Vacuum Sucks!
I saw that I had a few search engine referrals to this site in which people were seeking information on why electron microscopes use vacuum systems. I have mentioned vacuum systems here, but I haven’t explored it as a topic yet. I surely disappointed inquiring minds when google sent someone here and there ended up not being the requested information present. Well, this post is to remedy that and offer a basic overview into vacuum systems on electron microscopes.
So, why does an electron microscope need a vacuum system you ask? Well, an electron beam emitted in a chamber with no vacuum present would only travel less than one centimeter. We need to put the column of the microscope under high vacuum in order to increase the mean free path of electrons. High vacuum helps to keep the gun area clean as well, and also helps to reduce possible contamination of a specimen.
Vacuum on microscopes is usually measured in Pascals (Pa). I say usually, because there is no standard and you may see it measured in other formats. 1 Pascal = 7.52×10-3 torr. 1 torr = 1mm/Hg. 760 torr = atmospheric pressure at sea level (1 atmosphere). Click the link for more information on vacuum. The TEM and SEM that I use typically work at 10-4 Pa (TEM) and 10-3 Pa (SEM). It’s interesting to note that when your column is under high vacuum (10-4 Pa), there are still about ten billion gas molecules in a cm3 of space.
Vacuum systems on electron microscopes are typically kept running 24/7. This is beneficial to the lenses and seals, as particles won’t have a chance to enter the column. One loose particle, thread, or hair in the column could completely disrupt the vacuum system and may not let it get up to an operational level.
A TEM usually keeps separate areas of the scope under vacuum, and you can shut vacuum off to the separate areas in order to change your sample or retrieve film. This is handy because the column is quite huge on TEMs and it needs to be under higher vacuum than an SEM. If you had to devacuum the whole scope every time you had to change a sample it would probably over an hour to change out your sample. By only having to break the vacuum of the sample holder, you can change out samples in about a minute on a TEM. On the other hand, with the SEM, it takes about three or four minutes between sample changes.
Explore posts in the same categories: Electron Microscopy (general), Scanning Electron Microscope, Transmission Electron MicroscopeTags: electron microscopes, EM, high tech, high vacuum, Pascals, Scanning Electron Microscope, science, SEM, technology, TEM, torr, Transmission Electron Microscope, vacuum, vacuum system electron microscope
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October 14, 2011 at 3:18 am
“with the SEM, it takes about three or four minutes between sample changes”
At least on some FEI microscopes, when using LoadLock, you can change the sample in less than 100 seconds.