DIY Hornyak scintillator build

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DIY Hornyak scintillator build

Postby Doug Coulter » Sun Oct 23, 2011 11:35 am

It's come to our (well, everyone's) attention that there needs to be some standard way to measure neutron outputs between labs, and within a lab to see the results of tuning changes.
For the first, I've been an advocate of silver activation, as it is impossible to fool, and integrates over a decently long run, going to Indium for very hot or longer runs. There's going to be a little variability there depending on neutron oven build and placement, as well as counting the result, for which the solution is our Standard Counter - these are all guaranteed to count the same, and be particularly suited for activation counts as they are more beta sensitive than most counters.

However, activations aren't the end-all, be-all, though it's a great start. Silver has some metastable states so short lived that it is more sensitive to the neutron output at the end of a run than the rest, In takes a lot of neutrons to activate well out of background, and neither give real-time feedback to the operator as he tunes things. While we have a much better chance at getting to absolute numbers with activations (within say 20% or so), for tuning we really don't need that. For that matter, as long as everyone is using the same neutron counters, we don't need absolute numbers there either.

While my own lab is blessed with just about every type of neutron detector there is, from BTI's to 3He tubes, both kinds of boron tubes etc, I find that especially for newbs around EMI (which would include almost everyone who hasn't made a life study in it), EMI induced false counts in those tend to create significant errors, and people (even I sometimes) just wind up tuning for max EMI, not max neutrons out of their fusors. I've been using cross-checks with activations to make sure of that one, as well as monitoring the raw signal on scope and audio, but that's more work than many will likely do. So we need an affordable, stable, EMI resistant, neutron counter.

In recent tests with a pretty standard cheap photomultiplier, the Hornyak detector I got from Eljen has come though with flying colors in this regard. Stable, repeatable, pretty immune to EMI because the signal is pretty good and large, and slow, so it's easy to get the right counts in the presence of noise. Since the store bought ones are expensive ($150 for just the button) and this isn't rocket surgery, I propose to start making the button heads. They might not all be precisely the same sensitivity (same would be true of the bought ones) but they'll be stable. I've made one before that worked, despite having the "wrong" phosphor for it. Well, now I have the right stuff, so why not make up a few of improved design? This thread will document doing just that.

The plan is pretty simple. I will cut up a bunch of 2" sq pieces of 1/16" thick plexiglass for light guiding. I will stack these up with phosphor/epoxy slurry between them, and harden this under pressure in a jig, so we will have alternating layers of ZnS:Ag and clear plex light guide. This is required as ZnS:Ag has such a large index of refraction that there's nothing you can cast it in that will result in the unit being transparent to its own light output. A standard Hornyak tries to get around this by interleaving rings of phosphor/resin with just resin in a bullseye pattern.
That's hard to make, and it doesn't permit too many layers for a given phototube face size, here desired to be 1" (so the tube is not expensive or rare -- those are common from old PET scanners and easy to find). I plan to make a 2" by 2" by 1" thick one, then dice it up into 1x1x1" pieces to get 4 made per run on the glue jig. After polishing the plex edges, I'll coat the non phototube facing ones with something reflective, like space blanket material. This actually works better than white stuff, or metallizing the plastic, as if there's a tiny air gap, you get total internal reflection in the plexiglass for the angles that works for, wheras if things are touching, they can suck light out and only return it with some losses. (This is well known in detector lore, I'm just repeating it here for the non experts). Plex and most casting resins or epoxies have an index of about 1.5. ZnS:Ag is in the 4's, which is why it always looks white when cast into things, and Hornyak worked hard to find the right mix for best performance. Later, someone else came along with the interspersed light guide idea, and I'm doing my own version of that here.

Assuming this works out, I'll be able to provide neutron detector heads for the standard counter to complete the kit for fusor radiation monitoring, which is the goal. I expect the main issue will be some variations in sensitivity, as the interlayer slurry thickness will not be super easy to control, but we'll see how that works out in practice vs theory.

So, I've already cut up some plex, now I'll make a jig to hold them in alignment and under pressure while whatever I use as the adhesive gets hard. I plan to go for a fairly heavy concentration of phosphor in those layers, since the rest is clear anyway, and I'd never have a working thing with that concentration low enough that much light would get through the dispersion inside that layer anyway - it will have to get out into the plexiglass and go to the tube from there no matter what.

