V. Tioukine

Pulse response of thin III/V semiconductor photocathodes

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.

A Mott polarimeter operating at MeV electron beam energies

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 <1% relative variation. Compared to low energy polarimeters of similar construction an improvement of a factor 2-3 in absolute accuracy seems possible.

High average photocurrent research at MAMI

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.

Status of the polarized source at MAMI

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.

Hydrogen cleaning of superlattice photocathodes

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.

Vertical Beam Polarization at MAMI

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 An, which is a direct probe of higher-order photon exchange beyond the first Born approximation, has been measured in the reaction C12(e→,e′)C12. 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.

New results from the Mainz polarized electron facilities

The polarized electron source at the Mainz microtron has improved its performance in every parameter during the last year. Run times have been performed with simultaneously 20 Mikroamps of beam current and more than 80% of polarization. We demonstrate that the limited beam-current lifetime product is due to a transmission loss of about several 10−4. The loss can be minimized by the usage of photocathodes with only partially photosensitive surface, thus resulting in a nearly one order of magnitude higher beam-current lifetime product.

Ab Initio Calculation of Effective Sherman Functions in MeV Mott Scattering

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.