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Highlights • Line-focus X-ray tubes are suitable for clinical microbeam radiation therapy (MRT). • A modular high-voltage supply safely enables high electron beam powers. • An electron accelerator was designed to generate an eccentric focal spot. • We simulated a peak-to-valley dose ratio above 20 for single-field MRT. • Microbeam arc therapy spares healthy brain tissue compared to single-field MRT., Background and purpose Microbeam radiotherapy (MRT) is a preclinical concept in radiation oncology with arrays of alternating micrometer-wide high-dose peaks and low-dose valleys. Experiments demonstrated a superior normal tissue sparing at similar tumor control rates with MRT compared to conventional radiotherapy. Possible clinical applications are currently limited to large third-generation synchrotrons. Here, we investigated the line-focus X-ray tube as an alternative microbeam source. Materials and methods We developed a concept for a high-voltage supply and an electron source. In Monte Carlo simulations, we assessed the influence of X-ray spectrum, focal spot size, electron incidence angle, and photon emission angle on the microbeam dose distribution. We further assessed the dose distribution of microbeam arc therapy and suggested to interpret this complex dose distribution by equivalent uniform dose. Results An adapted modular multi-level converter can supply high-voltage powers in the megawatt range for a few seconds. The electron source with a thermionic cathode and a quadrupole can generate an eccentric, high-power electron beam of several 100 keV energy. Highest dose rates and peak-to-valley dose ratios (PVDRs) were achieved for an electron beam impinging perpendicular onto the target surface and a focal spot smaller than the microbeam cross-section. The line-focus X-ray tube simulations demonstrated PVDRs above 20. Conclusion The line-focus X-ray tube is a suitable compact source for clinical MRT. We demonstrated its technical feasibility based on state-of-the-art high-voltage and electron-beam technology. Microbeam arc therapy is an effective concept to increase the target-to-entrance dose ratio of orthovoltage microbeams.

Physical review accelerators and beams 24(4), 042803 (2021). doi:10.1103/PhysRevAccelBeams.24.042803, We have designed a tool to measure the bunch length of an electron beam in a minimally invasive way by means of coherent Smith-Purcell radiation (SPR). The technique has been employed successfully at a test apparatus for the Mainz Energy-recovery Superconducting Accelerator MESA, demonstrating it is possible to determine the bunch length while losing less than 0.6% of the electron beam. The impact of the space charge on the bunch length can be reduced while tuning the longitudinal bunch preparation system during a live measurement at beam currents up to 1 mA. Doing so, RMS bunch lengths of 70 ?m can be achieved in a typical operating mode of the low-energy beam transport system of MESA. In addition to the bunch length measurements, typical properties of the generated SPR are demonstrated., Published by American Physical Society, College Park, MD

One aim for the new electron accelerator MESA is to measure the weak mixing angle in electron proton scattering with high precision. This results in a requirement for beam polarization measurement of deltaP/P approx 0.5%. The Moller polarimeter proposed by V. Luppov and E. Chudakov opens the way to reach a sufficiently accurate measurement. The polarized atomic hydrogen target is under construction. The current status including recent modifications is presented.

A low-energy test set-up for the measurement and optimisation of secondary electron currents from the collector in electron coolers is presented. A variable-field Wien filter and current-measuring aperture plates grant insight into the trajectory and energy distribution of the secondary particles. While secondary electrons flowing back from the collector can be bent away and dumped, these electrons in turn create new secondaries (“tertiaries”), and the resulting cascade is what ultimately limits the performance of the whole collector assembly, i.e. collector, optics, and Wien filter as a complete unit. Our set-up aims at understanding the behaviour of secondary and tertiary electrons inside this assembly. Using the included diagnostic devices, the efficiency of the collector itself can be measured as a function of the beam parameters and optics; some measurements of a COSY-style collector are given as an example.

