New neutron moderation oven build

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New neutron moderation oven build

Postby Doug Coulter » Thu Jul 17, 2014 6:42 pm

Well, with all the new shielding, and the difficulty of handling heat, and leaks near where we have to kind of have them to let some out to measure, I thought I'd do a new take on the old neutron oven. This one will be easier to shield around, and will help with the heat issues as well - at least for the top half of the fusor, I'll still need a fan blowing across the bottom half - and that will have to be converted from a "hole" to an "elbow" so as to stop the leak through the original (small but straight) hole. At any rate, Carl Willis once computed for us the thickness of the initial layer, assuming it was HDPE, to get a neutron energy spectrum centered around the big resonances in silver, indium, and gold. It was ~~ 1.5". This version will use water plus ethylene glycol (pure, not antifreeze since I have no idea what the neutron capture characteristics are of those secret ingredients it contains), and will allow for boiling and re-condensation to help keep at least the top half of the fusor not much above 100c. It will be made of copper, and silver soldered, with an exhaust tube near the top going to a vertical condenser about 5' high made of 3/4" Cu pipe - the idea being to let that condense anything the boils and let it drip back in there. Since water+glycol isn't going to be quite as good a moderator as HDPE (oxygen is heavier than carbon) - I made the thinnest part 2" for this one. The top will be plain old HDPE, which I now won't have to worry about melting and so forth - and encased in some of our thickest lead (capture gammas, power supply X rays, and so on). There will still be some getting out, but it's far from the operator position at least.

Here's where it starts:
NewNOven.JPG
The sides for the new oven sitting on a piece of SS pipe that's just like the one in the fusor, ready for initial silver soldering. I haven't put in the hole for the elbow for the copper pipe yet, it's pretty floppy now for that.


More when I have more. I'm alternating between writing software (data aq, scope control and so on) and machine projects depending on how I feel that day.
As usual, there are about 10 things "in progress" so I always have something to do that matches my current motivation. Or almost always...
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: New neutron moderation oven build

Postby Doug Coulter » Fri Jul 18, 2014 6:51 pm

Another day, a little progress. I cut the bottom piece, shaped it, and soldered it on. Silver solder for some spots, hi temp solder for the rest, and I'll probably go back and touch up leaks with regular plumbing solder. I thought I might want the strength of silver solder and the high temp stuff, as regular solder gets weak fast when hot, not that this will be under more than about 2" of water pressure. Still to do is add the copper angle fitting (there will be a tubing straight up from this for filling, and condensation so it will drip back into the thing), and of course, the top (of this part, which is the bottom of the entire thing).
While far from perfect, this will be a lot easier to get the lead tight fitting around to stop the other X rays from leaking, and simple to make a lead-encased HDPE top part for, with our usual silver sample inbetween. Actually, we may have to move to indium or gold with the new output levels we're getting (way over 10m neuts/second is now all too easy for one's health). The odd RFI/EMI in the shack means we need to retain this so as to be able to trust our "quick feedback" detectors we log with a computer, along with results of activation. If it doesn't all agree - we know what to trust, and it's this. Of course, there will have to be a calibration phase here...a pain, but worth doing.

Here it is, sitting right over the place where the action is. It should also help keep the fusor shell cooler - I may even put in some heat xfer compound between them as once in place - it's going to stay right there anyway.
NewOvenFit.JPG
Fit on the real thing. On an identical piece of SS pipe, there's not a lot of light between them - it's a decent fit despite my sloppy work.


