Bismuth Low Temp Shielding Alloys

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Bismuth Low Temp Shielding Alloys

Postby lutzhoffman » Tue Aug 03, 2010 4:48 pm

Bismuth Alloys: Cerrobend which Doug has used a lot, is just one of these alloys. Another one is the specific alloy used to form the beam shaping blocks for radiation therapy called "Radiation Therapy Shielding Alloy - Bismuth", this one melts at 250 F, and it will reproduce pretty fine and thin mold details, when it is fresh. There is a seller selling a bunch of this stuff on ebay for about $3.00 a pound.

Getting the fresh alloy is worth all of the extra effort, since upon repeated re-melting, and re-use, some component metal in the alloy oxidizes, and the Bi crystals which form on cooling get larger, and greater in number, until it becomes very difficult to get decent castings with slow cooling, or with a large mass. This stuff is real useful for so many things, since you are free to use many types of plastic, as a mold material. In the rad. therapy setting they use styrofoam blocks as mold stock. This alloy also has a very low background radiation count, my most sensitive probe sees nothing from a block of it. Maybe Doug has some thoughts about what metal is being lost due to oxidation? This may provide a method to re-fresh the alloy, when it starts to behave more, and more like pure bismuth. The MSDS on at least one companies alloy version indicated that it contained cadmium, which may in part explain why it works so well as a gamma shield for fast neutron probes. Taking this into consideration propper hand washing etc. would be prudent just like for lead.

One thing about these alloys is that when you have it around, you tend to keep finding new, and more uses for it, so on several occasions: What I had thought to be a sufficient stash turned out not to be enough. My father wanted some to do chamber casts in firearms, for which it worked great by the way. Currently there is a seller on ebay who has the fresh rad. shielding alloy for about 3 bucks a pound, see the trading post entry. Thank god for USPS flat rate boxes : )
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Re: Bismuth Low Temp Shielding Alloys

Postby Doug Coulter » Tue Aug 03, 2010 6:20 pm

The stuff we got was labled "cerro-shield" but by measuring things, we found it was the same as the older version of cerrobend.
It can be melted under boiling water, just barely. It tends to expand on cooling, but this seems to vary sample to sample with the stuff
we got from a radiology dept. (about 500 lbs, free).

Though called "safe" this stuff is loaded with Cd, and that's most likely what is oxidizing the most, it's the most chemically active component of
the alloy by far, the tin being second. And Cd is not like lead -- it's thousands of times worse to get in and on you. One has to laugh that the hospitals
bought this as "safe" due to the low melt point, but actually it's one of the more poisonous things in my lab! Overheating it will
cause the Cd in it to burn and fume into the air! I think of this whenever some doctor tells me about being safe and
healthy -- they know so little in general it seems. Or I look at my wall poster "more doctors smoke Camels than any other brand".

Cd has no special cross section for neutrons at high energy at all -- it may as well be any other substance for fast neutrons.
In fact, it's used as a filter to only stop slow ones, but let fast ones through, frequently. It's a good trick if you want to find out
if you've got fast or slow neutrons into a moderated detector tube -- if a thin piece of Cd makes it stop, they were slow, if not,
they were fast. Mentioned all over in the old nuclear physics text books as a slick trick to find that out.

Cross_Cd.gif
Cd cross section, log plot


Here is a plot of the neutron cross section of Cd. Click the pic to make it readable. Note how at low energies the cross section is 10k barns, but only
less than 10 to the point the plot stops.

Despite some people freaking, I even machine this stuff with high Cd content -- it machines nicely and just throws the usual stringy swarf onto a sheet of paper
I lay on the lathe ways to recover it for later use. But yes, you wash your hands, and when I do this on a higher Cd alloy, as in the
ones I used in the neutron camera to stop slow neutrons after I slow them down in a moderator, I wear nitrile gloves, and then
wash my hands anyway. There is a reason that Cd plated hardware is no longer available, and skin acids make it human-soluble.
Caswell plating even sells a process that makes things look like they are Cd plated, without the Cd, as that look is important to some
who restore old stuff. All the metal books are replete with statements like "the most dangerous element there is" about it.

But Rotometals will of course sell it to you. But you have to mind it catching on fire when you make alloys -- you melt the other stuff first, let
it cool some, then stir in the Cd and let it dissolve, being patient. Even so, I've had it catch on fire in a melt and just ran away till it stopped, as evil
yellow-brown fumes evolved, one breath of which would be certain death. Now I am the proud owner of half a lb of Cd oxide I don't know what to do with.

There are some other variations. One is called cerro-safe, and I use this all the time in gunsmithing, though it's expensive. It has a nice property of shrinking right
after casting, then regaining the exact mold dimensions in half an hour or so. This lets me measure things like chambers and insides of barrels "safely" at least insofar
as damage to the firearm goes. That seems to be going for about $11/ half pound, but of course, you reuse it forever.

