Floor detectors

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

Floor detectors

Postby Doug Coulter » Sun May 29, 2011 3:42 pm

Bill recently scored a number of large flat ionization chambers (kind of) that were originally mounted on the bottoms of carts to check for contamination of floor surfaces in places that handle nasty things. As such, they came with very thin al-mylar diaphragms, and being old, they leaked. They were designed to run at STP with flowing gas, and maybe hold a charge for an hour, but these were pretty beat up, repaird with scotch tape etc -- which of course blocks the alphas anyway. So I put a new thin Al diaphragm on one for testing it further w/o alpha sensitivity. It was no big trick finding super thin Al foil -- go to the grocery store and get the cheapest stuff, which is about 1/5th the weight per square of the good stuff.

Here's what they look like:
FloorDet.jpg
Det with new diaphragm.
OldDia.jpg
Old diaphragm


At first I kinda turned up my nose at these -- I'm real space limited here, but then I started thinking about what could be done with them. Even with a thick plate there (so it wouldn't leak and you could seal it off) you've got a large area cosmic ray detector for background anti-coincidence things. And -- these have 5 long .5 mil or smaller wires, so they can be used as proportional or geiger counters -- you need that thin wire to get the avalanche field near it. It's so fine I can only sometimes see it if things are just right and I'm wearing my 5x stereo magnifier - it's far thinner than the 1 mil wire I have in stock.

And, it's an ideal place to play with gas mixes. I tried various things -- air stinks. Ar+CO2 is a great counter gas (welding gas mix) at STP for example. A + CO2 + butane? Have to try it.

So this lead me to think we might try with hydrogenous gas and have a neutron-recoil detector that wouldn't need a moderator. Or maybe even more fun, coat one plate with uranium or a compound and make a fission detector - they are just the right size for that - you want a gas path length shorter than an alpha range so it's easier to tell a 4 mev alpha from 70 mev of fission products - and these are only half an inch thick, with the wires in the middle of that. So, stay tuned, I may have some super cool tricks coming off these. I've already run one in unmodified form with the above welding gas and it works quite well - nice big signal, and you can run it up into the gas-gain region and get big pulses out, always a bonus.

Edit:
CarlW shows how to plate U oxide on things here:
http://www.fusor.net/board/download_thr ... 1203413452
And here's that other paper on the other technique for this, found on the web.

I've already tested one of these in its leaky state with various sources and the original diaphragm -- they'll be plenty sensitive enough to make a large area fast-neutron detector. U 238 will fission on any neutron over 2 mev -- and we have 2.5. The oxide I have has both isotopes in it...
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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Location: Floyd county, VA, USA

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