# Reduced $E$1 $S$-factor of $^{12}$C($\alpha$,$\gamma_0$)$^{16}$O

## TL;DR

This study uses R-matrix theory to analyze the astrophysical E1 S-factor for the $^{12}$C($ extalpha$,$ extgamma_0$)$^{16}$O reaction, revealing a reduced S-factor influenced by channel radius and molecular structure effects.

## Contribution

The paper introduces a non-linear R-matrix approach to determine the E1 S-factor, emphasizing the impact of channel radius and pole proximity on the reaction's nuclear dynamics.

## Key findings

- The E1 S-factor is not significantly enhanced by the subthreshold 1$^-_1$ state at a channel radius of 4.75 fm.
- The proximity of the 1$^-_2$ and 1$^-_1$ poles suppresses interference effects.
- A small channel radius results in low penetrability and a reduced E1 S-factor below the Coulomb barrier.

## Abstract

The astrophysical $S$-factor of $E$1 transition for $^{12}$C($\alpha$,$\gamma_0$)$^{16}$O is discussed in the $R$-matrix theory. The reduced $\alpha$-particle widths of the 1$^-_1$ ($E_x= 7.12$ MeV) and 1$^-_2$ ($E_x= 9.59$ MeV) states are extracted from the result of the potential model. The formal parameters are obtained without the linear approximation to the shift function. The resultant $E$1 $S$-factor is not strongly enhanced by the subthreshold 1$^-_1$ state if the channel radius is 4.75 fm. The calculated $\beta$-delayed $\alpha$-particle spectrum of $^{16}$N and the $p$-wave phase shift of $\alpha$+$^{12}$C elastic scattering are also found to be consistent with the previous studies. The small channel radius leads to the low penetrability to the Coulomb barrier, and it makes the reduced $E$1 $S$-factor below the barrier. Owing to the large reduced width from the molecular structure, the $R$-matrix pole of the 1$^-_2$ state is shifted in the vicinity of 1$^-_1$. The proximity of the two poles suppresses the interference between the states. The transparency of the $\alpha$+$^{12}$C system appears to be expressed as the shrinking strong interaction region.