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Old 05-15-2019, 12:20 AM
KidCharlemagne is offline
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Why do radioactive atoms eject helium nuclei and not hydrogen nuclei?


I'm watching Chernobyl on HBO and so decided to reacquaint myself with basic nuclear chemistry. Why do alpha particles consist of 2 neutrons and protons? Why don't atoms also eject a single neutron/proton pair? Thanks!
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Old 05-15-2019, 12:31 AM
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Atomic nuclei do undergo beta decay, where a proton or neutron loses a positron or electron.

Nuclei that become sufficiently unstable will fission into two smaller nuclei, plus alphas, betas, electrons, single short lived nucleons, and gammas.

Wikipedia has good articles on decay and fission that I shouldn't copypaste (and cannot explain as well as) here, but I do recommend them.
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Old 05-15-2019, 05:46 AM
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As noted, neutron emission is possible, as well as proton emission or a proton/neutron pair or two protons, etc., but these processes are less likely than alpha decay.
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Old 05-15-2019, 06:49 AM
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Here's the back-of-the-envelope reasoning as to why alpha particles are a lot more commonly ejected than lone nucleons or other types of nuclei:

Nuclei are stuck together by the strong nuclear force. This means that to full off a single neutron or a single proton from a nucleus usually requires some amount of energy input. If a process requires a net energy input, it can't happen spontaneously. So if it would require energy to pull a single nucleon off of a nucleus, then the nucleus can't eject a nucleon on its own.

But notice that I said net energy input above. The helium-4 nucleus turns out to have a particularly high binding energy of its own. This means that you can often pull a helium-4 nucleus out of a heavier nuclide and have a net energy output. You can imagine pulling out two neutrons and two protons (which requires some energy input). But then you could assemble those into a helium-4 nucleus, at which point you "get back" more energy than you put in to remove the nucleons. Since this process would release energy if it happened on its own, it can (in principle) happen spontaneously.

This also explains why we don't see deutrons or tritons ejected from nuclei very often: their binding energy is less, and so we don't get back as much energy when we assemble such a nucleus from their constituent nucleons. The energy balance requires a net input, and so such decays don't happen spontaneously.
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Old 05-15-2019, 07:47 AM
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As to why the helium nucleus has such a high binding energy, it's for basically the same reason why it's chemically stable (though unfortunately, the correspondence doesn't go any higher than helium).

The Pauli exclusion principle says that you can't have more than one particle (at least, particles of a certain sort, which includes protons, neutrons, and electrons) in the same state. But there are a number of ways that particle states can differ. If nothing else, you can put two particles in different states based on their "orbital parameters", which relate to position and momentum: These states generally require more and more energy. So if you're placing electrons around a nucleus, you start by putting one in the ground state (the state with the lowest energy), and as you put in more, you have to start using higher and higher energy states.

But before you have to start going to higher energy states, you have another option. Particles like protons, neutrons, and electrons have a fundamental property called "spin", that can be in either of two different states (usually referred to as up and down). So if there's room for one electron in the ground state, there's also room for another one there, one each of spin-up and spin-down. So helium has as many electrons as it can possibly get into the really cheap state, which is highly efficient, so it's chemically stable (chemistry cares about how the electrons are arranged).

So now look at the states of the protons and neutrons in the nucleus. "Orbital states" of nuclear particles are much more complicated, so much so that we don't know all of the details, but the ground state is pretty simple. So you can put a proton in the ground state. And then you can put a neutron in the ground state, too, because it's a different kind of particle, and so they don't exclude each other. And then you can put in another proton and another neutron, in spin states opposite of the first one, and they're all still in the ground state. So you can get a total of four nuclear particles all into the cheap ground state, and that's very efficient, so the helium nucleus is very stable.
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Old 05-15-2019, 10:06 AM
KidCharlemagne is offline
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Quote:
Originally Posted by Chronos View Post
As to why the helium nucleus has such a high binding energy, it's for basically the same reason why it's chemically stable (though unfortunately, the correspondence doesn't go any higher than helium).
Funny, as I was reading you're whole post I was starting to wonder if if your explanation is related to the fact that helium is inert, while hydrogen isn't. Before I asked I decided to reread your post and there it is in the first frickin line.

Thanks to you and everyone else for your explanations.
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Old 05-15-2019, 04:18 PM
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For nuclei, the evens win. Isotopes with an even number of protons and an even number of neutrons just dominate. And He4 is wonderfully even for both.

Look at the aforementioned curve of binding energy.

Note the big jump for H1 to He4. (And how the curve goes down a little after He4.) That's a lot of energy to tap into for what was described above.

Note on the right side of the diagram where you find U238. Note how small of a gap it is to compared to the energies of the isotopes to its left. Think of how powerful a standard fission bomb is. Compared to fusion into He4 that's a big meh. Hence H-bombs are much scarier.

Sure, there are neutron and proton sources around based on other forms of decay, but they are just ... odd and so don't have all the right things going for them compared to alpha decay.
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