The alchemists' dream (one of them, anyway) was always to make gold. We now know there are very good reasons they couldn't: since gold is an element, making it from anything that doesn't contain gold(*) requires you to change the nuclei of the atoms involved, while all the alchemists had access to was the electron shells around the atom(**). So their efforts were basically hopeless. Now, though, we do have the ability to manipulate nuclei, and in fact we do so on industrial scales. So could we make gold? In fact, let's be more ambitious: could I make gold in my basement? The answer is, surprisingly, yes.

First of all, the easiest nuclear reaction to go for is to transmute mercury into gold. Mercury is commercially available (though somewhat encumbered by very sensible environmental concerns) and not actually very expensive - on the order of $18 per kg (***). Gold, by comparison is more like $40000 per kg. So there's room for some profit here.

How could I make the nuclear reaction happen? This particular reaction needs so-called "fast neutrons", that is, neutrons that are still zipping around at the high energies typical of nuclear reactions, as opposed to neutrons that are bouncing around at energies consistent with room temperature. I could stick the gold in a "fast breeder reactor", but I don't actually have one in my basement, and they're kind of hard to build. I could use a particle accelerator to generate some neutrons (basically by bashing nuclei around until some neutrons fall off) but while I do have a particle accelerator in my basement, it takes one a lot more serious than I can reasonably build to get neutrons out. Nuclear fusion reactions give out neutrons, though, so all I'd have to do would be to build a fusion reactor in my basement. Improbable as it sounds, this actually is feasible, provided I'm not trying to get any energy out.

The trick is that there's a fusion reactor, called the Farnsworth-Hirsch fusor, that is surprisingly simple to build. It is actually something of a cross between a fusion reactor and a particle accelerator: I'd set up an electrical potential in a spherical configuration, accelerating deuterium ions towards the center, where they'd crash into other deuterium ions, occasionally hard enough for fusion to happen. This fusion would release a fast neutron.

To make gold, then, all I'd have to do would be to build a fusor, surround it with a blanket of mercury, run it for a while, and then extract the gold from the mercury. Simple, really.

Let's look at the economics, though.

Suppose we want to make a kilogram of gold, giving us $40000. We need about a kilogram of mercury, costing $18. We also need about 5 g of deuterium (assuming perfect efficiency), which would cost about $30. Finally, we need the power to run the fusor. That's not going to be cheap. An optimistic number for the best fusor ever built is about 10^12 neutrons per second from about 4 kW input. That amounts to 9*10^14 neutrons per kilowatt-hour. Assuming perfect efficiency again, we need about 3*10^24 neutrons for our kilogram of gold, or 3 terawatt-hours, about the world's total energy production for an hour and a half. At $0.10 per kilowatt hour (I live in the land of cheap hydroelectricity) that's three hundred billion dollars.

There's a somewhat more disturbing possibility, though. Gold is easily obtained; you can just buy it. But as a global society, we try very hard to make sure you can't easily get plutonium, particularly plutonium-239, since that is well-suited to building atomic bombs. (You can make bombs out of uranium too, but that requires you to separate the different isotopes, which are very nearly chemically identical. Plutonium, on the other hand, can easily be separated from uranium since it is a different element.) Uranium isn't too hard to come by, especially "depleted uranium" (uranium with most of the uranium-235 removed) - armies fire the stuff at each other, for example. And if you had lots of U-238, a fusor would let you make plutonium out of it. The cost would be high, hopefully prohibitively so, but you could do it without doing anything that would put you on the radar of the IAEA. Fortunately, the power use is so outrageous we don't really need to worry about it.

So, in short, I could make gold in my basement, but not any appreciable quantity, and not for any kind of sensible price.

(*) Since gold is a noble metal, there aren't many chemical compounds that contain gold; unlike, say, iron, gold is often found on Earth as lumps of raw gold. So while in principle alchemists could have started from some gold compound and gotten the original gold back, this would not have been a very interesting accomplishment.

