International audienceThe Polarized Electrons for Polarized Positrons experiment at the injector of the Continuous ElectronBeam Accelerator Facility has demonstrated for the first time the efficient transfer of polarization fromelectrons to positrons produced by the polarized bremsstrahlung radiation induced by a polarized electronbeam in a high-Z target. Positron polarization up to 82% have been measured for an initial electron beammomentum of 8.19 MeV=c, limited only by the electron beam polarization. This technique extendspolarized positron capabilities from GeV to MeV electron beams, and opens access to polarized positronbeam physics to a wide community

Laser light with photon energy near the bandgap of GaAs and in LaguerreGaussian modes with different amounts of orbital angular momentum was used to produce photoemission from unstrained GaAs. The degree of electron spin polarization was measured using a micro-Mott polarimeter and found to be consistent with zero with an upper limit of ~3% for light with up to ±5ħ of orbital angular momentum. In contrast, the degree of spin polarization was 32.3±1.4% using circularly-polarized laser light at the same wavelength, which is typical for bulk GaAs photocathodes.

Nuclear physics experiments at Thomas Jefferson National Accelerator Facility's CEBAF rely on high polarization electron beams. We describe a recently commissioned system for prequalifying and studying photocathodes for CEBAF with a load-locked, low-voltage polarized electron source coupled to a compact retarding-field Mott polarimeter. The polarimeter uses simplified electrode structures and operates from 5 to 30 kV. The effective Sherman function for this device has been calibrated by comparison with the CEBAF 5 MeV Mott polarimeter. For elastic scattering from a thick gold target at 20 keV, the effective Sherman function is 0.201(5). Its maximum efficiency at 20 keV, defined as the detected count rate divided by the incident particle current, is 5.4(2) x 10 -4 , yielding a figure-of-merit, or analyzing power squared times efficiency, of 1.0(1) x 10 -5 . The operating parameters of this new polarimeter design are compared to previously published data for other compact Mott polarimeters of the retarding-field type.

Two K 2 CsSb photocathodes were manufactured at Brookhaven National Lab and delivered to Jefferson Lab within a compact vacuum apparatus at pressure $10 À11 Torr. These photocathodes were evaluated using a dc high voltage photogun biased at voltages up to 200 kV, and illuminated with laser light at wavelengths 440 or 532 nm, to generate dc electron beams at currents up to 20 mA. Some conditions produced exceptionally large photocathode charge lifetimes, without measurable quantum efficiency decay, even from the center of the photocathode where operation using GaAs photocathodes is precluded due to ion bombardment. Under other conditions the charge lifetime was poor due to extensive ion bombardment under severely degraded vacuum conditions, and as a result of localized heating via the laser beam. Following beam delivery, the photocathodes were evaluated using a scanning electron microscope to determine surface morphology.