Questions about nuclear fission and fusion

Factual Questions
1

What causes nuclear fusion? Is it when nuclei get close enough together that the strong force overcomes the electrical repulsion of the protons? How does that work if nucleons are color neutral? Is fission simply caused by the kinetic energy of an incoming particle overcoming the strong force? If a neutron has sufficient energy to penetrate the electron cloud, but not enough to cause fission, can the neutron fuse with the nucleus? Do you have to use a neutron? Does it always fuse causing an unstable state which immediately decays? In a fusion reaction, where does the energy released come from? What gets annihilated? The nucleons aren’t less massive, are they? What is energy anyway? I used to think that photons were energy, but I have come to realize that they are not, they just carry it. Is it possible for a particle to have energy and be totally at rest?

Thanks for your help,
Rob

2

Yup, that’s it.

The nucleons are color neutral (colorblind?) but the quarks composing them are not.

Wow, ok. One at a time. Fission is caused by the instability of a nucleus - it’s an energy level thing, analogous to how an atom has certain electron orbitals. Sometimes the nucleus is naturally unstable. Those elements are radioactive. Other times we need to fire something at it to create an instability, typically either a neutron or an alpha particle (two protons, two neutrons). In fact, whatever gets injected in does “fuse” with the nucleus first. That creates an unstable nucleus, which then fissions.

In a fusion reaction, nothing gets annihilated (you might be thinking of matter/antimatter collision). The energy comes from the two nuclei. The individual nucleons don’t need as much energy when they’re bound in a single larger nucleus, so the excess gets released. Think of it as the protons and neutrons dropping to a lower potential well and radiating away the extra energy. Yes, the nucleons are less massive, in an E=mc[sup]2[/sup] sense.

Now, onto the big ones. Energy is a very vague term in physics. There are three types of energy: kinetic, potential, and rest energy. The third one is energy that is “locked up” as mass in matter. Photons, when they’re just shooting around the Universe, fall into the potential energy category, because they are the manifestation of the electromagnetic field that exerts forces on charged particles.

No particle can ever be perfectly at rest (Einstein says rest is relative, and Heisenberg says there’s always uncertainty in the momentum anyway). However, particles with mass are those particles that have “rest mass” or equivalently, “rest energy”. But even more simply, if you go up to the top of a building and just stand there, you have gravitational potential energy simply by virtue of your location.

Hope this helps.

3

Fusion occurs when a nucleus aqcuires additional charged fermions and the resulting state is unstable. (Typically the fermions are protons or neutrons, but hypothetically it’s possible that exotic baryons could be included as well, though the lifetimes of these particles are so short that in practice we don’t see them outside of a cloud chamber attached to a particle collider.) The reason for instability is complex, but essentially the interactions between fundamental particles that make up the nucleons don’t all add up and to make the sheets balance, and repulsive eletrostatic forces cause the nucleus to decay, releasing the binding energies and (with several reaction chains) other particles (protons, neutrons, neutrinos, alphas, the stray positron, whathaveyou).

What you’re referring to as the strong force is technically called the nuclear force or residual strong force. Strong and weak interactions don’t occur directly on composite fermions but rather as exchanges between their constituant particles. The nuclear force acts only on a very short range whereas the electostatic force has a much larger field, so it takes a lot of kinetic energy (and a lucky shot) to get a nucleon or nucleus close enough to another nucleus to overcome the Coulomb repulsion and cause a reaction, and the larger one or both nuclei are, the more repulsive the forces are, hence why most naturally occuring fusion is of the D-D and D-T variety, with helium burning (alpha processes) and the C-N-O chain only occuring in older or more massive stars. “Penetrat[ing] the electron cloud” is typically a nonissue on the energy scale of nuclear fusion because in comparison to the repulsive forces of the postively charged protons because the charge of the electron is “spread out” along its orbital while the proton is denser and therefore more localized. (This isn’t strictly correct, but it’s close enough for the purposes of discussion without devling into the guts of quantum mechanics.)

In any case, a neutron, being charge neutral, won’t have any problem with repulsion; however, it’s hard to get free neutrons to “stick to” (neutron capture) low mass nuclei. I think the lightest weight nuclei they’ll stick to in quantity is lithium-6, which produces an alpha particle and a tritium nuclei. (This is typically used to “breed” tritium in thermonuclear weapons rather than having to store and replenish the unstable tritium as is done with boosted weapons.) When a nucleus acquires additional neutrons and becomes unstable and decays, this is considered fission, not fusion.

In fusion, the energy is released as kinetic energy of emitted particles and electromagnetic radiation (X-rays, gamma rays). Where does this come from? It comes from the “binding energies” that keep the nucleons together. When you stick more particles in the nucleus it requires less total energy, so some are either ejected or the energy is released as energetic photons. (From a power production standpoint we’d prefer that either the particles released are charged particles or photons because that makes it easier to capture and convert the energy to electricity than neutron products, but the reactions that require the lowest temperatures are neutronic.)

Photons are energy. So, in fact, are protons, neutrons, et cetera. With the latter set, the energy is bound up in a form that we call “mass” (particle physicists often refer to it as “mass-energy” to properly identify and distinguish it), so even a particle that is strictly motionless with regard to a refernce frame still has its “rest” mass(-energy). When you put a particle and its antiparticle together you’ll get a reaction in which the resulting product is a pair of photons (with equal and opposite momenutum) with frequencies corresponding to the energy bound up in the mass per Einstein’s famous result of E=MC[sup]2[/sup] plus whatever kinetic energy and momentum the original particle-antiparticle system had before.

Looking back over this I’m not sure that I’ve answered your questions very clearly or completely, but they’re not paying me to explain nuclear physics and I’m at the end of my lunch break so I’m going to cut this loose as-is. I hope this helps to resolve some of your questions.

Stranger