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Electron capture is a method of nuclear decay. It is only really common in elements where the the $Z'=Z-1$ element has a lower mass, but the difference is less than the mass of the positron. One of the atoms' electrons is captured into the nucleus, forming a nucleus with one less proton and one more neutron.

Atoms that do $\beta^+$ decay could also decay by electron capture, but I suppose the partial lifetime due to this is longer, so it's rare.

What I'm wondering about is stable nuclei turning into unstable nuclei (that will eventually $\beta^-$ decay) by capturing a high energy electron. That is, $100\,\mathrm{keV}-10\,\mathrm{MeV}$ or so electrons from a beam are absorbed by a nucleus to form an excited nucleus that decays by re-emmitting an electron (plus some neutrinos along the way). Such transformations never occur from ordinary electron capture from the bound electrons because the bound electrons don't supply the necessary energy.

Looking around online, I see some papers discussing such processes, but only as something that happens in stars. I can tell that the issue with this idea is that the cross section is low. Typical beta decay lifetimes are milliseconds to seconds (or longer), and typically the cross section for producing something is proportional to one over the lifetime of that thing. Also the production mechanism is the weak force. Normally in accelerators we aren't producing things through the weak force (unless the energy is high enough that the weak force isn't weak anymore).

But I can't find any resources online that calculate the cross section in these terms - like what would it take to stimulate such a process on earth. So I'm curious if anyone is aware of any experiments that have demonstrated such an effect - or any resources that explain why it's impossible by working out the cross section, and by extension the beam parameters that would be needed to do this. Of course, I've looked around and haven't found anything. But maybe I just haven't found the right search terms because I really can't find anyone even discussing this possibility in these terms.

AXensen
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1 Answers1

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In the context of isomeric transitions (the inverse of internal conversion), the keyword is "nuclear excitation by electron capture", usually abbreviated NEEC. This was demonstrated in 2018 in this Nature article: https://www.nature.com/articles/nature25483. You can find calculations for the cross-sections here: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.73.012715

For the case of transmutation (the inverse of $\beta^-$ decay), the keyword is "(transmutation by) induced electron capture" and this has been discussed in the following question: Electron beam induced electron capture?

As of yet it seems that it still hasn't been successfully observed. One complication is that at high beam energies the Brehmstralung upon hitting the target will in fact be high enough to lead to $\gamma$ induced reactions, and this will be the dominant interaction, which is used intentionally for transmutation experiments, i.e. https://link.springer.com/article/10.1007/s00340-004-1637-9

AwkwardWhale
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