Natural Science Forum / Physics / Research / October 2007

The most common detector is a photomultiplier (PM) tube.  This device
has a photocathode that is coated with a material with a low work
function.  This means that a photon reaching this cathode is more
likely to eject an electron.  This electron finds itself in a vacuum
chamber with an electric field.  The electric field sweeps up the
electron and sends it down a staircase like structure that alternately
accelerates the electron then stops it in an electrode.  The
electrodes also have a low work function so each electron that strikes
it is likely to eject several low energy electrons for each one
arriving.  Each of these electrons is accelerated by the electric
fields to the next electrod where they are "multiplied" again.  Such a
detector can easily detect a single photon of light.

Another common detector is an Avalanche Photo Diode (APD).  This is
actually a solid state analog of the vacuum photomultiplier described
above.  A PN junction is reverse biased to such a high voltage that it
is almost in breakdown.  When a photon arrives in the junction, it
creates an electron-hole pair which then accelerate in opposite
directions due to the extreme electric field in the junction.  They
each accelerate a short distance and collide with atoms in the
junctions, thus releasing several low energy electrons and holes,
which accelerate some more, collide some more, etc until they arrive
at the edge of the junction.  The result is that one photon can cause
the junction to rapidly transition from an open circuit to a short
circuit, which is observed by electronics and counted as one photon.
The advantage of APDs is that they typically have a much higher
probability of detecting a photon than a PM tube.

Does that answer your question, or did I miss your point?

Rich L.