Microchannel Plate Electron Multiplier for Mass Spectrometer Applications

A prototype microchannel electron multiplier has been tested for use in a satellite-borne mass spectrometer to be flown on the GIOTTO cometary mission. In the mass spectrometer application position, information is obtained both from discrete anoaes deposited on the back face of the microchannel plate (contact anodes), as well as from discrete remote anodes. Pulse height distributions from remote and contact anodes are found to be comparable. Crosstalk between contact anodes, although acceptably low (~1:50), was larger than expected for the amplifier gain, bandwidth, and integration periods used. Scans across the plate were made using a highly collimated beam of electrons that was small with respect to the channel diameter, allowing the boresighted response to be determined. The electron beam intensity was varied to measure gain G(n) and depression versus count rate n for single channel excitation. Where the channel recovery time ¿ is ~ 0.060 s, G(n) = G(o)/(1 + ¿n) The mass spectrometer configuration requires the use of a grid placed in front of the MCP to accelerate ions to detectable energy. A strong field for ions could diminish the gain of the MCP by extracting the initial secondary electrons produced at the front of the channel. The effect of accelerating fields on the efficiency of the MCP was investigated using energetic alpha particles and found to be small for fields of practical interest.