Wax moderators and casting info.

[Richard Hull]

Tom Dressel's can/wax moderator is really nice.

For those casting parafin. You should use a double boiler melter and NO OPEN FLAME ALLOWED!!

Molten parafin is a superb fuel similar to kerosene! It will flash to fire as the vapor is instantly flammable.

You should heat water on an electric grill, stove, or better still, a hot plate out of doors. In the hot water, float a second vessel with the wax to be melted. Have a garden hose at hand and a large can of sand or dirt incase of fire.

Wax, in bulk, is best purchased on the internet. I went to "sissy's candles" to purchase my 100lbs of parafin for about 39 cents/pound. (1999) Now, you might try for bulk buys...

http://www.candlewic.com/Store/

I plan on setting up my new little BF3 toys with HDPE barriers and wax barriers in a test to determine optimum thickness. Stay tuned on this, I move slowly.

My guess is 2" to 4" will be optimum for fast neuts. (Based on old eberline unit I own.)

Richard Hull

[Richard Hester]

I paid a visit to candlewic.com, and found it very interesting that there were several grades of wax available, sorted by melting point. My inclination would be to use the higest melting point wax, even if it is a little more expensive. The higher melting point wax will be a little more durable (though a touch more brittle), and will have less of a tendancy to soften up on hot days.
The assembly looks nice and businesslike painted flat black, but don't leave it out in the sun...
Battleship (primer) grey might be a better choice. For better scratch resistance, do all the mechanical things and paint the can before the paraffin is added. A hour or two in the oven at 225-250 degrees (Fahrenheit) with the door cracked open slightly will bake in the paint and make it much tougher. Of course, your spouse might not be too happy with you stinking up the house, but that's life...

[Tom Dressel]

That stuff about no open flames should be taken seriously. The box that the wax came in, warned aganst melting the stuff on an open flame, hotplate or oven!

Tom Dressel

[richnormand]

Found a few informative threads about wax or parafine as moderator. Just got my Russian corona tube today ( 0.5" diag by 5" long) and I am planning to use a large coffee container to contain the wax. I found enough candles around the house to fill at least two of these 6" diag by 8" tall cans. I'll use a hot plate outside to melt them all.

Questions:
1. They have various colours. Is there a possibility the coloring agents could have heavy metals or ingredients that would kill the neutrons.
2. Is borax added to candle parafin?

[Frank Sanns]

No borax or heavy metals. Candles are made to burn indoors and not poison the inhabitants. Organic dyes for the color.

Frank Sanns

[richnormand]

That was my impression too, but but wanted some feedback just in case before casting several dozen old candles to their new role in life.... after rapture.... tomorow.
Thanks.

[Richard Hull]

You would only ad borax to candle wax if you wanted to make a neutron SHIELD, (stop neutrons). For a moderator, it is tough to beat water, (cheapest), Parafin wax, (solid, but fully castable) or Polyethylene, (A little more costly, but easily sawed.)

Carl Willis posted a while back on the normal thicknesses needed to obtain thermal neuts from fast fusor neuts. In general, ~3 to 4 inches of water, wax or poly will do the trick.

UHMW Polyethylene is safest and not a fire hazard, (wax). You don't have to worry about a leaky tank or spills, (water).

I use water for my 3He detector as this detector "head" is fixed and unmovable. (To retain some sort of rather loose calibration in relation to fusor runs.)

Good luck on your efforts.

This was, indeed, an old resurrected posting.

Richard Hull

[richnormand]

Thx Richard.

"You would only ad borax to candle wax if you wanted to make a neutron SHIELD"
That is why I asked if borax or other critical impurities were somehow introduced in commercial candle making as this would have defeated the purpose since these corona tubes are not that efficient (but cheap). The whole thing will be poured in a large coffee grind metal can with a central hole for the detector with a metal top cap and a connector.


"This was, indeed, an old resurrected posting." Ya: some oldies pop-up using the search feature. That’s great as many are still pretty good for information and wisdom. Something I appreciate as I am getting older too

[Richard Hull]

I have seriously considered getting a UHMW cylinder for my 3He detector so that it can become somewhat luggable, but the local plastic place gets about $125.00 for the size I need and it would still have to be bored out. I may just break loose with the coin and snag one, yet.

Richard Hull

[Chris Bradley]

I have considered LDPE sheeting - waterproof membrane type - and wrap your tube in it.

LDPE is about the same density as UHMWPE, and both are lower density than HDPE. All are much-about-the-same [despite the names], depends more on who's made it. [LDPE can be found in grades (0.91 up to) 0.94g/cc, which is what HDPE is.]

You'd have to wrap tight to avoid the air gaps in the layers, of course (use a former tube to drop the 3He tube into so you don't crush it), but if you get the thickest grade I don't see why it'd not work out and be much the same sort of size. Also be softer and will protect the tube against shocks, maybe. The thickest stuff also means you aren't spending your life rolling it!!

