How 'Messy' are Fusion Reaction Chains in Stars?

[Drakkith]

TL;DR

How 'Messy' are Fusion Reaction Chains in Stars?

When looking up stellar nucleosynthesis and the various reactions that occur inside stars, I often see very straightforward reaction chains, such as this one for the Silicon burning process (isotope numbers and such left out):

$Si + He \to S$
$S + He \to Ar$
$Ar + He \to Ca$

And so forth down to Iron fusing with helium to make Nickel.

But surely this entire chain is MUCH more complicated ('messy'), right? I assume there are various side chains along with photodisintegration happening all along the main chain. Does anyone have any good references that go into a little more detail about this?

 

[Vanadium 50]

I think there's a problem involving this in Clayton. As I recall, it was...messy.

 

[sbrothy]

I fear It’s considerably more complicated and involved as even wiki will tell you. A quick search on arXiv using just the words “solar fusion” opens up an entire goldmine of info.

Happy reading. :)

 

[Vanadium 50]

Fundamentally, it's the same kind of coupled DE that we all leaned not know and love with the "tanks of brine" problems. I think the issues are a) an analytic solution is not very enlightening, and b) a real problem is probably attacked numerically anyway.

 

[phyzguy]

I once asked a scientist doing simulations of stellar fusion in stars which nuclei he included in the simulations. His answer, "All of them".

 

[Astronuc]

Remember, the stable isotopes for Si are ²⁸Si, ²⁹Si and ³⁰Si, and there are the odd n-capture, and if EM fields are strong enough, photonuclear reactions.

 

[BVirtual]

The fusion type you posted on with He atoms is formally called "Alpha Ladder" or sometimes "Alpha Process." It is the continuation of the "Triple Alpha Process" that creates Carbon from 3 Helium atoms. I am documenting the "messy" details for a project I am working on.

There are over 30 minor reaction equations to include in the 'basic' Alpha Ladder. I need the percentage probability of each, along with the released energy, that is the kinetic energy of the fused product, or kinetic energy of the fused atom when it fissions/fragments into two atoms (and rays), as is common with fusion. Or an atom with a neutron and a ray type ... is rare. I need the Gamma Ray energy emitted (when present), and optionally the much less energy of the emitted neutrino, when present. Wikipedia is getting more of these minor reaction equations and associated numerical values. Look for "element_name_here burning" for a few of the reaction equations.

EMPIRE 3.2 Malta software release has all the information I need, but writing an algorithm to fetch it out is very messy. I estimated/thought it would be easier to write code to extract it from ENDF database, but turned out that is much worse. Someone claims to have written an easier API, which I got and need to read its documentation. Next, I will try "Expect" to manipulate the graphic user interface, to fetch the dozens of reactions, along with many initial cross-sections/energy level of initial reactants. Why? The GUI output is more graphs, until you click to ask for numerical tables the graphs are based on. Sigh. Very messy indeed. Easier would be to hire an author of EMPIRE. Hmm, just thought of that.

Regarding photodisintegration that is one way to attenuate, erh, absorb the Gamma Rays created in the Alpha Ladder. The percentage is going to be rather low given the cross section of the Gamma Ray and nuclei. Gamma Ray attenuation is done mostly through ionizing collisions with electron shells. Any where from 1 to 3 or more electrons will be knocked lose from an atom, until the Gamma Ray is can be captured by a nucleus, or more likely is further attenuation to X-Rays and even to UV, and then ionization of inner electron orbitals. An occasional X-Ray will hit a nucleus, and cause various types of havoc there.

I do know if the process is hot enough that spontaneous fragmentation does occur. Modified Maxwell-Blotzmann Velocity Distribution curves predict this rarely happens.