Gas pressure dependence of prebreakdown conduction in high vacuum

Abstract

Secondary-electron pulse-height distributions induced by 29 keV positive ions impinging on a stainless steel converter of a Daly detector have been investigated. Some factors are discussed which influence the shape of this distribution and may sometimes determine the background current which remains after proper discrimination. In our case this background current is about 10−19 A for a detector vacuum of about 2 × 10−8 torr.The following factors are considered: (1) low-energy radiative background pulses generated by ion bombardment of the converter, and (2) ejection of electron clusters consisting of 1, 2, 3, etc. electrons from the converter when relatively high gas pressures (≿ 10−6 torr) prevail in the conversion chamber. It is suggested that the former effect is due to radiative de-excitation of excited particles emitted from the converter. The latter effect appears as a set of more or less pronounced peaks superimposed on the pulse-height distribution measured at lower pressures. The intensity of these peaks strongly depends on the gas pressure in the conversion chamber (∝ p2.7). A simple model is suggested which qualitatively accounts for this so-called “gas effect” and a new method is described for calibrating the system energy scale by means of it.The measured pulse-height distributions are compared with a Poisson distribution corrected for instrumental broadening. The good agreement enables us to obtain fairly precise values for the secondary-electron emission coefficient γ¯. For 29 keV argon ions incident on 304 stainless steel under an angle of 52° with the surface normal γ¯ appears to be about 7.8.