  The spot size of the x-ray source is a key parameter of a flash-radiography
facility, which is usually quoted as an evaluation of the resolving power. The
pinhole imaging technique is applied to measure the spot size of the Dragon-I
linear induction accelerator, by which a two-dimensional spatial distribution
of the source spot is obtained. Experimental measurements are performed to
measure the spot image when the transportation and focusing of the electron
beam is tuned by adjusting the currents of solenoids in the downstream section.
The spot size of full-width at half maximum and that defined from the spatial
frequency at half peak value of the modulation transfer function are calculated
and discussed.
