Weak interaction, fusion, and tunneling

[ephedyn]

I have 2 questions...

I was told that the transmutation of Li-7 to Be-8 by proton collisions at energies below the classical barrier verifies the QM description of the weak interaction cross section. Firstly, I found the leap in logic very confusing - does the weak force have a role in tunneling and fusion at all?

I only knew that the weak interaction is at work in the deuteron formation step in the proton-proton chain and beta decay.

I've been studying the subject only from Enge's Introduction to Nuclear Physics, which doesn't seem to help with my task. I need to figure what exactly is "the quantum mechanical description of reaction cross sections" for certain 1~2 MeV collisions... It seems to me that most introductory texts only briefly mention tunneling for collisions, and use a semi-classical description involving surmounting a Coulomb barrier, which you can say is the opposite of what I'm looking for. So, secondly, are there any texts one would recommend which are more relevant? And if the weak force is indeed implicated here, where can I start an understanding of the quantum description of the weak force?

Thanks in advance!

 

[bcrowell]

I was told that the transmutation of Li-7 to Be-8 by proton collisions at energies below the classical barrier verifies the QM description of the weak interaction cross section. Firstly, I found the leap in logic very confusing - does the weak force have a role in tunneling and fusion at all?

8Be is unstable with respect to breakup into two alpha particles. This is a strong-force decay, so its decay time is very short (less than a femtosecond). In the WP article on nucleosynthesis, for example, they list the reaction as 7Li+p -> 4He+4He, since the intermediate 8Be state is just a resonance, not a bound state.

I don't know if there's some small probability of 7Li+p -> 8Li or 7Li+p -> 8B. If so, then that would be an example of a weak-interaction process.

I need to figure what exactly is "the quantum mechanical description of reaction cross sections" for certain 1~2 MeV collisions... It seems to me that most introductory texts only briefly mention tunneling for collisions, and use a semi-classical description involving surmounting a Coulomb barrier, which you can say is the opposite of what I'm looking for.

I don't think there are two ways of looking at it, one semiclassical and one involving tunneling. For a given process, it's either classically forbidden or its not. If the energy is below the Coulomb barrier, then it's classically forbidden, and you have to use tunneling to get the cross-section. If the energy is above the Coulomb barrier, then there can be quantum-mechanical corrections, but basically the total cross-section for nuclear reactions is pretty close to what you expect classically.