First light on CHM-14 Russian B10 neutron tube

[Doug Coulter]

Well, it kinda worked on the first try. Not super -- there are issues, but it does work, and it does detect neutrons and not other stuff.

Temp lashup for testing. Neutrons coming out from about where the fan in the upper left is.

Here's the lashup. I pretty much used what the data sheet Joe found suggested. 5 ten meg resistors in series. 210 pf coupling cap, 10k load R to the scope probe.

The "issue" is that today, as sometimes happens, the nearby weather radar (normally a boon around here -- nice to have one in your backyard to get accurate pix from it) is really hammering us, and making every possible ground loop ring. That noise on both sides of the real pulse is there with the scope probe shorted to itself, not connected to anything else, and you can actually watch the big dish swing -- it comes and goes periodically. This will obviously require a preamp to get to TTL kinds of levels and low impedance to be reliable in the presence of things like that (like I've done for the other neutron tubes and counters/scintillators). I'll probably get better pictures later -- that radar doesn't normally bother things this badly, but it's a weird weather day, so maybe we're seeing extra reflections or beaming from that.

Screen shot from scope

So here's a typical pulse. I could have cleaned it up a bit more with scope settings (heck, it was in 1 ghz sampling peak detect mode which gets *everything* no matter how slow the sweep), but till I get rid of the radar interference, no point really. I got pulses from about 1 div up to about 150 mv or so, various backside ringing, nothing all that hard to deal with compared to any other detector. If we didn't know it ran in the mysterious corona mode, you'd never notice. The normal overshoot after the pulse due to cap charging, nothing special -- looked better than in that paper Carl found.

The thing is, it's not real sensitive at all. The B10 tube below it on the same rack was going 100hz or more while this was doing 1-2 hz counts (lousy eyeball estimates of course), and the 3He tube was cranking at hundreds of HZ on the same neutron output. On the good side - it has next to zero background, undetectable right now till I clean up that other noise. Note that this isn't way off scale from the estimate you'd make based on the comparison of tube volumes and surface areas -- the other B10 tube is enormous, and so is the 3He. There might be another 2-3x if we can pick off the marginal pulses from reactions oblique to the tube walls with a lower trigger threshold.

It's already a cubic crap-ton more sensitive than a BTI, and realtime -- bonus.

The datasheet says "1400-1600v" for this. I tried 1400, it worked, I tried 1500, it works better -- and now I'm up here typing. More to come, perhaps, since the fusor is ready to go with a single button press right now. But it really looks like I need a nice, tightly shielded loud output analog preamp before doing much more with this. Should be a matter of two transistors more or less, probably off a 9v battery, as that will all fit inside a little box I can mount on there tight, and give nicely big pulses out that will make this noise not an issue at all. I'll see if I can design one for this signal that won't run batteries down fast -- I'll shoot for 1 ma.

Looks like a qualified winner -- I'll see if I can get it better. If we can kill off that RF noise (which isn't this thing, as I said, the scope shows it with shorted probe just being on, and not even connected to system ground to pick up ground loop noise) maybe we can set a decently low threshold and pick up more sensitivity. It does count even when the big fusor is very much underperforming -- it would work fine for someone just starting out.

FWIW, I happened to have a spark plug wire connector sitting on my end table that fits the nub on the tube perfectly, so that's what I used for this. A type plate cap ought to fit too. I grounded the detector tube with a bit of spare flashing I shoved down in the hole with it for a tight fit.

Interestingly, even with the very sloppy looking lashup, even when I had arcs on the main HV very little of that got into the scope signal...that last modification to clean up any radiation from that at the source really works nicely.

And oh, there's those other two tube types to test now that I have the lashup basically going...

[Doug Coulter]

More pix so you can see another viewpoint. For these, I put the thing right where I normally put the neutron oven == right on the tank, but about an inch spacing as it's too big to fit inside the flanges. So, call it about 4 times closer to the "source" wherever that really is.

See the radar antenna turning?

It does clean up a good bit when not in peak detect mode on the scope, but with a slow roll, you don't see anything real either, so I left it that way. See the radar antenna going around? It has back and sidelobes, so it's never gone completely, but in a fast mode (as in the previous post) you can see the rep rate....The HV is on in this picture, just that there's no gas in there till I hit the inlet solenoid button. Plenty of X rays of course and HV for this is 50kv (since it's not in current limit). All the phosphors in the tank are glowing brightly and my geiger counter is about 5x background (behind 2" of lead). It will go to 10x background at full fusor power, but only was getting to about the same numbers for the picture below.

Now, here with the fusor running very conservatively ( 35kv, 15 ma) here's what it looks like.

Should be able to tell the difference, eh?

This was with the fusor not really cranking - I have a sort of stable, low output mode I use to test things like this. Boy, putting it up close to the action (no other changes) does make a difference.
Could be since it's near the 3He tube here it's seeing some neutrons reflected off its moderator too.

