Development of a thermionic diode detector for laser spectroscopy

This paper presents work on the development of a thermionic diode detector for laser spectroscopy of atoms. Basically, this is a positive ion-detector that exploits the negative space charge accumulated around the cathode of a diode. In this device, a thin and straight hot cathode wire is stretched axially into a long cylindrical heat-pipe that constitutes a device similar to an electronic vacuum diode. The ions in the diode space-charge region are produced as a result of photoionization or collisional ionization of lithium atoms. The design aspects and measurement of its electrical characteristics are described. A new method based on the resonant photoionization of lithium is used to measure the gain of the ion-detector. An application is also demonstrated to measure the photoionization cross-section (σ  =  7.8  ±  1.6 Mb) of lithium near its first ionization threshold for two-photon excitation at 639.145 nm. Important features of the thermionic diode are its easy, versatile and sturdy design with high gain (10⁴) and sensitivity that make it a valuable and cost-effective tool for spectroscopists. (paper)