Background: The formation of superheavy elements by fusion of two massive nuclei is severely inhibited by the competing quasifission process.

Accepted for publication in Physics Letters BInternational audienceThe role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries

Coincidence measurements of breakup fragments were carried out for the ${}^{7}\text{Li}+{}^{144}\text{Sm}$ and ${}^{6,7}\text{Li}+{}^{207,208}\text{Pb}$,${}^{209}\text{Bi}$ reactions at sub-barrier energies. Breakup modes in reactions of ${}^{6,7}\text{Li}$ were identified through the reaction $Q$ values, and the time-scales of each process inferred through the relative energy of the breakup fragments. Breakup was found to be predominantly triggered by nucleon transfer, with $p$ pickup leading to $$\alpha${}+$\alpha${}$ coincidences being the preferred breakup mode for ${}^{7}\text{Li}$, and $n$ stripping leading to $$\alpha${}+p$ for ${}^{6}\text{Li}$. Breakup triggered by $2n$ stripping was also found to be prominent in the ${}^{7}\text{Li}+{}^{144}\text{Sm}$ reaction. The breakup yields were separated into prompt and delayed components based on the relative energies of the breakup fragments. This enables the identification of breakup process important in the suppression of complete fusion of ${}^{6,7}\text{Li}$ at above-barrier energies.

Fusion excitation functions have been measured for the first time with rather good accuracy for S32+Zr90 and S32+Zr96 near and below the Coulomb barrier. The sub-barrier cross sections for S32+Zr96 are much larger than for S32+Zr90. A coupled-channels calculation considering the inelastic excitations is capable of describing sub-barrier enhancement only for S32+Zr90. The unexplained part for S32+Zr96 is found to be correlated with the positive-Q-value intermediate neutron transfers in this system. The comparison with Ca40+Zr96 suggests that couplings to the positive-Q-value neutron transfer channels may play a role in the sub-barrier fusion enhancement. Multi-neutron transfers are taken into account in Zagrebaev's semiclassical model to explain the discrepancies of the sub-barrier fusion cross sections for S32+Zr96.

The transfer reactions of ${}^{11}\mathrm{B}{\mathrm{}(d,p)\mathrm{}}^{12}\mathrm{B}$ and ${}^{12}\mathrm{C}{\mathrm{}(d,p)\mathrm{}}^{13}\mathrm{C},$ at incident energy of $11.8\hspace{0.5em}\mathrm{MeV},$ have been used to extract asymptotic normalization coefficients and root-mean-square radii for the last neutron in ${}^{12}\mathrm{B}$ and ${}^{13}\mathrm{C}.$ It is found experimentally that the second ${(J}^{\pi }{=2\mathrm{}}^{-})$ and third ${(J}^{\pi }{=1\mathrm{}}^{-})$ excited states in ${}^{12}\mathrm{B}$ and the first ${(J}^{\pi }{=1/2\mathrm{}}^{+})$ excited state in ${}^{13}\mathrm{C}$ are the neutron halo states, whereas the third ${(J}^{\pi }{=5/2\mathrm{}}^{+})$ excited state in ${}^{13}\mathrm{C}$ is a neutron skin state.