Speaker
Description
The physics underlying the fusion hindrance phenomenon [a] and, consequently, its features
in the various systems, as well as their link to different nuclear structure situations, may be related to the Pauli exclusion principle [b], but it has not yet been fully clarified. Furthermore, we know that its existence in the fusion of light systems may have significant consequences in astrophysics [c], since it impacts the stellar evolution and nucleosynthesis of heavier elements. The experimental study of fusion in light systems (Q$_{fus}$ > 0) is particularly challenging, especially when trying to identify the hindrance phenomenon, given the S-factor oscillations observed in some cases.
The behaviour of slightly heavier cases may allow a reliable extrapolation towards $^{12}$C+$^{12}$C and nearby systems. In recent years, we investigated the cases of $^{12}$C + $^{28-30}$Si,$^{24-26}$Mg, and for all of them, the hindrance phenomenon has been observed. In particular, the fusion excitation function of $^{12}$C + $^{28}$Si has been measured down to ~40 nb [d], using the $^{28}$Si beams from the XTU Tandem accelerator of LNL, and by detecting the evaporated charged particles with two DSSD in coincidence with the prompt $\gamma$-rays identified by the spectrometer AGATA. The figure on the left shows the excitation function, where the red dots were obtained by the electrostatic deflector PISOLO, allowing a useful normalisation of the AGATA data (blue dots). Very recently, the fusion of the lighter system $^{12}$C + $^{19}$F has been studied. The hindrance effect also shows up in this case, as reported in the centre figure, where the logarithmic derivative of the energy-weighted cross sections vs the energy reaches the LCS limit.
https://drive.google.com/file/d/15EvGpA3mjeEfJwnC7iGhNH-ACb6vHJyG/view?usp=sharing
Left: Fusion excitation function of $^{12}$C + $^{28}$Si; Centre: Logarithmic derivative of $^{12}$C + $^{19}$F fusion; Right: Hindrance systematics of medium-light systems
$^{12}$C + $^{19}$F has a very large positive Q-value (~23MeV), and no sub-barrier fusion data were available before the recent experiment. The threshold energy for the onset of hindrance has been obtained and included in the systematics shown in the figure on the right [e], which reports the threshold (E$_s$) for several medium-light systems with Q$_{fus}$>0. The abscissa is the parameter $\zeta$ characterising the system [c] ($\mu$ is the reduced mass). The magenta line is a phenomenological fit of the experimental systematics using the function shown. The thresholds for several light systems going from $^{10}$B + $^{10}$B to $^{16}$O+$^{16}$O (open dots, plotted with increasing $\zeta$) are extrapolations from higher energies using the hindrance model [a,c]. They were not included in the fit to the measured data for the heavier systems, which leads to an extrapolation to the lighter ones, very close to the hindrance model results. In the talk, the experimental evidence will be examined in depth, offering a basis for discussing the conclusions that can be drawn from these measurements.
[a] C. L. Jiang, et al., Phys. Rev. Lett. 89, 052701 (2002)
[b] C. Simenel et al., Phys. Rev. C 95, 032601(R) (2015)
[c] C. L. Jiang, et al., Phys. Rev. C 75, 015803 (2007); Phys. Rev. C 79, 044601 (2009)
[d] A. M. Stefanini et al., Phys. Lett. B 872, 140084 (2026)
[e] A. M. Stefanini et al., Phys. Rev. C 111, 064620 (2025), and to be published