Electron multiplier–scintillator detector for pulse counting positive or negative ions

A general-purpose ion detector has been developed for fast pulse counting in mass spectrometry. It consists of a six-stage electron multiplier joined to part of a scintillator detector. This novel arrangement combines the virtually noiseless pulse-amplification response of an electron multiplier with the bipolar voltage flexibility of a scintillator detector. The resulting hybrid detector is superior in performance to either an electron multiplier or a conventional scintillator detector. It is designed to detect a beam of positive ions or negative ions focused to a line image or to a spot at the focal plane of a mass analyzer. Background counting noise is less than 1 count/min when the detector is used in either the positive ion or negative ion detecting mode. Mass-resolved ions are post-accelerated through an electric field to give them an additional kinetic energy of 10 keV before they strike the ion-to-electron converter in the electron multiplier. This allows low-energy ions to be detected with an efficiency close to unity, over a range of ion masses from 1 amu up to several hundred amu. Randomly spaced pulses can be counted at rates from 0 Hz to 10 MHz. The design of the detector and its operating characteristics are discussed.