One big advantage of a Hornyak - it only sees fast neutrons. This means no moderator is required, and unlike say a 3He tube, the counting rate won't vary as a big bag of water moderator (eg me) moves around the lab during a run and changes the neutron flux for slow neutrons. We are also looking at other neutron-recoil type detectors, but this one is known to work, so it will be the standard the others are judged by. You can in theory just use a light gas - H2 or He, in a proportional tube and count that, but again you're getting into low signal output there, though you save having to buy a phototube. Those will get their own threads at any rate so we can do honest side by side comparisons with them all with the same source (fusor) running during our testing.

Hornyparts.jpg
Easy part is done, dice them up

Edgeview.jpg
Light gets through sideways fine


Even with unpolished sides and the brown paper on there, light gets through this stack just fine.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: DIY Hornyak scintillator build

Postby Doug Coulter » Sun Oct 23, 2011 4:16 pm

A few pix of the process. I tried one layer with 5 min epoxy and what seemed like a lot of ZnS, but it took a ton of pressure to squeeze out, and only got to about 30% opacity. So I mixed a batch of casting resin (much thinner stuff) and used a LOT more ZnS in that (darn, it's not exactly cheap stuff) for the main effort. It squeezes out easily, hopefully not too easily. Still takes some decent pressure to keep the edges from sucking air back in, so I hope that's not so much as to squeeze it all out in the long time it takes for this stuff to cure. (over night min).
Stack.jpg
Wit a little UV to make it glow

stack2.jpg
Another view of the jig

Had some leftover and squeezed out so I improvised a quick possible use for it with some microscope slides and cover slips to make the thickness uniform. Previous experience with the pinhole camera underlined how important that is.
Extra.jpg


This should be solidified enough to play with tomorrow night, though they mention 72 hours for a full cure. I may need to replace some parts in my fusor, as the last run it started arcing badly near the end of the feedthrough. I've had this kind of "self repair" before when things cooled off, but this might actually require breaking vacuum and putting in new parts. At any rate, once it is running again, I'll have a test horse for the blocks I'll cut from this blank. I have a standard holder for a type of phototube we got a bunch of, so I can switch these into a known working thing.
However, with ZnS, even ZnS:Ag which is the faster decay version (ZnS:Cu is very slow) you have to wait nearly 24 hours for it to get quiet in the dark before it really works right.

One nice thing about plexiglass vs scint plastic - you can polish it with power tools. The least contact of any power tool with the scint plastic crazes it, and the resulting cracks propagate through the plastic to depth about 1/4" and destroy the uniform light transmission by being reflective. Plex, heck, is easy compared to that. You can use a belt sander (gently) to get close, then go to the hand work.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: DIY Hornyak scintillator build

Postby JonathanH13 » Thu Oct 27, 2011 3:24 am

This looks very promising Doug! Nice work. EMI immunity is definitely worthwhile in this environment!

It would be nice to know that each head has an approximately similar interlayer thickness/sensitivity if we are going to use them as a standard.

I guess you could supply the button head with a phototube, precalibrated, then the head sensitivity would be less of an issue.

Do you know how they apply these phoshor coatings industrially?
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Re: DIY Hornyak scintillator build

Postby Doug Coulter » Thu Oct 27, 2011 7:44 am

Well, I hope they are promising. I've since cut the edges on the mill to get a feel for some of those issues. Seems I got the phosphor layers pretty uniform, if on the thin side. The beam of my UV flashlight gets about 60% of the way through, sideways, and the emitted light does come out the faces nicely to the eyeball. Interestingly the plastic is so clear that almost none of the light comes straight out the face, but at the angles a little off center where you are viewing the whole phosphor layer on either side, eg a double humped angular distribution. That won't affect a phototube of course. And yes, these will have to be at least relative-calibrated off say the original one, for which we have some good numbers from our own and HEAS runs.

And I want to thank those guys for the data they've provided, that's been a real force-multiplier to take advantage of their brains and effort to get a decent calibration - this is based on their joint efforts to get good numbers from Richard's fusor and I think that after a few false starts, they've fixed up any serious errors in their calculations at this point.

I'd guess we were inside 10% error, which I hear is quite decent in this game (from reading the literature in those expensive Neutron Physics books). Obviously it would go best if I do the whole probe head with a phototube. These do make pulses, though, so even that isn't as variable as you'd think -- a different phototube gain would only require a different preamp gain-threshold to get things the same at the counter - it doesn't look like threshold will affect things all that terribly much as long as you put it above the PT noise and say about 1/4 the max pulse height. It's not as sensitive as the B10 or He3 tubes, but it's very numb to noise, and the kicker is - I can make these forever at a decent price, which I can't yet do with the other technologies.

