Positronium negative ions for monitoring work functions of surface plasma source

Abstract

Cesiation is addition of a small admixture of cesium to a gas discharge to increase negative ion emission and decrease electron emission. Cesiation decreases the surface work function and increases the probability of back scattered and sputtered particles escaping as negative ions. It is difficult to control the surface work function during the discharge. A positronim negative ion is a bound system consisting of a positron and two electrons that is created when a positronium atom escaping from metal captures an electron from the surface. As in the case of H- formation, the probability of formation of a positronium negative ion strongly depends on the surface work function. The creation rate of positronium negative ions can be determined by measuring the shift of the annihilation gamma ray spectrum coming from accelerated positronium negative ions. In this report, it is proposed to use the emission of positronium negative ions to monitor the surface work function. The surface to be monitored is first irradiated by positrons from a positron emitter (for example radioisotope 22Na). The emitted positronium negative ions are then accelerated by an electric field. The gamma rays produced by the electron and positron annihilation are registered by a proportional detector (for example, pure Germanium). The number of Doppler shifted gamma rays generated by the accelerated positronium negative ions can then be used to determine the surface work function.