Reduction of deuterium content in carbon targets for 12C+12C reaction studies of astrophysical interest

The 12C(12C,p)23Na and 12C(12C,$\alpha$)20Ne fusion reactions are among the most important in stellar evolution since they determine the destiny of massive ($M \simeq 8-10 M_{\odot}$) stars. However, experimental low-energy investigations of such reactions are significantly hampered by ubiquitous natural hydrogen and deuterium contaminants in the carbon targets. The associated beam-induced background completely masks the reaction products of interest thus preventing cross-section measurements at the relevant energies of astrophysical interest, $E_{\mathrm{cm}} < 2$ MeV. In this work, we report about an investigation aimed at assessing possible deuterium reductions on both natural graphite and Highly Ordered Pyrolytic Graphite targets as a function of target temperature. Our results indicate that reductions up to about 80% can be attained on both targets in the temperature range investigated, $T \simeq 200-1200 {}^{\circ}$C. A further reduction by a factor of 2.5 in absolute deuterium content is observed when the scattering chamber is surrounded by a dry nitrogen atmosphere so as to minimise light-particles uptake within the chamber rest gas (and thus on target) through air leaks. The results from this study will inform the choice of optimal experimental conditions and procedures for improved measurements of the 12C + 12C reactions cross-sections at the low energies of astrophysical interest

Helium burning and neutron sources in the stars

Helium burning represents an important stage of stellar evolution as it contributes to the synthesis of key elements such as carbon, through the triple-alfa process, and oxygen, through the 12C(alfa, gamma)16O reaction. It is the ratio of carbon to oxygen at the end of the helium burning stage that governs the following phases of stellar evolution leading to different scenarios depending on the initial stellar mass. In addition, helium burning in Asymptotic Giant Branch stars, provides the two main sources of neutrons, namely the 13C(alfa, n)16O and the 22Ne(alfa, n)25Mg, for the synthesis of about half of all elements heavier than iron through the s-process. Given the importance of these reactions, much experimental work has been devoted to the study of their reaction rates over the last few decades. However, large uncertainties still remain at the energies of astrophysical interest which greatly limit the accuracy of stellar models predictions. Here, we review the current status on the latest experimental efforts and show how measurements of these important reaction cross sections can be significantly improved at next-generation deep underground laboratories

Electron screening in 7Li(p,α)α and 6Li(p,α)3He for different environments

The electronscreening in the 7Li(p,α)α reaction has been studied at Ep=30 to 100 keV for differentenvironments: Li2WO4 insulator, Li metal, and PdLi alloys. For the insulator a screening potential energy of Ue=185±150 eV was observed, consistent with previous work and the atomic adiabatic limit. However, for the Li metal and the PdLi alloys we find large values of Ue=1280±60 and 3790±330 eV, respectively: the values can be explained by the plasma model of Debye applied to the quasi-free metallic electrons in these samples. Similar results have been found for the 6Li3(p,α)He reaction supporting the hypothesis of the isotopic independence of the electronscreening effect. The data together with previous studies of d(d,p)t and 9Be6(p,α)Li in metals verify the Debye model scaling Ue∝Zt (charge number of target)