NetNotes
back. Te instrument are not used by electron microscopists so the idea behind this long life being that the service technician is able to fit a new filament on their six monthly service visits. We worked hard to obtain this life and one of the features that we found that was very important in our development was the vacuum level. Tus we run with an ion pump on the gun and the difference in filament life is considerable. Tis is no criticism of the basic instrument, which exhibits a gun vacuum level in its standard form the same if not better than its competitors. Another area that the gun vacuum level influenced was the high voltage stability. Te instruments need to run for up to 24 hours unattended and it is most important for the high voltage to remain absolutely stable over that entire period of time. Double plots of high voltage level and gun vacuum show that once the gun vacuum settles so does the high voltage. Tus the operators wait for this stability period prior to starting a sample run; proven to be the best practice. As a second point I know LaB6 users who have been forced to use tungsten whilst waiting for a new source and they have also commented on the increased filament life. Steve Chapman
protrain@emcourses.com Wed Jun 22 Te 11 µA emission current leads me to think that the lower
temperature enhances the effects of good vacuum. As for a more normal system, I’ve found Amrays follow the manual, 40 hrs, ion pump or not, unless the vacuum is especially poor. JEOLs seem about the same except their typical life is 100–200 hours. I did have one dry pumped ETEC that would run for weeks and months 24-7 in the 10–8 T range, but it was run slightly undersaturated because life was more important than stability for the particular experiment. Maybe you can shed some light on this thought. I’ve always wondered how much effect the physical gun design has on the temperature required for a particular emission current. Your example seems to indicate the same thing that TEMs seem to indicate and that is that the lower emission currents, and consequent lower temperatures result in oxidation being a larger factor than evaporation on filament life, so better vacuum has a greater effect on lower temperature filaments. Any thoughts? Ken Converse
kenconverse@qualityimages.biz Wed Jun 22 I seem to have spent most of my life playing with electron guns.
How oſten I have moved filaments forward or backwards, sharpened tungsten filaments to obtain LaB6 type performance, liſted up anodes and checked out different cathode shapes! To most people in laboratories a gun is what the manufacturer gives you, but with the good old tungsten hairpin that may be considered to be just a starting point. Total cathode geometry and the changes that you are able to make regarding filament life in one direction and performance in the other, are outside of the norm. Add to this raising the anode to improve performance and few appreciate how you may transform an instrument. On tungsten filament temperature I have a great deal to say but as usual from me it is probably controversial? I rarely fully saturate the gun! I am a great believer in under saturating. I see no point in throwing away filament life and with modern stabilities there is usually no point in doing so. An amazing amount of work may be accomplished and for most SEM analysis there is no point, it hardly can be said to improve X-ray resolution! I only saturate when working at the absolute limit at a particular kV and I challenge anyone to see a visual degradation in any images I record through using this technique; but only up to 100,000×. Te big bonus for most labs is this technique really does improve tungsten or even LaB6 filament life. Tus I agree with you with low filament currents it is oxidation that is the final tungsten filament killer not evaporation! Too hot a filament and it just evaporates but be more thoughtful and life is extended. We improve the vacuum and the life is extended even further; but
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probably at the cost ($) of an ion pump! Steve Chapman protrain@
emcourses.com Wed Jun 22 If you are working above 10 kV and at a maximum of 5,000×, I
see no point of moving to LaB6. LaB6 is brighter than tungsten and running at a lower temperature has less chromatic aberration. Te problem with LaB6 is that you must have a very good vacuum or the possibility of a discharge could be catastrophic! A discharge may seed a crystal in what is meant to be a single crystal tip. Unfortunately over time this seed crystal may grow and eventually you have a double emitting source. Nice for fancy images but not for science! Tus great caution is required when running up the LaB6 filament and changing kV in order to prevent discharge occurring. If you were always working below 5 kV and looking at images I would suggest LaB6 as due to the higher brightness and lower aberrations this source will always better tungsten at these levels. Steve Chapman protrain@
emcourses.com Fri Jun 24
TEM:
acquiring Holz lines I am trying to acquire HOLZ lines in [000] transmitted spot of
CBED patterns for point group determination purpose, but have been experiencing difficulties in acquiring/recording images of HOLZ lines. Te TEM I am using is Philips EM-20 with Gatan camera. On the phosphor screen, I was able to see HOLZ lines for higher index zone axis patterns and they seem to be clearer under 80 kV than 200 kV. However, when I inserted the Gatan camera to take pictures, I noticed that the quality of the diffraction images was too bad—the HOLZ lines were blurry, and some areas were too bright, making the nearby HOLZ lines invisible. Does anyone on this list have similar problem before? Any suggestions on how to record sharp/clear HOLZ images would be much appreciated! Chuan Zhang
zhangchuan827@gmail.com Tu Jun 16
Tere are four points to consider when taking convergent beam
electron diffraction patterns (CBED) on a digital camera: (1) Correct focus (diffraction lens). If your camera is far from the viewing screen, then the CBED pattern can appear to be out of focus and the HOLZ deficiency lines are quickly lost. I have used a Gatan Imaging Filter to acquire (energy filtered) CBED patterns and always have to fine-tune the diffraction focus to get a nice sharp (000) CBED disc—the condenser aperture should appear sharp when the diffraction focus is correct. (2) Sampling rate. If your binning is too high or the camera length is too small the modulation transfer function (MTF) of the camera may be blurring the HOLZ deficiency lines too much. Try using single binning and/or a higher camera length. You may have to try several different areas because HOLZ deficiency lines tend to narrow with thicker areas. (3) Dynamic range. Because of the highly inhomogeneous distribution of intensity in a diffraction pattern, it is easy to saturate the camera. I have found using multiple exposures with a small acquisition time to help tremendously. For example, I have taken diffraction patterns with >100,000 counts per pixel on a 16-bit camera (max. 16396) by using the multiple frames option (DM > Camera > Acquisition Parameters > Acquire > Process- ing > Frame Sum (No. of Frames). Ten or twenty frames can make a significant difference to the visibility of weak features. (4) Spot size (used in conjunction with point (3). I’ve found that using a smaller probe with less current leads to better CBED patterns (and using a larger exposure time). Most samples have a thickness gradient so the CBED pattern you see are an incoherent mixture of CBED patterns from each point on the sample that is irradiated, i.e., smaller probes “average out” the CBED pattern over a smaller area. However, if your
www.microscopy-today.com • 2011 September
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