K.-H. Steffens

THE MAMI SOURCE OF POLARIZED ELECTRONS

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.

A Moller polarimeter for CW and pulsed intermediate-energy electron beams

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.

A spin rotator for producing a longitudinally polarized electron beam with MAMI

Abstract The design and performance characteristics of a full 4 π-space spin rotator for 100 keV electrons are described. The spin rotator was developed as part of the acceleration scheme for polarized electrons in the MAINZ race track microtron cascade MAMI [1]. It allows to orientate the polarization vector in any direction before injection. Thus it is possible to optimize the longitudinal polarization component, required for experiments with polarized high energy electrons, at target position. With this scheme various experimental halls can be supplied with longitudinally polarized electrons in the full energy range of MAMI between 180 and 855 MeV.

PICOSECOND POLARIZED ELECTRON BUNCHES FROM A STRAINED LAYER GAASP PHOTOCATHODE

Abstract At the Mainz electron accelerator MAMI a facility has been set up that allows the measurement of the bunchlength, the energy spread and the averaged and phase resolved polarization of picosecond electron bunches from a source of polarized electrons for MAMI. Electron bunches of typically 10 to 30 ps duration are generated by illuminating a strained layer GaAsP photocathode with femtosecond laser pulses.

Photoemission of spinpolarized electrons from strained GaAsP

Strained layer GaAs.95P.05 photo cathodes are presented, which emit electron beams spinpolarized to a degree of P = 75% typically. Quantum yields around QE = 0.4% are observed routinely. The figure of merit P2 × QE = 2.3 × 10−3 is comparable to that of the best strained layer cathodes reported in literature. The optimum wavelength of irradiating light around 830 nm is in convenient reach of Ti:sapphire lasers or diode lasers respectively. The cathodes are produced using MOCVD-techniques. A GaAs.55P.45-GaAs.85P.15 superlattice structure prevents the migration of dislocations from the substrate and bottom layers to the strained overlayer. The surface is protected by an arsenic layer so that no chemical cleaning is necessary before installation into vacuum. The source of polarized electrons attached to the Mainz race track microtron MAMI works with such cathodes now. More than 1000 hours beamtime have been performed successfully.

Electromagnetic form factors of the nucleon

Abstract 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.

Determination of the neutron electric formfactor in quasielastic collisions of polarized electrons with 3He and 2D. Collaboration A3 at MAMI

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±...

First measurement of the electric formfactor of the neutron in the exclusive quasielastic scattering of polarized electrons from polarized 3He

A first measurement of the asymmetry in quasielastic scattering of longitudinally polarized electrons from a polarized 3He 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 GnE independent of binding effects to first order. At Q2=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 GnE=0.035±0.012±0.005.

Determination of the neutron electric form factor GEn in double polarized electron scattering from 3He

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.

Emission of polarized ps-electron bunches from III–V semiconductor cathodes

At the pulsed electron gun testfacility at MAMI we have measured electron bunches from GaAs samples with different layer thicknesses. We observe an increasing dc-polarization with decreasing layer thickness. From the pulse measurements this behaviour may be explained in terms of bunchlength and spin relaxation time.