Channeltron woes

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ChristofferBraestrup
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Channeltron woes

Post by ChristofferBraestrup »

I'm trying to get some channeltrons working for secondary electron spectroscopy, ion counting, TOF-MS etc., and there are some details i simply can't work out.

I've purchased 2 NOS ВЭУ-6 // VEU-6, and they're pretty standard MS detector units from USSR. Still being made, actually, by a Russian company.

datasheets at http://lampes-et-tubes.info/pm/pm179.php?l=e


Now, electronically I've hooked them up as in the first configuration seen below, taking the anode signal as output:
veu-6 hookup.jpg
With an 1M ohm resistor between "last dynode" and anode - the first dynode at chassis ground. A 4.7n cap extracts the signal to a BNC, and the "last dynode" is connected directly to an ortec 0-3kV psu.

Trouble is I get no output. at very high (<2500V) bias voltage it sometimes crashes and oscillates, and there is the occasional very slow pulse (1 second or longer), but no fast electron pulses.

with 50 ohm termination at the scope end (1 M 50 ohm cable), i see no signals at all.

See attached pictures for the setup. the pressure in the chamber is between 10^-6 and 10^-7 torr, and the channeltron is mounted on a KF40 feedthrough by screw unions and bare 1mm copper wire for structural support.

So why don't I see any signals? I was expecting dark count to be enough to test it out with.

-20 MHz scope too slow to see signals at all
-dark count too low to reliably pick up
-need current reg. resistor for bias voltage
-channeltron damaged/decayed/delaminated etc.


Anyone have any suggestions or comments? Unfortunately I don't have a beta source I could test it with.

Thanks in advance.
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Maciek Szymanski
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Re: Channeltron woes

Post by Maciek Szymanski »

I think that the bias voltage is too low. According to the data sheet it is 4kV, and event there is a note that it may operate at higher voltages but the parameters may then deviate from the data sheet. As regarding the scope - the channeltron maximum counting speed is stated as 10e5 pulses/s so it may be expected that 20 mHz should suffice. But from my experience the low bandwidth scopes sometimes have problems with reliable triggering with short pulses.

What are you using as the electron source?
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ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

I'm mainly drawing from my experience with PMT's, but it would surprise me a bit if there were no signal at 75% of maximum operating voltage. Another datasheet states 3500V as normal operating voltage.

There is no electron source, I was counting on being able to acertain if it worked at all or not just by background radiation, but maybe that was optimistic.
What would the easiest electron source for it be?

I'm not triggering on pulses with my scope, I was just hoping to see them roll in untrigered.
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Maciek Szymanski
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Re: Channeltron woes

Post by Maciek Szymanski »

At your pressures only a hot filament source can be used. It should be enough to have a heated filament at the ground potential (eg. small light bulb without the glass) and a anode in a form of a plate with a hole or a metal mesh at +600 to +1500V. It should extract and pass enough electrons to be detected.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

Thanks for the suggestion! I'll start crushing bulbs and extracting filaments...

I was really hoping to avoid it, since the situation of building an untested device to test another untested device is only good if it works - If I see nothing it could be either electron gun or channeltron being at fault.
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Richard Hull
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Re: Channeltron woes

Post by Richard Hull »

We are often forced to make tools to test tools or align them. We often find ourselves one tool or critical item short of the mark. We are forced to find workarounds or just give up.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Maciek Szymanski
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Re: Channeltron woes

Post by Maciek Szymanski »

And that's the fun pat for me. Finding the workarounds and deducting form coincidence of the indirect clues is the beauty of the experimental particle physics.
The low energy electrons are rater nice species to wok with. You can detect them with a simple screen made of a piece of glass painted with the mixture of the water glass and the ZnS:Cu craft store fluorescent pigment. Ad the charge can be easily proven with a moderate permanent magnet (even ferrite will do - no need for neodymium).
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
Dan Knapp
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Re: Channeltron woes

Post by Dan Knapp »

Were the channeltrons listed as used? I suspect the vast majority of channeltrons sold on eBay are bad ones that were replaced in mass spectrometers (a “consumable” in the MS business). Also, the surface will deteriorate over time with atmospheric exposure even on a new one. A weak multiplier removed from service and then left exposed to atmosphere is likely to be worthless. A channeltron is one item I would never buy on eBay unless it was sealed in original packaging with a relatively recent date stamp.
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

I guess I have the wrong glasses on. I have been looking forward to building electron guns as well! I've already installed a rudimentary one, results TBD.

