Indirect techniques for astrophysical reaction rates determinations

Abstract

Direct measurements of nuclear reactions of astrophysical interest can be challenging. Alternative experimental techniques such as transfer reactions and inelastic scattering reactions offer the possibility to study these reactions by using stable beams. In this context, I will present recent results that were obtained in Orsay using indirect techniques. The examples will concern various astrophysical sites, from the Big-Bang nucleosynthesis to the production of radioisotopes in massive stars.The main characteristics of the nuclear reactions involved in stellar nucleosynthesis is the low energy where they generally occur, between few keV to few MeV and at these energies the cross sections of these reactions are very small ranging from fbarn to hundreds of pbarn especially when it involves charged particles. This makes the direct measurements at stellar energies very difficult and often impossible. Hence direct measurement are usually performed at higher energies and then extrapolated down to stellar energies where the reactions occur. However the extrapolations can lead sometimes to wrong results when a possible very low energy resonance or the tail of a possible sub-threshold resonance are not taken into account. 201, 0 0 EPJ Web of Conferences

Primordial Nucleosynthesis

“…Leaving aside this unsolved lithium mystery, the precision of 1.6% (or better [19]) on the observed D/H value [4], requires that the uncertainties on the D(p,γ) 3 He, D(d,n) 3 He and D(d,p) 3 H rates that govern deuterium BBN destruction be reduced to a similar level. Indeed, a +1% variation of these rates induces a respectively -0.32, -0.46 and -0.54% variation of D/H [5].…”

“…We chose the second option, with Marcucci et al [20] [D(p,γ) 3 He] and Arai et al [21] [D(d,n) 3 He and D(d,p) 3 H] as theoretical S -factors, keeping the normalization (α) as a free parameter that has to be determined by comparison with experimental data. The procedure we followed [5] for the D(p,γ) 3 He reaction was i) to select experimental datasets [22][23][24][25] for which systematic uncertainties were provided, ii) determine for each data set, by χ 2 minimization, the normalization factor to be applied to the theoretical S -factor of Marcucci et al [20], iii) add quadratically the systematic uncertainties and iv) perform a weighted average of the normalization factor. We obtained α = 0.9900 ± 0.0368 for this factor, that was subsequently used to scale the Marcucci et al S -factor, and calculate the thermonuclear D(p,γ) 3 He reaction rate and associated uncertainty.…”

“…The procedure we followed [5] for the D(p,γ) 3 He reaction was i) to select experimental datasets [22][23][24][25] for which systematic uncertainties were provided, ii) determine for each data set, by χ 2 minimization, the normalization factor to be applied to the theoretical S -factor of Marcucci et al [20], iii) add quadratically the systematic uncertainties and iv) perform a weighted average of the normalization factor. We obtained α = 0.9900 ± 0.0368 for this factor, that was subsequently used to scale the Marcucci et al S -factor, and calculate the thermonuclear D(p,γ) 3 He reaction rate and associated uncertainty. This result is quite robust given the data and the theoretical S -factor, as verified using bayesian techniques instead [26].…”

“…However, an improved calculation of the S -factor by Marcucci et al (2016) [30] lies significantly above the previous calculation: if one applies the same renormalisation method one finds α = 0.915 ± 0.038. We used the same procedure for D(d,n) 3 He and D(p,γ) 3 He except that the theoretical S -factor is taken from Arai et al [21]. All three reaction rates are higher than previous evaluations at BBN temperatures leading to a decrease in the D/H prediction, as shown in Table I.…”

“…Experiments have now excluded a conventional nuclear physics solution (see e.g. [3] and references therein), even though a few uncertain reaction rates could marginally affect Li/H predictions. This lithium problem has recently worsened.…”