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The electric form factor of the neutron GEn has been determined in double polarized exclusive 3He(e,e'n) scattering in quasi–elastic kinematics by measuring asymmetries A⊥, A∥ of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular to the momentum transfer q in case of A⊥ and parallel in case of A∥. The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam and of the gaseous 3He target were each about 50%. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large solid angles. Quasi–elastic scattering events were reconstructed from the measured electron scattering angles ϑe, φe and the neutron momentum vector pn′ in the plane wave impulse approximation. We obtain the result (0.27 < Q2c2/GeV2 < 0.5)= 0.0334 ± 0.0033stat± 0.0028syst which is averaged over the indicated range of Q2, the squared momentum transfer. This GEn value is significantly smaller than measured from the D(e,e'n) reaction under similar kinematical conditions. To what extent final state interactions in 3He quench the GEn result is subject of calculations currently in progress elsewhere.

The electric form factor of the neutron G E , n has been measured in the quasifree 2 H ( → e , e ′ → n ) p reaction using the 855 MeV polarized cw electron beam of the Mainz Microtron MAMI. The polarization of the scattered neutrons was analyzed in a polarimeter consisting of two walls of plastic scintillators. The precession of the neutron spin in a magnetic field was used for the first time to circumvent the measurement of the effective analyzing power of the polarimeter and the beam polarization. In this way G E , n could be determined with little model dependence and experimental uncertainties. The result G E , n ( 0.34 GeV 2 / c 2 ) = 0.0611 ± 0.0069 stat ( { + 0.0069 } { − 0.0055 } ) syst is larger than previously assumed.

The electric form factor of the neutron GE,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q2= 0.15 (GeV/c)2 in a measurement of the recoil polarisation ratio Px/Pz in the quasifree reaction D(e,e′n)p. At this Q2 the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenhovel et al.. After the corresponding corrections a value of GE,n(0.15 (GeV/c)2) = 0.0481±0.0065stat±0.0053syst is obtained.

The measurement of the polarisation transfer to the proton in the reactions\(H(\vec e,e'\vec p)\) and\(D(\vec e,e'\vec p)\) performed with longitudinally polarised electrons in quasi-free kinematics is presented. The coincidence measurement was executed atQ2≈8fm−2 using the 855 MeV, c.w. beam of the Mainz Microtron MAMI. The recoil polarisation was determined by means of a carbon analyser. The experiment shows that the binding of the nucleon does not modify the polarisationPx of the recoil proton within an error ofΔPx/Px≈10%. The measured polarisation agrees with recent theoretical predictions. Implications for the measurement of the electric form factor of the neutron using the\(D(\vec e,e'\vec n)\) reaction are discussed.

The neutron electric form factor GEn has been measured at the cw electron accelerator MAMI in the double polarized exclusive reactions 3He(e,e′n) and D(e,e′n) in quasi elastic kinematics with one common detector system. Experimental set-up and analysis of a 3He(e,e′n)-measurement in the range of momentum transfer Q2=0.27−0.5 (GeV/c)2 are discussed in this article. For events with low missing momentum (|Pm|⩽100 MeV/c) the PWIA analysis yields GEn=0.0352±0.0033±0.0024 with statistical and systematical error, corresponding to a quadratically added total error of 11.6% relativ. This result will be compared with preliminary results obtained from the D(e,e′n)-reaction and with data from elastic D(e,e′) scattering.

The recent availability of polarized electron beams at electron accelerator laboratories adds another technique to probing the electromagnetic structure of nuclear matter [1]. The present paper discusses the application of polarized electron scattering to the determination of the electric formfactor of the neutron. Its value at medium energies is only poorly known hitherto. Electron scattering is dominated by interaction with the neutron magnetic moment. Therefore the contribution of the charge distribution to the scattering cross section is scarcely detectable in case of unpolarized collision partners. The elastic scattering of longitudinally polarized electrons from a polarized neutron target offers a new and better chance to measure the neutron electric formfactor.

A first measurement of the asymmetry in quasielastic scattering of longitudinally polarized electrons from a polarized 3 He gas target in coincidence with the knocked out neutron is reported. This measurement was made feasible by the cw beam of the 855 MeV Mainz Microtron MAMI. It allows a determination of the electric formfactor of the neutron G E n independent of binding effects to first order. At Q 2 =0.31 ( GeV /c) 2 two asymmetries A ∥ ( S He ∥ q ) and A ⊥ ( S He ⊥ q ) have been measured giving A ∥ =(−7.40±0.73)% and A ⊥ =(0.89±0.30)% . The ratio A ⊥ / A ∥ is independent of the absolute value of the electron and target polarization and yields G E n =0.035±0.012±0.005.

The determination of the neutron electric formfactor from quasielastic reactions 3H↘e(e↘,e′n) and D(e↘,e′,n↘) respectively is one of the present goals of experiments with polarized electrons at the Mainz race track microtron MAMI. A GaAsP‐photoelectron source is used at MAMI to get an 855 MeV electron beam spinpolarized to a degree of 35% at a current of 10 μA. Polarized 3He‐nuclei are produced by optical pumping metastable 3He. Scattered electrons are detected in coincidence with the recoil neutrons, the transverse spinpolarization of the neutrons may be analyzed by neutron‐proton scattering in a double wall plastic scintillator detector. A subset of the final detector set‐up has been tested successfully now by investigating the polarization transfer to the proton in reactions H(e↘,e′p↘) and D(e↘,e′p↘) and to the neutron in D(e↘,e′n↘) at a 4‐momentum transfer with −Q2=8fm−2. First data from the exclusive quasielastic collision 3H↘e(e↘,e′n) indicate a value of the neutron electric formfactor of GnE=0.035±...

An angular distribution of negative pions produced on 15N, leading to the ground state of15O, has been measured via the (e, π−)-reaction at Ee ≈ 170 MeV. Using virtual photon theory, single-differential cross sections are extracted from the measured double-differential cross sections. The measured data is in good agreement with theoretical calculations in the DWIA model. A clear signature of nuclear EO excitation is found in contrast to previous experiments in a similar reaction on 13C.

A silicon strip detector is described that is suitable for low energy pion identification ( T π = 9–25 MeV) in electropion-production experiments. The special features are high energy resolution and high background suppression capability. Details of the design are given. The properties of the detector are experimentally tested.

The internal structure of proton and neutron is manifested in a variety of observables. The form factors for elastic electron scattering which describe the distribution of charge and magnetic moments provide information vital for our understanding of the confined quark system. The form factors of the proton could be measured over a wide range of momentum transfer through the conventional method of Rosenbluth separation. The determination of the neutron form factors is more demanding; it will now benefit from coincidence experiments at modern cw accelerators. For the neutron charge form factor a measurement of the asymmetry in the scattering of high energy polarised electrons from polarised targets or the spin transfer to the recoiling neutron from unpolarised targets promises a big improvement. Experiments of this type have already been started and deliver first results.