A Very Important Process of Nucleosynthesis in Stars

At the pre-explosion stage of a supernova with M≈15-30M_⊙,the core with ρ≥5×10¹¹(g/cm³),R≥l0⁷ cm and M₁=4π/3R³ ρ≥M_⊙ collapses to a neutron star.The gravitational energy emitted by each nucleon E_n~140MeV transforms into the radiation energy which prevents the material outside R>10₇ cm from collapsing and causes the explosion of the-supernova.The γ-radiation is so strong that it makes the Fe⁵⁶-Ni⁶⁰ nuclei with the mass of~0.2M_⊙ within the inner shell dissociate and the proton-neutron gass with the neutron number density n_n~l0²⁸-l0³⁰/cm³ form.At the same time,the radiation pressure transforms into the kinetic energy of the dispersed material,which sets off shock waves.Thus the local density is ρ>10¹² g/cm³ and the neutronization is accelerated,which causes the Ca⁵⁶-Ca⁶⁰ nuclei with the masses of 1-2 M_⊙ to collide with each other and to form the nuclei with A~112-360.Finally,the nuclei(A140-440,Z~30-82) rich in neutrons can-be formed by adding a large amount of neutrons.When these nuclei are free from the strong gravitational field,they,will be unstable but will becom stable nuclei through the competitions of the following processes.(1) neutron-evaporation,(2) spontaneous fission and(3)β^--decay.At the initial stage,(1〉and(2) are important and(3) can be ignored.The qualitative results are as follows;(1) It seems that the nuclei with A>l00 come from the spontaneous fission and β^--decay of the neutron-evaporation nuclei with A~140-440,which can replace the γ-process.(2) The superheavy elements with Z=114-126(A~330-360) can be formed.They can be observed in cosmic rays if they have the half-time τ_1/2>l0⁷ years.(3) The peak in the rare-earth element comes from the symmetric fission of superheavy elements,(4) There are more neutron-rich nuclei in the-fragments.,(5) The abundaxicei of Z>83 elements in cosmic rays arc one order,of magnitude higher than that in the solar system.