Cathodes of medical x-ray tubes

Abstract

Cathodes of typical medical diagnostic X-ray tubes operate under adverse conditions. The X-ray focal spot on the anode in view of the cathode may generate metallic and other vapors. The residual gas pressure is comparatively high, as clinical tubes are not subject to active evacuation during operation. Subcomponents like the anode may release significant amounts of gas. Electron currents of up to 1 A or more produce ions under high electric fields, which may bombard the cathode. Ion energies may reach 150 keV. Occasional vacuum plasma discharges may appear close to or even invoking the electron emitter. Tubes for computed tomography may experience centrifugal acceleration of more than 30 times the gravitational acceleration. There exists an indisputable quest for utmost reliability. All these requirements conserved the classical thermionic tungsten emitter as the workhorse of medical diagnostic X-ray tubes, which will be the primary focus of this article.