On the nucleosynthesis of lithium, beryllium, and boron

Abstract

Listen

Translate article

The nuclear physics of the spallation reactions leading to the formation of lithium, beryllium, and boron from proton bombardment on light nuclei is reviewed. A large amount of experimental data is gathered, including in particular several measurements made at Orsay with the 150-MeV synchrocyclotron. A simple rule is devised which classifies quite neatly the cross sections in term of their isotopic spin parameters. This rule can be used to predict the results of unmeasured reactions. For 12C and 16O target nuclei, more accurate determinations are made with the help of calculations based on statistical models. All this information is used to determine the formation ratios of the Li, Be, and B isotopes in CNONe, first by protons of fixed energies, second by cosmic ray protons with an extended energy spectrum. We do not know of course the exact shape of the spectrum giving rise to these Li Be B, but we may assume that they are of the type commonly found in cosmic ray physics: exponential in rigidity or power of kinetic energy. Fortunately, the isotopic ratios thereby obtained are largely insensitive to the shape of the spectrum. Different ratios could of course be obtained from certain special types of spectra (for instance from much steeper spectra). This possibility is only briefly discussed here; it will be more fully discussed in a companion paper. The nuclear reactions possibly responsible for the alteration of the ratios after their formation are discussed next. Finally, the observations of these elements in natural settings are summarized, compared and discussed in terms of the formation and destruction rates, and some conclusions are drawn. The astronomical information gathered so far is consistent with the following picture: each star generates its own Li, Be, B, by proton irradiation of the stellar atmosphere. The isotopes 6Li and 7Li are later partly depleted by proton capture reactions, presumably at the bottom of the convective zone. One of the main results of this paper is that, within the uncertainty of the evaluation, the meteoritic 11B to 10B ratio is equal to its formation ratio on CNONe. This result therefore leads, for the formation of the light elements in the solar system, to a model somewhat different from the FGH one. The 7Li to 6Li formation ratio has a value smaller than the meteoritic one, but the alteration can be ascribed to ( p, α) reactions at temperatures between 2 and 4 million degrees at the bottom of the surface convective zone of the young sun, and this process would not alter the 11B 10B ratio. A tentative picture of the history of the solar system would go as follows: from energetic considerations, we are led to think that the Li Be B have been formed even before the Hayashi (fully convective) phase of the sun. The 7Li 6Li ratio would then have been altered during the Hayashi phase. This material would still later have been separated from the sun. An alternative picture is also sketched, based on the assumption of a special type of proton spectrum, of the kind mentioned above.