D. Boersma

The optical properties of the BigBite spectrometer at NIKHEF.

The optical properties of the BigBite spectrometer currently in use at the Internal Target Facility of the AmPS ring at NIKHEF have been determined. The spectrometer, which consists of a single dipole magnet, combines a large solid angle with a large momentum acceptance. The track of a particle is determined from the information of two sets of drift chambers behind the magnet. Tracing this track through the magnetic field to the target yields the position of the scattering vertex and the size and direction of the momentum vector of the scattered particle at the target position. These quantities are calculated using an analytical approximation of the spectrometer, followed by a refinement with the matrix method. The p-resolutions of the reconstruction for 600 MeV electrons are 3 mrad for the angles, 3.2 mm for the vertex position, and 8.4]10~3 for dp/p. ( 1998 Elsevier Science B.V. All rights reserved.

Investigation of the ExclusiveH3e(e,e′pp)nReaction

Cross sections for the 3 He(e,e8 pp )n reaction were measured over a wide range of energy and threemomentum transfer. At a momentum transfer q5375 MeV/c, data were taken at transferred energies v ranging from 170 to 290 MeV. At v5220 MeV, measurements were performed at three q values ~305, 375, and 445 MeV/c). The results are presented as a function of the neutron momentum in the final state, as a function of the energy and momentum transfer, and as a function of the relative momentum of the two-proton system. The data at neutron momenta below 100 MeV/c, obtained for two values of the momentum transfer at v5220 MeV, are well described by the results of continuum-Faddeev calculations. These calculations indicate that the cross section in this domain is dominated by direct two-proton emission induced by a one-body hadronic current. Cross section distributions determined as a function of the relative momentum of the two protons are fairly well reproduced by continuum-Faddeev calculations based on various realistic nucleonnucleon potential models. At higher neutron momentum and at higher energy transfer, deviations between data and calculations are observed that may be due to contributions of isobar currents.

Spin-dependent electromagnetic response of few-body systems.

We report on the experimental program with polarized electrons and polarized few-body systems at the internal target facility of the NIKHEF electron storage ring. With a polarized deuterium internal target, absolute measurements of unprecedented accuracy have been performed for the analyzing powers T20 and T22 in elastic electron-deuteron scattering at transferred four-momenta 1.1 < Q < 2.8 fm−1. Results for a first measurement of the tensor analyzing power in quasi-elastic scattering are also presented. These data provide new constraints for descriptions of the deuteron spin structure and for reaction mechanism effects. Quasi-elastic electron scattering from polarized 3He may provide precise information on the S′ and the D-wave parts of the 3He ground-state wave function, the neutron form factors, and the role of spin-dependent reaction mechanism effects. We have started an experimental program at NIKHEF where polarized 3He and polarized electrons are used, in combination with large acceptance electron and hadron detectors. Data were taken for the transverse (Ay0) and longitudinal (A′z) asymmetries at Q ⋍ 2 fm−1. We outline the performance of the target and detectors.

Recoil detection with a polarized He-3 target

Abstract The ultra-thin gas targets used in storage ring internal target experiments allow low energy, heavy nuclei to emerge from the target region. A detector capable of analyzing these nuclei provides unique access to many nuclear reactions. We report here the first use of such a detector in conjunction with a polarized 3 He internal target and a polarized electron beam. The results of using the detector as a luminosity monitor and as a polarimeter to measure the product of beam and target polarizations are presented. The ability to study coherent pion production via the reactions 3 He → ( e → , e ′ 3 He )π 0 and 3 He → ( e → , e ′ 3 H )π + is shown along with the ability of the detector to unambiguously separate the two- and three-body breakup channels of the reaction 3 He → ( e → , e ′p) .

Electron scattering at NIKHEF with polarized beam and targets

We present the Internal Target Facility of the NIKHEF 900 MeV polarized electron storage ring. We give some results which illustrate the presently unique opportunity offered by this facility to study the spin structure of the nucleon, 2-body and 3-body system by the measurement of spin-dependent electron scattering observables.

Electron-induced two-proton knockout from 3He

Electron-induced two-nucleon knockout reactions at intermediate electron energies are driven both by one-body currents (coupling of the virtual photon to correlated nucleon pairs) and two-body currents (intermediate Δ-excitation and meson exchange currents (MEC)). In addition, final state interactions (FSI) contribute to the cross section. Recent experiments [1] have shown that the 160(e, e’pp)14C reaction is dominated by the direct knockout of strongly correlated proton pairs via one-body currents.

SPIN EFFECTS IN MEDIUM-ENERGY ELECTRON-3HE SCATTERING

Abstract New physics can be accessed by scattering polarized electrons from a polarized 3He internal gas target. It is discussed how the asymmetries for the reactions 3 He ( e , e′), 3 He ( e , e′p) 3 He ( e , e′n) 3 He ( e , e′d) and 3 He ( e , e′pn) may provide precise information of the S′ and the D-wave parts of the 3He ground-state wave function, the neutron form factors, and the role of spin-dependent reaction mechanism effects. The experiment uses up to 900 MeV (polarized) electrons from the AmPS storage ring in Amsterdam, Netherlands, in combination with large acceptance electron and hadron detectors.

Measurement of spin correlation parameters in the Δ region for the 1H⃗(e⃗,e′) reaction

Sensitivity to the quadrupole transition form factors (C2 and E2) of the N-Δ excitation can be obtained by measuring double-polarization electromagnetic observables in the Δ-region. Such measurements have been carried out with the internal target facility at NIKHEF. Radiative corrections are an important ingredient of the data interpretation. They need to be evaluated carefully, taking into account their spin-dependence, in order to compare the data to theoretical models of pion electroproduction. From these comparisons the E2/M1 and C2/M1 ratios can be extracted.

Longitudinally polarized electrons in a storage ring below 1 GeV

We report on the results of studies of the longitudinal electron polarization in the AmPS storage ring at NIKHEF. The ring was operated using a partial Siberian snake at the first magic energy, and a full Siberian snake between 440 MeV and 720 MeV. We have investigated the effect of high beam currents in the ring, and found that some of the electron polarization is lost if the beam current becomes larger than ≈120 mA.

Quasi-elastic electron scattering from polarixed 3He

Quasi-elastic electron scattering may provide precise information on the S′ and the D-wave parts of the 3He ground-state wave function, the neutron form factors, and the role of spin-dependent reaction mechanism effects. An experiment is being performed at the AmPS storage ring at NIKHEF (Amsterdam, the Netherlands), where polarized electrons (up to 900 MeV) are used in combination with large acceptance electron and hadron detectors. Preliminary results from data at four-momentum transfer squared Q2=0.15 GeV2 are presented.