Scintillating!

The ``workhorse'' of experimental is the scintillation counter. This simple device works as follows: the ionization of certain types of molecules causes photochemical reactions that liberate visible light called ``scintillation'' light. This light is conveyed through a clear liquid, plastic or crystalline matrix, bouncing off polished exterior surfaces total internal reflection until it reaches the photocathode of a vacuum tube where the photons liberate electrons the photoelectric effect. These electrons are then accelerated by high voltages in the tube until they strike a ``first dynode'' where each electron knocks loose about ten additional electrons which are accelerated in turn to the ``second dynode'' where they in turn each knock loose another ten electrons each, and so on down a cascade of up to 18 dynodes. As a result, that one electron originally liberated by the incoming photon can produce a pulse of electrons at the ``anode'' or the tube, which is (mnemonically, for once) called a photomultiplier tube. These amazing devices have been refined over a period of nearly half a century until some have ``quantum efficiencies'' approaching 100% (they can fairly reliably detect single photons) and (most importantly) generate electrical pulses a few ns (nanoseconds, billionths of a second) wide whose arrival at a bank of fast electronics is correlated with the time the original ionizing particle hit the detector within a fraction of a ns. This means that high energy physicists can routinely do timing with a resolution comparable to the length of time it takes light to go 10 cm! Without this impressive timing capability it would be very difficult to do any modern experiments. Interestingly enough, this part of the technology has not improved significantly in several decades.