Gamma Ray Detectors
Following a capture reaction in the gas target, the product nucleus
emits one
or more gamma rays very quickly (typically before it has travelled more than a
few micrometers). The gamma rays pass readily through the thin wall of the
target vacuum box and into the bismuth germanate (BGO) detectors, where their
probability of interaction is high. Most of the energy of a gamma ray is
converted into kinetic energy of an electron or an electron-positron pair.
As electrons lose energy in BGO they produce light-- about 3 photons per keV
of energy loss. Photomultipliers attached to the BGO counters convert the light
into an electrical current which is registered by the data acquisition
electronics.