The original ones are all cast in two steps - one for the resin plus phosphor, then a step that fills in the spaces with clear resin. That requires a fancier mold than just stacking these up, and I am currently thinking the way I did it is better as it gives finer interdigitation of the light producer and light guides. Obviously, some tests are in order to determine that and will be coming soon.
Since these need shielding from X rays, and a non magnetic probe enclosure (ideally) I'll probably make some of those out of lead sheet, and use the mu metal that Geo so kindly provided to shield the phototubes - I will use the ones we got out of a PET scanner because I like those best in this size - they are real quiet when there's no signal, but will nearly count single photons - very nice little tubes. They have a separate anode connection and so run off negative HV, and give a nice pulse that can DC couple right into the preamp input. Nice clean signals without undershoot on the backside of the pulses - great!

We'll just have to see how this design works in practice. I'll be reporting in a few days (I hope). I kind of managed to fry my HV FT in the fusor in doing that massive In activation run, and it may be fine now that it's cooled off, or not (it has kind of self-healed before). If I have to break vacuum and replace the glass and quartz, that will add some time-to-test.

Another variable not yet optimized is how "long" to make the diced up sensor heads. The commercial ones are about 1/2" long, the theory I think being that longer than that and the plastic has moderated any neutrons below the energies that will give useful further light in the phosphor from knock-on protons. If that's the case, I get almost 8 heads out of that one big blank I made (saw kerf might cut that to 6 or so, and I'll probably bork one doing this).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: DIY Hornyak scintillator build

Postby Doug Coulter » Sun Oct 30, 2011 8:33 pm

I got a new camera to play with, just learning how to do things with it (and gheesh, cut down the size of these huge movies so they don't take an hour to upload 2 min worth). Sorry about my ugly mug in this one - perhaps I should put in my teeth and clean my fingernails? Nah, the star here is the Hornyak blank. Enjoy!
http://www.youtube.com/watch?v=KC8Tq9yd7W0
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: DIY Hornyak scintillator build - first light

Postby Doug Coulter » Sun Nov 06, 2011 4:23 pm

Got this tested a little, not very well, but not the fault of the Hornyak. I really did fry my HV feedthrough on the fusor, so for real tests that's got to be fixed first - I was only able to run in a low-output, unstable, mode for this "first light". Since I just put the thing in simple kludge for the phototube light-tightness, no preamp inside, I had some noise issues, but some experience can easily tell one which pulses are "real" - though you might not believe me if I took scope shots and published them - it was fairly bad due to the intermittent arcing going on. Of course, one big clue is "does the waveform go negative and then decay back to baseline exponentially, rather than just be a burst of +/- noise at 10x faster rise/fall times...

This looks comparably sensitive to the "official" ones from Eljen. The button was cut out of the master at 1x1" by .66" thick, so I should be able to get 6 total out of that master if I don't bork any sawing and polishing them. I didn't coat the outside with more resin/phosphor like they do the official ones, being in somewhat of a hurry to see if this was going to work. I just polished all the faces and wrapped it in space blanket for reflector, and black tape to hold it together, using silicone coupling grease to the photo-tube face as I usually do. Once something is known to work right, I use an optical glue that reminds me for all the world of crazy glue, even though it's a two part mix. Crazy glue also seems to work fine, if you get your positioning right immediately(!).

So the next thing to do is to build up another preamp like I used for the other version of this, get some Al tubing to fabricate a non-magnetic housing, and put the preamp in the housing with the tube so you put in power, and out comes a nice ttl (at least) level signal, and we're rolling with this one. I suspect they won't all be exactly alike, they are pretty uniform for a hand job without fancy jigs, but...

At any rate, I can calibrate them off the official one we calibrated here and at Richard Hull's now, so that won't matter very much - and this gave plenty signal even when the other ones were reporting pretty low fusor output, so it will do for the fusor hobbyist, though it probably won't see a very low output, as from a Po/Be source out of the cosmic rays very well.

Should be a nice adjunct to the standard counter - on even a fusor barely limping along it's way out of the noise, so it should be good even for someone just getting going on neutron production, easily seeing as low as 10k neuts/second out of a fusor when near the outer shell.

Now I gotta break vacuum on the big boy and cut some glass and quartz to fix that FT. It actually shattered in there....not good. I may try a mod on the design to protect it better from stray ions, which seem to be what's doing it in. I'll report more on this project once I have a really nice neutron source going again...
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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