We present a method for determining the characteristics of an intense laser
pulse by probing it with a relativistic electron beam. After an initial burst
of very high-energy γ-radiation the electrons proceed to emit a series
of attosecond duration X-ray pulses as they leave the field. These flashes
provide detailed information about the interaction, allowing us to determine
properties of the laser pulse: something that is currently a challenge for
ultra-high intensity laser systems.Comment: 9 pages, 8 figure