T. Mijatović

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.

Oscillations above the barrier in the fusion of 28 Si + 28 Si

Abstract Fusion cross sections of 28 Si + 28 Si have been measured in a range above the barrier with a very small energy step ( Δ E lab = 0.5 MeV ). Regular oscillations have been observed, best evidenced in the first derivative of the energy-weighted excitation function. For the first time, quite different behaviors (the appearance of oscillations and the trend of sub-barrier cross sections) have been reproduced within the same theoretical frame, i.e., the coupled-channel model using the shallow M3Y + repulsion potential. The calculations suggest that channel couplings play an important role in the appearance of the oscillations, and that the simple relation between a peak in the derivative of the energy-weighted cross section and the height of a centrifugal barrier is lost, and so is the interpretation of the second derivative of the excitation function as a barrier distribution for this system, at energies above the Coulomb barrier.

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.

Relevance of single-particle and collective excitations in zirconium isotopes populated by neutron transfer reactions in the 90Zr+208Pb system

Multineutron transfer reaction {sup 90}Zr+{sup 208}Pb has been studied at the energy close to the Coulomb barrier energy by using the PRISMA + CLARA set-up. In this fragment-γ coincidence measurement, the selective properties of the reaction mechanism in the population of the specific states have been discussed. Based on the observed γ transitions of neutron transfer channels, namely {sup 89–94}Zr isotopes, their level schemes have been constructed and updated.

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.

Sub-barrier transfer reactions studied with the magnetic spectrometer PRISMA

Transfer reactions in the sub-barrier region have been studied at Laboratori Nazionali di Legnaro (LNL) by using the large solid angle magnetic spectrometer PRISMA. In particular, excitation functions of multinucleon transfer channels have been measured for the systems 96Zr+40Ca (closed shell nuclei), 116Sn+60Ni (super-fluid nuclei) from the Coulomb barrier to 25% below. Results of these measurements are presented in this contribution. An ancillary detection system has been designed and built in order to perform kinematic coincidence measurements in conjunction with PRISMA. It will allow to study the effects of secondary processes, such as evaporation and fission, which strongly affect the final yield of the reaction products. The status of this development is also reported.

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.

Analysis of states in {sup 13}C populated in {sup 9}Be + {sup 4}He resonant scattering

Measurements of 9Be+α resonant scattering have been performed using the thick-target approach with a 4He gas volume and a large area silicon strip detector. 9Be beam energies in the range 12 to 21.4 MeV were used to measure the 13C excitation energy spectrum between 13.2 and 16.2 MeV. An R-matrix analysis has been performed to characterize the spins and widths of 13C resonances, some of which have been proposed to be associated with a 3α+n molecular band.