New ion counting system for laser sputtered neutral mass spectrometry

In sputtered neutrals mass spectrometry (SNMS) with a laser for post-ionization a bunch of ions is generated within a short period corresponding to a laser pulse width (typically, a few tens of nanoseconds). The number of ions exceeds 100 under some conditions, so that a peak counting ability close to 10¹⁰ is necessary to detect all of them. The authors used an analog detection system for this purpose, but it was found to suffer from signal fluctuation, particularly when the primary ion beam current was low. They therefore developed a new counting system in which a bunch of ions incident on a microchannel plate is converted into photoradiating spots on a fluorescent screen. A charge coupled device camera records these spots and accumulates the signal for a (shutter opening) period. The signal is then transferred to a frame memory. Subsequently, a personal computer counts the 'events' in the frame memory.