R. Neuhausen

The three-spectrometer facility at the Mainz microtron MAMI

Abstract A set-up of three high-resolution magnetic spectrometers, for simplicity named A, B and C, has been built as the central facility for the precise determination of double and triple coincidence cross sections of hadron knock-out and meson production through the scattering of electrons at the Mainz microtron MAMI. The spectrometers A and C with point-to-point optics in the dispersive plane and parallel-to-point optics in the non-dispersive plane have a solid angle of 28 msr and a momentum acceptance of 20 and 25%, respectively. They each consist of a quadrupole, a sextupole and two dipole magnets, reaching maximum momenta of 735 and 550 MeV/c, respectively. The spectrometer B has a solid angle of 5.6 msr and a somewhat reduced momentum acceptance of 15%, but it reaches a maximum momentum larger than that of the MAMI electron beam (855 MeV/c). It consists of a single-clamshell dipole magnet with point-to-point optics in both planes. Each spectrometer is equipped with a position-sensitive detector system consisting of four planes of vertical drift chambers, two planes of plastic scintillators and a threshold gas Cherenkov detector. The operational experiences demonstrate that all three spectrometers exceed the specifications. Selected results of double (e, e′ x ) and triple (e, e′ x 1 x 2 ) coincidence experiments, x 1 and x 2 standing for charged hadrons, are presented, which demonstrate the performance of the whole set-up.

Precise neutron magnetic form factors

Precise data on the neutron magnetic form factor Gmn have been obtained with measurements of the ratio of cross sections of D(e, en) and D(e, ep) up to momentum transfers of Q 2 = 0.9 (GeV/c) 2 . Data with typical uncertainties of 1.5% are presented. These data allow for the first time to extract a precise value of the magnetic radius of the neutron.  2002 Elsevier

Precise measurements of the neutron magnetic form-factor

Abstract The neutron magnetic form factor Gmn has been determined via a measurement of the ratio of cross sections D(e,e′n) and D(e,e′p). The absolute detection efficiency of the neutron detector was measured with high accuracy using tagged neutrons produced from H(n,p)n elastic scattering by means of a high intensity neutron beam. This approach minimizes the model dependence and improves upon the weakest points of previous experiments. Data in the range q2=0.2–0.8 (GeV/c)2 with uncertainties of

Improved limits on the weak, neutral, hadronic axial vector coupling constants from quasielastic scattering of polarized electrons.

Abstract 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.

The neutron charge form factor and target analyzing powers from 3He(e→,e′n) scattering

The charge form factor of the neutron has been determined from asymmetries measured in quasi-elastic 3 (He) over right arrow((e) over right arrow, e`n) at a momentum transfer of 0.67 (GeV/c)(2). In addition, the target analyzing power, A(y)(0), has been measured to study effects of final state interactions and meson exchange currents.

Measurement of the electric form factor of the neutron at Q 2 = 0.3-0.8 (GeV/ c) 2 ⋆

Abstract.The electric form factor of the neutron, GE,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e’¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q2 = 0.3, 0.6 and 0.8 (GeV/c)2. Corrections for nuclear binding effects have been applied.

Polarization transfer in the 4He(e→, e′ p→)3H reaction

We have measured the proton recoil polarization in the {sup 4}He(polarized-e, e-prime, p){sup 3}H reaction at Q{sup 2} = 0.5, 1.0, 1.6, and 2.6 (GeV/c){sup 2}. The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a predicted medium modification of the proton form factors based on a quark-meson coupling model. In contrast, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.

Form factor of the M1 transition to the 10.23 MeV state in 48Ca and the role of the Δ(1232)

Abstract The form factor of the 10.23 MeV 1 + state in 48 Ca has been measured by inelastic electron scattering up to a momentum transfer of 1.4 fm −1 . The state is known to have a relatively simple shell-model structure. A shell-model calculation is, however, in severe disagreement with the data. We have attempted to analyse this discrepancy in terms of non-nucleonic degrees of freedom. The mechanism of Δ-hole polarization is important for the reduction of the form factor at low q if the Δ-hole interaction is sufficiently strong. The remaining quenching may be attributed to nucleonic core polarization while corrections from meson-exchange currents turn out to be small.

Electron scattering from the 3.65 MeV (0+, T = 1) state in 6Li at high momentum transfer

Abstract The form factor ofthe 3.56MeV(0 + , T = 1) state of 6 Li has been measured for momentum transfers q = 1.0–3.0 fm −1 , and the 2.18 MeV (3 + , T = 0) and 5.37 MeV (2 + , T = 1) states have been measured up to q = 2.5 fm −1 . The 3.56 MeV form factor is analysed in terms of a phenomenological shell model with l = i valence nucleons. The radial wave functions are found to have a greater radial distribution than given by the harmonic oscillator, more closely resembling Woods-Saxon functions. The M1 form factor is found to decrease at high momentum transfer somewhat more slowly than the models predict. A technique for determining the Mλ transition current density based on the Fourier-Bessel analysis is developed and applied to the M1 transition. The M1 transition current density is obtained within a moderate error band and compared with the harmonic oscillator and Woods-Saxon densities. The M1 radiative width is 8.18 ± 0.25 eV, in agreement with previous measurements.

Transition charge densities of low-lying collective states in even Zn isotopes

Abstract The inelastic electron scattering cross sections for the quadrupole transitions to the 2 1 + and 2 2 + states in the even Zn isotopes 64 Zn, 66 Zn and 68 Zn and for the hexadecapole transition to the 4 + 1 state in 64 Zn have been measured in a momentum transfer range up to q = 2.2 fm −1 . In the frame-work of the vibrational model these states are considered as one- and two-quadrupole-phonon states. The measurements are characterized by high statistical accuracy and by an overall resolution of δE / E 0 = 10 −3 which permitted separation of almost all members of the two-phonon triplet. The measured cross sections are analyzed with phenomenological models as well as with a Fourier-Bessel expansion of the transition charge density. The latter analysis yields realistic error bands for the transition charge densities and model-independent values for the reduced transition probabilities and transition radii.