Workshop on Polarized Electron Sources and Polarimeters

total exposure as high as 5800 L. Within some uncertainties, no degradation of the QE
was observed. Photocathodes from the MOCVD group showed a different evolution of
the quantum efficiency. With these photocathodes higher initial QE of about 25 - 30%
were routinely obtained. Moreover, the first AH cleaning typically resulted in an
increase of the QE with an obtained highest value of 35 %. However, further AH
cleanings (21 with total exposure of 5100 L for the sample with longest history) and
reactivations of these photocathodes lead to a slow, steady decay of the QE with a
saturation value at about 15 %. We varied hydrogen exposures within 100 - 440 L but
did not see strong improvements. The refreshing of the surface with HCL treatment
inside glove box did not rise QE value proving that the surface cleanness is not
responsible for the steady QE degradation along with the number of AH treatments.
The possible reason of the degradation is believed to be due the insertion of a
dislocation network inside the semiconductor structure during multiple heat-cleaning
cycles of up to 450 0 C. Indeed, transmission – mode GaAs photocathodes on glass or
sapphire substrates are mechanically strained and potentially have a risk to develop a
dislocation net along with heat-treatment cycles even at moderate temperature
treatment due to the different thermal expansion coefficients of the semiconductor
heterostructure and the sapphire base plate. Study of the surface morphology of
degraded photocathodes with an atomic force microscope indeed revealed the
existence of a mesh – like structure resulting from the dislocation network which is
inserted into the heterostructure during multiple high temperature treatments under the
influence of mechanical strain. More studies are needed to minimize the dislocation
density in strained photocathodes.
TSR ELECTRON TARGET PERFORMANCE
Using the cryogenic photocathode source, ultra cold electron beams were obtained,
with transverse and longitudinal temperatures of