  In state of the art photoinjector electron sources, thermal emittance from
photoemission dominates the final injector emittance. Therefore, low thermal
emittance cathode developments and diagnostics are very important. Conventional
thermal emittance measurements for the high gradient gun are time-consuming and
thus thermal emittance is not measured as frequently as quantum efficiency
during the lifetime of photocathodes, although both are important properties
for the photoinjector optimizations. In this paper, a single shot measurement
of photoemission transverse momentum, i.e., thermal emittance per rms laser
spot size, is proposed for photocathode RF guns. By tuning the gun solenoid
focusing, the electrons transverse momenta at the cathode are imaged to a
downstream screen, which enables a single shot measurement of both the rms
value and the detailed spectra of the photoelectrons transverse momenta. Both
simulations and proof of principle experiments are reported.
