Measurement of the 30Si(n, γ) and 64Ni(n, γ) cross sections at CERN n_TOF

Neutron capture reactions play a crucial role in nuclear astrophysics as they are at the base of the stellar nucleosynthesis processes that synthesize the elements heavier than iron. The cross sections of these reactions represent therefore important inputs for stellar nucleosynthesis models and their accurate knowledge is essential to predict reliable stellar yields and isotopic abundances. In particular, the neutron capture cross section of 30Si plays a pivotal role in explaining the silicon isotopic abundances observed in presolar components of meteorites (SiC grains), while the capture cross section of 64Ni has been found to significantly affect the predicted abundance of many isotopes produced by neutron-capture nucleosynthesis in massive and Asymptotic Giant Branch (AGB) stars. The small cross sections of these isotopes make their measurement particularly challenging and therefore, despite their importance, data available in the literature are scarce and discrepant. For this reason, new time-of-flight measurements have been performed at the n_TOF facility, a pulsed white neutron source at CERN characterized by a wide neutron energy range, high instantaneous neutron flux and excellent energy resolution. Highly isotopically enriched samples have been employed in the measurements, together with a setup of deuterated benzene liquid scintillators optimized to minimize the background induced by scattered neutrons. The measured reaction yields have been fitted with the R-Matrix code SAMMY to extract resonance parameters and the Maxwellian Averaged Cross Sections (MACS) of astrophysical interest at different stellar temperatures have been finally computed. The results obtained from both measurements show important discrepancies with respect to the cross sections recommended in most of the evaluated nuclear data libraries: huge resonances expected in the energy range of astrophysical interest are not observed and the measured resonance kernels are significantly different from the recommended values. As a consequence, our MACS at 25 keV for 30Si(n,γ) lies in between the values recommended by different evaluations and between the values reported by the two latest measurements, while our MACS for 64Ni(n,γ) at 8 keV is significantly lower than all the current evaluations, but similar to the results of previous measurements.