(**) There are actually situations where you can affect the nucleus by manipulating the electron shells. For example, if an isotope decays by electron capture, you can drastically slow down its decay by stripping away all its electrons. But stripping away all the electrons from a reasonably heavy element is one of those things that's virtually impossible under terrestrial conditions but not too rare astrophysically. In any case this has no effect on stable isotopes.

(***) Canadian dollars and American dollars are equivalent to astronomical accuracy.


Popup said...

It's been over ten years since I did any particle physics, but as far as I can make out, I don't think your scheme would work.

The article you linked to claims that irradiating Hg with fast neutrons will generate Au-198 and Au-199, both of which decay under β-emission with a half-life of a couple of days. Unless I'm mistaken the result ought to be Hg-198 and Hg-199, both of which are stable.

So, all you have achieved is a very expensive space-heater, and a basement that will keep the haz-mat guys busy for foreseeable future.

Or would you first separate out the Hg-196 (about 0.15% according to the Wikipedia), which might transmute straight to Ah-197? Sounds cumbersome, to say the least.

(Or have I missed something? Always a possibility, because, as I said, I haven't done any particle physics this century. (But I was at CERN for a couple of years during the last millennium.))

You don't allow < sup > tags in comments. Could that be added easily?

mvc said...

Interesting. But while I knew you had a particle accelerator, I didn't know you had a basement.

Popup said...

Aha! Here is more about gold synthesis from the wikipedia. One way it suggests is to use Hg-198 and a 6.8MeV γ-ray, which can generate Au-197 and a neutron and a positron. Of course, you'd need to separate out the Hg-198 first, but 'normal' Hg contains about 10% of Hg-198, so it ought to be easier.

Of course, I have no idea how feasible it is to generate 6.8MeV γ-photons in a basement.

Anne M. Archibald said...

@popup: you're quite right, I didn't read all that carefully. The reaction I pointed to won't work. Since you get fast neutrons out of the fusor, you can convert Hg-198->Hg-197->Au-197. It's only 10% of the mercury, though, and if its cross-section for fast neutrons is smaller than that of all the other isotopes, you're going to have a miserable efficiency. (Plus, of course, all sorts of nasty radioisotopes as side effects, but the power requirements are really the issue.) The main point of the calculation was really to confirm that this wasn't a feasible way of making plutonium.

(I'll look into the sup tags, but I don't know how much flexibility blogger offers.)

jstults said...

Nice scheme. I did a similar back-of-the-envelope when I lived down on the gulf coast to see how much sea water I'd need to process to get a kilogram of gold; needless to say the economics were not compelling...

Anne M. Archibald said...

@jstults: If you want to talk seawater, the way to go is to build your plant somewhere they have cheap solar power and desperately need fresh water. The gold is just a nice side effect of the desalinization. (Though I guess one doesn't normally get very concentrated brine, let alone extract all the solutes.)

Anonymous said...

Anybody hear of this before? Having trouble finding more info to see if it's efficient transmutation.

Anonymous said...

(The person who wrote this article...) your math is completely fudged up.

3 X 10 (to the 24th) is about the number of nuclei in a volume of mercury the size of a penny, or less than that. You have to learn about MOLES. The math seems foreign at first, but you can learn it.

SECOND: You need slow neutrons.

Your math didnt consider cross sections of the mercury nuclei.

Fusing two atoms requires 1 MEV of energy (per atom), not total. To fuse massive amounts of nuclei, you would need a serious rig!

Finally: you would have to ionize the mercury, or else you wouldnt hit anything but electrons.


Anne Archibald said...

Last anonymous: I do know what a mole is, thank you. The point of this back-of-the-envelope calculation was to see whether the number of neutrons coming out was sufficient to be of any interest; the answer is no, which makes neutron moderators and the thickness of the mercury shell needed basically irrelevant. If you're going to criticize beyond that, Popup has the right of it: I got my nuclear reactions wrong.