Breakup of helium nuclei in high-temperature plasmas

The rate of breakup of helium nuclei by particle-induced reactions is computed. It is shown that the rate is determined by the endothermic reaction p+4He->3He+d, becoming effective at kT> or approx. few MeV; the characteristic breakup temperature has a logarithmic dependence on density and available time. The rate is computed from experimental data on the reaction cross section, extrapolating to lower energies near threshold with the insertion of Coulomb-barrier penetration factors. The Coulomb factor for the outgoing particles, being essentially an exponential function of energy, results in a ''Gamow peak'' for the reaction rate and, effectively, a shift of the threshold energy from 18 MeV to about 24 MeV. Competing particle-induced breakup reactions are considered, in particular, the process p+4He->3He+p+n. It is found that the energy dependence of the cross section (sigma3) for this reaction can be understood in terms that (sigma2) of the former reaction (->3He+d) from phase space considerations. The magnitude of the calculated sigma2/sigma3 also agrees well with experimental data, being determined by the 2- and 3-body phase space factors and the short-range deuteron wave function usub(d)(0). A brief discussion is given of the possibility of the occurrence of helium breakup in astrophysical plasmas. The phenomenon might be seen in matter ejected from the vicinity of black holes, for example. (orig.)