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different phenomenon and I can’t see how they can be discussed as if they were interchangeable. Te non-background light is shiſted in path and phase either by refraction alone, diffraction alone or refraction of diffracted light or diffraction of refracted light and I am unclear which is the most accurate description. Dave Knecht david. knecht@uconn.edu Wed Apr 13 Abbe defined the formation of a microscope image as the result


of interferences between diffracted wave fronts. But you also have refraction due to the difference in density (refractive index). Actually you get both phenomena if you observe cells. Both phenomena must be taken into account for phase contrast because they are responsible for different parts of the technique: Te phase-shiſt, responsible for the contrast, is due to the refraction (as I said it is not enough to create a good contrast image though, you still need to phase-shiſt and dim the background light). Te separation of background light from diffracted light is made possible by the diffraction phenomenon taking place in the specimen and by the presence of the phase ring at the objective (which specifically modifies the background light without affecting the diffracted light). In summary, both terms are definitely not interchangeable but they are still both playing a role in the formation of a phase-contrast image. Stephane Nizets nizets2@ yahoo.com Tu Apr 14


EM:


chamber vacuum problems We have EPMA SX100, in which there is problem in its chamber.


Aſter opening the chamber because of sample trapping (I opened the bottom of chamber and took out the samples), the vacuum isn’t completed. Does anybody have an idea to help me to solve this problem. Kazem artur.irani@yahoo.com Tue Apr 19 First action is to wipe your finger all of the way round the “O”


ring surface; do not clean with a solvent. Ten clean the flat plate surface in the area where the “O” ring would sit; here you may use a solvent. Do not grease the “O” ring. Steve Chapman protrain@ emcourses.com Tue Apr 19


EM:


poor vacuum We are trying to figure out what is wrong with our Zeiss TEM. Te


vacuum reads 5 × 10–3 milliBar and goes no further down on the gauge on the console. We have checked the seals on the gun chamber and camera chamber and they seem to be o.k. We checked the forepumps by disconnecting the vacuum lines from the column, and both are pulling over 30 lbs/sq in. Diffusion pump heater is going on, and the cooling water is cooling the outside. Te Penning gauge was cleaned and that didn’t change the reading. We think the main valve is opening because the gauge on the console only reads the column vacuum. We are running out of ideas. Perhaps there is something obvious that is preventing the column from pumping down. Ideas, anyone? Carol Heckman heckman@bgsu.edu Fri Mar 11 I once had a leaky aperture changer, but only leaking in one of


the three possible positions. Found it only by chance. Did you check if the pressure gets down when you are changing positions on one of the three aperture changers? Other possible points of interest: —all movable parts (plate camera, viewing screen, specimen movements)— what happens when you set the plate camera to atmospheric pressure? Fastest solution for a fix: get a leak tester. Stefan Diller stefan.diller@t- online.de Fri Mar 11


EM:


asynchronous electromagnetic interference We’re getting a new FEI650FEG SEM and just had the site survey. We’ve been running an ancient SEM in the room for many


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years. Tey have informed us that our room failed the asynchronous electromagnetic interference (ASYNC EMI) requirements. We can go ahead and get it installed with a waiver without fixing the ASYNC EMI issues and then try to fix them later if they are a problem. Alternatively, we can try and get it fixed now. Do any of you have experience with this sort of problem? Is it really a big problem or a little one? If it is a big problem that needs fixing, do you have recommendations on how to fix it? Robin Foley rfoley@uab.edu Wed Apr 20 About a year ago we had faced the same problem before


installation of FEI Magellan in a newly prepared room. Te room has ASYNC EMI a bit higher than the pre-installation requirement states. We made several efforts with more than one EM expert, failed to identify the source of this EMI, but we mapped all the EMI sources neighboring the room and found that we can decrease their effect by installing EM-dissipating metal plates over surrounding walls. Ten we installed the instrument and found no problem of interference at any scan condition, and the specs. were OK as well. Before the installation we asked FEI about possible effects of this over-spec ASYNC EMI. According to FEI, there could be flagging at specific scan condition set. Our ASYNC EMI was about 10% higher than specs. in only one direction. We decided to proceed with installation first, because the EMI was not too large and because FEI Magellan has extremely wide set of scan conditions at which we hoped to escaped flagging even if it will occur. Hope this will help you to make you decision. Inna Popov innap@savion.huji.ac.il Tu Apr 21


TEM:


beam stability What is a reasonable length of time to wait for a beam to stabilize?


I take a series of 256 images with a 120 kV TEM and have found that I have to wait one to two hours for the beam current to stabilize sufficiently to provide equidense images for montaging or analysis. Te beam instability occurs even without apertures or a specimen inserted and with both tungsten and LaB6 filaments—the LaB6 does seem to take longer to stabilize. Larry Ackerman larry.ackerman@ucsf.edu Mon Mar 21 Tere are two areas that need to stabilize, both thermally. One is


the high voltage power supply. Te components change value slightly with temperature so the output (actual kV and filament current) may driſt slightly until everything equilibrates. Te other area is the gun. Te cathode to Wehnelt distance changes as the gun changes temperature (so does the Wehnelt to anode distance). Tat spacing is far more critical for LaB6, hence the longer time to stabilize. My SEM customers who do e-beam lithography generally don’t even reduce their LaB6 cathodes to the standby temperature. Tey leave the gun fully at temperature and saturated. Te cost is that the cathodes get replaced every 6 months. Since they oſten operate at 40 kV (high for an SEM), I always had them reduce the kV to 20 when they weren’t using the system. Aside from protecting the high voltage power supply and some of the gun components, this also keeps the lenses warm. Yes, that’s a third area that needs to stabilize thermally, but it doesn’t seem to be as critical as the first two areas. If your TEM is stabilizing in 1–2 hours from a cold start, I’m stunned! Te thermal masses involved in both the power supply and the gun are probably far greater than most SEMs. If it takes that long from a semi-warmed up standby situation, then there might possibly be some kind of issue, but maybe not. 1–2 hours for high stability doesn’t strike me as at all unreasonable. Ken Converse kenconverse@qualityimages. biz Mon Mar 21 Te time to stabilize that I mentioned starts when the filament is heated aſter a sample change. Te high voltage is never off—for


www.microscopy-today.com • 2011 July


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