B10 lined neutron tubes

[Doug Coulter]

Are supposed to be hard to deal with, but tonight I kludged up the simplest imaginable circuit and the two I have worked great with it, using the fusor as a test source for neutrons, and a very hot torbermite rock as a source of gammas to make sure I was only counting neutrons.

Here's the circuit I used:

Simple TLO-84/CCFL inverter B10 driver

Like I said, simple.

This opamp "shouldn't be fast enough" but it works fine. I get 1-5v pulses on neutrons, roughly 50 uS wide, and about 250mv on hot gamma rocks. I used 10 meg R's in the opamp feedback and positive input for balance. This could easily drive an NPN transistor for a digital count output (use a 10k or so R in series with the base to limit base current). I used my standby CCFL inverter for the HV, about 680v or so, and for that, the nominal 12v input inverter only needs about 3.5 volts. I'm going to fool around with this a little more and see if I can get it going with a 5v opamp next, so maybe I can even make a portable version -- that CCFL can't be drawing too much at that voltage input level, they are fairly efficient, so maybe I can go battery operated. I tried with both short and long BNC cables to the tube -- it just works. The unlabeled R's on the opamp inputs are 10 megs. The only issue I had (built this on a push-in kludge board) was noise from the CCFL, which radiates a fierce amount of 43khz -- but a copper shield fixed that right up -- and is something you usually need for those. I used my old standby, Digi part number
289-1025-ND, which is a JKL BXA-12529 type. I find these are a ton better than the TDK made ones for quality and efficiency. They run nicely all the way down to about 1 Vbe at reduced input/output, and are quite stiff -- you can regulate the output well enough by regulating the input. For now, I'll make a fixed preamp/power box and feed the counts into our multigeiger PIC project for logging during runs.

Scope trace, 1v/div, 10ms/div while fusor running.

As you can see, I'd not miss many real neutrons running with a more or less fixed .6 volt theshold (eg a transitor BE junction) as my "comparator". I guess a nit picker would say it would be temperature dependent, but I'd reply that if you think any of these count neutrons accurately enough for that to matter -- I want some of what you're smoking. I see about 300mv max out of this on the hot rock I use for testing; some 20k cpm or more on a geiger from a good distance -- torbermite ore with lots of daughters. That one I keep in a baggie and a pig outdoors when not in use, and wash after touching the baggie (radon daughters build up quick in the pig).

That was not a particularly hot fusor run, and the moderator/tube were about a foot away from the action. So don't despair if you can't find a "better" 3He tube -- you're good to go for fusors with this ancient tech just fine, thanks. In fact, I have to "numb" my 3He tube to use it at all close to the fusor, setting the theshold at about twice what a single neutron hit inside a "tube time" gives, so I only see multiple hits within say 10us of one another -- else it counts at over 10kcps and I can't use that on audio to tell how I'm doing!

Ancient tube in modern moderator

I use a 6.5" diameter piece of UMHW HDPE for the moderator on these kinds of things. It's an absolute bear to machine/drill etc, but the best there is otherwise.
I had to use a chainsaw to cut the 3' rod to length, and it took a couple hours with a drill and extension to drill out an 18" by 1.5" hole in that stuff. I now have one very strong arm from running the drill press table up and down -- the only way to cut this is to melt it, and you have to pull the drill real often and clean it....about once per half inch.
A hand saw just skips over this stuff, and my metal bandsaw melts it and grabs. It nearly ruined my chain saw! So much for a precise cut. I could have lathe cut it had I been willing to change chucks, maybe.

With some adjustments, this would probably work well with BF3 tubes too. I'll try it soon.

[Doug Coulter]

After seeing how well these work, and how well they reject gammas from a piece of torbermite ore (20k cpm, from that natural reactor in Africa), I am looking at actually making some of these from scratch now. I am finding recent research on them that indicates this isn't a completely dumb idea, and that there are ways to improve the performance further. The main issue here is having as much surface area/volume ratio on the boron as you can get. Just "more" boron isn't what you want, as it will absorb neutrons internally but the reaction products don't get out to run the counter. A recent book I read mentions that BF3 isn't really that great, it makes a lousy gas for a counter, and though I could no doubt get it (from the same place I got the D), it's nasty stuff to handle to boot. Unenriched boron is still 20% or so of B10, and I am not seeing a convincing case for paying a zillion times more for the enriched stuff -- B11 just doesn't eat the neutrons, so they are free to find their way to the B10 in any reasonable design.

All this leads to the idea of coating one or more fairly fine mesh but open screens with boron, boric acid, some boron compound, as though it were vitreous enamel. Heck, you might make that out of pyrex (very thin fibers) which already has boron in it, and just enrich the surface....you need to have "holes" to let the ions through someplace, so just coating the outside of the tube with an insulating boron compound won't do for this. One possibility to get that perfect screen with a lot of surface/volume ratio is simply to wind a coil of whatever you're going to coat with B compound around a glass (or quartz) form that looks like a cylinder fusor grid, kind of in the manner of those old miniductors, so all that's needed there is something to wind that will take the heat of melting boric acid onto it. The lathe here makes a great coil winder for thin stuff, with up to about 140 tpi density, and thin glass/quartz fibers are pretty easy to make, maybe during the winding process itself.

