Fast Neutron-Gamma Conversion Detector

[lutzhoffman]

Hello:

The conventional activation neutron detection methods tend to use a moderating system, to thermalize the neutrons, for better target isotope absorption. This got me thinking about some more direct method, which takes advantage of a suitable fast neutron reaction, to produce the betas, gammas etc. While the fast neutron cross sections are much lower, the elimination of the thermalization step can compensate for the lower cross section of the fast neutron reactions. Cu, and Mg is used for this purpose, but the half life of the products formed are to long.

I looked at the tables for fast neutron reactions, and one which stood out was the As + n > gamma reaction. The isotope formed has a half life of only 16ms using D-D neutrons, with a reasonable cross section. The emitted gammas would be easy to detect with either a NaI(Tl) or even a plastic scintillator. The advantage of NaI(Tl) would be a lower sensitivity for direct neutrons, thus it would not be hard to discriminate for only the gammas from the As emission.

An aprox. 1" layer of As should be plenty, I also found a cheap easy source of As at the hardware store. Several vegetation killers contained up to 37% by weight elemental arsenic, in the form an organic herbicide compound, named Cacodylic acid:(CH3)2AsO2H. If the water were to be carefully removed from this product via evaporation etc, then it should contain enough As to do the job. The best part is that the As would still remain in a lower toxicity form, since apparently the As is tightly bound up in the organic compound, so that most of the As does not get absorbed via the digestive tract. First I thought of a way to pyrolyticaly decompose this material to yield a denser As material, but why risk the amplified toxicity, unless you obtained and used elemental As in epoxy, which would seem to be ideal?

The advantage of this method if it works, would be to enable real time fast neutron flux measurements, due to the extreme short half life of the As product. Plus your trusty old NaI(Tl) unit could be used as is, with just a slip on As cover. Maybe someone else could add if this is realistic or not, on the surface it does look interesting though. I am sure there may be better methods, but this approach could offer some unique advantages.

[Doug Coulter]

Well, the reason I activate silver and so on is pretty simple -- you can't fake it, you can't cheat, it always works. As soon as you add electronics to the chain, that's not true anymore
in the experience of many. Noise can get in, thresholds drift, you name it. Activation isn't the prime realtime indicator, but it's the check on all the rest of the gear. I think of it as a checksum or an error correcting code. I count it with a very reliable geiger counter because these are more stable than just about any other rad detector, and mine gets checked regularly with standard sources and background radiation that is well characterized. The half lives of silver and indium are "just right" for this work, they stay hot long enough, but cool back down quick enough to be ideal. Gold for example has too long a half life to see much activation on unless the neutron output was very fierce, or a hyper sensitive detector is used. At this point I don't have anything like an HPGE/cryo detector for things like that.

At least my own fusor is quite electrically noisy (something I am working on, but it's what it is now). Noisy enough to have fried a nearby computer not hooked to it on Thursday last (ow, $300 repair bill) -- the various cables were enough antenna to allow that. In that kind of environment I don't trust electronics for my prime source of information, and in fact, use the silver activation to check the electronics count numbers -- if they don't scale together, I know which one wasn't right. It can be hard to look at millivolt high impedance signals in that stuff, even with what looks like TEMPEST grade shielding. The fact is, that with the odd megawatt-class noise pulse, even that standard isn't even close to good enough. These can be very hard to shield against at the source, as the high voltage needs some "air" between it and ground at some point to avoid arcing -- there's always some inner conductor showing, and my screen just doesn't stop it all, and never will unless I make things such that I can't get in there easily when I need to. And of course, some HF will leak out of any viewport.

I am curious about what the cross section is -- just saying decent with no numbers isn't illuminating -- remember near resonance the silver and indium cross sections are huge, and those are barely sensitive enough to be well out of the noise. You can't use NaI:Tl near a neutron source, the various components themselves get activated. That's an expensive mistake as of course it's hyper sensitive to any radiation produced right inside it.

If you have plastic scint, you already have a very high probability of intercepting a fast neutron and getting a count. Sensitivity seems pretty good comparing a 1 sq in piece to much larger proportional tubes and their large gathering area of moderator, which is what I'm using for "almost real time" measurements that mainly tell me with way I should be adjusting this or that parameter.

In my applications, even the moderation time to a proportional tube is far too slow for the time accuracy/resolution I want -- that gives microsecond-class random errors. A plastic scint on a phototube is more in the few-nanoseconds -- much better if you are driving something in the tens of mhz and wanting to know the *phase* when the neutrons came out. 60 ms (and that's only an average) is far too slow for realtime and too fast for after the fact, so at least in my lab, not ideal. That would put it into the same class as the proportional tubes, but worse.

Remember too that when real fusion is happening, there are some super hot gammas from that third reaction pathway to 4He -- and the amount of lead that stops those also will seriously time-smear and scatter neutrons -- inches thick is what that takes. I've not worried about that terribly much, as it doesn't seem to affect the proportional tubes (they are not great at absorbing very high energy photons), and it still means fusion is happening if it falses a phototube kind of thing -- though if you were going to be doing pulse height sorts of things it would be a big problem.

I'd be curious to know what process this would be good for or problems it solves that isn't already solved better and cheaper another way.
A working demo of it would be interesting -- there's room for curiosity here, but of course this is for people who do things, not just speculate.

[lutzhoffman]

Well in looking for the numbers again to post, I discovered as is often the case, that some one else has been there before, but instead of a "candy wrapper on the ground", I found a nice juicy patent with some real test figures, here it is below. I agree with Doug, about the activation issue of NaI, activating your counter would not be a good thing, and if you ever got anywhere near to the 10e6 n/s level for example, then this would be a serious concern. Am I going to build one of these, not yet. If however the chance presents in the near future, I may put a jug of As weed killer next to the entrance window of my gamma probe, when with some work and luck, I get some fusion neutrons going after we move : )

[Doug Coulter]

Here's a link to some data on the gammas given off during thermal neutron capture.

http://www.nndc.bnl.gov/capgam/indexbyn.html

Now if I can tie that in with capture cross sections....