Neutron detector tubes

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Re: Neutron detector tubes

Postby Joe Jarski » Sat Apr 23, 2011 7:16 pm

Doug, most of the stuff that I found so far is in old scientific journals that you have to pay for a copy of, but I'm still looking.

That being said, here's my crack at translating Russian again for a similar type of tube...

CHM-16.jpg

...a little different than a GM tube and the series resistor is about 10X higher resistance. Also, notice the 500kHz amplifier requirement.

The other data that I found is here, which looks like the same data posted on the auction. For some reason when you flip back and forth between the original and translated page the tube number changes from CHM-14 to SNM-1914... I don't know where the "19" is coming from, but the specs are the same.

It looks like there may be 2 different operating modes for these tubes based on the 2 charts. The first one being just as a typical B10 "Neutron counter" and the second one being the "Proportional and Crown neutron counter" which is the coronal discharge mode.

Edit: Here is the link to where I found the datasheet posted above.

Edit again: Somewhere in my searches I saw a reference to 100M series resistors used also, so they should be big whatever they are, which seems logical. And the gstube.com site in the link above has a nice variety of tubes that are apparently for sale so it's worth a look.
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Re: Neutron detector tubes

Postby Doug Coulter » Sat Apr 23, 2011 9:01 pm

Excellent work, Joe! A round of applause, y'all :D

That'll get me started down the right road -- let's not enrich the journal rip-off guys any more than required (instead, cultivate some college students and get them to download them - most of the ones we used to have have since graduated, rats). Now I wish I hadn't given Richard all those super high value Victoreen resistors (which he's now selling at high prices, even though he barely acknowledged the gift... I guess he knows now I wasn't giving him trash). But mere tens of megs, not so big a deal, and not so hard to come up with. I suspected that from the start, because any of these gas tubes get real messed up if there's a lot of current available - they tend to devolve into an arc (negative resistance at some point) -- it's just a matter of where on the curve that happens. The 3He tubes from the same place are also listed like this, but he says they are 3He + argon -- so should work in the normal proportional mode too. It's only the more modern tubes of any kind that can hack low series resistance values.

I almost wonder how you'd even see a particle in the presence of a large (25 uA is what one listing says) corona current. Coronas tend to be more than a bit noisy, and that's a ton of power compared to a single few-MeV event. In corona, you'd think most of the gas was ionized already, so what would change? That's interesting from a merely intellectual point of view, of course.

I note they were also mentioning 100's of millivolts signal -- maybe that's a reason to run the weird mode, so they don't need as nifty an amp. For that matter, I've run most gas "proportional" tubes right up at the edge of geiger mode to get big signals without that complication (5v pulses across 3 megs from a 3He tube run right at the limit). And of course, nowadays, a preamp is a cheap chip away, so maybe we don't need the fancy and dodgy mode to get the job done. After all, they all have gas in them, and are proportional counters if set up that way -- physics is physics, and a label can't change that. The fact that it has low pressure gas and a B10 lining are all we really need, no matter the mode we run them in. These are simple enough we were discussing making them ourselves, but at this price? No reason to bother.

In fact, in my own looking, I did find some places where regular tubes were called corona tubes, but weren't -- most of the "real" corona tubes I found were those old shunt regulators they used in geiger counters as power supply parts. And as you surely know, tons of garbage that has nothing to do with this topic -- gotta get my google-fu up to better speed.

FWIW, the translation at this link (which is otherwise interesting) would convince some people that this is all a translation issue. Not that it is, but evidently not everyone "gets it" if it isn't.
http://www.electropedia.org/iev/iev.nsf ... m&part=394
corona counter tube
counter tube in which a corona discharge effect is maintained by a passing ionizing particle which produces a sharp current change


So the corona is only maintained by the passing of an ionizing particle? Whaaa? Guess we'll just have to see, I tend to trust the data sheet Joe found a lot more than this.

My guess is that the other R and C are also crucial here. The C and low R would provide an avalanche current to make a really nice pulse, once triggered -- and then that extra current would go away as the cap charged again, in time to keep from frying the tube. Cute.
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Re: Neutron detector tubes

Postby Joe Jarski » Sat Apr 23, 2011 10:00 pm

I've been mulling over the operational theory of these and my thought is that with the high resistance value and the requirement for a 500 kHz amplifier is that maybe they operate as an oscillator and you end up with a fairly consistent noise floor of ~500kHz pulses until an alpha particle from the B10 decay goes plowing through and ionizes more gas than the corona discharge would by itself. Their immunity to gamma rays is probably because they aren't going to plow the field like alphas and they just get lost in the noise. That's just a guess, though.

I ended doing a cut-n-paste of Russian text to find some of this stuff because I wasn't having much luck in English. The first several papers listed on the InfoBridge looked good here, but there aren't available online. :(

Well, my link to the InfoBridge doesn't work properly and take you right to the list, so if you want to check it out - search on corona+neutron+counter... I know, that's a tough one to figure out! ;)

Oh, and one more thing - there were some Russian GM tubes that just sold on ePay, I think from the same seller that had a "+" marked on the tube too, so based on that and the datasheet I would guess that the "+" means probably something other than positive.
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Re: Neutron detector tubes

Postby Doug Coulter » Sun Apr 24, 2011 10:20 pm

CarlW posted this paper on fusor.net.
Here's a copy. Thanks to Bill for the head's up. Evidently both groups are reading one another -- from my pov, that's great.