[Joe Jarski]

Nice work Doug! I wasn't expecting a whole lot of progress on this for a little while. You made pretty quick work of getting these up and running. I like the idea of putting the whole thing in a canister too.

[Doug Coulter]

I have a wife-free weekend, which is why I thought I might pull it off this quick. No distractions. (other than the new hotrod computer -- when she sees the bill for that, well, it's going to be noisy for awhile)

Yes, in this case a case is going to be required due to the rather small signal. Even at that, I usually "push" detectors to put out as much as I can (eg overvolt them etc) for noise reasons. Here, there's not much to gain that way. Raising the load R just makes noise pickup worse, and doesn't give much more signal. The tube regulates its own voltage, so no go there either once you're in a good place.

I already control as much noise as possible at the source -- believe it or not, despite a little arcing (50kv at an amp peak) nearby, that darn radar was worse. The impedance here is really the 10k to ground, not the 50 megs in the power line. But even then, that little CCFL has to be in a tight box or the 50khz comes out and gets into things -- there's no noise on its output wire at all with a decent filter (.047uf/10k final stage) but that's not the whole story.

What I plan to do in this case is make what amounts to the top of a bud minibox that can screw to the top of the moderator can and hold the preamp, battery, HV input and signal output jacks. Since it doesn't have to be strong per se, copper flashing should do fine for that and make it nice and tight for RF and other trash (and be the work of seconds to get it cut and bent). I plan to try a circuit like this except for swapping the transistor sexes and flipping the polarities around and diddling the values so the transistors are just barely turned on at all. The idea there is to get faster response at turn-on ( initial edge of the pulse turns them both on, which is faster than turning them off), and to be able to bias the second stage almost turned off so it draws almost no power between pulses and sits high so it can swing lower relatively. That trick has worked just fine with much faster signals, so I ought to be able to get the power really low but have it still be fast -- and low output impedance due to the feedback, even though the actual R values would be fairly high. I just got to a point in the day where it was time for the beer and hamburgers on the grill...(ummm, grilled beer).

On closer examination than is possible from these posts (sometimes you just gotta be there), there are quite a few neutron signals that are lower level than the current noise floor. This would be expected (and I see it on the other B10 tube as well) as the reactions are at the tube wall -- and sometimes the reaction products fly tangent and don't plow the detector gas much. Only the lucky few that fly straight across the bore really make those larger pulses. So being able to get down there should raise the useful sensitivity up to a real decent place, not that it's bad now.

If someone wanted to run the entire thing on batteries, you'd use more voltage multiplication on the CCFL, as it's just drawing quiescent current now, which goes up with it's supply volts. In essence, cut the input voltage requirement down by 1/2, the current it needs also goes down about that much...power needed is then 1/4 of what it is now. That would work out to roughly 40 ma at roughly 4v, or < 200 mw for the HV supply part. Here I just have a bulk 12v supply that runs all the stuff around the rig, so I don't care myself -- but you may have noticed all those large ferrite common-mode chokes I have to use to keep ground noise out of that setup. Supply volts don't seem that critical, though in this test I did have a regulated supply. You'd probably want something better than batteries driving one of those CCFL's directly -- they change quite a bit over their life in output voltage. (typical alkaline cell is 1.56v new, .9v at eol -- big ratio)

But for the preamp part -- batteries for sure, no noise at all, crucial probably. And that's actually the main design challenge -- to make the transistors bias reasonably over the entire voltage range of a 9v battery life while retaining the ability to have a decently large output signal. 5v peak over the entire life range is my design goal, might have to accept 4, we shall see.
At any rate, enough output to drive things directly no sweat.

With the high pass needed anyway to eat the ringing of the corona, you're well out of the 1/F noise of decent transistors, particularly low noise audio types (which nevertheless go to tens of mhz), so down in the single digit millivolts should be a snap -- with a good box to keep the external garbage at bay. At any rate, that's next. I have to do some other repair -- my pulser for the vacuum outlet valve failed today, the cheap cap I scrounged from somewhere opened up and it's only putting out microsecond pulses to that valve....oops, too cheap to work reliably

I think this is going to work out fine, and finally take the lock$ off neutron detectors for people just getting started at this. It's plenty sensitive for starters, and would even want to be moved farther away from a hot fusor to avoid pulse pileups - not bad at all. I suspect that with the good preamp, you'd be in the several hundred Hz count range with my fusor hitting 10% output and it close in -- plenty good to learn how to tune fusors with. In other words, even a pretty crappy fusor would make it count enough to know it's working and what makes it work better or worse.
At some point we can cross calibrate these with activations or some other more fundamental standard.

For now, they'll be fine and work in conditions where you'd never get enough activation to be sure.

[Joe Jarski]

So, I have 4 of these tubes that I was going to use to build up some neutron detectors and I started thinking why not put 2 tubes in ones chunk of moderator? I can think of a few possible advantages like more detection area, anti coincidence and things like that. And a few disadvantages like only having 2 separate detectors and maybe any neutrons detected would be divided between the 2 tubes.

