Recovery of quantum efficiency in spin-polarized photocathodes by atomic hydrogen cleaning.

We demonstrate a new method of cleaning superlattice (SL) photocathodes using atomic hydrogen that allows an increased number of repeat activations. GaAs/GaAsP SL photocathodes were activated with either conventional heat cleaning or atomic hydrogen cleaning. Repeated heat cleaning was found to gradually lower the quantum efficiency (QE) of the photocathodes, while a relatively constant QE was maintained over repeated atomic hydrogen cleaning. These results show that atomic hydrogen cleaning allows a greater number of photocathode cleanings without a loss of performance. Analyses of SL photocathodes indicated that the degradation resulting from heat cleaning can likely be attributed to the build-up of residual Cs compounds as well as damage to the GaAs cap layer. The present study also determined the 1/e lifetime for a GaAs/GaAsP SL photocathode to be 7 h at an initial current of 2.2 µA.