A miniaturized non-radioactive electron emitter including a vacuum pressure gauge based on electric retarding field ion currents

We present a miniaturized non-radioactive electron source based on thermo emission for generating free electrons at atmospheric pressure. Most gas sensor systems based on atmospheric pressure chemical ionization (APCI), such as ion mobility spectrometers, use radioactive beta minus sources to provide free electrons with high kinetic energy to initiate a chemical gas phase ionization of the analytes to be detected. Instead of a radioactive beta source we generate free electrons by classical thermo emission from a tungsten filament in vacuum. The emitted free electrons are then accelerated towards an electron transparent Si₃N₄ membrane, through which the electrons are transferred from vacuum to atmospheric pressure. For both the development process and operation of this non-radioactive beta minus source pressure monitoring inside the vacuum chamber is required. Our setup already comprises all necessary components for measuring an ion current proportional to the pressure. These ions are generated within the residual gas inside the vacuum chamber through electron bombardment. However, if the electron emission in the vacuum chamber is not constant, both the ion current and the electron emission need to be measured to calculate the pressure. This increases the instrumental effort. Therefore, we present a new method for monitoring the pressure that is based on electric retarding field dependent ion currents.