Question

I'm trying to understand what would really happen when large quantities (e.g., 10g) of anti-matter collide with matter. The normal response is that they'd annihilate each other and generate an expanding sphere of gamma ray photons.

However, thinking about it in more detail, what I see is that the anti-electrons annihilate first against the electrons. Let's assume the energy release in that case is not sufficient to noticeably change the momentum of the projectile. Then the nuclei penetrate the electron-annihilation plasma, and since antiprotons attract protons, their trajectory is changed. However, the nuclei are so small and so widely separated that presumably they just orbit each other as the electron clouds annihilate, and eventually enough energy is generated that the ionic plasma of nuclei and anti-nuclei just expands, with a small fraction of them actually ever combining.

In other words, we don't actually see total conversion happening -- only a small fraction of the total mass in an anti-matter/matter collision is turned into gamma rays.

Is that what actually would happen? (The ideal answer would be a video!)

Answer 1

I believe this is where your thought experiment goes wrong: However, the nuclei are so small and so widely separated that presumably they just orbit each other as the electron clouds annihilate

Why would they be widely separated? Solids touch in order of nanometers. The protons and antiprotons would attract each other with the 1/r^2 electric field, which is very strong. Even if the energy of the projectile is small enough to form "protoniums", ie. have antiprotons in the place of electrons in the quantum mechanical solution, the wave function will be so large that the protons antiproton wave function will overlap and annihilate. The lifetime is of the order of .1 to 10 microseconds.

To get an idea of the annihilation Here is a bubble chamber photo of an antiproton entering from below annihilating on a proton at rest int he chamber. The multiplicity is on pions and kaons which will then decay with their characteristic decays.
The multiplicity is high even for annihilation at rest.

If the projectile has high enough energy, it will be a scattering with the cross section of antiproton on proton, depending on the area, to get the percentage annihilated. It is very difficult to get a plasma of protons and antiprotons anyway. Annihilations have a very large crossection. The antiprotons will either scatter elastically if they have enough energy, and leave the samples or annihilate if in the region of protons.

If the two bits of matter are in space with only their own gravity, then some will escape a first scatter and then fall back and annihilate or escape if they have escape velocity.