A Sensor Employing an Array of Silicon Photomultipliers for Detection of keV Ions in Time-of-Flight Mass Spectrometry

Pixellated scintillation detectors have the potential to overcome several limitations of conventional microchannel-plate-based detectors employed in time-of-flight mass spectrometry (ToF-MS), such as extending detector lifetime, reducing vacuum requirements, or increasing the ion throughput. We have developed a prototype comprising a fast organic scintillator (Exalite 404) coupled to an array of 16 silicon photomultipliers (SiPMs), with read-out electronics based on the FastIC application-specific integrated circuit (ASIC). Each SiPM signal processed by FastIC is fed into its own time-to-digital converter (TDC). The dead time of a single channel can be as short as ∼20 ns. As a result, our system have the potential to process ion rates above 109 cm^-2 s^-1. We have evaluated the performance of our prototype using a velocity-map imaging ToF-MS instrument, recording the time-of-flight mass spectra of C₃H₆ and CF₃I samples. We achieved time resolutions of (3.3±0.1) and (2.5±0.2) ns FWHM for ions of mass-to-charge ratio (m/z) values of 196 and 18, respectively. This corresponds to a mass resolution of ∼1000 for m/z<200, which we found to be dominated by the spread in ion arrival times.