SNM-10 pulses

This is bound to get mixed up with things in Electronics, check both. Physics-specific stuff here, mostly.

SNM-10 pulses

Postby chrismb » Sun Aug 28, 2011 9:51 am

I have two of the SNM-10 B10 lined corona tubes, and tried them out with the same setup as in viewtopic.php?f=11&t=430&p=2756#p2756

The noise was approx twice the height of the same circuit running on the SNM18-1. It was difficult to threshold and problem-solve because, unlike the SNM18-1 that has around 6 counts a minute on background only, this only seems to fire 3 counts an hour.

The pulses are shorter;

and much less frequent (by factor 100);
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Re: SNM-10 pulses

Postby Doug Coulter » Sun Aug 28, 2011 12:18 pm

That will of course change with a moderator. A large one will convert more incoming cosmics to knocked-out neutrons to detect, and more of them will be at energies that have a high cross section in the B10 reaction. Try it!

One reason people don't usually use things like that Li6 detector you mentioned at without moderators is that they have a much larger cross section at thermal energies than for fast neutrons, but lack the ability to tell you what energy each count happened from; there's therefore no way to weight to the cross section curve. You'd think, fine, I can adapt/calibrate for that. But reality is that there is no no-moderator situation on the planet! The air itself is a moderator. Your body is pretty good if anywhere near, the table top, and so forth conspire to add serious calibration errors when any of that changes at all. Don't move that coffee cup (or pint of stout)!
Just because stainless steel (or lead) stinks as a moderator compared to dense hydrogen compounds doesn't mean it doesn't have an effect that is quite noticeable.

So if there are fast neutrons about, one either uses a fast-only detector (knock-on protons in scint plastic or similar scheme) or a fully moderated one that then detects the thermal neutrons, and is always working at a particular narrow band of energies so the cross section variability doesn't come into play much. At least then the position of the operator in the room doesn't make for huge sensitivity changes...It's quite a large effect, and my neutron metrology books all make the joke "for accurate results, first remove all the air from the lab". Some even mention the use of helium filled balloons to "subtract" neutrons from beams at certain energies where the He has a big cross section. Tricky stuff!

Lucky for us, no working fusion device needs anywhere near "limits of detectability" neutron instrumentation, except maybe to get it basically working at first. Even a crummy fusor will make 100k/second or more. It only gets difficult when trying to get past single digit millions/second output from a fusor - which is getting well into trivial to detect and starting to become a safety issue instead. None of these counters really see my little 30n/sec source, (which is getting near the limits of easy) but they all work great on a fusor, even a somewhat malfunctioning one.
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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