A closeup. I have no idea whether we gain much of anything (I doubt it) neutron wise from the long sides, but they should help with cooling if nothing else, and it being rectangular (well, nearly) will make putting the lead around it that much simpler. The 2" of moderator (ethylene glycol/water) won't quite get neutrons to thermal here - but it will get them down to the 1 volt range where the silver and friends are most sensitive. A thicker top cap of HDPE will thermalize any that happen to reflect back into the sample.
NewOvenClose.JPG
Closeup, I hope I never see the innards again. A few cuts from sharp burrs will do that for ya.
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: New neutron moderation oven build

Postby Doug Coulter » Sat Jul 19, 2014 5:13 pm

A litle more progress today. I've been making extensive use of the fact that I have solders available with very different melting points (protip) to do parts of this. Most structural integrity stuff was done with BAG-7 silver solder. I used high melt (mostly lead, 1% each tin and silver) for most of the soldering where sheer bulk was required (priced BAG-7 lately?). I then used regular plumbing solder (50-50) to fill in any little bad looking places. I will use that to solder the other fittings and the top, once I get there. While I solder pretty well, both electronics and plumbing, well, I just don't want to have any leaks or any leak testing process to speak of - time is money, hence the permatex blue (about the only kind you can use with copper, the ammonia the other types give off corrodes it and destroys any seal).

There will be a removable couple foot long 3/4" copper standpipe, then another few feet of quartz tubing (not because it's quartz, it's just the only clear stuff I have that fits right) to the ceiling, where it'll be cable tied to a screw sticking out of a joist. That way, I'll be able to see any evaporation/condensation that happens and know if things are getting too hot. I'll stuff some stainless steel wool into the top end to keep stray insects and so forth out of it.

Once the Permatex cures, I'll leak-test the bottom, solder in the threaded joint so the standpipe can be removed when wanted (filling, level check etc), and solder the top on. Then I get to make the other half, which should be easier. Just a bunch of 7" by 4.5" by 1" stacked HDPE blocks inside thick lead which will stick down to over-lap this part and keep the gammas down. I'll probably make that part 4-5" thick to get all the way to thermal neutrons reflected back into the sample. This should be a pretty good "oven" for silver and other activation "baking". And it might just help keep the fusor shell cool. We see some fusion at the tank walls when the shell isn't too hot - any is appreciated, and before we had to seriously limit (runtime * power) to keep the old oven from melting and stinking. This has a lot more thermal mass, and a way to dump heat as well. We'll just have to see.
Oven_front.JPG

Oven_back.JPG


To make the 90 degree angle fitting more robust, I actually made a too-small hole in the copper, then "flanged" it by driving the right sized peg through it while it was resting over that hole in the piece of wood in my vise. That one's mostly silver soldered as well, so that joint won't do something funky when I plumbing-solder in the threaded fitting.
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: New neutron moderation oven build

Postby solar_dave » Sat Jul 19, 2014 5:51 pm

haha pretty cool sitting next to the Perl bible! LOL

Wish mine looked that new and clean, the pages are starting to fall out!
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http://phx-solar.no-ip.info:8081/Footprints.html
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Re: New neutron moderation oven build

Postby Doug Coulter » Sat Jul 19, 2014 6:59 pm

I hoped someone would get it! The secret is sharing the wear around a lot of perl books - I think I have a stack a few feet thick. I admit that Mastering Regular Expressions makes my head hurt, though. They are here because one of the other main projects I'm working on is a remote control for the new scope, and of course, perl is the ideal duct-tape for things like this. Though I could whine just a little. The recent explosion of new desktop "ideas" in linux - eg make it all look like a tablet, even though I have a mouse with buttons, a real keyboard and a screen that isn't touch sensitive, has left CPAN a bit behind - GladeXML won't eat the output of the newer Glades (a GUI layout tool for those who don't know), which come with all the newer distros (this machine is runing Mint-mate since it's about like the old days, the way I like stuff - and customizeable out the wazoo). But the Glade on it makes incompatible XML GUI layouts that perl can't eat. So...virtual box to the rescue, with ubuntu 10.04 (also before they screwed up the UI) and that version of glade is where I'm developing that - which in turn, runs just fine on any newer distro - or windows if you're a masochist and want to install all kinds of stuff (perl, gnome...). 32 or 64. You gotta love perl...it's not Larry's fault that no one bothers to try and keep up with all the new UI's most of which are gonna (actually, have, why is there Mate, after all, or Elementary) fail miserably anyway.

