Interruption of a dc x-ray beam by a projectile has been used to produce a timing pulse for light-gas gun work. A finely collimated x-ray beam from a conventional dc x-ray generator intersects the projectile path at the muzzle. This beam is detected by a detector consisting of a thin NaI(Tl) scintillator, a beam splitter, and two photomultiplier tubes. The projectile, as it leaves the gun barrel, interrupts the x-ray beam; this results in a current drop from the photomultiplier tubes. Associated electronics convert this drop into a single electrical pulse. Response time of the electronics is approximately 3 μsec.