NetNotes
appreciated with eyes especially at high intensities, but when they are presented in histograms it is much easier to see. It may be that both signals cannot be collected at a similar intensity as to observe a nice mix of both like a nice yellow from red and green mixing. For profile scan it doesn’t matter because you just want to show that the position of each peak is similar, whatever the height of the peaks. But for mixed images it is of course better if the structure has a well balanced mix of both signals. Say the red is much stronger than the green, the merged image shows a red structure; well it is not a very convincing demonstration of co-localization. Tus it is oſten required to increase the intensity of one signal. Take care to increase the overall intensity of the whole picture and not only part of the histogram like playing with the contrast or even worse: only a part of the image since this is a misconduct. If playing with the contrast is acceptable, do not forget to state it but I would avoid it. Stephane Nizets nizets2@yahoo. com Fri Jun 11
TEM:
filament life I work with two Philips CM series TEMs. I am trying to help
diagnose some filament life issues, and I need to know what the temper- ature I am heating the filament to is, in a real unit (kelvin), instead of just knowing it is heating stage 21. Does anyone know of a conversion procedure? Te same question for the vacuum readings, the emission current scale, and is it possible to give meaningful units to what the emission level setting? Ben Micklem
ben.micklem@pharm.ox.ac.uk Wed May 2 May I help on filament life issues? I am not too familiar with
the CM Series but I would have thought they provide an emission current (beam current) meter as do other manufacturers. However I do not know any manufacturer in the EM business who would even guess at the filament temperature under the wide range of operating conditions offered. What puzzles me with your question is the heat applied to the filament is just part of the gun saturation process, filament current, bias setting (emission current required) and filament position being the three factors which relate to the saturation point. I have analyzed filament problems on EMs for over 40 years and have never had cause to relate to actual filament temperature so may I offer some ideas? 1. As the filament ages it becomes thinner and will eventually break at the side which is slightly thinned during the bend. Te normal break in a TEM should be through a gentle taper being shown on the failure area. 2. A poor vacuum in the gun will cause high levels of evaporation and contamination and the shortening of the filament life. Do not take notice of the vacuum gauge reading as this is almost certainly well away from the gun area. 3. From time to time all manufacturers suffer from being supplied with poor quality wire made up of metal and a considerable amount of rubbish. Short life, a smelly chamber and a very dirty cathode assembly are indica- tions of this problem. 4. Finally we have situations where the operator just does not know what they are doing when saturating the gun and they overheat the filament. I wrote an article for Microscopy Today some years ago that related to “Te Life and Death of the Tungsten Hairpin Filament” which may add information to your fault finding procedures? Steve Chapman
protrain@emcourses.com Wed May 26 Tanks for your reply Steve. I’m using LaB6 and CeB6 filaments,
not Tungsten. Tey prematurely failure at around 700 hours, when the manufacturer says they should be lasting 2000+ hours. Te manufacturer has been helpful so far, and I just wanted to answer their questions about operating environment. Tey may not be that familiar with TEM applications. We increase the bias setting as the filament dims over its life, usually requiring an increase in
2010 September •
www.microscopy-today.com
heating current to maintain saturation- I think this is why most fail eventually from the insulators melting. But I don’t know how else to maintain the required beam strength when the filament becomes dimmer. Te emission current is always fairly low—again I only have the scale the microscope has, but it is around the 5% mark for most of its life, then this may increase to a maximum of 10% when it has got really dim, and we are on a high bias setting (e.g. 4 out of 6—see how useful these numbers are to a non-Philips user!). We are using 500 µm wehnelt apertures, and set the filament tip 150 µm below the outer surface of the wehnelt aperture. I have tried using a greater distance to the Wehnelt, but the beam was not strong enough for our uses. We recently tried CeB6, but they had about the same life as LaB6. I don’t think we have unusual requirements for beam intensity, so I am puzzled as to why the filaments are not lasting as long as the manufacture claims. Ben Micklem
ben.micklem@pharm.ox.ac.uk Wed May 26 I have been using the LaB6 mini-Vogel mount you show for many
years on a CM30 300KV TEM. My observations are: 1. Almost all failures are operator induced. Generally failure is due to heating too rapidly. Te LaB6 chip is very brittle and subject to damage by thermo shock. Do your routine run up as you would a new filament, i.e. Slowly. 2. Te LaB6 is very sensitive to the vacuum level. Evaporation is markedly increased with a slightly degraded vacuum level. Te most likely leak is SF6 which is most easily detected by the smell when you open the gun chamber and by the amount and color of the deposits on the anode. 3. Improper conditioning may result in a hard flash over. Consider the voltage and the HV cable capacitance and you can see a large quick discharge carries a high energy density, resulting in actual filament shiſt or work function damage. Either way it requires mechanical realignment of the tip or most of your beam will be coming from the side facets rather than the tip. Of course there are a lot of other possibilities but these are the main culprits in our facility. To answer your direct question, we expect and get about 2 years per filament. Fran Laabs
fclaabs@iastate.edu Wed May 26
SEM:
backscatter electron intensities From a backscattered electron (BSE) image, can the yield or
intensities be quantified to give a fairly accurate number? It will be on a binary alloy of metals. Does anyone know if that can be done? Tommy Derflinger
gtuser@comcast.net Sat Jun 12 I would think so but only for that particular capture. Which
would mean that it is actually relative to the other elements in the image. One would need a known standard along with the specimen to make any headway towards a meaningful quantification. In any case, quantification would just mean what the pixel intensity is at any point on the image. I don’t see what use this would be other than to say that element x is twice the intensity of element y. Interesting, but so what? Just as with film and a densitometer, contrast and brightness will change all elements’ intensity. Of course one can obtain the same type of “quantitative” results but again, they are relative to that piece of film and/or print. What is the end purpose? I would think that EDS mapping is going to provide true quantitative results for a specimen. Having counts provided at each pixel is quantitative. Gary Gaugler
gary@gaugler.com Sat Jun 12 You may want to check this paper: Validation of quantitative
backscattered electron imaging for the measurement of mineral density distribution in human bone biopsies P. Roschger, P. Fratzl, J. Eschberger and K. Klaushofer, Bone, Volume 23, Issue 4, October 1998, Pages 319–326. Vladimir M. Dusevich
dusevichv@umkc.edu Mon Jun 14
67
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76