User`s guide

General Precautions
There is no defined cutoff where the scintillator will be damaged. Rather,
damage can result from prolonged exposure of the scintillator to a very high
beam intensity (above 30,000 counts in binned by 1 mode and 60,000 counts in
binned by 2 and higher modes). If you have no saturation with a short exposure,
then there is no danger. You can then increase the exposure time and still not
harm the scintillator as it is the beam intensity on the scintillator that causes the
problem and not the accumulated signal in the CCD.
It is recommended to use beam stop to block the central spot when acquiring
SAED diffraction patterns.
If you think you have damaged the scintillator, take an unprocessed image of
uniform illumination. If the spot turns out dark, you have damaged the
scintillator! If it is bright, spread out the beam and set the illumination so you
have about 10,000 counts/sec (in binned by 2 mode) and switch the shutter
switch on the controller to Open. Expose the scintillator for 3 min, switch back
to Auto, and take another picture. The bright area should be gone and the
scintillator should operate normally.
• Warm the scintillator if oil contaminated.
Some microscopes have a significant vapor pressure of diffusion pump oil in
the viewing chamber. Since the camera is maintained at temperatures below
-20ºC during use, it can act as a cold trap and accumulate an oil film. While thin
this film does not affect imaging, but if it becomes thicker or coalesces into
droplets, stronger and stronger gain correction will become necessary. Other
problems caused by the oil film are charging and the sticking of dirt which
could otherwise be blown off easily. If left for too long, the hydrocarbons
chains in the oil will be cracked by the electron beam making them non-volatile
and un-dissolvable. Then it is necessary to send the camera back to have the
scintillator serviced by Gatan. Fortunately, these problems can be avoided by
adhering to the practice described below.
If you have a clean vacuum in the camera chamber area, warm the scintillator
once or twice a month. But if you have backstreaming from your diffusion
pump, you will need to do it once a week. The camera should only be warmed
for 15 hours or overnight, but not longer. If the camera is used sufficiently
without warming, the electron beam can harden the trapped oil making the
scintillator difficult to clean.
Switch the Peltier cooler to the “Warm” position.
It is OK to run the Peltier cooler in the warm position overnight,
but do not run it when the camera is at air pressure or if the cooling
water is not flowing.
Check the vacuum system.
If oil accumulates even with regular warming of the scintillator, it
is recommended that the vacuum system be checked. Note that the
oil will not be imaged as soon at 120 kV as at a lower voltage (40
kV), so that a problem can exist for some time before it is noticed,
UltraScan User’s Guide Reference Series Rev 1.0 5-3