Dan, these channeltrons came vacuum packed in their original box, along with bespoke data/test sheets, so definitely NOS. "Relatively recent date stamp" is well.. relative. 1987 vintage!

I think one is safer getting the "high pressure" multipliers - typically linear ones - as it seems they're made to take more of a beating.
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

Thank you for all your suggestions!

I rigged up the simplest electron gun I could; a 2 pin KF40 feedthrough with a lightbulb filament in the end of a KF40 T, and a rod extending down with a copper foil disc I'd punched a hole in with a paper punch. Filament was supplied by a bench supply, at between 20-60 mA at 12V, anode was connected to a ratemeter for cordless high voltage : 1.2kV.

And sure enough, at about 40 mA filament current, pulses started to appear in the channeltron! At 50ish mA they were too closely spaced to count by eye.

The channeltron gave good output pulse heights down to 1900V bias. at higher than 2500V I saw lots of 60Hz on the waveform, and the occasional crash where it just went into oscillation or something - this may very well be my crusty trusty ortec 3kV supply being a bit unhappy at higher voltages.

Either way the channeltron definitely does what it sais on the tin!

One quirk though: I see nothing if I terminate it in 50 ohm by my scope. I wonder what the deal is with that. I don't think it states in the datasheet what impedance to terminate in, but perhaps 1K would be worth trying.

Either way I'll go dancing on my roof for a bit.
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

Oh and some imagery
IMG_20210402_175739_301.jpg
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Maciek Szymanski
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Re: Channeltron woes

Post by Maciek Szymanski »

Great! That's the real experimenter's spirit! And nice to see an analog scope.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

I just realized something strange is going on - Why are my output pulses positive?! I've set up the channeltron for negative ions/electrons, and with signal tapped from the anode.

See below for an actual schematic of my setup. If electrons collect on the anode, then the bias should momentarily drop, producing negative pulses. This looks more like "last dynode" pulses.

Can anyone spot what Is going on? I sure can't. the 1M resistor is just a normal 0.25W, perhaps it's too conductive at HV to not just act as a short?

There is large variance in the internal Rcm, it seems.
electrontest.jpg
John Futter
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Re: Channeltron woes

Post by John Futter »

1 meg is very low and that is your source impedance so terminating the scope to 50 ohms will be a very big attenuator
I would use 10 meg or higher and then use a preamp in your metal box as an impedance convertor ie Jfet frontend and 50 ohm output impedance so you can use the scope termination. This stops cable reflections on your signals
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

For bias a meg seems pretty standard for electron multipliers, but I could be mistaken.
Here's an article featuring a neg bias VEU-6 for positive ion detection, and to me it looks like they terminate the output directly in 50 ohm, and get 100-200 mV pulses out.

http://www.mathnet.ru/links/da376d78129 ... tf4571.pdf

All of my channeltron preamps has an input impedance of 50 ohm as well, but I haven't tried adding them in yet.
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Re: Channeltron woes

Post by John Futter »

Again
an input impedance of 50 ohms is going to attenuate your signal by 20,000 times if it has a source impedance of 1 megohm.
so .2volts out multiplied by 20000 gives 4000 volts coming from your channeltron (I do not think so)
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

I don't think that's right. There's no voltage drop over the anode resistor, it's purely a current limiting thing. One can also ground it, and take the inverted signal from the last dynode.

http://winspa.de/help101/images/Channeltron.jpg
http://winspa.de/help101/hardware.htm

Here's an example of a bias network for a (different) channeltron. I also plan on using the Ortec VT120's as preamplifier - they have an input impedance of 50 ohm too.

1M is a pretty common anode resistor for PMT's as well. Not sure what's going on.
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Re: Channeltron woes

Post by John Futter »

ChristofferBraestrup
your included circuits provided all the clues
the VT 120A preamp is fed via a 10pf cap so for about 100kHz pulse repetition this equals about 3 meg input impedance into the 50 ohm preamp. Yes your preamp is 50 ohms optimized for extremely low noise current input and with a gain of 200
If you use your scope direct no termination you will attenuate by about 3 your signal in peak to peak and have to put up with lots of reflections from a not matched source / load and your high impedance will be slowing up your pulses and your noise will have risen because of 4ktbR in tis case the R

so far I see little wrong with the info I gave you so far
ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup »

Oh I'm sure you're right, and I'm very grateful for the help!

I will try adding in the preamp and see if there is a difference. Perhaps the 3.6n is too large a coupling cap as well.

The wrong polarity of my pulses could be my scope being too slow to see the original negative component, leaving only some wierd double differentiated tail to be seen. Or perhaps it's just the cable reflections
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