Shields up!

For Farnsworth type designs.

Re: Shields up!

Postby Donovan Ready » Sat Aug 09, 2014 4:44 pm

Boron or Mo2S3 have large capture cross-sections, but may not be available (enough)... Got any Hafnium? :|
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Re: Shields up!

Postby Doug Coulter » Sat Aug 09, 2014 7:49 pm

Thanks mainly to Bill, I have a pretty decent collection of just about every element (including gadolinium - not much, but some). The issue is more complex. When boron eats a neutron, it gives off a 570-something kev gamma, which due to the way lead absorbs photons, takes way over 10 times as thick lead to "eat" as a 50kv one. I'm using boric acid and borax dissolved in ethylene glyclol 5" thick in my viewport (glass block, hollow), after 2 thicknesses of borate glass, and before one thickness of lead glass - and those 570kv gamms created by the moderation and subsequent capture of neutrons in the boron are one of the big worries here. Other things are worse, but have a lower cross section, so the gammas aren't as numerous (but are hotter, though in general, they come from further off - no moderator to slow them down till the neutrons are ~25 feet away, so square law helps with that).

Like I said, it's a classice multi-dimensiional engineering tradeoff. Do I accept a few neutrons barreling through me where I'm mostly air (lungs) - or do I stop them and then eat hot gammas as a result? Where's the sweet spot in terms of human damage? To me, it looks like slowing the neutrons down - a little - to where the multiplier in the Sievert calculation is lower, but not so slow as thermal (where many things have a 1/V cross section and the resulting capture gammas) looks right, but I've been wrong before.

We have numerous easy to stop X rays - I'm almost done there, at 50kv or so (power supply voltage). We then have some from fast protons etc hitting tank walls (if they happen to hit an iron nucleous or something) - those are in the almost megavolt range. Then there are capture gammas, from 570kev up. Well, if I had a ton of 6Li...they'd be alpha particles, but that stuff ain't cheap. And moderating neutrons slow enough (using usually some hydrocarbon or water) to where the 6Li gets them means that some are captured in H, which gives me a nice sweet 2.2 MeV gamma. It's a hard game to win.

TL:DR - you can't win, you can't cheat, you can only tilt the field so far without going to "heroic" measures, like thick concrete and losing ability to actually observe the thing in person. I've gained so much via observing I'll not give it up easily. So many of the armchair theorists have been so dead wrong when compared to actually testing and observing it's not even funny anymore (and I'm kicking their asses with real results). So I'm loathe to give that up. I'd be much more worried about a couple percent increase in cancer likelyhood in 30 years if I wasn't already 60+. But I am - that'll have to get a digger to get me if that's all it is.
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: Shields up!

Postby Doug Coulter » Sun Aug 17, 2014 12:46 pm

Uh oh. This is getting worse fast. When Bill was last here, about a week and a half ago, we were at half a bubble off level back behind the fusor (and all the new lead).
This is bad. This is sitting on a big X ray supply we've not yet used, too dangerous without finishing the shielding yet...heck, the Spellman SL2KW was already too dangerous, this is twice the volts and current at least.
It appears the new lead, in conjunction with the milling machine, and the X ray supply (it takes a winch to lift the transformer alone) are just too much for my floor.
LevelNOT.JPG
Now off by a whole bubble. Oops.

It now looks like this underneath. Since my normal helper is too...em...rotund to even fit in there at all, looks like I'm going to be doing some crawling, digging, and installing a new pier right there. That beam is pretty bent, and it will take about a year to get straight again if previous experience is any guide. You jack it up more than a quarter inch or so, and the entire building comes off the other piers - it's pretty stiff on a short term basis.
BentBeam.JPG
Less headroom than it looks like over there where it needs work.


Not really looking forward to sniper-crawling with a couple cap blocks, a shovel (super ugh place to have to dig) and a big bottle jack and large angle iron to spread the force well. But looks like I'd better find the motivation fairly quickly.
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: Shields up!

Postby Donovan Ready » Sun Aug 17, 2014 8:25 pm

Oh, damn, I feel for you. I was a builder for a good long time, and the only thing worse than being underneath a pier-and-beam to jack it is having a slab, like we have (mostly) in the Austin area.

Foundation repair companies have too easy a time when the tract houses are built on thin slabs without enough beams or wire and steel tied right. We poured footings with steel in them, P&B on that, and a 100-year warranty. Sorry if that sounded braggish, but nobody has called yet, and that's been 30 years. Maybe they're dead? :mrgreen:
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Re: Shields up!

