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Author: Subject: homemade mass spectrometer
phlogiston
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[*] posted on 2-4-2015 at 02:44


What if you were to include only a scintillating material in the vacuum, and a PMT outside (viewing the scintillator through a window)?



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neptunium
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[*] posted on 15-5-2015 at 12:00


updates,
finally ! i had the money to go get my mass spectrometer !
here is a few shots and a first vacuum test...
0515151532.jpg - 576kB
this is the coil and ion gun area followed by the ion beam focusing rings
0515151533.jpg - 636kB
at the other end the detector will take its place between the two electrodes (one for the signal and one for the ground)
and a general view of the pumping system by my washing machine!
I am working on upgrading my lab to have a better work area the laundry room is not equiped for fragil glass expansive vacuum pumps and high voltage wires...
so i will have a bench in a few weeks i hope in a different part of the house! this is NOT a cheap hobby guys!




0515151533a.jpg - 627kB

0515151533b.jpg - 587kB




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smaerd
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[*] posted on 19-5-2015 at 16:42


It's beautiful :)



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neptunium
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[*] posted on 23-2-2017 at 10:29


latest development
https://www.youtube.com/watch?v=-S7i2a3IN8g

major vacuum problems, i have to re engineer the glass/electrodes connections
i can get it to about 5e-5 Torr with this pump without any problems, which would give me a mean free path above one meter (longer than the spectrometer)
still working on it ( when my job doesnt get in the way...)

[Edited on 23-2-2017 by neptunium]




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wg48
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[*] posted on 23-2-2017 at 12:49


Hi Neptuneium. Wow a serious project. I noticed in your youtube video you had the focusing electrodes floating. I would expect that to stop any electrons getting down the tube as they will build up a negative charge..

Is the long valve in the video part of the mass spectrometer? How is the sorting of the different masses done? I don’t see any big magnets or quadruples. Is it a time of flight mass spectrometer?
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Sulaiman
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[*] posted on 23-2-2017 at 13:29


if your vacuum is only sufficient for a mean free path just a little longer than the physical path length
I would expect your spectrum to have very poor resolution;
i.e. if the mean of ions just about make it to the detector without collision, there must be a statistical spread of collisions.
I do not know, but you should check,
by how many orders of magnitude should the mean free path be longer than the physical path to achieve your desired resolution ?

I do not see why you can not just use the detected beam current directly into an electronic amplifier.
In the '70's I was working in the fA range, technology must have moved on since then, if you like I could have a look for a suitable amplifier ?

Regarding magnetic deflection with permanent magnets;
N55 sounds sexy,
but I think that you would do better with a greater volume of a lower grade magnetic material,
use a calculator like this https://www.dextermag.com/resource-center/magnetic-field-cal...
to try different configurations.

also, I suspect that for good resolution you may need to shield your spectrometer tube from electric and magnetic fields, especially 50/60 Hz magnetic fields.

If you do go for spectral scanning by varying the field of an electromagnet, I2.R heating is always a problem
if you have permanent magnets creating a magnetic field of half your peak desired field
then varying an electromagnet in series with the magnets only requires +/- 0.5 times the current .... 0.25 times the heating.




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neptunium
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[*] posted on 23-2-2017 at 14:48


Wg48, no the focusing copper ring are not floating they are pressed between the glass and the electrode with a small spring.
I don't know what valve you a reefering to but I was planning on using the electromagnet as a deflection way . based on preliminary test I might need a better magnet even if my tube seems long enough...




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neptunium
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[*] posted on 23-2-2017 at 14:52


Sulaiman,
Thanks for your help . this was just a preliminary test . I did plan on using an electromagnet for scanning the mass spectrum but upon review I might use one of my N55 ... Or a much much stronger electromagnet... This is really a mass spectroscope . if I can just get a visible trace of deflected ion on the phospor I'll be a happy camper....but any Improvement is indeed very welcome !!





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wg48
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[*] posted on 23-2-2017 at 16:49


naptunium:

The glass tube in your youtube video I was referring to as a valve.

The two rings (I think you called them focusing rings) to the right of the cathode area on the left end of the valve glass tube thing. The two rings are not connected to anything, they are electrically floating. You should at least connect them to the filament or they will charge up and electrons or for that matter ions will be stopped from going down the tube.


So if this tube is the mass spectrometer/spectroscope tube then the deflection can be no more than the diameter of the long tube. I have got that correct?

Do you have the one side of the filament supply connected to the negative of the HV supply? It should be connected.

No glow discharge is visible in the tube. When you apply the 1000v you should get a glow discharge if your vacuum is not low enough but still less than about 20torr.

If you run the tube with filament off and apply the HV between two electrodes then start evacuating the tube you should get a glow discharge as the pressure drops eventually as the vacuum get very good the glow will stop. I suggest you try that but limit the HV current to say a few mA to avoid damage to your electrodes. See image below.

What was the pressure in the video.


You should be able to creat a glow like this as the vaccum increases

https://en.wikipedia.org/wiki/File:Glow_discharge_regions.jp...

Sorry the image would not display for some reason

[Edited on 24-2-2017 by wg48]

[Edited on 24-2-2017 by wg48]
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neptunium
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[*] posted on 23-2-2017 at 18:57


The pressure was around 2e-3 Torr as shown .
I see what you mean with the focusing ring.. This was just a proof of concept and you are correct I did not connect them to any thing yet. I have to fix my vacuum leakage problem first. I do get a glow discharge from 10 Torr to about 1e-3 or so then the filament supply the electron by thermo electro generation like in a X ray tube. It does not have to be supply with a negative HV




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[*] posted on 24-2-2017 at 02:22


Quote: Originally posted by neptunium  
The pressure was around 2e-3 Torr as shown .
I see what you mean with the focusing ring.. This was just a proof of concept and you are correct I did not connect them to any thing yet. I have to fix my vacuum leakage problem first. I do get a glow discharge from 10 Torr to about 1e-3 or so then the filament supply the electron by thermo electro generation like in a X ray tube. It does not have to be supply with a negative HV


Your HV supply should have two output connections. One commonly designated positive and connected to the anode at the phosphor end of the tube. The other connection of your HV supply commonly called negative which I assume is connected to the electrode at the far end of the tube near the filament. That must be connected to one side of the filament so that there is a current path from the filament back to the HV supply.
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[*] posted on 24-2-2017 at 14:48


https://www.youtube.com/watch?v=MQV3BWt4G9A&feature=yout...

still work in progress but i got a trace of electrons on the phosphor! and the vacuum leaks are fixed!
i need a much better filament ( i messed it up while fixing the vacuum leak on the electrodes)but i do commonly get down to 1e-4 Torr and below easily now!
unless i manufacture an ion gun and perfectly line it up to the detector (which has to be made as well)
this phosphor is just an alpha scintillator (ZnS )


[Edited on 24-2-2017 by neptunium]




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