Software makers know that most people use whatever's on their computer from then on, and don't change the opsys. PC sales are down, mobile up. It hasn't yet occurred to them that there's a perfectly good reason - PC's have been more than "good enough" for a long time. Just like the color TV market, at first it was growth, now it's just replacement. Doesn't mean they aren't out there (might be over 20 of them here, haven't counted lately). I have a nice Nexus 10 - and screw typing on that or developing anything, it's great for consuming content, but horrible for making any. Which is what I do - I'm a maker. And where in heck is right-click on a tablet? It's kind of hard to give that one up.

I even buy new PC's now and then (well, the parts, then assemble them - probably doesn't show up on the software guy's radar, just the hardware parts makers). This one's an Intel NUC which I just adopted and it's truly fantastic how far things have come. It boots virtualbox and ubuntu 10.04 in ~9 seconds, and yes, I just tried it while having a few things open in this 64 bit Mint while doing it. Add to that - 9w power drain while doing a video chat, less at idle. The monitor draws well over twice that (because I'm too cheap to get a LED one till this one ages a bit more). The upshot is, I finally can have an always-on computer on my off-grid system and just plain not notice the extra drain. Most of the time the monitor is asleep, drawing diddly itself, so...I was able to save that much by replacing my old-tech whole-house led nightlights with newer ones...Man, if I do the same for my crawlspace vent fan (save some power via more efficient fan) then I can have another NUC!
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: New neutron moderation oven build

Postby Doug Coulter » Mon Jul 21, 2014 6:35 pm

Not much, but some. After putting on the top last night, I let it sit full of water all night - no leaks at all, even with some pressure, including around the top where I hadn't used permatex.
Had homestead chores today, so I only got the top and sides made for the top part. Front and back soon. The HDPE will be screwed in via the handle holes, and through the sides for the bottom piece, so I can lift this part off the sample, which would be sitting on top of the copper. I'm going for a low-rad diet here, so I'm not skimping on the lead anywhere - especially here, where "the action is" - this is only one of several layers of shielding back there - but this one I have to be able to get to quickly for measurment of sample activation, at least if I keep using silver (I may "graduate" to indium or gold, since our output is now so much more and expected to continue on that trend).
OvenTopnSides.JPG
The remove-able part of the moderator assembly, which will also have some X ray shielding. This won't do much for really hot gammas, but less of anything is good.


Obviously, the front and back will have cutouts for the fusor tube, as well as various clearances for the pipe and the fusor flange bolt heads...that's going to take time to get done, just a lot of fiddly bits and fjords.
I realized that with this design (after I did it, it wasn't planned) that we can have our choice of hotter or more epithermal neutrons via just putting less water in there...cool, or well, warm.
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: New neutron moderation oven build

Postby Doug Coulter » Sat Jul 26, 2014 3:05 pm

Getting there, and now it's seen "first light" and it seems to work just fine. It would be better if I had more-intimate thermal contact with the tank, working on that, and need some magic heat-sink grease or the moral equivalent of it (but something that won't creep away).

Looks like this now:
NewOven_NUC.JPG
New oven in place. That top is HEAVY due to the thick lead, and a bit sticky to the bottom, so I painted it all with graphite paing and greased it. Still going to want a pulley for that rope instead of a simple loop stuck in the ceiling.


Tiny X ray leaks around this are nasty. I'm going to have to make some more leak stoppers that will still allow the top to come off for sample access. But it worked well on first light, can't really tell any difference from the previous one, which is good, I think I can assume the calibration is about the same.
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Re: New neutron moderation oven build

Postby johnf » Sat Jul 26, 2014 8:36 pm

Doug
You could use sil-pad to help heat flow
you know the grey silicon rubber washer material that all use to replace the mica washers on semi-destructors
you can get it in big sheets, dodgykey or similar should have it
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Re: New neutron moderation oven build

Postby Doug Coulter » Sat Jul 26, 2014 8:57 pm

I've got some of that stuff. I may not be compressible enough for this low-force application, but I'll try it. Bill suggests high temp epoxy (use a piece of wax paper or saran wrap to keep it from getting glued to the fusor, then remove that). Almost anything is better than an air gap...
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