Directional Neutron Detector

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This is for tales of woe that teach. We often learn quickest through failure that shows us how our thinking was off, and of course, sometimes it's funny at least in hindsight. Share your "Doh!" moments here.

Directional Neutron Detector

Postby Doug Coulter » Wed Sep 01, 2010 11:28 am

Well, it was a nice try, but....now that I'm shaming myself here I've got to get this working so I can show how it's done correctly too.

I had this thought that it would be really nice to be able to image neutron sources, even if only one sloppy pixel at a time, and I'm not alone in wanting to do it (even here I think, as I got going on this due to something John Hendron said). Since any moderation scatters neutrons by definition, I figured I would start with a fast neutron detector that is known to work, and selected a plastic scintillator/phototube combination I'd tested as just a straight fast-N detector with success. That part was and is fine, though I could use a much thinner piece of scintillator for just neutrons, the one I built is thick and real good for gammas too -- no need for that here. In fact, it's a problem, a large one.

OK, so I have a fast-only neutron detector. I know no way to actually image or focus fast neutrons (still) so what I figured I'd do is simply stop the ones from whatever direction I didn't want to see -- a hole in a neutron stopping pipe, more or less. The size and length of this hole represent a tradeoff between resolution and sensitivity, and based on my flux range and how well the detector worked "naked", I chose about a 1/2" hole 6" long as being somewhere in a useful range. So far so good. I then determined I would use alternating washers of borated wax and Cd alloy washers to stop the neutrons not going my way -- huge mistake. All neutron captures in these materials make gamma rays, and my detector sees them fine -- some of them are high enough energy to fly right through a fair thickness of Cd alloy, or the cerroshield I used around the tube (which itself has Cd in it, rats).

I will now have to build another (lucky I made the tube/scint easy to remove as a piece) from scratch, w/o the boron or the Cd anywhere around it. The resulting thing had zero directionality, and mean none I could detect at all! It did count....

For reference (some of this is from a recent thread on fusor.net) the numbers look like:

Cd113+n -> 558 keV gammas (source -- google search)
B10+n -> 400 keV (Carl W)
H+n -> 2 MeV (Carl W)

And of course, in a fusor, there are these few truly high gammas from that third DD->He reaction, rare but there. The Q on that one is about 16 MeV, don't know how hot the gamma is there, but it's got to be way up near that -- and therefore take some very thick shield to stop.

Zowie, the last two are going to be real hard to stop. I am now wondering what could be done with carbon, and what if any gammas that may produce if/when it eats any neutrons rather than just scattering them. I'd suppose this would be low, as they were able to use it in moderators for the original piles with un-enriched U, correct? I guess that really hot gamma from real fusion might be OK to let through, because it's fusion I'm trying to image anyway -- but - what if that reaction tends to happen under different conditions than the neutron producing one?
That would indeed be a piece of data I'd like to have too! Might need a special design just for that one, separately.

XRayShielding.gif
X ray shielding from a medical reference.



I've been doing some fishing around, and while carbon also emits some hot gammas when it captures a neutron, it also has a far lower capture cross section than hydrogen, so I will use graphite rod, drilled, for the next attempt on this one.
http://www.nndc.bnl.gov/capgam/indexbyn.html
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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Doug Coulter
 
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Re: Directional Neutron Detector

Postby Remy Dyer » Mon May 28, 2012 1:15 am

Maybe you could do it the way gamma-ray astronomy is done, and use two detectors lined up, and a coincidence counter?
Place one close at the "aperture" and scan the other around at the "imaging plane" and just accept counts where both go off at the same time (accounting for the distance between the two if you can, probably a delay line?).
Could probably make a xy scanner a-la the whiteboard drawing robots, using two lines attached to controlled reels. Have the detector hanging in between the two reels, and move it about by controlling the lengths of the two lines.
-- Remy
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