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High energy states in the self-conjugate 16O nucleus were investigated by analyzing 13C(3He,α)12C reactions in the bombarding energy range 1.4–2.2 MeV. Thanks to a low-threshold and high-resolution array, we succeed in measuring cross sections of the decay channel where an α particle is emitted in coincidence with a 12C nucleus in the Hoyle state, leading to 4α particles in the final channel. The analysis of angular distributions indicates the occurrence of two low-spin resonant states, respectively with Jπ=2+ and 3−, at energies of ≈ 24.1 and 24.5 MeV. In this energy window, the observed branching ratios for the transition involving the emission of the Hoyle state are well larger than calculations based on barrier penetration, signaling the possible occurrence of high-energy states in 16O with a large cluster component.

The 19F(p, α0)16O reaction at low energies plays an important role in fundamental physics. In particular in nuclear astrophysics it represents, together with the 19F(p, γ)20Ne reaction, the crossing point between the CNO and the NeNa cycles in stars. Further, in hydrogen-rich stellar environments, it is the most important fluorine destruction channel. In this paper we report new measurements on the 19F(p, α0)16O reaction at deeply sub-Coulomb energies (0.2–0.6 MeV), a region where, despite the key role of this reaction, very few and old data are reported. The deduced astrophysical S-factor is ≈1.5–2 times larger than currently adopted extrapolations with possibly important astrophysical consequences.

Light charged-particle multiplicities in the evaporation residue (ER) and fission (FF) channels from the reaction 200MeV 32S + 100Mo as well as ER and FF channel cross-sections have been measured and compared to the predictions of the statistical model (SM) to estimate the fission time scale. The statistical model fails in reproducing the whole set of data and no convincing estimate is possible. In particular, while pre-scission multiplicities can be reproduced, the model strongly overestimates proton and alpha particle multiplicities in the ER channel, irrespective of the SM input parameters and prescriptions used for the level density and the transmission coefficients. Same calculations performed on data from literature in the A $ \approx$ 150 and excitation energy Ex $ \approx$ 100-200 MeV region, for the ER channel, provide similar conclusions. These findings repropose the problem of the reliability of the SM in describing the compound-nucleus decay and have a relevant impact on the extraction of the fission delay time through the use of the SM. [ABSTRACT FROM AUTHOR]

The total cross-section of 12C(α,γ)16O was measured for the first time by a direct and ungated detection of the 16O recoils. This measurement in inverse kinematics using the recoil mass separator ERNA in combination with a windowless He gas target allowed to collect data with high precision in the energy range E = 1.9 to 4.9 MeV. The data represent new information for the determination of the astrophysical S( E) factor. [ABSTRACT FROM AUTHOR]

: The 32S + 100Mo and 36S + 96Mo fusionlike reactions were studied at incident energy of Elab = 298 MeV and 320 MeV, respectively, with the aim of probing the influence of the entrance channel charge asymmetry on the dipole γ-ray emission. The excitation energy and spin distribution of the compound nucleus created in these reactions were identical, the only difference being associated with the unequal charge asymmetry of the two entrance channels. High-energy γ-rays were detected in an array of 9 seven-pack BaF2 clusters. Coincidence with fusionlike residues detected in four PPAC ensured the selection of central reaction events. By studying the differential γ-ray multiplicity associated with the two reactions it was shown that the dipole strength excited in the compound nucleus increases with the entrance channel charge asymmetry. From the linearized spectra, the increase of the GDR γ-ray intensity was found to be ∼ 25% for the more charge asymmetric system. The results are discussed and compared with those of previous data obtained at different incident energies. [ABSTRACT FROM AUTHOR]

We report on the results obtained from the study of the32S +64Ni and32S +58Ni peripheral reactions at incident energies Elab = 288 MeV and Elab = 320 MeV, respectively. High-energy γ-rays were detected in an array of 8 seven-pack BaF2 clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. All of the relevant reaction parameters were kept constant with the exception of the different initial dipole moment caused by the different entrance channel charge asymmetry. While for quasi-elastic events no N/ Z effect was observed in the differential γ-ray multiplicities of the two reactions, for deep-inelastic events a larger dipole γ-ray emission occurs during the more N/ Z asymmetric reaction. A theoretical interpretation based on a collective Bremsstrahlung analysis of the reaction dynamics is presented. [ABSTRACT FROM AUTHOR]

High energy states in the self-conjugate 16O nucleus were investigated by analyzing 13C(3He, α)12C reactions in the bombarding energy range 1.4–2.2 MeV. Thanks to a low-threshold and high-resolution array, we succeed in measuring cross sections of the decay channel where an α particle is emitted in coincidence with a 12C nucleus in the Hoyle state, leading to 4 α particles in the final channel. The analysis of angular distributions indicates the occurrence of two low-spin resonant states, respectively with J π = 2 + and 3 − , at energies of ≈ 24.1 and 24.5 MeV. In this energy window, the observed branching ratios for the transition involving the emission of the Hoyle state are well larger than calculations based on barrier penetration, signaling the possible occurrence of high-energy states in 16O with a large cluster component. [ABSTRACT FROM AUTHOR]

The 19 F(p,  α 0 ) 16 O reaction at low energies plays an important role in fundamental physics. In particular in nuclear astrophysics it represents, together with the 19 F(p,  γ ) 20 Ne reaction, the crossing point between the CNO and the NeNa cycles in stars. Further, in hydrogen-rich stellar environments, it is the most important fluorine destruction channel. In this paper we report new measurements on the 19 F(p,  α 0 ) 16 O reaction at deeply sub-Coulomb energies (0.2–0.6 MeV), a region where, despite the key role of this reaction, very few and old data are reported. The deduced astrophysical S -factor is ≈ 1.5 – 2 times larger than currently adopted extrapolations with possibly important astrophysical consequences. [ABSTRACT FROM AUTHOR]

Abstract: The reactions are studied at 14.5 A MeV. Evidence is found for important odd–even effects in isotopic observables of selected peripheral collisions corresponding to the decay of a projectile-like source. The influence of secondary decays on the staggering is studied with a correlation function technique. It is shown that this method is a powerful tool to get experimental information on the evaporation chain, in order to constrain model calculations. Specifically, we show that odd–even effects are due to interplay between pairing effects in the nuclear masses and in the level densities. [Copyright &y& Elsevier]

Abstract: The reactions and are studied at 14.5 A MeV. After a selection of the collision mechanism, we show that important even–odd effects are present in the isotopic fragment distributions when the excitation energy is small. Close to the multifragmentation threshold this staggering appears hidden by the rapid variation of the production yields with the fragment size. Once this effect is accounted for, the staggering appears to be a universal feature of fragment production, slightly enhanced when the emission source is neutron poor. A closer look at the behavior of the production yields as a function of the neutron excess , reveals that odd–even effects cannot be explained by pairing effects in the nuclear masses alone, but depend in a more complex way on the de-excitation chain. [Copyright &y& Elsevier]

The reaction 12C(α, γ)16O plays a key role in determining the C/O ratio in the stellar core at the end of helium burning, which in turn influences the further evolution and nucleosynthesis. The determination of the cross section at the relevant astrophysical energy , which is of the order of 10−17 b, requires extrapolation of the S factor determined from data collected with high precision and accuracy at higher energy. A new experimental approach has been undertaken in order to study this reaction for the first time by the direct detection of the 16O nuclides using the recoil mass separator ERNA. [Copyright &y& Elsevier]