Does anyone else have any thoughts on wether 2 tubes per moderator would be a good idea or not? Or maybe there's some other configuration that would be more useful - 2 singles and 1 double?

[Doug Coulter]

I'd think that would certainly be fun to try and informative, actually. Good thought. If you didn't like the results, you could always pour wax back into the extra holes anyway. I'd space them around a circle in the middle with some moderator between them, but mainly because that feels good and would look nicest, no particular evidence it would help - but it might make packaging and shielding the whole thing easier.

When we made our first big moderator for the 3He tubes, I deliberately set the tube in the wax casting well off center to see if we might learn something by then rotating the thing vs the fusor source of neutrons -- the idea being maybe to find out how to make the next version work better. No real noticeable change was observed and we were looking for one.

Neutrons come from the back side in the moderator about the same as from the front (remember that video CarlW put up on fusor.net a couple years back? I tried the same thing here, and yup, you can just moderate behind the tube and it's fine - only a little worse than all around it), and the model I see mentioned in Fast Neutron Physics (a super expensive two volume book set we got) is one of a sort of milky substance for light. Once there's light in there anywhere, it pretty much evens out everywhere, as it would in some translucent substance. Those experts say it's about the same for neutrons. Since they come in fast, but go out slow, neutrons actually build up in a moderator in terms of flux per CC and kind of just fart around in there for quite some time until either something captures them, or they get lucky and fall out at the boundaries. In other words, your moderator eventually gets enriched in D vs H(!).

This makes me think that yes, you could use multiple tubes in a moderator as long as that didn't involve removing too much moderator, and that yes, anti-coincidence might help lower the background from otherwise summed counts due to cosmic showers (which is pretty low on these anyway, not a big problem actually). At the fairly low count rates, fake co incidence ought to be rare for the case of a non-bursting fusor. If you suspected that case, you could look at both co and anti signals to confirm bursting above what you normally get with cosmics anyway....

I would definitely use the preamp design in the other thread about the Russian 3He tubes -- I've tested it with these and it works quite well with them. Really nice pulses that are well out of the noise, flat baseline, nice linearity, all the good stuff. The big advantage the boron tubes have over 3He is that they can use a better counter gas - He and BF3 are good for detecting neutrons, but lousy as proportional counter gases...the breaks of the game, I guess. I'll have to post more scope pix of that preamp with the B10 tubes -- that coffee can shield really cleans things up hugely and gives textbook perfect signals out of it.

Of course, the issue with any moderated detector is time-smear, so I'd think that the time in moderator being on the order of 100us would smear out any fast bursts from the source, and is why I'm also looking at other techniques if I can work out their issues. The plastic scintillator/phototube thing does indeed see fast neutrons fine (actually, better QE than the B or He tubes) -- but it also sees everything else too. Including 2.2 mev gammas from any hydrogenous moderator you use to collimate it to a "pixel", eg, make it directional. Enough lead to stop those gammas from other moderators (including the human and the lab walls and water in the air) might mess up the neutrons - I'm working that one now (well, it's on my rather long list anyway). Any capture gammas would also be time-smeared from a neutron burst event, so might be discriminated by time, or by amplitude, sorta-kinda. In a good scintillator, 2.2 mev gammas ought to produce a particular pulse height, where as proton-knock on light from a fast neutron would be a continuum from zero energy on up -- so you might just be able to get rid of the big pulses and get rid of most of the capture gammas? That's a work in progress....and it will take work to produce progress.

[Joe Jarski]

I'll have to give some different configurations a try. I'll setup one with a single tube for a control reference and try a double and triple tube variation to see what the differences are, if any. I just ordered some of the stuff so that I can build the preamps for running all of these.

[Joe Jarski]

Doug, I've been trying to tell by the pictures here, and on the preamp and 3He tube post, but I haven't been able to figure it out. Are you using a ~210pF cap between the ~47M and ~16k resistors (like on the datasheet) and feeding that into another ~210pF coupling cap on the preamp? Or are you hooked up like a GM tube with a single cap coming off from the voltage divider in the tube circuit?

[Doug Coulter]

Sorry 'bout that. In the pix I'm doing it just like the circuit you found, but with a single 210pf cap. I've since added another in parallel, and I'll post a pic next time I take off the coffee can. I did the parallel so that the HV side of the caps is in the middle, and the outsides are going to the preamp, so as to cut down corona in the can. Seemed to work. I just arranged it all better since I (hoped?) thought that might be my excess pulse problem. It wasn't, but now it's nicer and nicer looking too. Adding the extra capacity there helps the pulse look slightly better, but not a ton. Just a little less differentiation which cuts down the post pulse overshoot a little, but not a heck of a lot. This seems to be "not that picky", and especially with those B10 tubes not picky at all.