The Small Thermalized Electron Source at Mainz (STEAM) is a dc photoemission source. It is designed to operate at up to 200kV bias voltage with an accelerating field of up to 5 MV/m at the cathode surface. In several experiments, the properties of GaAs operating under the conditions of spin-polarized photoemission were investigated. Its performance, quantum efficiency lifetime and surface charge limit observations for bulk-GaAs will be discussed., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Proceedings of the 10th International Particle Accelerator Conference The 10th International Particle Accelerator Conference, Melbourne, Australia, 19 May 2019 - 24 May 2019; JACoW Publishing, Geneva, Switzerland 898-901 (2019). doi:10.18429/JACOW-IPAC2019-MOPTS024, At the GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt, Germany, the HElmholtz LInear ACcelerator (HELIAC) is currently under construction. The HELIAC comprises superconducting multigap Crossbar H-mode (SC CH) cavities. The input beam is delivered by an already existing High Charge Injector (HLI). For the further development of the accelerator a detailed knowledge of the input beam parameters to the SC section is necessary. A method for beam reconstruction is incorporated, which provides for longitudinal beam characteristics using measurements with a beam shape monitor and a particle simulation code. This finalizes the investigations on 6D beam parameters, following previous measurements in transversal phase space. The reconstruction of the longitudinal phase portrait is presented., Published by JACoW Publishing, Geneva, Switzerland

For future experiments with heavy ions near the coulomb barrier within the super-heavy element (SHE) research project a multi-stage R&D program of GSI/HIM and IAP is currently in progress. It aims for developing a supercon-ducting (sc) continuous wave (CW) LINAC with multiple CH cavities as key components downstream the High Charge State Injector (HLI) at GSI. The LINAC design is challenging due to the requirement of intense beams in CW mode up to a mass-to-charge ratio of 6, while covering a broad output energy range from 3.5 to 7.3 MeV/u with unchanged minimum energy spread. Testing of the first CH-cavity in 2016 demonstrated a promising maximum accelerating gradient of E_{a} = 9.6 MV/m; the worldwide first beam test with this sc multi-gap CH-cavity in 2017 was a milestone in the R&D work of GSI/HIM and IAP. In the light of experience gained in this research so far, the beam dynamics layout for the entire LINAC has recently been updated and optimized.

The Mainz Energy-recovering Superconducting Accelerator (MESA) will be an electron accelerator allowing operation in energy-recovery linac (ERL) mode. It provides the opportunity to operate scattering experiments at energies of ~100 MeV with thin gas-targets. The MESA Internal Gas Target Experiment (MAGIX) aims to operate windowless jet targets and different gases up to Xenon to search for possible dark photon interactions, to precisely measure the magnetic proton radius and astrophysical S-factors. Investigations on the impact of the target on beam dynamics and beam losses are required for machine safety and to examine limits to ERL operation. The goal of this work is to understand target induced halo in the different experimental setups, track halo particles through downstream sections to examine beam losses and include a suitable collimation system and shielding into the accelerator layout to protect the machine from direct and indirect damage through beam losses and radiation. The present status of the investigations is presented., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

An important part of the new accelerator MESA (Mainz Energy-recovering Superconducting Accelerator) is the low energy beam transport system connecting the 100 keV electron source with the injector accelerator. The present setup includes the chopper- and bunching system. The devices are of most importance in order to achieve sufficient bunch compression particularely at higher bunch charges and currents. With the circular deflecting cavity of the chopper system it is possible to measure the longitudinal dimension of the bunches upstream of the buncher whereas downstream the longitudinal size will be measured by Smith Purcell radiation. Based on experimental results obtained from this setup we will discuss the beam parameter and compare them with simulations of the beamline., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Physical review accelerators and beams 21(2), 020102 (2018). doi:10.1103/PhysRevAccelBeams.21.020102, Published by American Physical Society, College Park, MD

This article describes the future P2 parity-violating electron scattering facility at the upcoming MESA accelerator in Mainz. The physics program of the facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries stemming from two-photon exchange and a possible future extension to the measurement of hadronic parity violation. The first measurement of the P2 experiment aims for a high precision determination of the weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 = 4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z pole. It comprises a sensitive test of the standard model up to a mass scale of 50 TeV, extendable to 70 TeV. This requires a measurement of the parity violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150 \micro A polarized electron beam impinging on a 60 cm liquid H_2 target allowing for an extraction of the weak charge of the proton which is directly connected to the weak mixing angle. Contributions from gamma Z-box graphs become small at the small beam energy of 155 MeV. The size of the asymmetry is the smallest asymmetry ever measured in electron scattering with an unprecedented goal for the accuracy. We report here on the conceptual design of the P2 spectrometer, its Cherenkov detectors, the integrating read-out electronics as well as the ultra-thin, fast tracking detectors. There has been substantial theory work done in preparation of the determination of the weak mixing angle. The further physics program in particle and nuclear physics is described as well., Comment: Invited EPJ A Manuscript, many figures, large file size