Better still, buy a roll already rolled factory-tight, with the right core diameter in the middle of it so you don't have to bother rolling it at all, and just hack-saw the whole roll to length.

[Richard Hester]

I don't see why a little air space between PE layers would be a problem for the rolled PE sheet option, as long as the requisite thickness of moderator is in place - an interesting and cheap solution.

Another experimenter a few years ago colllected junk CDs to use as a moderator.

I got my HD/UHMPE cylinders on Ebay. I think it was Online Metals that had them. They were reasonably cheap at the time, but I think you'lll pay close to $100 for a foot of 6" diameter rod nowdays. Stacking up pieces of polypropylene sheet might be another way to assemble a moderator.

I just got my Russian boron-lined corona tubes yesterday, and they look to be the right size to use with a coffee can. I'll do a little more checking later on.

[Doug Coulter]

For what it's worth, I used wax for my big He3 tube here, and it works fine cast into a 6" PVC pipe (even though that eats a few neutrons) with a 1" PVC pipe used to make the "hole" for the tube. I put the hole off axis in a misguided attempt to see if that would tell me what the ideal thickness would be, but due to the nature of things, rotating the assembly vs the fusor showed basically no change. I use HDPE now for everything, but since that one works fine, and has almost too much sensitivity for my big fusor (pulse pileups become an issue) -- kept that one the way it is.

One thing of note -- paraffin shrinks quite a lot on hardening, and keeps shrinking for many minutes after that. You'll get a pocket and even voids if you try to pour a lot at a time. I overcame this by doing small pours (required by my limited melting pot size anyway) and making each pour hot enough to re-melt the previous one some, since it was still warm itself.

If you use HDPE, don't get the UMHW variety of it - there is no advantage and it's the very devil to work with. Razor sharp Japanese hand saws just skip over the stuff, it melts and seizes in my metal bandsaw, and you have to be real careful about frequent chip removal when drilling or you're going to lose a bit and the workpiece when it freezes in there.

I wound up cutting it with a chainsaw, which it nearly ruined with molten chips getting in the clutch.

And, the stuff is indeed pricey. I bought a 3 foot length of 6.5" diameter stuff at McMaster and paid something around $300 for that. It's a great moderator, make no mistake, but a real pain to fabricate.

I wouldn't think tiny voids would make much difference, but if they make the thing bigger, you'd lose more neutrons scattered out of it. But for future things, I think I'll go with smaller pieces that are easier to handle and just stack them up. For example, 1" thick plate is easy to drill a hole in the middle of, and just make enough to stack them the height of the tube -- modular and reusable. Other things, like just building boxes inside boxes seem reasonable and avoid how hard it is to chew through a really thick piece.

FWIW, a spade wood bit is the thing to drill this stuff. A twist drill will sometimes grab and pull itself in like a screw, very fast -- and it's way not easy to get back out. Forstner bits work, but have other issues of being easy to get frozen in there.
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I did use an open flame to melt my wax (propane turkey cooker outdoors) but I was very careful on heating rate, and -- this is important -- had ready a tight fitting cover for the melting vessel to quench any fire that might have started -- the amounts that are practical for one pour aren't that big a danger in my estimation if the practitioner is skilled and proper precautions are taken. I have a 30lb bottle of CO2 right there as well. I used an IR thermometer to watch the temperature.

BillF found us a large load of "used" candle wax, but let me warn you - don't get the scented stuff, not that it eats neutrons, but it makes that sickening smell (nasty when it's loud) forever in the lab. You'll dread warm days if you use that stuff.

[JohnCuthbert]

I'm glad someone has pointed out that melting wax with a flame isn't going to instantly turn the whole neighbourhood into a burned wasteland.
If anyone asks nicely I will send them the video I just took of me melting a large (about 10 cm diameter) candle directly with the flame from a propane torch.

You really can point a blowtorch flame directly at a candle and the only thing that will happen is that you will light the wick.
It's not a good idea to do it inside. A double boiler makes sure you won't scorch anything. Considering that this is a group of people who plan to do nuclear physics, I wonder about the warnings in this thread.

Anyway, the polythene sheet is a neat idea. you could also wet the polythene with oil and fit the coil into a pipe to get rid of air bubbles, but as far as I can see, the air won't scatter neutrons nearly as well as the polythene, and that's the whole point of a moderator.

[Richard Hester]

The point was, the air won't help, but a little won't hurt. Winding the sheet tight will make it easier to handle. Melting the edges on one end together will help keep it wound up tight.

On another note, I did a short stroll on the web, and polyproplylene sheet is not all that expensive, and available in thicknesses up to 2". It'll probably be a little easier to cut and drill than polyethylene. A modular moderator made of drilled polypropylene squares or donuts might be a viable approach. Finding the polypropylene as scrap is also a possibility, as it's commonly used in fabricating things like chemical containers, plating tanks, etc.

