Scanning Electron Microscope - Scintillator
The scintillator is the portion of the secondary
electron detector which absorbs the impinging electrons and emits photons,
which are then converted to electrical signals by the photomultiplier tube
and pre-amplifier. There are several types available, the most common
being formed from a disk of clear substrate (plastic, glass or quartz)
coated on one side with a scintillating medium ("P47", cerium doped yttrium
silicate, Y2SiO5:Ce) and the less common (read much
more expensive) type formed from a single crystal of clear scintillating
material (cerium doped YAG, yttrium aluminum garnet, Y3Al5O12:Ce
or YAP, yttrium aluminum perovskite, YAlO3:Ce).
The first step is to move the light shield (rubber boot) in order to
gain access to the set screws holding the pre-amplifier to the photomultiplier
tube housing.
Then the set screws can be loosened and the pre-amplifier and photomultiplier
tube extracted from the housing.
With the PM tube and pre-amp out of the way, the screws holding the
detector to the sample chamber can be removed and the detector extracted
from the chamber.
Removing three small screws which hold the grid (a hemispherical, wire
mesh, electrode biased to accellerate the secondary electrons toward the
scintillator) in place allows access to the scintillator itself.
A small metal ring (soldered to a ground wire) holds the scintillator
disk (phosphor side towards the sample chamber) to the end of the light
pipe (an acrylic rod which conducts the photons generated by the scintillator
through the detector bulkhead to the photomultiplier tube). Lifting
(with gloved hands) the ring off of the light pipe allowed the old scintillator
to drop off. The new scintillator (P47 on quartz, from M.E.
Taylor Engineering) was placed over the end of the light pipe and the
ring pressed down over both and three (120º apart) small drops of
silver paint were brushed onto the junction between the ring and the phosphor.
The o-ring (AS 568A size 229), o-ring groove and mating flat were cleaned
(cotton swabs, paper towels, white gasoline), the o-ring lubed with Krytox(RTM)
GPL perfluorinated grease and the detector reinstalled.
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