Proceedings of the 9th International Particle Accelerator Conference 9th International Particle Accelerator Conference, IPAC2018, Vancouver, Canada, 29 Apr 2018 - 4 May 2018; JACoW Publishing, Geneva, Switzerland 955-958 (2018). doi:10.18429/JACOW-IPAC2018-TUPAK002, A multi-stage program for the development of a heavy ion superconducting (sc) continuous wave (cw) linac is in progress at HIM (Mainz, Germany) and GSI (Darmstadt, Germany) under support of IAP (Frankfurt, Germany). In 2017 the first section of the CW-Linac has been successfully commissioned at GSI. Beam acceleration at the CW-Linac is foreseen to be performed by up to twelve multi-gap CH cavities. The linac should provide the beam for physics experiments, smoothly varying the output particle energy from 3.5 to 7.3 MeV/u, simultaneously keeping high beam quality. Due to a wide variation of the input- and output -beam energy for each cavity, a longitudinal beam matching to every cavity is of high importance. An advanced algorithm for an optimization of matched beam parameters under variable rf-voltage and rf-phase of each cavity has been developed. The description of the method and the obtained results are presented in the paper., Published by JACoW Publishing, Geneva, Switzerland

The European physical journal / Web of Conferences 138, 01026 (2017). doi:10.1051/epjconf/201713801026, Published by EDP Sciences, Les Ulis

International audience; New measurements of the beam normal single spin asymmetry in the electron elastic and quasielastic scattering on the proton and deuteron, respectively, at large backward angles and at ⟨Q2⟩=0.22 (GeV/c)2 and ⟨Q2⟩=0.35 ( GeV/c)2 are reported. The experimentally observed asymmetries are compared with the theoretical calculation of Pasquini and Vanderhaeghen [Phys. Rev. C 70, 045206 (2004).PRVCAN0556-281310.1103/PhysRevC.70.045206]. The agreement of the measurements with the theoretical calculations shows a dominance of the inelastic intermediate excited states of the nucleon, πN and the Δ resonance. The measurements explore a new, important parameter region of the exchanged virtual photon virtualities.

The P2 Experiment at the new Mainz Energy-recovering Superconducting Accelerator (MESA) aims at measuring the weak mixing angle $\sin^2 \theta_\text{W}$ at low Q$^2$ with high precision. Therefore the polarization of the incident electron beam has to be known with a very high accuracy (< 0.5 %). A conventional Mott-polarimeter requires a lot of effort in calibration to be able to acieve this goal. A Double Scattering Polarimeter (DSP) promises to meet these requirements with less effort.

The transport of polarized electrons in photocathodes based on highly strained semiconductor superlattices is studied experimentally and theoretically. In the experiments, the electron emission from the photocathode excited by a femtosecond laser pulse is studied by time-resolved measurements. The response time and the spin relaxation time are experimentally determined. The photoresponse of the photocathodes is calculated, and the results are correlated with the experimental data. A conclusion regarding the partial localization of photoelectrons in the superlattice is drawn.

NEA-GaAs based photocathodes allow the production of spin-polarized electron beams for fundamental research. We demonstrate how semiconductor properties influence salient parameters such as polarization and beam brightness. We present techniques that provide remedies for the extreme sensitivity to environmental factors. These are discussed for the specific installation that has provided spin-polarized beam at the MAMI facility during the last decade.

We suggest to start an accelerator physics project called the Mainz Energy recovering Superconducting Accelerator (MESA) as an extension to our experimental facilities. MESA may allow to introduce an innovative internal target regime based on the ERL principle. A second mode of operation will be to use an external polarized electron beam for parity violating experiments.

Polarized electron beams have played an important role in scattering experiments at moderate to high beam energies. Historically, these experiments have been primarily targeted at studying hadronic structure — from the quark contribution to the spin structure of protons and neutrons, to nucleon elastic form factors, as well as contributions to these elastic form factors from (strange) sea quarks. Other experiments have aimed to place constraints on new physics beyond the Standard Model. For most experiments, knowledge of the magnitude of the electron beam polarization has not been a limiting systematic uncertainty, with only moderately precise beam polarimetry requirements. However, a new generation of experiments will require extremely precise measurements of the beam polarization, significantly better than 1%. This paper will review standard electron beam polarimetry techniques and possible future technologies, with an emphasis on the ever-improving precision that is being driven by the requirements of electron scattering experiments.

