# What is an 'S-factor' in nuclear physics?

I have seen the "S-factor" in many places, but I've never read an explanation of what it actually is. I have read that it is related to the cross-section of a reaction, but that's about it.

• Can you link to an example of the context? (Also, where did you look to try to figure this out yourself?) – David Z Jan 21 '15 at 5:51
• Can't it be that you make a confusion with the S-matrix? – Sofia Jan 21 '15 at 8:14

The S-factor separates out the Coulomb interaction (barrier) energy term from the cross-section $\sigma$, e.g.,

$$\sigma(E) = E^{-1} e^{-2\pi\eta} S(E)$$ where $\eta$ is a dimensionless factor relating to the Coulomb barrier.

It can be useful for finding a small resonance within a large electric field (image from here):

• What's that grey tail at the left-hand side of the S-factor vs. energy graph? – Arturo don Juan Jan 21 '15 at 14:58
• Also, after reading a few articles online, I have seen that most people write it as $\sigma (E) = S(E) E^{-1}e^{-2\pi \eta}$. What do you have to say about that? – Arturo don Juan Jan 21 '15 at 15:54
• My error, that is the correct form - will edit. – Kieran Hunt Jan 21 '15 at 15:54
• Thanks. Also, a quick question - a resonance and the Gamow Window are unrelated, right? (I think you can tell why I'm asking) – Arturo don Juan Jan 21 '15 at 16:40
• I could open a new question for that if it would be more fitting. – Arturo don Juan Jan 21 '15 at 16:47