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

¹ Dipartimento di Fisica e Astronomia, Università di Catania, Italy
e-mail: llamia@lns.infn.it
² Laboratori Nazionali del Sud-INFN, Catania, Italy
³ Centro Siciliano di Fisica Nucleare e Struttura della Materia (CSFNSM), Catania, Italy

Received: 10 February 2012
Accepted: 15 March 2012

Abstract

Context. The charged-particle induced reactions on lithium have been studied by several works. In particular, several direct measurements of the ⁷Li(p, α)⁴He reaction have been performed to extrapolate its low-energy astrophysical S(E)-factor and evaluate the electron screening potential, U_e.

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 ⁷Li(p, α)⁴He bare-nucleus S(E)-factor down to energies of  ~10 keV, rendering unnecessary the extrapolation procedure typically used by the direct measurements. The ⁷Li(p, α)⁴He S(E)-factor is deduced from the ²H(⁷Li, α⁴He)n reaction by selecting the quasi-free contribution to the reaction yield. The plane-wave impulse approximation has proved an effective approach as distortions appear negligible.

Results. The THM enable us to measure the ⁷Li(p, α)⁴He S(E)-factor over the energy region of interest for astrophysics, namely 0.01 < E_cm < 0.4 MeV. The zero-energy S(E)-factor and the U_e electron screening potential have been measured and compared with the available direct data. From the TH measure of the ⁷Li(p, α)⁴He 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.