At the Institut fur Kernphysik of Mainz University a harmonic double-sided microtron (HDSM) has been built to extend the experimental capabilities for nuclear and particle physics experiments to higher excitation energies. This novel microtron variant accelerates the 0.855 GeV continuous wave (cw) electron beam of the established three-staged race track microtron (RTM) cascade MAMI B up to 1.5 GeV. It consists of two normal conducting linear accelerators (linacs) through which the electrons are guided up to 43 times by a pair of 90°-bending magnets at each end. For beam dynamical reasons the linacs operate at the harmonic frequencies of 4.90 and 2.45 GHz. The extended facility is called MAMI C. The relatively strong vertical defocussing due to the 45°-pole face rotations (Fig. 1) at both the entrance and exit of the segment-shaped bending magnets is compensated for all recirculations by a suitable field decay in the magnets towards higher orbits. As a consequence, the energy gain of the electrons has to decrease with increasing turn number to maintain coherent acceleration. This occurs by an appropriate phase slip of the electron bunches downwards the rf-waves during the acceleration process. In this paper the functional principle and the beam dynamical concept of the double-sided microtron (DSM) as well as the design and development of its main components are described. Finally, the results of first beam measurements taken after starting up in December 2006 are discussed.

A new measurement of the parity-violating asymmetry in the electron-deuteron quasielastic scattering for backward angles at $⟨{Q}^{2}⟩=0.224\text{ }\text{ }{(\mathrm{GeV}/c)}^{2}$, obtained in the A4 experiment at the Mainz Microtron accelerator (MAMI) facility, is presented. The measured asymmetry is ${A}_{PV}^{d}=(\ensuremath{-}20.11\ifmmode\pm\else\textpm\fi{}0.8{7}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0ex}{0ex}}1.0{3}_{\mathrm{sys}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. A combination of these data with the proton measurements of the parity-violating asymmetry in the A4 experiment yields a value for the effective isovector axial-vector form factor of ${G}_{A}^{e,(T=1)}=\ensuremath{-}0.19\ifmmode\pm\else\textpm\fi{}0.43$ and ${R}_{A}^{(T=1),\mathrm{anap}}=\ensuremath{-}0.41\ifmmode\pm\else\textpm\fi{}0.35$ for the anapole radiative correction. When combined with a reanalysis of measurements obtained in the G0 experiment at the Thomas Jefferson National Accelerator Facility, the uncertainties are further reduced to ${G}_{M}^{s}=0.17\ifmmode\pm\else\textpm\fi{}0.11$ for the magnetic strange form factors, and ${R}_{A}^{(T=1),\mathrm{anap}}=\ensuremath{-}0.54\ifmmode\pm\else\textpm\fi{}0.26$.

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.

Negative electron affinity (NEA) GaAs photocathodes show different pulse responses depending on the wavelength of photoexcitation. The pulse response at 800 nm shows a long and relatively intense tail, whereas at 400 nm, a tail of similar shape but with an intensity lower by around two orders of magnitude is observed. We explain this behavior with the specific properties of NEA photocathodes and compare it with the response of a positive electron affinity photocathode.

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.

Even though theoretical estimates predict response times for the photoemission process of electrons from a negative electron affinity GaAs photoemitter 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 photocathode 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 photoemitter.

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.

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.

The MESA accelerator will operate for particle and nuclear physics experiments in two different modes. A first option is conventional c.w. acceleration yielding 150-200MeV spin-polarized external beam. Second, MESA will be operated as a superconducting multi-turn energy recovery linac (ERL), opening the opportunity to perform experiments with a windowless target with beam current of up to 10 mA. The perspectives for innovative experiments with such a machine are discussed together with a sketch of the accelerator physics issues that have to be solved.

