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±...

Even though theoretical estimates predict response times for the photo emission process of electrons from a negative electron affinity GaAs photo emitter in excess of hundreds of picoseconds, recent measurements found electron bunch durations of 40 ps or less. This work presents precise measurements of picosecond electron bunches from a negative affinity bulk GaAs photo cathode and develops a model which explains the measured bunch durations as well as the observed bunch shapes. The bunch shape turns out to be independent from the quantum efficiency of the photo emitter.

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 Arenhövel 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 electric form factor of the neutron GE,n has been measured in the quasifree 2H(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 GE,n could be determined with little model dependence and experimental uncertainties. The result GE,n(0.34GeV2/c2)=0.0611±0.0069stat({+0.0069}{−0.0055})syst is larger than previously assumed.

The electric form factor of the neutron G(E.n) has been measured in the quasifree H-2((e) over right arrow, e'(n) over right arrow)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 GeV2/c(2)) = 0.0611 +/- 0.0069(stat) ((+0.0069)(-0.0055))(syst) is larger than previously assumed.

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 MAMI source of polarized electrons is based on electron photoemission from III-V-semiconductors. Strained layers of GaAsP are used at present. Typical degree of spinpolarization of emitted electrons is 75 % at a wavelength of light irradiating the cathode of 830 nm. Recent work has been focused on the improvement of source operational reliability and the improvement of the capture efficiency of the interface between source and accelerator. Appreciable gain in stability was obtained by shortening the injection line and setting up a new source right at the injection point of MAMI and by replacing the Ti:Sapphire laser used so far by an infrared AlGaAs-diode-laser as a light source. installation of a new harmonic prebuncher in the injection line to MAMI increased the capture efficiency by more than a factor of three to 44 %. Beam currents up to 10 mu A and spinpolarized to 75 % are available at MAMI for long term experiments now. First tests of driving the source with a 2.45 GHz pulsed laser synchronized to the accelerator-RF were successful already. Implementation of this technique may bring the capture efficiency to a value near unity in not too far future.

A semiconductor modelocked diode laser has been used to produce picosecond spin-polarised electron bunches from strained GaAsP photocathodes and inject them into MAMI, synchronised to the 2.45 GHz accelerating field. The laser meets the operational requirements of MAMI producing stable electron beams, with a polarisation purity of 72% and a transmission efficiency of 52% at an accelerated beam current of 10.1 μA.

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 lime.

The neutron electric form factor G(En) has been measured at the cw electron accelerator MAMI in the double polarized exclusive reactions 3 (H) over right arrow e((e) over right arrow,e'n) [1-3] and D((e) over right arrow,e'(n) over right arrow) [4-7] in quasi elastic kinematics with one common detector system. Experimental set-up and analysis of a (3) (H) over right arrow e((e) over right arrow e'n)-measurement in the range of momentum transfer Q(2)=0.27-0.5 (GeV/c)(2) are discussed in this article. For events with low missing momentum (\(P) over right arrow(m)\less than or equal to 100 MeV/c) the PWIA analysis yields G(En)=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) over right arrow, e'(n) over right arrow)-reaction and with data from elastic D(e, e') scattering [8].

A fast analog trigger processor is being developed for the measurement of the parity violating asymmetry of the cross section of elastically scattered polarized electrons on unpolarized protons. The total event rate expected is about 100 MHz in the whole detector which consists of 1022 PbF2 crystals. This requires fast cluster recognition, spatial and temporal pile-up rejection and fast data storage. First results of a prototype are discussed.

Optical properties of large size lead fluoride (PbF2) crystals of three different manufacturers and their degradation caused by 60Co γ-radiation have been investigated. Transmission losses have been systematically studied at absorbed energy doses between 0.1 and 7kGy. Several radiation induced absorption bands have been observed. Optical bleaching with light of wavelengths ≳365nm has been found very effective to restore the original characteristics even after repeated irradiations. This observation together with the high density and the ultraviolet extended transmission make PbF2 an excellent choice for high rate and high resolution e.m. calorimetry.

The present work describes the source of polarized electrons that is run at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universität 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′ p̄), 2D(ē, e′ p̄), 2D(ē, e′ n̄), and 3H̄e(ē, e′ n). More than 1600 h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI up to now.

At the Mainz electron accelerator MAMI a selfstarting Kerr lens modelocked Ti:sapphire pulselaser with a repetition rate of 1.039 GHz was designed and tested. A laser of this type will be used at the source of polarized electrons to increase the lifetime of the photocathodes. The compact lasersystem, which contains an etalon for the compensation of group velocity dispersion, emits light pulses with a pulse length of 210 ps FWHM at an average output power of 520 mW.

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.

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.

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 polarisationP x 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.

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 GEn independent of binding effects to first order. At Q2=0.31 (GeV/c)2 two asymmetries A∥(SHe∥q) and A⊥(SHe⊥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 GEn=0.035±0.012±0.005.