While looking at a table of helium isotopes it said that helium-8 turns into Tritium(hydrogen-3) and helium-5 through fission and beta decay.

But the pathway to helium-5 isn't so direct. It is more like this

helium-8 -> hydrogen-3 and hydrogen-5 (fission) -> helium-5 (beta decay)

But the table of hydrogen isotopes said nothing about hydrogen-5 going through beta decay. All it had there was double neutron emission producing tritium which after 12 years turns into helium-3 via beta decay.

Does this mean that I discovered that hydrogen-5 does go through beta decay but not nearly as often as double neutron emission?


1 Answer 1


Sorry, you haven't. I don't know where your information is from but I think there is an error in your decay chain. For the 0.9% of $\ce{^8_2He}$ decays that proceed via spontaneous fission your equation should look more like this: $$\ce{^8_2He ->[fission] ~^3_1T + ^5_1He + \beta ^{-}}$$ $$\ce{ ^5_1He ->[neutron~emission] n + ^4_2He} $$

  • $\begingroup$ Looking at the A=8 energy level diagrams (tunl.duke.edu/nucldata/figures/08figs/menu08.shtml) gives a slightly different picture. 0.9% of the time you get He-8 undergoing beta decay to Li-8, which than can fission in to He-5 and T. 16% of the time the beta decay instead lands on Li-8 levels that decay to Li-7 + neutron. 84% of the time it decays to an even lower Li-8 energy level, drops to the ground state, and then will beta decay to Be-8. Nuclear energy level diagrams are quite useful. $\endgroup$
    – Jon Custer
    Dec 15, 2015 at 23:00
  • $\begingroup$ I am aware of the other decay modes. I chose to focus on the one pertinent to the question. I have updated the question to be more explicit about this. $\endgroup$
    – A.K.
    Dec 15, 2015 at 23:06
  • $\begingroup$ For clarity it seem that there should be two separate equations. One for $\ce{^8_2He}$ and one for $\ce{^5_1He}$. $\endgroup$
    – MaxW
    Dec 15, 2015 at 23:23
  • $\begingroup$ But fission of helium produces 2 hydrogens. So the larger hydrogen(hydrogen-5 in this case) goes through beta decay. $\endgroup$
    – Caters
    Dec 16, 2015 at 2:59
  • 1
    $\begingroup$ This is because fission not only splits the nucleus but it also splits the electrons. So to get helium-5 from helium-8 it first has to go through fission into tritium and hydrogen-5. Then it has to go through beta decay and capture that electron in the 1s orbital to turn into helium-5 $\endgroup$
    – Caters
    Dec 17, 2015 at 21:04

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