Scintillators are materials that produce light
when ionizing radiation passes through them.
These materials can be solids or liquids or
gases.
Scintillator materials absorb incident gamma
radiation by one of the three mechanisms we
discussed before.
As the kinetic energy of the reaction electrons
is deposited in the material, it raises the
materials electrons to excited states.
During the subsequent de-excitation, the scintillator
usually emits a photon in the visible light
range.
The light emitted from the scintillator is
guided to a photomultiplier tube, where it
interacts with a photocathode, releasing electrons.
These electrons from the photocathode are
guided with the help of an electric field
toward the first dynode.
The dynodes are coated with a material that
emits secondary electrons, usually more than
one.
The secondary electrons from the first dynode
move toward the second, and so on down the
dynode string to the anode.
More than a million electrons can be created
for each electron starting from the photocathode.
Hopefully, many light photons were created
as the photoelectron moves through the scintillator,
so that a respectable signal arrives out of
the PM tube.
Many different scintillation materials are
used for detectors.
Some of the more common are sodium iodide,
cesium iodide, BGO, which is chemist lingo
for bismuth germanium oxide, zinc sulfide,
which is the great-granddaddy of all scintillating
materials (This was the original material
that Rutherford used to do his scattering
experiments.), and lithium iodide, which is
used as a neutron detector.
The main reason that photomultiplier tubes
are needed is that we need a large amplification,
because the primary signal out of the scintillation
material is low.
Unfortunately, this large amplification leads
to poor energy resolution.
To see the battle that we’re trying to fight,
let’s pick typical values here.
Scintillator efficiencies are on the order
of 13% or less.
Scintillator efficiencies are the ratio of
the photon energy to the kinetic energy of
the electrons put into the scintillator material.
Light losses are in the order of 2/3.
It’s a good day when we can steer 1/3 of
the light to the photocathode and the multiplier
tube.
The quantum efficiency, which is the frequency
in which the light hitting the photocathode
is converted back to an electron, is on the
order of 7-20%.
These factors taken together lead to values
of 230eV or so per electron fed into the dynode
strings of the photomultiplier.
If we go calculate the full width half max
of the signal from this ideal detector, we
get 29keV, and this is as good as it gets
for cesium-137, which emits a 662keV gamma
ray, and, believe me, it’s generally much
worse than this.
