Some time ago, I read about a certain isotope that is stable when neutral but decays with electron emission (beta) when being completely ionized, but I can't find which one it was.

Which isotope decays when fully ionized?

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    $\begingroup$ The structure of the nucleus has nothing to do with the structure of the atom so I dont think there is such a isotope $\endgroup$
    – Jun Seo-He
    Jan 29, 2022 at 17:19
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    $\begingroup$ Are you it was not the other way round (decaying when neutral, but stable when completely ionized) ? $\endgroup$ Jan 29, 2022 at 17:37
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    $\begingroup$ Yes, I am sure. $\endgroup$
    – iblue
    Jan 29, 2022 at 18:14

2 Answers 2


I guess you have read about an isotope decaying via electron capture (which is kind of an inverse $\beta$ decay). There are quite many radio-isotopes decaying in this way, for example: $${}^{59}\text{Ni} + e^- \to {}^{59}\text{Co} + \nu$$ $${}^{40}\text{K} + e^- \to {}^{40}\text{Ar} + \nu$$

In this decay the atomic nucleus captures one of the surrounding electrons (usually one of the innermost $K$ shell). Of course this process can only happen if the atom or ion has at least one electron. It cannot occur if the ion is completely ionized, i.e. it has no electrons at all.

Another quite rare phenomenon is bound-state $\beta^-$ decay. Here the created anti-neutrino takes almost all of the decay energy, and the created electron gets very little energy so that it fails to escape the atom, and instead integrates into the atomic orbital.

You probably have read about the completely ionized rhenium ion which decays quickly (with half-life $32.9$ years) by bound-state $\beta^-$ decay $${}^{187}\text{Re}^{75+} \to {}^{187}\text{Os}^{76+} + e^- + \bar{\nu}$$

whereas the neutral rhenium atom is almost stable (with half-life $42$ billion years) $${}^{187}\text{Re} \to {}^{187}\text{Os}^+ + e^- + \bar{\nu}$$

This particular rhenium isotope ${}^{187}$Re has a very small $\beta^-$ decay energy (only $3$ keV). This energy (or more precisely: the energy part delivered to the electron, not to the antineutrino) is not enough for the electron to escape the neutral atom. And it cannot find a place in the shell because all electron orbits of the atom are already occupied. But when the ion is fully ionized (i.e. all electrons stripped off), then the energy is enough for the electron to reach a low electronic orbital of the ion.

See also the original article by Bosch et al. (1996)
"Observation of Bound-State $\beta^-$ Decay of Fully Ionized ${}^{187}$Re: ${}^{187}$Re-${}^{187}$Os Cosmochronometry".

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    $\begingroup$ @iblue You have probably read about Re-187. $\endgroup$ Jan 29, 2022 at 18:45
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    $\begingroup$ YES! That's it! By searching for the bound state β-Decay I found the Dy-163 I was thinking of. Dy-163 is stable, where the ion has a half-life of 47 days. journals.aps.org/prl/abstract/10.1103/PhysRevLett.69.2164 $\endgroup$
    – iblue
    Jan 29, 2022 at 19:19
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    $\begingroup$ Excellent find on the rhenium decay. You inspired me to actually do a database query, rather than just skimming; so far as I can tell, rhenium-187 is the only isotope whose $\beta^-$ decay energy is less than tritium decay to hydrogen. $\endgroup$
    – rob
    Jan 29, 2022 at 19:27
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    $\begingroup$ @rob Surely you mean tritium decay to helium?! $\endgroup$
    – PM 2Ring
    Jan 31, 2022 at 3:54
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    $\begingroup$ @PM2Ring That is what I meant. Whoops. $\endgroup$
    – rob
    Jan 31, 2022 at 10:38

I’m nearly certain you are thinking of beryllium-7, but that you have remembered the condition backwards.

Neutral $\rm^7Be$ can decay to $\rm^7Li$ by electron capture, with energy about $\rm 860\,keV$. Positron-emission decays are always disfavored relative to electron-capture decays, because the final state with an electron missing is lower in mass than the final state with the positron added. Since the total $\rm^7Be$ decay energy is less than the mass difference $2m_\mathrm e = \rm 1022\,keV$, the positron-emission mode is completely forbidden. Beryllium-7 is not found in beryllium ores on Earth, but completely-ionized $\rm^7Be$ is a stable component of cosmic rays.

A read through all the NNDC dataset finds a number of other nuclei with electron-capture $Q$-values below $\rm1\,MeV$, starting with $\rm^{41}Ca$. However, cosmic ray populations are heavily skewed towards the low-mass end of the chart of isotopes; I’ve only ever heard people talk about beryllium-7 having this property.

For the condition you describe, where an ionized parent nucleus can decay but the neutral parent atom is stable, the decay energy would have to be less than the electron binding energy for the daughter atom. If that were the case, the ionized nucleus could decay to a bound state of the daughter and the beta electron, with the antineutrino carrying away the energy. But the neutral atom would be “Pauli-blocked” from decaying, with its bound electrons already occupying the possible final states for the $\beta^-$.

I believe there are no $\beta^-$ emitters with energies this low. If such a decay existed, it’d be an interesting place to try and measure the mass of the electron antineutrino, by doing precision mass spectrometry on the parent ion and the daughter ion, to be compared with recoil measurements on the daughter following the decay. Instead, that experimental energy has gone into the KATRIN experiment, which analyzes the $\beta^-$ decay of tritium to helium-3, with endpoint energy $\rm17\,keV$.

However, as revealed in the comments on another answer, my belief was incorrect. Neutral dysprosium-163 is stable against $\beta^-$ decay, with $Q$-value $\rm-2.6\,keV$; the linked paper observes the beta-decay of the bare nucleus. The next candidate would be $\rm^{148}Eu$, with $Q$-value $\rm-27\,keV$.

  • $\begingroup$ Tangential question on the 7Be being a component of cosmic rays - I can find papers on the production of 7Be in the atmosphere by cosmic rays, but none on 7Be being a cosmic ray itself. Do you have a citation that I could read? $\endgroup$
    – Jon Custer
    Aug 18, 2022 at 13:18
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    $\begingroup$ Nevermind, found a nice paper in the Astrophysical Journal using data from Voyager. Between the two, they saw about twice as many 7Be as 9Be nuclei. No Earth atmosphere involved! $\endgroup$
    – Jon Custer
    Aug 18, 2022 at 15:03

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