Tavendale_A_J.pdf
Paper on corona tubes
(304.11 KiB) Downloaded 226 times
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Re: Neutron detector tubes

Postby Doug Coulter » Fri May 06, 2011 11:54 am

Got all the tubes we ordered today, they look to be in good shape, new old stock.

1 each chm-14 (boron, short)
1 each chm-11 (boron, long)

2 each chm-17 (3 He)

The specs and operating conditions of course will be a bit different from type to type. What did others here get, so I know what to make work first?
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Re: Neutron detector tubes

Postby Joe Jarski » Fri May 06, 2011 12:53 pm

I just got mine too - they're of the CHM-14 variety.
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Re: Neutron detector tubes

Postby Doug Coulter » Fri May 06, 2011 1:02 pm

Ok, that one goes first then. I have the big R's in stock and so on already. I've noticed that at that sort of impedance level, I'm going to have noise pickup issues if I just air-wire on the bench, so probably I need to make a little box for that end of the tube to stick into for shielding, and put the preamp right there. If the Russian circuit works (I'm guessing that cap is 390 pf, not nf) then the noise might not be so bad but it pays to get that kind of thing right. Heck, that corona current isn't going to be easy to measure floating at HV, and it does seem to set the basic gain and ringing frequency of the setup (and maybe that's how you know you're coming into range). Hopefully I can coax enough neutrons out of the upstairs setup to test all these (where it's easier to work). Now to go out and get a long shank 3/4" drill or two for a chunk of that hard to handle HDPE moderator I have. Next time, don't get UHMW!
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Re: Neutron detector tubes

Postby Joe Jarski » Fri May 06, 2011 1:35 pm

Cool, I'm interested in knowing how these will work out. That's probably a good call on the cap. The translation of "pico" begins with a character that looks like a "n".

I'll have to get myself some moderator too. About 1-1/2" between source and tube (wall thickness) should be about right, right?
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Re: Neutron detector tubes

Postby Doug Coulter » Fri May 06, 2011 2:30 pm

Yes, you'll need moderator for these. B10 has a 1/v sensitivity (so does 3He), so the slower the neutrons, the better. That 1.5" figure is ideal for getting neutrons in to the "resonance range" for activating sliver and such -- a couple ev. For this, we want thermal, or about .025 ev. Typical would be a 6" diameter rod you'd drill into. 3" isn't really enough -- but some reflect back into the tube after collisions past it. The bigger size lets more neutrons hit it as well. Due to the random walk nature, they still have a chance to get into the tube about as well as the straight on ones do. Next time I do this, I probably won't buy the fat rod, it's expensive and hard to deal with. Some 1" plates would be a lot easier....and there's nothing magic about it being round. Slightly cooler looking is the only reason to have it like that.

Avoid PVC -- the chlorine captures neutrons too. You can use wax, but thicker, it's not as many H's and C's per cube (ditto oil). Polyethylene is about the best, but don't get UMHW -- it's too hard to work with, the plain HDPE should be as good, but easier to handle by far. That other stuff -- I'm tempted to heat a rod and try to burn through. I almost wrecked a chainsaw cutting it, and nothing else would. Even the shop metal saw just bound the blade in there. You have to melt it to make it cut at all (and that stuff wound up in my chainsaw clutch).

They seem to want a very short RC time constant, and according to the paper that's because there is a lot of LF noise from the corona -- might as well high-pass that right out from the start from what I infer there. Even 390 pf sounds on the big side for that, I'll try some 270's I've got that are quiet at HV, and drive an emitter follower off the load R (or a jfet or something, I'll try a few things). Should at least get long battery life on the preamp.

CCFL + doubler to run it - I had just built one up to test those panel type alpha/beta detectors Bill had just found anyway. More on those later -- they are huge, sensitive, and you provide the gas in a flow system, they leak -- on purpose or at least, with a large area super thin window, it can't be helped. As a result of that I found a standard welding gas mix of 75% Ar, 25% CO2 works wonderfully for a counter gas (at least 10 times better than air)...and the price is right. I'll probably just string together some 10 meg R's in a glass (or quartz) tube for the big R needed here.
That's about 100 ma at about 9v needed to run that (with no load on the output)...a lot for batteries.

I gotta go do an errand run, but by the end of the weekend, I ought to have something to show here. If they're real numb, it will take some running between the upstairs and downstairs labs to get things tuned up (that fusor is ready to go with the flip of a switch or two), and I'm just recovering from whatever I did to my back, so I gotta go a little slower than usual.
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Re: Neutron detector tubes

Postby William A Washburn » Fri May 06, 2011 4:52 pm

Thanks guys,

Just got home from work and picked mine up from the post office today (CHM14).
Imagine my surprise when I found these posts!
I do have some 1 & 2 watt rsistors up to 1 gig-ohm so I'm OK there.

Bill
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