Why hasn't the universe turned into a big mass of neutrons?
June 2, 2012 11:20 AM Subscribe
In Electron Capture, an electron falls into the nucleus of an atom and combines with a proton to form a neutron. A neutrino and a gamma ray are emitted. It's one form of nuclear decay, but relatively uncommon. Why hasn't this happened to every hydrogen atom in the universe already?
Response by poster: So you're saying that a neutron is a higher energy state than a proton plus an electron. Is that it?
An electron and a proton are opposite electric charges and should attract one another. Why would it require energy for them to do that?
posted by Chocolate Pickle at 12:25 PM on June 2, 2012
An electron and a proton are opposite electric charges and should attract one another. Why would it require energy for them to do that?
posted by Chocolate Pickle at 12:25 PM on June 2, 2012
Response by poster: So a proton is two up quarks and a down quark. A neutron is an up quark and two downs. The down quark has more mass, so a neutron is inherently a higher energy state. And so an electron might crash into the proton in a hydrogen atom, but not with enough energy to make a neutron. Therefore it would bounce off.
Is that it?
posted by Chocolate Pickle at 12:36 PM on June 2, 2012
Is that it?
posted by Chocolate Pickle at 12:36 PM on June 2, 2012
Best answer: Yes. The mass of a proton is 938.27 MeV, the neutron is 939.56 MeV, and the electron 0.5 MeV. So there's an extra 0.8 MeV of energy you need to make up for in the electron capture process, which has to come from the kinetic energy of the particles involved. The quarks can be abstracted away by just saying that the excess energy is the "binding energy" of a neutron.
Fun fact: free neutrons have only a 14 minute half life before they beta decay.
posted by kiltedtaco at 12:44 PM on June 2, 2012 [1 favorite]
Fun fact: free neutrons have only a 14 minute half life before they beta decay.
posted by kiltedtaco at 12:44 PM on June 2, 2012 [1 favorite]
My very dim memory of physics is that one reason this doesn't happen spontaneously is that it takes energy to confine an electron (or any particle) to a smaller space— an electron in its ground state bound to the proton is still occupying enough space that the overlap with the proton is tiny.
Explaining why atoms don't spontaneously collapse like this is one reason the Rutherford atomic model was supplanted by quantized models (Bohr, etc).
posted by hattifattener at 2:38 PM on June 2, 2012
Explaining why atoms don't spontaneously collapse like this is one reason the Rutherford atomic model was supplanted by quantized models (Bohr, etc).
posted by hattifattener at 2:38 PM on June 2, 2012
To add to the above: the only reason that electron capture can happen in some nuclei is that the candidate daughter nuclide is more tightly bound than the parent nuclide, by at least 0.8 MeV. The nuclear binding energy is a complicated function of the number of protons and neutrons, so it's altogether conceivable that the change in binding energy between the nuclides would make up for this difference. But there's no binding energy involved with a lone proton + electron (well, OK, "negligible binding energy on nuclear energy scales.")
posted by Johnny Assay at 8:30 PM on June 2, 2012
posted by Johnny Assay at 8:30 PM on June 2, 2012
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Additionally, the cross-section, and thus the reaction rate, for this process is likely very slow, but I don't have the data to calculate it quickly. It's dependent on the abundance of high energy electrons in the universe.
posted by kiltedtaco at 11:26 AM on June 2, 2012