GEM structure makes self-portrait

In 1996 Fabio Sauli at CERN introduced the gas electron multiplier (GEM) – a new idea for gas amplification in particle detection. The concept has since seen increasing use in particle physics and other applications. Recently Ronaldo Bellazzini and his team at INFN/Pisa have used a GEM-based pixel detector illuminated by ultraviolet (UV) light to produce a “self-portrait” of the GEM amplification structure.

Bellazzini uses a UV lamp to illuminate a caesium-iodide photocathode that is also the entrance window of the gas pixel detector. The light intensity is sufficiently low that the device detects only one photon at a time, each producing a single electron. The electron drifts into a single GEM hole where it knocks further electrons from atoms in an avalanche effect. The avalanche due to the single electron is extracted and a fine-pitch pixel CMOS analogue chip, which is also the charge-collecting electrode, provides a direct reading of the GEM charge multipliers, measuring the centre of “gravity” of the avalanche. If the resolution is good and the noise is low, the centre of gravity corresponds to the centre of the GEM hole.

Accumulating thousands of such events produces a map, in effect a “self-portrait”, of the GEM amplification structure with individual dots only 50 μm apart. The charge-collecting chip has 100,000 pixels arranged in a honeycomb pattern also at a pitch of 50 μm, providing an intrinsic resolution of the read-out system of only 4 μm, in response to a single primary electron.