Negative Electron Affinity Detector (NEADs)

In the late 1970s, NEADs were discussed as 'a most promising new technology for X-ray detection'. A NEAD, or negative electron affinity detector, is a device with highly desirable properties: high spatial resolution, high quantum efficiency, and moderate energy resolution. In the current literature, however, you will rarely even find a reference to a NEAD.

Semiconducting compounds composed of elements from the 3rd and 5th columns of the periodic table (GaAs, for instance) can be activated to a state of negative electron affinity by treatment of the surface with cesium and oxygen. That is to say, the top of the conduction band in the bulk can be made to lie above the vacuum level. Conduction band electrons can therefore escape from the surface when an X-ray is absorbed, even if the photon interaction takes place deep within the material. This is what makes the quantum detection efficiency be near 1. The moderate energy resolution follows from the fact that the most probable number of emitted photoelectrons increases monotonically with energy.

Although heavily investigated and modeled at the Smithsonian Astrophysical Observatory, a position-sensitive NEAD was never demonstrated. The stumbling block seems to have been the need to maintain the NEA surface in an extremely high vacuum (10^-10 Torr) in order to prevent contamination which would kill the negative electron affinity property. For the large area detectors needed in X-ray astronomy, this was indeed a very hard state to attain and keep... at least at the time of the 1970s and 1980s when they were being investigated. Maybe NEADs will become possible again as technology continues to develop.