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Re: [Phys-L] electron microscope rehab



On 11/17/2017 05:47 AM, Peter Schoch wrote:

Amazingly there's no vacuum pump as the distant from electron gun to
sample holder is <1mm. The resolution is not great compared to
modern machines.

I still think the device needs a vacuum pump.

1) Quoting from https://www.smecc.org/rca_emt_tabletop.htm

Dr. Reisner said that a unique advantage in time-saving and
convenience is afforded by an engineering advance which for the first
time permits insertion of specimens into the evacuated column, and
^^^^^^^^^
their removal, without breaking the vacuum. In addition, he said,
^^^^^^
photographic plates may be changed without admitting more than a
small amount of air to the column. As a result, pumping time between
^^^^^^^
plates is reduced to only 90 seconds, which is just about enough time
for the photographic development normally carried out between
exposures.


2) And then there's the plain old physics. Ordinarily an
electron gun has a filament, and if it didn't operate
in vacuum (or perhaps inert gas) it would burn out even
quicker than it already does.

Also, an electric discharge in air tends to produce ions,
not just O2(-) but also O(-) which is insanely reactive,
and even excited states thereof, which are even more
insanely reactive. I suspect the sample would be quickly
destroyed, or grossly altered, by the "intense beam".


3) I'm not convinced the <1mm number is relevant. Here's
the diagram:
https://www.smecc.org/rca_images/emt8.jpg

After passing the sample, the electrons have to travel
the whole length of the instrument, which is hundreds
of times longer than 1mm, and the air would surely
capture all the electrons and turn them into ions,
which would be impossible to focus.

The mean free path of electrons in air is microns,
not millimeters, let alone decimeters.

==========================

Also note that this is a transmission electron microscope
(TEM). That means that sample preparation is difficult,
because the sample must be verrrry thin. Sample prep is
bad enough for a modern scanning electron microscope (SEM),
but this is way worse.

99% of the time you'd be happier with a scanning /tunneling/
microscope (STM). Almost completely different physics, and
drastically different engineering.
https://www.nobelprize.org/nobel_prizes/physics/laureates/1986/