PImMS, a fast event-triggered monolithic pixel detector with storage of multiple timestamps

PImMS, or Pixel Imaging Mass Spectrometry, is a novel high-speed monolithic CMOS imaging sensor tailored to mass spectrometry requirements, also suitable for other dark-field applications. In its application to time-of-flight mass spectrometry, the sensor permits ion arrival time distributions to be combined with 2D imaging, providing additional information about the initial position or velocity of ions under study. PImMS1, the first generation sensor in this family, comprises an array of 72 by 72 pixels on a 70 mu m by 70 mu m pitch. Pixels independently record digital timestamps when events occur over an adjustable threshold. Each pixel contains 4 memories to record timestamps at a resolution of 25 ns. The sensor was designed and manufactured in the INMAPS 0.18 mu m process. This allows the inclusion of significant amounts of circuitry (over 600 transistors) within each pixel while maintaining good detection efficiency. We present an overview of the pixel and sensor architecture, explain its functioning and present test results, ranging from characterisation of the analogue front end of the pixel, to verification of its digital functions, to some first images captured on mass spectrometers. We conclude with an overview of the upcoming second generation of PImMS sensors.