atomic_fungus (atomic_fungus) wrote,
atomic_fungus
atomic_fungus

#2631: Beta decay

...now that just blows my mind: beta radiation happens when a neutron turns into a proton.

Tritium--which is two neutrons and a proton--turns into Helium-3 (two protons and a neutron) through beta decay. One of the neutrons turns into a proton and the nucleus emits a beta particle, an electron, because charge is conserved the same way matter and energy are.

I really want to understand this, because nuclear decay is governed by the weak force. Yet it seems to me that if a neutron changes into a proton, the quarks inside the neutron have to change flavors. This change in flavor (that's the actual term, BTW, "flavor") results in the neutron emitting an electron and turning into a proton.

How?

I mean, a neutron existing in free space doesn't spontaneously decay into a proton and electron. Does it? Has anyone observed that? I've never heard of neutron decay until just now (here) and it's making me crazy.

It's got to mean that the weak and strong forces interact, somehow. There must be some kind of boundary conditions that switch on the neutron's decay function. When too many neutrons are bound into a nucleus, somehow their interaction can trigger a flavor change in one neutron's quarks (it would be indeterminate which neutron would undergo the change until it happened--Heisenberg, you know) and the nucleus changes to another element and spits out an electron.

Eh? "What did you think beta radiation was, anus?" I never really considered it. Heck, when you've got atoms changing into different elements, you could easily find yourself with an atom that had too many electrons, and one or two would naturally go zinging off in some random direction at high speed.

It makes me think I need to dig out my physics books and re-study the sections on nuclear decay, for crying out loud. What else did I miss?

It's bad enough that I don't really understand how the weak force works. The strong force--it's quarks exchanging gluons; that's easy. Electromagnetism is charge and magnetism; it's mediated with photons and electrons, and that's also pretty easy. Gravity is its own set of troubles but no one really understands it, so that's okay. But the weak force--

No one has pointed to something and told me, "This is what holds nuclei together!" There's binding energy involved, but I don't get how it works; I don't know if particles carry it around or if it's something like an electromagnetic field, except that it only works on particles with postive or neutral charge.

Electron orbitals--that's easy. Electrons are attracted to the positively-charged nucleus, but of course electrons can only exist in orbits which are integer multiples of their wavelengths. (That's why nearly all the mass in the universe is made of protons, neutrons, and electrons, instead of everything being neutrons.)

But why the hell does the helium-3 nucleus stay together? Two protons and a neutron? What keeps those two protons from flying apart?

The idea that the nucleus of an atom isn't solid doesn't bother me. It makes sense for a nucleus to be this churning maelstrom of particles all jumbling around, never actually colliding; but I can't grasp what keeps them from all going their separate ways.

Wikipedia sez "the exchange of W and Z bosons". That leads me to W and Z bosons and that article contains a Feynman diagram:



That explains nearly everything I need to know about beta decay, anyway. One of the neutron's down quarks turns into an up quark by emitting a W- particle, which then further decays into an electron and an electron antineutrino.

Except for the most critical thing: what the fuck makes it do that?

How does having too many neutrons in a nucleus make a random down quark inside a random neutron suddenly decide it was born to be an up quark and spontaneously undergo flavor reassignment surgery??

The answer is the bosons, W and Z.

Okay: inside a particle like a proton, there are three quarks. These quarks are constantly exchanging gluons; and the gluons carry flavor information with them--so an "up" quark emits a gluon and turns into a "down" quark, but the "down" quark next door is so close that it absorbs the gluon and turns into an "up" quark. The short distance is critical. Move the quarks too far apart and the gluon will "time out" and decay. It takes an enormous amount of energy to move the quarks apart, such that when you get them too far apart, the gluon "time out" results in the formation of new particles. Like stretching a rubber band until it breaks, only to find you now have two whole rubber bands rather than two pieces of one rubber band. E=MC2

Seems to me, then, that the churning maelstrom that is the nucleus actually works the same way: grab one particle at random and tag it (somehow) and keep an eye on it, and you find that it's constantly changing from proton to neutron and back as W bosons zing around the nucleus. The W particles keep everything corralled, because they decay if they get too far away and then you lose charge/energy/mass/whatever; this keeps everything more-or-less close to hand. But if you have too many neutrons, sometimes a W is emitted and doesn't get scooped up in time, and escapes the nucleus. Beta decay happens.

...I don't know if that model is right, but it makes sense to me.

It doesn't happen in a hydrogen atom (single proton with one electron, I mean) because there are no other particles; that's also why a neutron in free space can't spontaneously turn into a proton and electron: there's nothing to exchange the W with, and mass, charge, and energy must be conserved.

I think I've learned enough about physics for one night; any more and my head asplode.
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