Abstract

In 1992, it was demonstrated that hard x-rays can be produced in the high-intensity, >10¹⁸ W/cm², focus of a relatively low energy, ~40 mJ, ultrashort pulse laser system.¹ Measurements of x-ray yield through various filter materials provided a crude spectrum of the x-rays and indicated that the majority of the x-ray flux was at energies suitable for diagnostic radiography. A comparison of the 1992 laser-generated source with a conventional x-ray tube yields two distinct differences. First, the conventional source has an emission area that is limited both by space charge and by backstreaming of secondary electrons onto the anode. This source size is typically several hundred microns to 1 mm. Because the laser-generated source only produces hot, x-ray generating electrons in the focus of the laser field, the source size can be on the order of laser focal spot-size, i.e., 10 microns or less. The source size of the x-ray emitter determines a lower bound on the resolution diameter than can be obtained in any imaging application. Recently Herrlin et al. have performed resolution tests of a laser-generated x-ray source and have demonstrated imaging with 60 micron resolution. Their measurements were limited by the minimum feature size of the test object and not the x-ray source.² The second difference between conventional and laser-generated sources is the time duration of the x-ray emission, which can be on the order of a picosecond or less in the laser-generated case. The ultrashort x-ray emission time allows consideration of new imaging techniques. In particular, time-gated detection can be employed to reduce the effects of scattered radiation during radiography and to produce equivalent signal-to-noise images with less dose to the patient. Recently Gordon et al. demonstrated a contrast improvement of five in a first-generation, time-gated detection experiment.³ This experiment utilized a gated and imaged microchannel plated detector, which was originally designed for diagnostics of nuclear explosions⁴ to provide a ~100-ps shutter for ballistic x-rays. Resolution and contrast in this experiment were limited by the design of the detector. New detector designs are under development, which should provide ~0040 micron resolution and jitter-free gating.

© 1996 Optical Society of America