GEM Detectors


Designed by CERN, these GEM detectors will aid in muon tomography at Los Alamos National Lab. Muon tomography can be used to detect spent nuclear fuel or image for nuclear weapons/devices without obtaining specific details. Tomography has can also image architecture, such as the IL Duomo in Florence Italy. Due to their much smaller size compared to drift tube chambers, GEM detectors are a portable option when performing muon tomography.


On one end of the Gas Electron Multiplier (GEM) detector, there is a square grid of wires that reads out electrical signals. On top of this grid there are three GEM foil electrodes. These electrodes are kept apart by spacers to prevent arching between components. These electrodes contain high density of small holes that are around 70 micrometers in size. The electrodes lie below a different electrode that provides a drift area. This configuration is encased in a sealed enclosure that is filled with an inert gas such as pure nitrogen.

Operational Principles

Gas Electron Multiplier (GEM) detectors operate through a process referred to as electron avalanche. The incident charged particle generates electrons in the drift region through ionization. The holes in the GEM electrodes then act as edges for a capacitor and generate a strong electric field. These strong fields accelerate electrons, which increase the energy of these electrons. This added energy allows for interactions between the gas and the electrons, which produces further ionization. Since each stage produces more electrons, the process is called an electron avalanche. This electron avalanche gives a readable signal that gives information about the particle path.

Two views of a completed GEM Detector