just the model space treated in the subsequent calculation of the self-energy. The energy dependence of the HF energies induces an additional reduction of the spectroscopic factors for quasiparticle states close to the Fermi level by about 10%. Experimental data may signal the need of some further improvement in the treatment of intermediate-and long-range correlations. ͓S0556-2813͑96͒04105-2͔
A procedure for the calculation of the two-body spectral function of a finite nucleus is presented. This spectral function is used to calculate the longitudinal part of the 16 O(e,eЈpp) cross section assuming plane waves for the outgoing nucleons. Short-range correlation effects are included in the pair-removal amplitudes by adding corresponding defect wave functions that are obtained from the solution of the Bethe-Goldstone equation in the finite nucleus. The associated G matrix is used as the effective interaction in a large but finite model space to calculate the pair-removal amplitudes in a random-phase approach. The resulting spectral functions exhibit clear differences between different realistic interactions in the momentum range 2-5 fm Ϫ1 for the initial proton momenta. ͓S0556-2813͑96͒01309-X͔
The reaction 16 O͑e, e 0 pp͒ 14 C has been studied at a transferred four-momentum ͑v, jqj͒ ͑210 MeV, 300 MeV͞c͒. The differential cross sections for the transitions to the ground state and the lowest excited states in 14 C were determined as a function of the momentum of the recoiling 14 C nucleus and the angle between the momentum of the proton emitted in the forward direction and the momentum transfer q. A comparison of the data to the results of calculations, performed with a microscopic model, shows clear signatures for short-range correlations in the 16 O ground state. [S0031-9007(98)07083-5] PACS numbers: 21.10. Pc, 21.30.Fe, 25.30.Fj, 27.20. + n In recent years, studies on short-range correlations (SRC) in nuclei have made striking progress. Microscopic many-body calculations in nuclear matter  and nuclei  have shown that SRC can account for a sizable fraction of the depletion in the occupancy of the valence orbits, observed in (e, e 0 p) proton knockout reactions . Furthermore, these calculations predict an enhancement of the high-momentum components in the nucleon wave functions. Signatures of admixtures of highmomentum components in the nuclear ground state are expected to be found in the (e, e 0 p) reaction at high missing energies and in two-nucleon knockout (e, e 0 NN) studies [8,9]. Although experimentally more involved, the latter reactions have distinct advantages as a probe for studying SRC in nuclei.In an exclusive (e, e 0 NN) reaction both ejectiles are identified and the excitation energy of the residual nucleus is determined by energy conservation. This allows the measurement of the cross section for transitions to discrete states, as has recently been shown for the 16 O͑e, e 0 pp͒ 14 C reaction [10,11]. Furthermore, the reaction mechanism for two-nucleon knockout by virtual photons depends on the spin and isospin of the nucleon pair in the initial state. This implies that complementary information on SRC can be extracted from (e, e 0 pp) and (e, e 0 pn) reaction studies.In Ref., we have presented the first results of a triple coincidence 16 O͑e, e 0 pp͒ 14 C experiment. The excitation energy spectrum up to 20 MeV of the residual nucleus 14 C and the corresponding missing-momentum distributions were compared with calculations performed within a simple factorization approximation of the cross section. In this Letter the differential cross sections are presented as a function of the excitation energy, the missing momentum, and the emission angle of the forward proton. The data are compared to the results of calculations performed with the microscopic model, recently described in Ref. .The measurements were performed with the high dutyfactor electron beam extracted from the pulse-stretcher AmPS at NIKHEF. The measurements were performed with 584 MeV electrons and the scattered electrons were detected at an angle of 26 ± . The central values of the energy transfer v and three-momentum transfer jqj were 210 MeV and 300 MeV͞c, respectively. Protons, with momenta p 1 a...
The level scheme of " Sn has been studied by combining the results of '"Sn(n, y)" Sn and " Sn(n, n'y}" Sn experiments. Both experiments were performed using isotopically enriched samples and Ge y-ray detectors. Based on the thresholds of y-ray excitation functions measured for the " Sn(n, n'y) reaction and the precise y-ray energies from the capture reaction, 100 levels were observed below 4.3 MeV excitation energy. Approximately half of these were not known previously. Forty-eight of these levels have unique or tentative spin-parity assignments, and for ten more the spin has been restricted to a single value. The spin-parity for most other levels below 4.3 MeV excitation has been restricted to a few values. These spin-parity assignments and limitations were derived mainly from (n, n y) angular distribution measurements, together with additional information obtained from the cross section magnitudes in both experiments. Above 4.3 MeV excitation energy, 55 additional levels are proposed, based only on the '"Sn(n, y) results. No J information is available for these higher-lying levels beyond the fact that they most probably all have J~4. The level scheme below 4.3 MeV from the current work, together with known high-spin levels up to 5.4 MeV seen in other experiments, are compared to the combined predictions of the two-broken-pair model, the interacting boson model, and the deformed collective model. In addition, several states have been phenomenologically identified as proton 1p-1h and collective quadrupole-octupole two-phonon excitations. It is concluded from the good agreement between experiment and these models that all levels in " Sn with J~6 up to an excitation of 4.0 MeV and J 3 up to 4.3 MeV may have been experimentally identified. The nearest-neighbor spacing distribution is intermediate between that of a Gaussian orthogonal ensemble and that of a Poisson distribution, with a slight preference for the former. The neutron separation energy was determined to be 9563.47+0.11 keV.
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