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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 present work describes the source of polarized electrons that is run at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universitat in Mainz. The source is based on photoelectron emission from (III–V)-semiconductors. Presently strained layer InGaP- or GaAsP-cathodes are used, which are processed to negative electron affinity by coverage of the surface with a submonolayer of caesium and oxygen. Electron beams spin-polarized up to a degree of P = 55% at a quantum efficiency of QE = 2% or P = 75% at QE = 0.4%, respectively, are obtained. The well-known but hitherto unsolved problem of limited cathode lifetime has been sidestepped by the attachment of an UHV load lock system to the source electron gun. It allows quick replacement of cathodes without breaking the gun vacuum. Availabilities in excess of 85% are obtained regularly in beamtimes longer than 100h. The source was mainly applied in measurements of nucleon form factors via the reactions 1H(ē, e′ p), 2D(ē, e′ p), 2D(ē, e′ n), and 3He(ē, e′ n). More than 1600 h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI up to now.

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 Moller polarimeter was mainly designed for the cw electron beam of the Mainz microtron (MAMI). The described polarimeter covers an energy range between 25 and 185 MeV and can relatively simply be upgraded to the maximum MAMI energy of 840 MeV. The Moller-scattered electrons are momentum-analyzed in the defocusing plane of a quadrupole magnet and both Moller electrons can be detected in coincidence for symmetrical scattering with ⊖cm=90°. All polarization components of the electron beam can be measured by suitable choices of the orientation of the target polarization. For pulsed electron beams with a small duty factor and a high peak current the polarimeter can also be operated with single arm analog detection of the Moller-scattered electrons.

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.

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 polarization of high-energy electrons can conveniently be measured by means of Moller scattering from polarized electrons in a magnetized ferromagnetic target. This process exhibits in the limit of high energies (E ≥ 100 MeV) still relatively large, energy independent and well known analysator strengths. Furthermore, the laboratory cross section in the kinematical range suitable for polarimeter applications is large $$ \frac{{d\sigma }}{{d\Omega }}({\Theta _{cm}} = 90^\circ ) = Z \cdot 0.18{\text{ }}barn/sr$$ and energy-independent.

The source described is based on photoemission of electrons from 100-GaAs0.62P0.38 activated to negative electron affinity. It is built to inject a beam of polarized electrons into the 350 MeV linear accelerator in Mainz. It is capable of delivering a mean current of 28 μA spin-polarized longitudinally to a degree of 0.44. The lifetime of the cathode under operational conditions is better than 200 h. The source was successfully run in a parity experiment, in which the analysing power of quasielastic scattering from beryllium for longitudinally polarized electrons was measured.

In scattering polarized electrons (P1 = 44% by 9Be at an energy of 300 MeV at angles 115°⩽ϑ⩽145° a parity violating asymmetry of Acorr = (−3.5 ± 0.7 ± 0.2) × 10−6 was measured. After correction for finite electron polarization and background we deduce an experimental asymmetry of Acx = (−9.4 ± 1.8 ± 0.5) × 10−6. The quoted errors indicate the statistical and the systematic uncertainties, respectively. The asymmetry, which is dominated by the quasielastic cross section, is interpreted in terms of model-independent electron-nucleon coupling constants of the weak neutral current. The error limits in the sector of axial vector coupling constants have been improved by a factor of 3 over previous results. A model-dependent analysis for the Weinberg angle yields the result sin2θw = 0.221 ± 0.014 ± 0.004.

Precise ratios of elastic electron scattering cross sections for the nuclei /sup 54,56,58/Fe, /sup 58,60,62,64/Ni, and /sup 64,66,68,70/Zn have been measured in the momentum-transfer region 0.6 fm/sup -1/ < or = q < or = 2.3 fm/sup -1/. The data were analyzed with a Fourier-Bessel parametrization of the charge distribution. Charge distribution differences were determined nearly model independently for the ..delta..A=2 isotope and isotone pairs. The ..delta..Z=2 isotone charge distribution differences show a strong shell effect. For the ..delta..N=2 isotopes we observe a nearly constant increase of the ''half density radius'' in the investigated region of the 2p/sub 3/2/, 1f/sub 5/2/, and 2p/sub 1/2/ neutron shells. However, the skin thickness of the charge distribution increases strongly at the beginning (/sup 56/Fe-/sup 54/Fe) and decreases at the end (/sup 70/Zn-/sup 68/Zn) of these shells by adding two neutrons. The rms radii differences deduced from a combined analysis of the electron scattering data and present muonic x-ray data show the same trend. These differences, determined model independently with a typical accuracy of some 10/sup -3/ fm, decrease nearly linearly with increasing neutron number in the investigated region 28 < or = N < or = 40. These isotope shifts are nearly independentmore » of the proton configuration of the involved nuclei, hence, that the added neutrons interact primarily with the proton core rather than with the valence protons. A comparison of the experimental charge distribution differences with results of density dependent Hartree-Fock calculations indicates the importance of deformation changes. The core rearrangement due to added protons, deduced from the measured isotone charge distribution differences, and the core rearrangements due to added neutrons, directly measured by the isotope shifts, are similar. Core polarizations result in a smaller charge core radius.« less

Torsional vibrations of (CH3)3—X type molecules having C3v point group symmetry have been analyzed by using a periodic potential function, and by taking into account mutual top—top interaction terms. The energy scheme was calculated by perturbation methods, and the selection rules have been derived. For the high‐barrier case, two allowed transitions from the ground state to the first excited state can be predicted.

The Mainz electron scattering facility provides energies between 80 and 300 MeV and scattering angles up to 158° which corres-pond to a useful momentum transfer region 0.4 ≤ q ≤ 3.0 fm−1. The accelerator and analyzing system produce approximately 1 μA average current with the overall resolution in the final spectra of usually 0.15% to 0.30%.

Nuclear current distributions for rotational states of heavy nuclei obtained in realistic cranked shell-model calculations with BCS correlations are used to calculate the transverse PWBA form factors for inelastic electron scattering. By keeping the same density but changing the velocity fields the sensitivity of the form factors to the dynamics of nuclear motion is explored. It is found that experiments on the 0 + → 2 + transition alone will not allow a satisfactory determination of the nuclear flow but that an analysis also of the 0 + → 4 + transition will be necessary. For these transitions optimal q -windows are identified.