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The 12 C þ 12 C fusion reaction plays a critical role in the evolution of massive stars and also strongly impacts various explosive astrophysical scenarios. The presence of resonances in this reaction at energies around and below the Coulomb barrier makes it impossible to carry out a simple extrapolation down to the Gamow window-the energy regime relevant to carbon burning in massive stars. The 12 C þ 12 C system forms a unique laboratory for challenging the contemporary picture of deep sub-barrier fusion (possible sub-barrier hindrance) and its interplay with nuclear structure (sub-barrier resonances). Here, we show that direct measurements of the 12 C þ 12 C fusion cross section may be made into the Gamow window using an advanced particle-gamma coincidence technique. The sensitivity of this technique effectively removes ambiguities in existing measurements made with gamma ray or charged-particle detection alone. The present cross-section data span over 8 orders of magnitude and support the fusion-hindrance model at deep sub-barrier energies.

Prompt-fission γ -ray spectra (PFGS) have been measured for the 238 U(n, f ) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission γ rays were measured using a number of LaBr 3 scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission γ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for 238 U(n, f )andalsofor 252 Cf (sf ) as a reference. Spectral characteristics of PFG emission, such as mean γ multiplicity and average total γ -ray energy per fission, as well as the average γ -ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at E n = 1.9 MeV were found to be the mean γ multiplicity M γ = 6.54 ± 0.19, total released energy per fission E γ,tot = 5.25 ± 0.20 MeV, and the average γ -ray energy ǫ γ = 0.80 ± 0.04 MeV. Under similar conditions, the values at E n = 4.8 MeV were measured to be M γ = 7.31 ± 0.46, E γ,tot = 6.18 ± 0.65 MeV, and ǫ γ = 0.84 ± 0.11 MeV.