Nuclear transparency and the onset of strong absorption regime in the 12 C + 24 Mg system

The elastic scattering of ${}^{12}\mathrm{C}{+}^{24}\mathrm{Mg}$ has been studied by means of a phase-shift analysis of 21 angular distributions ranging from ${E}_{\mathrm{lab}}=16\mathrm{MeV}$ up to ${E}_{\mathrm{lab}}=40\mathrm{MeV}.$ A tridimensional plot of the reflection coefficient of the S matrix as a function of the angular momentum and energy shows a well-defined region of energy, which separates two regimes: strong absorption for higher energies and the so-called ``anomalous transparency regime,'' recently observed in this system at low energies. The Argand diagrams of the S matrix in angular momentum space also present very contrasting behaviors in the two regions with very rapidly varying phases in the low energy region, which we associate with a parity dependent term in the S matrix directly related to significant coupling to the elastic transfer of a ${}^{12}\mathrm{C}$ nucleus.