  Collisions between high intensity laser pulses and energetic electron beams
are now used to measure the transition between the classical and quantum
regimes of light-matter interactions. However, the energy spectrum of
laser-wakefield-accelerated electron beams can fluctuate significantly from
shot to shot, making it difficult to clearly discern quantum effects in
radiation reaction, for example. Here we show how this can be accomplished in
only a single laser shot. A millimeter-scale pre-collision drift allows the
electron beam to expand to a size larger than the laser focal spot and develop
a correlation between transverse position and angular divergence. In contrast
to previous studies, this means that a measurement of the beam's
energy-divergence spectrum automatically distinguishes components of the beam
that hit or miss the laser focal spot and therefore do and do not experience
radiation reaction.
