Re: Tek 7000 Series deflection plate precision?


Don Black <donald_black@...>
 

I'm no expert on electron microscopes but you should be aware of two types, TEM (Transmission Electron Microscopes) and SEM (Scanning Electron Microscopes). A TEM shines a beam of electrons through the specimen as if though a photographic slide and then expands the beam onto a fluorescent screen to obtain the image (often viewed with an optical microscope to see small details). A SEM scans the target with a raster of s fine electron beam. The beam in a SEM is relatively large since it's like an illuminating light source but the SEM needs a very fine beam, smaller in diameter than the smallest detail you are trying to see. I believe the high school microscope is a TEM and so doesn't need as fine a beam as a SEM.
Others who really know what they are talking about please jump in and correct any mistakes I've made here please.

Don Black.

On 21-May-13 2:36 PM, iglesia_cristiana_arpas_eternas wrote:
 

Hi Damian,Here my two cent about:
You need to put an extra objective lens "magnetical prefereble" in order to obtain a very small dot at least half what minimum size yo like to see.and ..or.. increase the tube length, for the same reason.

Most people use a Cristal plate with phosphor where the second emission electron emitted from the target impact and release a photon ,which in turn will be detected by the photomultiplier itself, I believe that such optical conversion is not necessary , just remove the cap on photomultiplier and allow the electron hit the dinodes directly ,If this approach work, surely will be give you a enormous gain and increase in S/N ratio, which in turn allow to use less beam intensity with all benefits.obviously all in high vacuum, inside the chamber.
End of my two cent.
Gabriel.

--- In TekScopes@..., "cheater00 ." wrote:
>
> Hi guys,
> could someone chime in on how precise the deflection in a Tek 7000 can be?
>
> I'm very new to vacuum tubes and the idea of electrostatic deflection,
> so please bear with any stupid questions...
>
> My main consideration is: with modifications, would it be suitable to
> jury rig an SEM (Scanning Electron Microscope)?
>
> I am trying to do something inspired by Ben Krasnow's SEM design. If
> you don't know it, he has some info here on how he made an SEM
> himself:
>
> http://benkrasnow.blogspot.de/2011/03/diy-scanning-electron-microscope.html
> http://benkrasnow.blogspot.de/2011/03/diy-scanning-electron-microscope_26.html
>
> He made the raster generator and deflection assembly and amplifiers
> himself. It was my idea to use a broken 7000 series tube and hook it
> up to a working 7000 scope configured with two time bases in a raster
> generator setup. Of course I'd need a Tungsten filament, the one in
> the 7000 CRT would be broken by having been exposed to air.
>
> What happens in an SEM is that the electron beam *scans* a rectangular
> area on the thing you're zooming in on. It moves in a raster image,
> just like on a TV, or an oscilloscope in raster config. This rectangle
> has to be very small, let's say 2mm across. You'd start out with a
> fairly big rectangle, say 2x2 cm, to see the overall shape of the
> object, then you zoom in a bit so that the rectangle is 1x1cm and pan
> a little, then zoom and pan again, and so on. Think CSI: Miami
> ("enhance"). You might end up with a square of 2mm or even smaller.
>
>
> Here's my question: the deflection needs to be precise, so e.g. if
> you're horizontally 5% of the way, it still needs to be this far
> across, and not erratically in some other place.
>
> It would be bad if it e.g. jumped from 0% to 10% to 20% and so on to
> 100% while skipping the intermediates.
>
> It would also be bad if it e.g. centered around that 5%, but noise and
> interference meant the beam would randomly err between 1% and 9% with
> 5% at its center.
>
> The rectangle as a whole needs to be moved at least in this area of
> 1cm or 2cm as well. It would be fairly bad if the rectangle as a whole
> drifted or if the position were shaky.
>
> How would one modify the deflection circuitry and assembly in a Tek
> 7000 to achieve such a small scan area? Is the circuitry going to be
> low-noise and/or linear enough so that deflection precision is
> retained?
>
> How *wide* across can the electron beam be?
>
> Thanks,
> Damian
>


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