Proton-Capture Rates on Carbon Isotopes and Their Impact on the Astrophysical

The ^{12}C/^{13}C ratio is a significant indicator of nucleosynthesis and mixing processes during hydrogen burning in stars. Its value mainly depends on the relative rates of the ^{12}C(p,γ)^{13}N and ^{13}C(p,γ)^{14}N reactions. Both reactions have been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy down to the lowest energies to date (E_{c.m.}=60 keV) reaching for the first time the high energy tail of hydrogen burning in the shell of giant stars. Our cross sections, obtained with both prompt γ-ray detection and activation measurements, are the most precise to date with overall systematic uncertainties of 7%-8%. Compared with most of the literature, our results are systematically lower, by 25% for the ^{12}C(p,γ)^{13}N reaction and by 30% for ^{13}C(p,γ)^{14}N. We provide the most precise value up to now of 3.6±0.4 in the 20-140 MK range for the lowest possible ^{12}C/^{13}C ratio that can be produced during H burning in giant stars.

Creation of pure non-crystalline diamond nanostructures via room-temperature ion irradiation and subsequent thermal annealing.

Carbon exhibits a remarkable range of structural forms, due to the availability of sp3, sp2 and sp1 chemical bonds. Contrarily to other group IV elements such as silicon and germanium, the formation of an amorphous phase based exclusively on sp3 bonds is extremely challenging due to the strongly favored formation of graphitic-like structures at room temperature and pressure. As such, the formation of a fully sp3-bonded carbon phase requires an extremely careful (and largely unexplored) definition of the pressure and temperature across the phase diagram. Here, we report on the possibility of creating full-sp3 amorphous nanostructures within the bulk crystal of diamond with room-temperature ion-beam irradiation, followed by an annealing process that does not involve the application of any external mechanical pressure. As confirmed by numerical simulations, the (previously unreported) radiation-damage-induced formation of an amorphous sp2-free phase in diamond is determined by the buildup of extremely high internal stresses from the surrounding lattice, which (in the case of nanometer-scale regions) fully prevent the graphitization process. Besides the relevance of understanding the formation of exotic carbon phases, the use of focused/collimated ion beams discloses appealing perspectives for the direct fabrication of such nanostructures in complex three-dimensional geometries.

Optical and surface enhanced Raman scattering properties of Au nanoparticles embedded in and located on a carbonaceous matrix.

Au nanoparticles (NPs) on the surface and embedded in a matrix have been the subject of studies dealing with a variety of spectroscopic and sensing applications. Here, we report on low energy Ar ion induced evolution of the morphology of a thin Au film on a polyethylene terephthalate (PET) substrate along with thermodynamic interpretations, and corresponding unique surface plasmon resonance (SPR) and photoluminescence (PL) properties. These properties are linked to the variation of surface nanostructures and the surface enhanced Raman scattering (SERS) effect of methyl orange (MO) dye molecules adsorbed on the surface. Ion induced thermal spike and sputtering resulted in dewetting of the film with subsequent formation of spherical NPs. This was followed by embedding of the NPs in the modified PET due to the thermodynamic driving forces involved. The surface and interface morphologies were studied using atomic force microscopy and cross-sectional transmission electron microscopy. X-ray photoelectron spectroscopy was used to study the chemical changes in the system upon irradiation. The optical properties were studied by diffuse reflectance UV-Vis spectroscopy and PL using a 325 nm He-Cd laser. The red shift of the SPR absorption and the blue shift of the PL emission have been correlated with the surface morphology. The blue PL emission bands at around 3.0 eV are in good agreement with the literature with respect to the morphological changes and the blue shift is attributed to compressive strain on the embedded Au NPs. Enhancement of the SERS signals is observed and found to be correlated with the SPR response of the Au nanostructures. The SERS analyses indicate that MO molecules may be adsorbed with different orientations on these surfaces i.e. Au NPs located on the surface or embedded in the modified PET. These polymeric substrates modified by NPs can have a potential application in solid-state light emitting devices and can be applied in SERS based sensors for the detection of organic compounds.

First direct measurement of the 17O(p,γ)18F reaction cross section at Gamow energies for classical novae.

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive (18)F, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The (17)O(p,γ)(18)F reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes, but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range E(c.m.)~/= 200-370 keV appropriate to hydrogen burning in classical novae. In addition, the E(c.m.)=183 keV resonance strength, ωγ=1.67±0.12 µeV, has been measured with the highest precision to date. The uncertainty on the (17)O(p,γ)(18)F reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.

Reflectance measurements and optical constants in the extreme ultraviolet-vacuum ultraviolet regions for SiC with a different C/Si ratio.

Reflectance versus incidence angle measurements have been performed from 5 to 152 nm on samples of SiC with a different C/Si ratio deposited with rf magnetron sputtering. The optical constants of the material at different wavelengths have been determined by using a curve-fitting technique of reflectance values versus incidence angle. Complementary measurements of the incident beam polarization, film thickness, surface roughness, and stoichiometry were performed to complete the analysis of the samples.