Neutron stars, neutron physics, and the transmutation of mercury into gold. [View all]

The paper to which I'll briefly refer is this one: Xilu Wang, Nicole Vassh, Trevor Sprouse, Matthew Mumpower, Ramona Vogt, Jorgen Randrup and Rebecca Surman, MeV Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers, The Astrophysical Journal Letters, volume 903, number 1, pages L3

It open sourced, there's no need to excerpt it; I only wish to refer to why I looked it up:

My son has been attending the ANS Student meeting and told me a cute story about talking to an undergraduate who had a nice poster about the history of mercury coolants in fast nuclear reactors. This was investigated in the early days of nuclear reactor development, but didn't get very far.

The kid's poster said that cracking in Hastelloy caused by mercury caused the abandonment of the project, whereupon my son explained that the issue was probably more connected with the fast neutron spectrum, specifically, resulting after the formation of 59Ni from neutron capture in 58Ni, the 59Ni(n,α )56Fe reaction as opposed to specific amalgam formation reactions. This reaction leads to interstitial helium in the alloys, causing cracking.

Our phone conversation on this topic caused me to riff on the fact that mercury coolants actually transmute tiny amounts of the minor mercury isotope 196Hg into gold via an 196Hg(n,γ )197Hg. The resultant radioactive 197Hg decays, with a half-life of around 64 hours to gold's only stable isotope, 197Au, thus small amounts of toxic mercury are transmuted into nontoxic gold.

After the call, I got to trying to remember how much of this minor isotope, 196Hg is actually found in mercury so I decided to look it up.

I'm often too lazy to go to the BNL nuclear data pages for isotopic distributions, since the Wikipedia pages are quick to access and are actually quite good for fast largely accurate info. So I went to the page for mercury isotopes to find that 196Hg is represents only 0.15% of mercury. The page identified the isotope 201Hg as "observationally stable" whereupon, clicking on the footnote to see if it is believed to decay by α emission to Pt-197, (albeit with an extremely long half life) to 197 Pt which would then decay by β emission (half-life just shy of 20 hours) to gold I found this text about 198Hg:

I wasn't aware of that.

I am generally aware of the gamma emissions of major fission products because I often muse about using these emissions to address certain intractable chemical pollution problems associated with organohalide persistent pollutants. These are rarely much higher than a few MeV. It occurred to me that although I seldom think about them, that the gamma energy of fission itself should be much higher, and so when I went to Google scholar to look up these energies the first paper I came across was the one listed at the outset of this post. Fission gamma rays are apparently strong enough to result in 198Hg(γ,n)197Hg reactions leading to gold.

It would appear that mercury coolants in nuclear reactors - and I'm certainly not recommending this - would end up transmuting more than 196Hg into gold.

That's interesting, if probably impractical. The next time I talk to my son though, I will mention it.

It is quite possible that the uranium and thorium now found on Earth originated in neutron star collisions as opposed to r process formation.

Cool stuff.