Vacuum bench for the characterization of thermoionization ion sources

Abstract

We have designed a vacuum bench to study the parameters of thermoionization sources with the ultimate goal of obtaining high spatial resolution for biomedical applications of secondary ion mass spectrometry. In the bench, the source ionizer can be directly heated with an electron gun positioned perpendicular to the axis of the ion beam and focused with an optical system including slit lenses and a magnetic sector. The source cross over diameter is measured by forming the image of the source using an Einzel lens at a 1×magnification. The ion beam current is measured in a Faraday cup placed after a movable diaphragm. The temperature of the diverse elements of the ionizer assembly is measured through a mirror with a micropyrometer. Using the vacuum bench with a cesium carbonate source, we measured a 35 μm minimum cross over size, and we calculated a 400 A/cm2/sr maximum brightness. We obtained an intense cesium ion beam when heating the ionizer with the electron gun. The vacuum bench will be used to compare the effect of the heating mode of the ionizer (i.e., indirect by filament electron emission or direct by electron beam) on the brightness of the cesium source, and to develop a thermoionization iodine negative ion source.