We study features in the optically detected magnetic resonance (ODMR) signals associated with negatively charged nitrogen-vacancy (NV) centers coupled to other paramagnetic impurities in diamond. Our results are important for understanding ODMR line shapes and for optimization of devices based on NV centers. We determine the origins of several side features to the unperturbed NV magnetic resonance by studying their magnetic field and microwave power dependences. Side resonances separated by around 130 MHz are due to hyperfine coupling between NV centers and nearest-neighbor C-13 nuclear spins. Side resonances separated by approximately {40, 260, 300} MHz are found to originate from simultaneous spin flipping of NV centers and single substitutional nitrogen atoms. All results are in agreement with the presented theoretical calculations., Comment: 12 pages, new data and discussions in updated version

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

The ENC project attempts to realize an electron-nucleon collider at the upcoming Facility for Antiproton and Ion Research FAIR at GSI Darmstadt by utilizing the antiproton high-energy storage ring HESR for polarized proton and deuteron beams. The addition of a 3.3 GeV storage ring for polarized electrons will enable electron-nucleon collisions up to a center-of-mass energy of - s - 14 GeV. In such a configuration peak luminosities in the range of L = 10(32) to 10(33)cm(-')(2)s(-')(1) are feasible . Beam-beam effects in a space-charge dominated regimes in conjunction with high-energy electron cooling represents one of the main challenges for this project. In this paper beam- and spin dynamics simulations are presented, together with the required mo difications and extensions for a collision mode of the HESR storage ring and the conceptual design of this new collider complex.

International audience; This article reports stable photoluminescence and high-contrast optically detected electron spin resonance (ODESR) from single nitrogen-vacancy (NV) defect centers created within ultrasmall, disperse nanodiamonds of radius less than 4 nm. Unexpectedly, the efficiency for the production of NV fluorescent defects by electron irradiation is found to be independent of the size of the nanocrystals. Fluorescence lifetime imaging shows lifetimes with a mean value of around 17 ns, only slightly longer than the bulk value of the defects. After proper surface cleaning, the dephasing times of the electron spin resonance in the nanocrystals approach values of some microseconds, which is typical for the type Ib diamond from which the nanoparticle is made. We conclude that despite the tiny size of these nanodiamonds the photoactive nitrogen-vacancy color centers retain their bulk properties to the benefit of numerous exciting potential applications in photonics, biomedical labeling, and imaging.

The assumption of a constant charge per lifetime of NEA photocathodes was tested at polarized electron currents of several mA. Polarized electrons were produced from bulk GaAs photocathodes by laser illumination with a photon energy close to the band gap. We demonstrate that cathode heating, ion trapping and increased relative beam loss due to space charge effects may lead to reduction of charge lifetime. We also find that cathode lifetime increases by a factor four when we increase the photon energy to 2.3 eV.

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.

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.

The helicity dependence of the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}\stackrel{\ensuremath{\rightarrow}}{p}\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{0}$ reaction has been measured for the first time in the photon-energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a $4\ensuremath{\pi}$-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the ${D}_{13}(1520)$ resonance and are used to determine its helicity amplitudes.

For the parity-violation experiment at the Mami cw electron accelerator facility at the University of Mainz, we need a stable polarized electron source able to deliver a current >150 μA with polarization >60%. We develop the Orsay polarized electron source SELPO as an injector for MAMI. SELPO source based on a flowing helium afterglow is now operating in a high voltage platform. The terminal will be tested completely at Orsay and is foreseen to be installed at MAMI before the end of this year. Recent results obtained with an array of capillaries to generate the helium flow and a longitudinal spin direction are described.

The results of experimental and theoretical studies of spin polarized electron transport in semiconductor SL used for photoemitter application are presented. The experimental study is based on the time resolved measurements of electron emission from the cathode after its photoexcitation by fs laser pulse. The response and spin relaxation times have been determined by means of measured time dependent intensity and polarization of electron emission. We also performed theoretical calculations of photocathode pulse response and compared the obtained results with experimental data. Our analysis testifies the presence of partial electron localization in SL. Electron capture by localized states leads to unavoidable losses of quantum efficiency and polarization.

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.

This investigation covers first feasibility studies for an electron ring in the tunnel of the High Energy Storage Ring (HESR) of the Facility for Antiproton and Ion Research (FAIR). This electron ring should accumulate and store polarised electron s at an energy of 2.8 - 3.3 GeV and should provide for collisions of longitudinally polarised electrons with the longitudinally polarised protons at the PANDA detector and at a centre-of-mass of around 14 GeV. Electron-ion collisions would also be possibl e. In order to attain longitudinally polarised electrons in the interaction region, two different concepts have been considered and the optical constraints and depolarising effects have been analysed numerically. Both concepts use spin rotators based on s olenoids. For the first concept the polarisation is in the horizontal plane, while in the second concept the polarisation is vertical in the arcs. The electrons would be pre-polarised before injection. The aim is then to obtain the highest possible depola rising time.