Postby David McKee » Wed Oct 08, 2014 11:15 am

For a longer term solution, you could build a "box" under the workshop directly under the fusor and other equipment, and fill that box from the top (via a hole in the floor) with liquid concrete. As long as your ground compaction and stability is good enough, this should provide an ideal foundation for any additional shielding needs.

Then again, with the success you are having you might just have to throw in the towel and build a proper underground shielded bunker for the fusor - so you don't start glowing!
"Mad Science" means never stopping to ask "what's the worst thing that could happen?"
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Re: Shields up!

Postby Doug Coulter » Fri Oct 10, 2014 2:32 pm

We've been looking at options, to be sure. I did jack the floor up, though. Good for the moment.

One would simply be to build another building, remote control, and in line of sight so I could still observe via a telescope (cameras just don't do it these days unless you can get full-manual exposure and find a way to keep the gammas out of the CCD).
That might have to happen, but those tend to be really spendy so far - I've not found a hack to use a generic auto-everything camera and make it all manual. The auto stuff just doesn't fly in a fusor - too much optical dynamic range is required, so it totally blooms out the light part trying to show me the dark parts I don't need to see. That is, when you get a picture at all - the gammas make a lot of "snow" in the picture.

Expensive and difficult to build another - not much flat land around here. And it's not like the fusor rack would be an easy thing to move at this point without breaking some really expensive stuff.

However, only about 25 yards away, I have a house trailer that I used to live and work in, that is now used for storage and reloading ammo. I could move back, and make this place "all lab" since it almost is anyway. The trailer would want to have a few small improvements to make it easier to heat...and it has aluminum wiring, mostly bad after being re-done twice (would have to rip out the walls to replace it, or just wire in conduit on the inside, which looks a lot easier) - every outlet box there has had a fire in it, even after I used the "magic goop" that is supposed to make the Al work out. It does, for a couple years, but no longer. If I build a bit of an addon, with another woodstove nearer the bedroom, and good insulation, it becomes a nice place to live, and then the square law gets me the attenuation I need for this. It's a dinky trailer and needs more space anyway...another room in front with a floor around 5 feet lower than the trailer "ground floor" is possible to build, about 8x20 feet, which could have a wood heater in it and it'd be really a nice place, though it has too many benches built against the walls now - easily fixed by removing them. Might be nice to have less of a man-cave 100% of the time, and that trailer is my only building with plumbing anyway. Looks like the lowest cost/energy solution at the moment.

We also do have what amounts to a cave behind that trailer - and would have 30 feet of rock between the fusor and us, but again - hard to get to, would need to be climate controlled, and would need a rather large amount of construction for the front wall (eg cover the "cave" mouth" which is partway down a very steep hill in the woods. Pretty hard to get people to carry cinderblocks etc down that hill when it's hard to even walk down it without grabbing trees etc to keep from going all the way to the bottom of the hill (160 feet). That just looks like too much work and expense at this point. I can't do it, and I doubt even an out of work Mexican would tolerate the conditions required.

The constraint is that I have to do all that, and along with that, increase the remote control/data aq capabilities a lot. Which I'm doing anyway, just a matter of time/effort. I could really use a full time lab or general purpose assistant!
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: Shields up!

Postby Donovan Ready » Fri Oct 10, 2014 3:33 pm

I forgot that you have a real winter there. If I weren't tied up with building a house this "winter" in Texas, I'd come up for the assistant position myself.

How about putting in a pulley system and sledge for moving the blocks downhill?
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Re: Shields up!

Postby Doug Coulter » Fri Oct 10, 2014 7:50 pm

I might take you up on that offer later on. For now, it seems easier to move me. I can walk, the other stuff, not so much.
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: Shields up!

Postby Jonathan Schattke » Sun Oct 12, 2014 10:01 pm

D-D fusion creates a 2.45 MeV neutron; D-T (which can happen with the D-D=>T reaction) a 14.1 MeV.
That's a pretty energetic neutron, and lead is NOT the correct way to stop it.
You want ~30 Mean-free paths through a hydrogen bearing compound like water (MFP ~4 cm for fast, ~0.5 cm for thermal).
If you want, I can do an MCNP shielding verification, but I''d need prints.
Easiest: put a 30-50 gal fish tank in the way. Next best is putting a foot thick slab of HDPE, then your lead.
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