[Chris Bradley]

My comment about winding it tight is merely that if you try to do it by hand you'll find that the 'roll' can end up twice as wide as the equivalent thickness of solid moderator! So '4cm' worth of 'polythene sheet' moderator ends up 20 cm wide in total (once your tube is in the middle of it as well!) which is a bit of a pain handling and positioning it!

[Doug Coulter]

I believe Carl Willis stated that HDPE gave you the most H's per CC, eg the best moderator with a given surface area/volume, which I took at face value. He ran the simulations on some fancy software he's got just for that sort of thing. And thanks again, Carl, the neutron oven you helped me design; works just great here!

In practice, I can tell that PVC isn't as good -- the chlorine in it eats some neutrons -- my first activation oven was made of it, and things really improved going to HDPE for that, in the correct thickness calculated by Carl to get most neutrons into the resonance-capture region for the things I activate (Ag, In). Any non-halogenated hydrocarbon seems to be fine unless you're fighting for that last fraction of a percent.

Wax is noticeably not quite as good as HDPE, but barely (less dense and more carbon per H I believe).

The bigger the volume needed, the more surface area to allow neutrons out, as well as in (and lower flux of slow neutrons inside it), and the more atoms needed, the more chance of a neutron capture in them. Or at least, that's how I understand the situation. I do seem to see some hot gammas from neutron capture near my moderators, but I've yet to do a "real" measurement. My fancy huge NaI really doesn't want to find itself activated! The smaller one I saw this on doesn't handle 2-something MeV photons well, so the results require more interpretation. All I know is I see one peak height on a scope that corresponds to power supply volts when there's no fusion, and a hugely larger peak height during fusion, consonant with what capgam reports for neutron capture in H.

Whether the hot gammas come from the moderator, or just the fusion reaction is open to question based on what I've measured -- maybe someone else has made the determination better than my puny and short lived single attempt (which was mostly accidental, I was looking for something else).

One might guess the ideal moderator would be a frozen block of pure deuterium (much rarer neutron capture than plain H), but I'll settle for the plastic or wax, both of which work well here.


One thing I've noticed is that it's not a very good solution to just move a neutron detector farther away when it's starting to become saturated with too many neutrons from the fusor. At that point, other moderators in the room (like your own body) get into the act more, and just walking around affects the readings....

[Richard Hester]

I did a search, and polyproplyene is very, very slightly inferior in density to the two hot-shot polyethylene options, indicating that it will be pretty much equivalent to polyethylene as a moderator. The difference is the methyl side chains, which make polypropylene stiffer, with a higher melting point. Reportedly, it's much easier to cut and drill using bits and blades for woodworking. I bought a sheet of 3/4" thick polypropylene on E-pay for a relative pittance, so I'll be finding out sooner or later if the stacked and drilled disc/square is a good option as a moderator. I might use it with my He3 corona tube, as it is relatively short, compared to the monster He3 tubes other folks have.

[Doug Coulter]

I'd bet it will work great for you. I just tested one of those Russians in a too-short block of HDPE (I'd cut it for a shorter b10 Russian tube) and it works fine. Remember, the reduced density is on top of having a higher carbon to hydrogen ratio -- but it's not going to matter much, in my experience.

http://www.coultersmithing.com/forums/v ... f=11&t=348 (moderator build)

http://www.coultersmithing.com/forums/v ... f=11&t=358 (Russian tube tests and preamp)

Note on the scope traces that I'm getting nearly the same count rate on the little tube as I do on the big 22" one only a couple inches farther away, in wax/PVC....nice, and the price can't be beat. Also far easier to lift!

(note, if anyone objects to me linking my own forums, I'll quit -- it's just the easiest way for me to "give something back" to this place that helped me get going. We're not trying to steal members and in fact aren't taking any now unless they are already in the neutron club here and they ask me -- doggone spam bots and dumb people forced us to shut down new registrations)

[Chris Bradley]

Doug, polypropylene is a saturated polyolefin, just like polyethylene. Same C:H ratio, 1:2.

[Richard Hester]

That's right, the ratio is the same, as the hydrogen taken from the backbone carbons in order to attach the methyl side groups to the backbone is included on the methyl side groups.

BTW, I just got my 3/4" polypropylene sheet not 15 minutes ago. I'll be asking my neighbor for a little time on his table saw soon (my blade, of course).

[Dustin]

Anyone tried water saturated silica gell?
Steve.

[Richard Hester]

Doesn't sound like you'd get enough water to make it worthwhile...

[JohnCuthbert]

Most of the silica gel I have seen has quite a lot of other junk in it, notably iron, but probably most of the common elements (Na, Ca, Cl etc)

[Dustin]

I was thinking of the stuff you stick in pot plants, That swells to 100 times it's size
and has the consistency of jelly. Not the dessicant ("with the do not eat" that makes you wander what it tastes like)
This may be useful for an adjustable moderator by moving the detector in a tank with less concern of leakage for a water moderator.
It would look cool too as you could see the detector in the tank.
Steve.

EDIT: Sodium polyacrylate, also known as waterlock, is a polymer with the chemical formula [-CH2-CH(COONa)-]n