[en] RADLAC II is a high current linear induction accelerator for electrons. The beam is produced in a field-immersed foilless diode injector. We report the first time-resolved characterization of a similar high-brightness immersed diode source using a time-gated, 2-D x-ray imaging technique. The experiments were performed on the 4-MeV IBEX accelerator and produced currents exceeding 40 kA in a 6-mm radius, thin annular beam with a measured thermal transverse velocity v/sub /perpendicular// = β/sub /perpendicular//c ∼ 0.1c. For currents of 30 kA, even brighter beams with β/sub /perpendicular// = 0.07 were obtained. At lower currents, beams as small as 2 mm in radius were produced with a smaller cathode tip. In all cases, the measured parameters were consistent with 2-D PIC simulations. The experimental results will be discussed and compared to theory and simulations. 7 refs., 10 figs., 1 tab