V. Tokić

First Measurement of the 19F(α, p)22Ne Reaction at Energies of Astrophysical Relevance

The observational 19F abundance in stellar environments systematically exceeds the predicted one, thus representing one of the unsolved challenges for stellar modeling. It is therefore clear that further investigation is needed in this field. In this work, we focus our attention on the measurement of the 19F(α, p) 22Ne reaction in the astrophysical energy range, between 0.2 and 0.8 MeV (far below the Coulomb barrier, 3.8 MeV), as it represents the main destruction channel in He-rich environments. The lowest energy at which this reaction has been studied with direct measurements is ∼0.66 MeV, covering only the upper tail of the Gamow window, causing the reaction-rate evaluation to be based on extrapolation. To investigate lower energies, the 19F(α, p) 22Ne reaction has been studied by means of the Trojan horse method, applied to the quasi-free 6Li (19F, p22Ne)2H reaction at Ebeam = 6 MeV. The indirect cross section of the 19F(α, p) 22Ne reaction at energies ≲1 MeV was extracted, fully covering the astrophysical region of interest and overlapping existing direct data for normalization. Several resonances have been detected for the first time inside the Gamow window. The reaction rate has been calculated, showing an increase up to a factor of 4 with respect to the literature at astrophysical temperatures. This might lead to potential major astrophysical implications.

The thick target inverse kinematics technique with a large acceptance silicon detector array

An experimental technique for studying elastic scattering using a thick gas target is described, with a measurement of the α(24Ne,α) reaction used as an example. Advantages such as ease, detector efficiency, and the possibility of measuring the cross section at 180° in the centre-of-mass are discussed. It is shown that a resolution of tens of keV is practical at zero degrees, and that the dominant contribution to the resolution for large angles is angular straggling of the beam in the entrance window. The use of helium gas as the target allows direct measurement of a-cluster states.

AGB nucleosynthesis: The 19F(α, p)22Ne reaction at astrophysical energies

Learning how 19F is produced and destructed in AGB-stars is crucial. Fluorine abundance is in fact important, given that it is strongly tied to standard and extra-mixing processes taking place in AGB-stars. This kind of objects are considered to be the main sources of fluorine in galactic environment, in which experimental abundances are far overestimated. For this reason the reaction 19F(α, p)22Ne, that represents the main destruction channel in He-rich environment, was studied at energies corresponding to T∼2·108 K. Such reaction has been studied with direct method at Ebeam = 1100 keV for alpha particles impinging on a fluorine target, corresponding to EC.M. ∼ 900 keV, still far from the Gamow window, placed at 390÷800 keV, below the Coulomb barrier (3.81 MeV). An experiment was performed at Rujer Boskovic Institut (Zagreb), applying the Trojan Horse Method. With this experimental procedure we were able to select the quasi-free contribution coming from 6Li(19F,p 22Ne)2H at Ebeam = 6 MeV at kinematically...

First evidences for 19F(α, p)22Ne at astrophysical energies

19F experimental abundances is overestimated in respect to the theoretical one: it is therefore clear that further investigations are needed. We focused on the 19F(α, p) 22 Ne reaction, representing the main destruction channel in He-rich environments. The lowest energy at which this reaction has been studied with direct methods is E C.M. ≈ 0.91 MeV, while the Gamow region is between 0.39 ÷ 0.8 MeV, far below the Coulomb barrier (3.8 MeV). For this reason, an experiment at Rudjer Boskovic Institute (Zagreb) was performed, applying the Trojan Horse Method. Following this method we selected the quasi-free contribution coming from 6Li(19 F,p22 Ne)2 H at Ebeam=6 MeV at kinematically favourable angles, and the cross section at energies 0 < EC.M. < 1.4 MeV was extracted in arbitrary units, covering the astrophysical region of interest.

The pairing correlation study in the 40Ar+208Pb reaction

The multinucleon transfer reaction 40Ar+208Pb has been measured at Elab = 260 MeV with the large solid angle magnetic spectrometer PRISMA. Mass and charge yields, differential and total cross sections, total kinetic energy loss distributions of different channels were simultaneously studied. Angular distributions were measured in a wide angular range by matching three different angular settings of the spectrometer. Absolute cross sections were obtained after the response function of the spectrometer was evaluated. These cross sections for different transfer channels allow the discussion of the role played by nucleon-nucleon correlations.

Study of interstrip gap effects and efficiency for full energy detection of Double Sided Silicon Strip Detectors

In this work is reported a study on the response of double sided silicon strip detectors. In order to investigate the effect of the electrode segmentation on the detector response, two experiments were performed aimed to measure the efficiency for full energy detection. Results show that the efficiency for full energy detection, that is directly related to effective width of the inter-strip region, varies with both detected ion energy and bias voltage. The experimental results are qualitatively reproduced by a simplified model based on the Shockley-Ramo-Gunn framework.

The sneaky interstrip gap of double sided silicon strip detector

In this work the effects of the segmentation of the electrodes of Double Sided Silicon Strip Detectors (DSSSDs) are investigated. Two experiments were performed with the aim to caharacterize the response of the DSSSDs. Results show that the efficiency for full energy detection, that is directly related to effective width of the inter-strip region, varies with both detected energy and bias voltage. The experimental results are qualitatively reproduced by a simplified model based on the Shockley-Ramo-Gunn framework.

Study of 24Mg resonances relevant to carbon burning nucleosynthesis

We have studied the decays of the resonances of 24Mg at excitation energies 1–6 MeV above the 12C+12C decay threshold, using the 12C(16O,α)24Mg* reaction at E(16O) = 94 MeV. Some preliminary results are presented and further analysis is in progress.

13C+4He resonant elastic scattering on a thick gas target

Resonant elastic scattering of 13C on a thick 4He target has been measured. Reaction products from 13C(4He, 4He) were detected at several beam energies and gas pressures. Raw yield was efficiency corrected using Monte Carlo simulations and an averaged yield spectrum was produced. The observed peaks correspond well to the previously measured elastic scattering data.

Study of 24Mg resonances relevant for carbon burning nucleosynthesis

We have studied decays of the 24Mg states at excitations 1-6 MeV above the 12C+12C decay threshold, using the 12C(16O,α)24Mg* reaction at E(16O) = 94MeV. Some preliminary results are presented and further analysis is in progress.