nuclear physics - What is the "penetrability factor"?

I have read/heard this term a few times in nuclear physics papers. I'm guessing it has something to do with the Coloumb barrier of a nucleus. Could you maybe explain what this "penetrability factor" is?

[See page 63 of this paper]

Answer

The penetrability factor is a carefully defined factor that separates the coulomb-force and the interesting nuclear-forces in the relationship between the partial decay width and the reduced width (for R-Matrix analysis) of a particular resonance.

$$\Gamma = 2\gamma^2P(l,\rho,\eta)$$

Here $\Gamma$ is the partial decay width, $\gamma$ is the reduced width, $l$ is the orbital angular momentum of the state, $\rho$ is defined as $k$ (wavenumber) multiplied by $r$ the target particle radius, $\eta$ is the sommerfeld [nuclear] parameter, and $P(l,\eta,\rho)$ is the penetrability factor. The penetrability factor is defined as,

$$P(l,\rho,\eta)=\frac{\rho}{\left(F_l (\eta,\rho)\right)^2+\left(G_l (\eta,\rho)\right)^2}$$

where $F$ and $G$ are the regular and irregular coulomb wave functions, respectively.

Source: http://www.scholarpedia.org/article/The_R-matrix_theory_in_nuclear_and_atomic_physics (equation 10 and 13)