J. Hose

SiPM Avalanche Size and Crosstalk Measurements with Light Emission Microscopy

Diodes operating in reverse bias emit visible light. This light emission process is important in silicon photomultipliers where microcells, arranged in a matrix, can trigger avalanches in adjacent microcells. One of the main goals toward improving silicon photomultipliers as light detectors is to design sensors with significantly higher photon detection efficiency than the classical photomultipliers. To achieve this goal, one needs to operate the silicon photomultiplier under a high relative overvoltage, saturating its Geiger efficiency. This means operating it under high gain and a correspondingly high level of optical crosstalk. In this paper, we describe a new and precise visual method for measuring the crosstalk as well as the transverse size of avalanches in a silicon photomultiplier microcell. This size turns out to be significantly smaller than the usually used microcell size, practically independent from the overvoltage or type. It also has implications for designing small, fast microcells that could reach a naturally lower size limit.

Light Induced Afterpulses in Photomultipliers

Various spurious effects limit the effectiveness of Photomultipliers when used in low-light-level coincidence counting experiments. One of these effects is afterpulses from light emission in the Photomultiplier. Here we present a study on the light emission in Photomultipliers using a variety of methods. First we image the light emission directly. Second we characterize the light emission spectrum. Finally we investigate the time resolved light emission. In particular we find that, not only the dynodes “glow,” but also the support structure emits light. A fluorescent effect is found. The experiments performed help to understand the fundamental effects of light emission and afterpulses in Photomultipliers.

Light Sensor Candidates for the Cherenkov Telescope Array

We report on the characterization of candidate light sensors for use in the next-generation Imaging Atmospheric Cherenkov Telescope project called Cherenkov Telescope Array, a major astro-particle physics project of about 100 telescopes that is currently in the prototyping phase. Our goal is to develop with the manufacturers the best possible light sensors (highest photon detection efficiency, lowest crosstalk and afterpulsing). The cameras of those telescopes will be based on classical super-bi-alkali Photomultiplier tubes but also Silicon Photomultipliers are candidate light sensors. A full characterisation of selected sensors was done. We are working in close contact with several manufacturers, giving them feedback and suggesting improvements.