Experimental evaluation of a megavolt rod-pinch diode as a radiography source

The rod-pinch diode is a cylindrical pinched-beam diode that provides an intense pulsed small-diameter bremsstrahlung source for radiography. For this work, the diode consists of a 1- to 6.4-mm-diameter anode rod that extends through the hole of an annular cathode. After exiting the cathode, wider anodes taper down to a 1 mm diameter. All of the anode rods then have a 1-mm-diameter tungsten tip that is usually tapered to a point. Rod-pinch diodes with anode rods of different materials, lengths, and diameters were powered by the Gamble II generator at peak voltages of 1.0 to 1.8 MV and peak currents of 30 to 60 kA. The radiation was characterized with temporally and spatially resolved X-ray diagnostics. Pinhole-camera images and time-resolved pin-diode measurements indicate that the radiation is emitted primarily from the vicinity of the rod tip. The dose measured with thermoluminescent detectors through a plexiglass transmission window ranges from 0.6 to 2.8 R at 1 m from the rod tip and the dose/charge scales faster than linearly with the diode voltage. The full-width at half-maximum (FWHM) of the radiation pulse is 30 to 50 ns. The size of the radiation source-is determined by measuring its edge spread function. The source diameter, defined here as the FWHM of the derivative of the edge spread function, decreases from 2 mm for a 6.4-mm-diameter rod to 1 mm or less for a 1-mm-diameter rod. Analysis suggests that the central portion of the radiation distribution at the source can be approximated by a uniformly radiating circular disc.