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We report on the first Q^{2}-dependent measurement of the beam-normal single spin asymmetry A_{n} in the elastic scattering of 570 MeV vertically polarized electrons off ^{12}C. We cover the Q^{2} range between 0.02 and 0.05 GeV^{2}/c^{2} and determine A_{n} at four different Q^{2} values. The experimental results are compared to a theoretical calculation that relates A_{n} to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the Q^{2} behavior of A_{n} given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus.

For the first time a vertically polarized electron beam has been used for physics experiments at MAMI in the energy range between 180 and 855 MeV. The beam-normal single-spin asymmetry $A_{\mathrm{n}}$, which is a direct probe of higher-order photon exchange beyond the first Born approximation, has been measured in the reaction $^{12}\mathrm C(\vec e,e')^{12}\mathrm C$. Vertical polarization orientation was necessary to measure this asymmetry with the existing experimental setup. In this paper we describe the procedure to orient the electron polarization vector vertically, and the concept of determining both its magnitude and orientation with the available setup. A sophisticated method has been developed to overcome the lack of a polarimeter setup sensitive to the vertical polarization component.

A compact spin rotation system based on a Wien filter has been installed at the Mainz microtron accelerator (MAMI). Under operation with varying spin rotation angles a significant change of focal length together with a shift of the central beam trajectory is expected. We demonstrate that these effects can be kept under control. As a consequence operation with spin rotation angles between 0° and ±90° has been achieved without compromising the beam quality and operational stability of MAMI.

The response time and spin relaxation time of thin unstrained and strained III/V-semiconductor photocathodes installed in sources of polarized electrons have been investigated. Cathodes of various active layer thicknesses have been studied. An upper limit for the response time of a 150 nm thick strained layer photocathode has been found to be 2.5 ps. As a consequence, the average depolarization during transport in the conduction band to the surface is estimated to be lower than 3% and does not contribute substantially to the upper limit of about 80% for the spin polarization of the emitted electrons. The results indicate a high surface recombination velocity of S>1.2×107 cm/s at the surface band bending region.

We plan to have two independent polarimetry systems at MESA based on totally different physical processes. A first one tries to minimize the systematic uncertainties in double polarized Mo/ller scattering, which is to be achieved by stored hydrogen atoms in an atomic trap (Hydro-Mo/ller-Polarimeter). The other one relies on the equality of polarizing and analyzing power which allows to measure the effective analyzing power of a polarimeter with very high accuracy. Since the status of Hydro-Mo/ller is presented in a separate paper we concentrate on the double scattering polarimeter in this article.

Moller and Mott polarimeters have been compared concerning their measurement accuracy in a dedicated "spin dance" experiment. The results obtained with the different polarimeters at vastly different energy ranges were in agreement with each other within the given systematic uncertainties of a few percent. We discuss investigations of background conditions which may allow to reduce this important contribution to the uncertainties in the Mott polarimeter to a value below 1.0%.

We have developed a Mott electron polarimeter for the Mainzer microtron (MAMI) accelerator in Mainz, Germany. At beam energies ranging between 1.0 and 3.5 MeV two double focusing magnet spectrometers collect elastically backscattered electrons from gold targets. In spite of the small spectrometer acceptance, a sufficient statistical efficiency is achieved to provide very fast measurements if beam currents typical for experiments at MAMI are being used. High reproducibility is achieved, demonstrating that the results of asymmetry measurements are independent of the primary beam current in a range from 0.004 to 45 μA at a level of

A Mott polarimeter operating at energies from 1 to 3.5 MeV and at backward scattering angle of 164 degree has been installed at the MAMI accelerator. Its detection system is based on magnetic spectrometers allowing for efficient background suppression. The useful range of primary currents is between 10 nA and 100 μA, hence adapted to the typical beam currents at MAMI. The reduction of analyzing power due to multiple scattering can be calculated directly from the cross sections with the help of Monte Carlo simulation.

Surface cleaning with a beam of thermally dissociated hydrogen atoms was applied to superlattice cathodes which resulted in a considerable improvement of quantum efficiency. Compared to untreated samples no significant reduction of polarization of the emitted electrons was observed.

The conventional methods for measuring the polarization of electron beams are either time consuming, invasive or accurate only to a few percent. We developped a method to measure electron beam polarization by observing the light emitted by argon atoms following their excitation by the impact of polarized electrons. The degree of circular polarization of the emitted fluorescence is directly related to the electron polarization. We tested the polarimeter on a test GaAs source available at the MAMI electron accelerator in Mainz, Germany. The polarimeter determines the polarization of a 50 keV electron beam decelerated to a few eV and interacting with an effusive argon gas jet. The resulting decay of the excited states produces the emission of a circularly polarized radiation line at 811.5 nm which is observed and analyzed.

