Reflex triode x-ray source research on Gamble II

Reflex diode and triode x-ray sources are investigated on Gamble II (2 MV, 2 Ω, 100 ns) to understand the coupling of high-power (TW), MeV-electron beams to thin converters in order to optimize emission of sub-100- keV x-rays. Thin converters reduce self-absorption of low-energy x-rays, but the beam electrons must pass many times through the converter for efficient x-ray production. The triode configuration was found to be superior to diode configurations for this purpose. A large range of converter thicknesses were tested, from 2.5-μm gold to 250-μm tantalum. The highest dose was obtained from 25-μm tantalum, although the spectrum became softer for thinner converters. Computer modeling is in very good agreement with the experimental data, and indicates that electron loss to large radius determines the optimum converter thickness. The coupling efficiency for thinner converters could be improved by decreasing the triode impedance, a motivation for developing similar diodes on higher-current generators.