C. Spitaleri

The Bare Astrophysical S(E) Factor of the 7Li(p, α)α Reaction

The astrophysically important 7Li(p, α)α reaction has been studied via the Trojan horse method in the energy range E = 10-400 keV. A new theoretical description, based on the distorted-wave Born approximation approach, allows one to extract information on the bare astrophysical S-factor, Sb(E), with Sb(0) = 55 ± 3 keV barns. The results are compared with direct experimental data leading to a model-independent value of the electron screening potential energy, Ue = 330 ± 40 eV, much higher than the adiabatic limit Uad = 175 eV.

Indirect techniques in nuclear astrophysics: a review

In this review, we discuss the present status of three indirect techniques that are used to determine reaction rates for stellar burning processes, asymptotic normalization coefficients, the Trojan Horse method and Coulomb dissociation. A comprehensive review of the theory behind each of these techniques is presented. This is followed by an overview of the experiments that have been carried out using these indirect approaches.

Indirect investigation of the {ital d}+{sup 6}Li reaction at low energies relevant for nuclear astrophysics

The indirect investigation of low-energy charged-particle reactions relevant for nuclear astrophysics is considered, employing statistical tensor analysis. Data from the quasi-free contribution to the {sup 6}Li({sup 6}Li, 2{alpha}){sup 4}He reaction at projectile energies above the Coulomb barrier are analyzed in order to extract the excitation function of the {sup 6}Li({ital d},{alpha}){sup 4}He reaction at very low relative energy. The results are compared with an astrophysical factor extracted from the free-reaction cross section data. The limits of the method are discussed. {copyright} {ital 1996 The American Astronomical Society.}

AN UPDATED 6Li(p, α)3He REACTION RATE AT ASTROPHYSICAL ENERGIES WITH THE TROJAN HORSE METHOD

The lithium problem influencing primordial and stellar nucleosynthesis is one of the most interesting unsolved issues in astrophysics. 6Li is the most fragile of lithiums stable isotopes and is largely destroyed in most stars during the pre-main-sequence (PMS) phase. For these stars, the convective envelope easily reaches, at least at its bottom, the relatively low 6Li ignition temperature. Thus, gaining an understanding of 6Li depletion also gives hints about the extent of convective regions. For this reason, charged-particle-induced reactions in lithium have been the subject of several studies. Low-energy extrapolations of these studies provide information about both the zero-energy astrophysical S(E) factor and the electron screening potential, Ue . Thanks to recent direct measurements, new estimates of the 6Li(p, ?)3He bare-nucleus S(E) factor and the corresponding Ue value have been obtained by applying the Trojan Horse method to the 2H(6Li, ? 3He)n reaction in quasi-free kinematics. The calculated reaction rate covers the temperature window 0.01 to 2T 9 and its impact on the surface lithium depletion in PMS models with different masses and metallicities has been evaluated in detail by adopting an updated version of the FRANEC evolutionary code.

A Novel Approach to Measure the Cross Section of the 18O(p, α)15N Resonant Reaction in the 0-200 keV Energy Range

The 18O(p, ?)15N reaction is of primary importance to pin down the uncertainties, due to nuclear physics input, affecting present-day models of asymptotic giant branch stars. Its reaction rate can modify both fluorine nucleosynthesis inside such stars and oxygen and nitrogen isotopic ratios, which allow one to constrain the proposed astrophysical scenarios. Thus, an indirect measurement of the low-energy region of the 18O(p, ?)15N reaction has been performed to access, for the first time, the range of relevance for astrophysical application. In particular, a full, high-accuracy spectroscopic study of the 20 and 90 keV resonances has been performed and the strengths deduced to evaluate the reaction rate and the consequences for astrophysics.

Recent evaluation of the 7Li(p, α)4He reaction rate at astrophysical energies via the Trojan Horse method

Context. The charged-particle induced reactions on lithium have been studied by several works. In particular, several direct measurements of the 7 Li(p, α) 4 He reaction have been performed to extrapolate its low-energy astrophysical S (E)-factor and evaluate the electron screening potential, Ue. Aims. In view of recent direct measurements, we discuss our estimate of the bare-nucleus S (E)-factor and an evaluation of the corresponding reaction rate. Moreover, we present the extracted electron screening potential. Methods. The Trojan Horse method (THM) allows us to measure the 7 Li(p, α) 4 He bare-nucleus S (E)-factor down to energies of ∼10 keV, rendering unnecessary the extrapolation procedure typically used by the direct measurements. The 7 Li(p, α) 4 He S (E)-factor is deduced from the 2 H( 7 Li, α 4 He)n reaction by selecting the quasi-free contribution to the reaction yield. The planewave impulse approximation has proved an effective approach as distortions appear negligible. Results. The THM enable us to measure the 7 Li(p, α) 4 He S (E)-factor over the energy region of interest for astrophysics, namely 0.01 < Ecm < 0.4 MeV. The zero-energy S (E)-factor and the Ue electron screening potential have been measured and compared with the available direct data. From the TH measure of the 7 Li(p, α) 4 He S (E)-factor, the reaction rate calculation has been performed in correspondence with the temperature window of 0.01 < T9 < 2, which is typical of several astrophysical sites where Li burning could shed light on some open questions, such as mixing phenomena. A variation of ∼13%, with respect the adopted NACRE one, has been found at temperatures of about T9 = 10 −3 , while a variation of ∼5% has been found at higher temperatures. By considering in the calculation the upper and lower limits to the TH reaction rate, no significant variation in the Li abundances of low mass giant stars follows or, if any modification occurs, this is negligible in comparison to the uncertainties in the free parameters considered in the extra-mixing model of Palmerini and collaborators.

Electron-screening effects on fusion reactions

It is in the nature of astrophysics that many of the processes and most of the objects one tries to understand are physically inaccessible. Thus, it is important that those aspects that can be studied in the laboratory be rather well understood. The electron-screening effect on fusion reactions addresses one such aspect: the observed effect under laboratory conditions is presently not understood.

6He + α clustering in 10Be

In a kinematically complete measurement of the 7Li(7Li,α6He)4He reaction at Ei = 8 MeV it was observed that the 10Be excited states at 9.6 and 10.2 MeV decay by 6He emission. The state at 10.2 MeV may be a member of a rotational band based on the 6.18 MeV 0+ state.

First application of the Trojan horse method with a radioactive ion beam: Study of the 18 F(p,α)15 O reaction at astrophysical energies

Measurement of nuclear cross sections at astrophysical energies involving unstable species is one of the most challenging tasks in experimental nuclear physics. The use of indirect methods is often unavoidable in this scenario. In this paper the Trojan Horse Method is applied for the first time to a radioactive ion beam induced reaction studying the

On the Measurement of the 13C(α, n)16O S-factor at Negative Energies and its Influence on the s-process

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