L. Lamia

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.

NEW DETERMINATION OF THE 2H(d,p)3H AND 2H(d,n)3He REACTION RATES AT ASTROPHYSICAL ENERGIES

The cross sections of the 2H(d,p)3H and 2H(d,n)3He reactions have been measured via the Trojan Horse method applied to the quasi-free 2H(3He,p 3H)1H and 2H(3He,n 3He)1H processes at 18 MeV off the proton in 3He. For the first time, the bare nucleus S(E) factors have been determined from 1.5 MeV, across the relevant region for standard Big Bang nucleosynthesis, down to the thermal energies of deuterium burning in the pre-main-sequence (PMS) phase of stellar evolution, as well as of future fusion reactors. Both the energy dependence and the absolute value of the S(E) factors deviate by more than 15% from the available direct data and existing fitting curves, with substantial variations in the electron screening by more than 50%. As a consequence, the reaction rates for astrophysics experience relevant changes, with a maximum increase of up to 20% at the temperatures of the PMS phase. From a recent primordial abundance sensitivity study, it turns out that the 2H(d,n)3He reaction is quite influential on 7Li, and the present change in the reaction rate leads to a decrease in its abundance by up to 10%. The present reaction rates have also been included in an updated version of the FRANEC evolutionary code to analyze their influence on the central deuterium abundance in PMS stars with different masses. The largest variation of about 10%-15% pertains to young stars (≤1 Myr) with masses ≥1 M ☉.

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

Big Bang nucleosynthesis revisited via Trojan Horse Method measurements

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New measurement of the 11B(p,α0)8Be bare-nucleus S(E) factor via the Trojan horse method

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Measurement of the 20 and 90 keV resonances in the 18O(p,alpha)15N reaction via the Trojan horse method.

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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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Measurement of the 20 and 90 keV resonances in the

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