Cerrobend as it shrinks, is dandy in the machine shop to hold fragile stuff so you can clamp it in a vise and mill it or generally have your way with it, as
well as the intended use for bending pipe without it kinking. Insta-jig!
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: Bismuth Low Temp Shielding Alloys

Postby lutzhoffman » Tue Aug 03, 2010 8:04 pm

I Just checked the link to Rotometals, which has many of these low melt alloys listed. It seems that most of the Cd bearing ones are at the lower end of the melting ranges. The Rotometals Low 255 is the one which peaked my interest, since it has no Cd at all, its just 55.5% Bi and 45.5% Pb. this should even give it a slight edge for Gamma and X-ray shielding, over the ones having lower atomic number metals in them. The one for sale on ebay was listed with a MP of 250 deg, which puts it out of the range of the Cd bearing ones listed by Rotometals. I would still assume Cd content unless I knew for sure do to its higher toxicity.I suppose the safe route would be just to buy the Low 255 from Rotometals, and not have to worry about the Cd at all.
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Re: Bismuth Low Temp Shielding Alloys

Postby Doug Coulter » Wed Aug 04, 2010 9:11 am

I (and also other sources like wikipedia) have some fairly extensive data on various possible alloys and their properties, though the info is scattered all over and can be hard to collate. I will try and get some of it posted on this thread soon -- it's fascinating stuff, and with people like Rotometals as sources, you can pretty much make anything you want with a well known composition (though some is either toxic, not cheap, or both). I'd guess that Rotometals is getting more per pound for whatever than an ebay source, but -- they are reliable and will be there tomorrow. I have gotten Cd, In, Bi, and other things from then with complete satisfaction. In is fun to play with, BTW, and it and alloys of it have a lot of good uses in low melt things, vacuum sealing O rings that can be recast and reused, and as an element to be activated that is easy to measure later (slower decay than silver, so some of the issues of speed getting to the activation counter are less hassliferous). In is a little less sensitive than silver due to the longer half life, but since it hangs around longer, you still get a more accurate reading if you activate it decently out of the background noise, and don't have to get to the detector as fast to make the accuracy decent either.

As far as shielding goes, it's kind of a trick to get just what you want, and a lot of ingenuity has been expended on that area. Not all high Z metals have their X ray absorption peaks in the same places for example, so a mix might help an alloy absorb X or gamma rays over a larger bandwidth with less total poundage of it. If you're trying to keep gammas out of a detector that sees both neutrons and gammas, so as to only see the neutrons, you might really not want any Cd or boron in the mix at all, since they emit some pretty hot gammas when absorbing neutrons that then need to be stopped before they hit the detector. I discovered this doing my neutron camera, which is about to be torn down and re made to take this insight into account. Since the plastic scintillator I'm using only sees fast neutrons, there is no need to absorb the ones I don't want to see for a directional detector, for example -- just slowing them down will do, and things that emit gammas when they stop neutrons are anathema, actually. So I flubbed the first design rather badly. Next time I will simply use a hole through HDPE for that part of the design, rather than borated wax and 40% Cd alloy washers.

FWIW, we use a lot of plain old pure lead sheeting for shielding here -- wrapped essentially the entire fusor setup in it. To do this, we took advantage of lower melt alloys to join it, so as not to hae to worry about melting the pure lead while "soldering" the various pieces together. This, combined with a very thick piece of high-lead glass (from a hospital radiology room renovation) allows our geiger counters to "sit still" even when close to a fusor running, and it's relatively safe to put your face to the window and admire what is going on inside it. Most of the human exposure then become the neutrons, and a few higher energy gammas that the reaction rarely produces. This makes me feel better about having it proximate to my living space, and even mollifies my partner, who is a bit nervous about even the cosmic background -- at least somewhat.

I'll try and see what I can collate from the various sources I have here, including the Rare Metals Handbook, which isn't all about low melt alloys, but does have alloy information on all of the metals involved here -- useful stuff. Wiki has some things too, from low melt to babbits, to type metals to bullet alloys. To really put this together, one would want to look at the X ray absorption cross section vs energy of each alloy component to design the best alloy for a given use, or so I believe. Of course, things like melting point, ductility, and crystallization come into the trade off mix here too -- I made a high Cd alloy, that while having a lot of good properties radiologically in bulk, stinks to work with as it make huge crystals on hardening, meaning a thin layer might just have pure Cd here, pure Bi there and so forth -- so it would leak some radiation like a sieve at some points if used thin. In that case I got plate-like crystals of half inch and larger dimensions, even cooling the metal mold in water to keep the cooling rate high and the crystals smaller. Pretty, but useless. I guess if I get anything working really well with Cd, I'll start a thread on it.

At any rate, since we're talking about Bi here, this is a useful page from the Rare Metals Handbook.
BiAlloys.gif
Bi Alloys table


Many Bi alloys expand on hardening, or soon thereafter, which is something that makes them good holder for delicate things you want to machine. The following page of the same book mentions that an alloy of 56 Bi, 20 Sn, 24 Pb (most of these are by weight, not mole). This one expands 1% in linear size.
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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