Spin-polarized electron kinetics is studied by time-resolved polarized photoemission with picosecond resolution. The re-sponse time of strained layer photocathodes is found to be in a range of a few picosecond offering an ultrafast response andhigh spin-polarization of emitted electrons. The studies of the sub-picosecond spin dynamics are facilitated in high-intensityexcitation regime when the length of the emission pulse is enlarged due to the dispersion of acceleration time and Coulombrepulsion ofthe electrons in their flight in the vacuum.Keywords: optical orientation, spin kinetics, strained semiconductor layer, time-resolved emission. 1. INTRODUCTION GaAs, layers are known to be promising for design and fabrication of ultrafast room- temperature "spin-electronics" devices 2 for which spin dynamics of the carriers is of a particular interest . The strained layer GaAs-based structures are activelystudied for applications in highly polarized electron sources for accelerators in which uniaxially strained GaAs or GaAsPphotocathodes are used 4-6Forthe polarized r.f.-guns application the pulse-response times in the few pico-second range aredesirable. A scaling of the response time with d2 for the diffusive process provides ultrafast time response ofthin layers and,as a consequence of this, low depolarization because these effects happen on longer time scales. Several factors may modifythe behavior at low thicknesses: first, the diffusive response could be gradually replaced with a ballistic one in the sampleswith the active layer thickness comparable with the electron free path length. Then, multiple electron reflection at the sur-face can reduce the electron capture rate to the band bending region (BBR) which can be expressed in terms oflowering ofthe effective surface recombination velocity. Finally, the effects of the escape processes from the BBR through the surfaceregion could modify the diffusive behavior, e.g. by adding additional time dispersion. These electron-kinetic details are of aparticular importance for the efforts to increase the emitted electron polarization above 90 % so that the polarization lossesat the subsequent stages of the emission processes have to be distinguished and minimized by the appropriate structure de-sign.In this report we present a time resolved study of gradient-doped strained layer GaAs photocathode stmctures at vary-ing excitation intensity. The pulse response experiments enable us to estimate spin relaxation during electron extractionfrom the active layer and in the band bending region, as well as the electron capture rate to the band bending region.

This talks addresses the operation of the polarized source at the Mainz Microtron MAMI. The source is operating with selected photocathodes of modulation doped, uniaxially strained layer photocathodes, which results in an average spin polarization of 80% and a quantum efficiency of typically 2 μA/mW. The operative lifetime has been improved by employing a novel activation technique which reduces transmission losses in the vicinity of the cathode. In addition a considerable simplification of the laser system has become possible by improving the power output of laser diode seed lasers so that it is not necessary to employ power amplifier units. It was shown that the potential for increasing the laser power is limited in our setup because of the thermal resistance between cathode and the surrounding electrode.

Time resolved photoemission of highly spin-polarized electrons from thin strained and unstrained GaAs x P 1-x films of various thicknesses has been investigated. An upper limit for the response time of a photocathode has been found to be 1 ps for layer thicknesses less than 150 nm. We show that the electron depolarization during the electron extraction to the surface band bending region can be as low as 2% while the losses in the band bending region can contribute to 4% spin relaxation.

We have measured the extractable charge during one lifetime (Charge lifetime, Qτ) of a NEA-GaAs-photocathode for two beam diameters. A limitation of Qτ to 1100 Coulomb due to ion-back-bombardment was observed for a 1.5 mm diameter emission site. When increasing the laser diameter from .3 to 1.5 mm we observe an increase of Qτ by a factor ≈ 5, which is in disagreement with the assumed proportionality of the charge lifetime with emission area. Possible reasons for this discrepancy are analyzed.

We report on a new measurement of the beam-normal single spin asymmetry $A_{\mathrm{n}}$ in the elastic scattering of 570 MeV transversely polarized electrons off $^{28}$Si and $^{90}$Zr at $Q^{2}=0.04\, \mathrm{GeV}^2/c^2$. The studied kinematics allow for a comprehensive comparison with former results on $^{12}$C. No significant mass dependence of the beam-normal single spin asymmetry is observed in the mass regime from $^{12}$C to $^{90}$Zr., Submitted for publication to Physics Letters B