I have some 1 mil (24u) W wire for center anodes, and obviously the rest is more or less simple machine work. This and a mix of Ar, CO2, and Br should result in a counter that doesn't need real high voltage to work, and ought to be medium sensitive even in fairly small sizes for fusors, or any decently hot source of neutrons. The source we made from a staticmaster brush Po source, and a Be foil doesn't even convincingly count a 3He tube, although it does put bubbles in a BTI dosimeter if you leave it overnight, so as far as I see it, there's just about nothing out there that will really do for weak sources (<<100 neuts/second) in the presence of cosmic rays.

Of course, any gas-proportional counter doesn't satisfy my fast-fast criterion (fast neutrons detected quickly) but as something for the fusion hobbyist this looks like a possible winner, since while bemoaning the shortage of 3He, no one who has a bunch is willing to sell any to beginners to advance the field.

[johnf]

BF3 tubes are easy to make

the pics make it ovious

[Doug Coulter]

I'm curious about this. Yes, at some level, "easy", no question. When I've tried the pinched-off copper fill tube on neon tubes, I've had failures, but really haven't done it enough to know why, and it surely seems the way to go no matter the gas fill type. I may have forgotten to clean the insides off after annealing the tubing or something, but they didn't live long -- leaked, and neon tubes are very picky about contamination (not to mention pressure), maybe more so than counter tubes.

I can't tell how the glass to metal seal was done from this, quite. I do tungsten into pyrex tubing here no, problems, but then putting that into a metal tube or endplate is ???. Epoxy?

Both of those are common to any detector tube type.

Now, on a cost basis -- boric acid etc are in stock -- free.

Just the regulator for the BF3 is $400....and I hear it's real nasty to be around due to the HF that comes off if any gets into the air near you. So I'd be risking maybe $1000 USD or more just to try that, with potentially a pretty small audience for the product -- amateur fusioneers. I'd never get the k-dollars for an amateur tube making effort, they'd have to go for 100 bucks or less.

Just thinking out loud about monetizing some of this

[johnf]

Doug
The feedthroughs are commercial glass metal seal types that you just solder in.

by the way those tubes in the pic are 20 years old and still work

[chrismb]

Any thoughts on where to get such pre-made metal-in-glass parts in the UK, John? I might try a few applications....

[Doug Coulter]

I'll second that one -- the price of new weld-in FT's from Lesker is more than I want to sell a whole tube for, and these look cheap. I may putz around with my glass working gear and try to make some, seeing if I can get away with a compression seal (see Kohl) with pyrex, which I can easily and reliably seal W wire or rod through. I've also had some success using old vacuum tube bases glued into things to get multiple feedthroughs, but they won't take baking (the glue decomposes). The ceramic glues I've tried are too brittle to make up for the "tempco monster".

The reason I'm thinking of using B lined rather than BF3 is because they have surpassed the performance of the pure BF3 tubes -- even our DHS has signed on for them, as even they can't get enough 3He to suit them. The reasons are:

You can get more boron in the tube, in useful milligrams that way, while still having it thin enough to let the alpha out. You just can't pressurize BF3 past a certain point without having to have such a high voltage it's hard to use (corona and insulator losses look like real hits), and if you make the tube smaller to get in a better place in Paschen's law, well, you lost volume that was sensitive -- you can't win that game very easily.

Fluorine is released when the reaction occurs (which also loses boron of course). The free fluorine is a trouble maker at some point to other tube materials. BF3 is also not an ideal gas to make a proportional counter with -- not bad, but not as good as other fills. It has the advantage of being able to see both the Li and the alpha that come off, where boron on a surface will usually lose one to the interior of the support, but the better gas fill more than makes up for that (according to some recent patents I've found). I don't know if this is more than a theoretical issue, as I've never managed to wear out an alcohol quenched geiger tube myself...

Of course, the smaller I can make these, the better, so higher quantum efficiency from just having more boron in there is a fairly big deal. Enriched stuff is going to be real hard to find in either form, of course. But since b11 doesn't have a big neutron cross section, having some of that in there extra can be made up by sheer "more of it" if a scheme that gives more surface area can be created. The patents show a couple of ways that are basically coated screens or wires, and you can have a lot of those in there if you leave enough space for the ions and field to penetrate from the outer shell to the inner anode.

One wonders (and I'm going to try it for giggles) if you couldn't just use a pyrex tube, which has 15% or so boron in it, with a screen inside to be the cathode, and have that work. Sure would be easy to make! All the stuff I have is too thick-wall to be ideal, but who knows?

[William A Washburn]

Spoke with Brian at "Specialty Seal Group" and will get quotes and samples soon for
the part on the left side of this picture. I'll send the samples to you and the good/bad
news about pricing: