Nucleosynthesis of heavy elements in the r-process
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EMENTARY PARTICLES AND FIELDS Theory
Nucleosynthesis of Heavy Elements in the r-Process I. V. Panov* Institute for Theoretical and Experimental Physics, National Research Center Kurchatov Institute, Bol’shaya Cheremuskinskaya ul. 25, Moscow, 117218 Russia Received August 3, 2015
Abstract—The current state of the problem of heavy-element production in the astrophysical r-process is surveyed. The nucleosynthesis process in the neutron-star-merger scenario, within which the problem of free-neutron source is solved, is considered most comprehensively. A model that describes well the observed abundances of heavy elements is examined. Theoretical approaches used in this model to calculate a number of features of short-lived neutron-rich nuclei are described. The contributions of various fission processes to the production of heavy elements are assessed. The possibility of superheavy-element production in the r-process is demonstrated. DOI: 10.1134/S1063778816020137
INTRODUCTION
1. CONDITIONS AND SCENARIOS OF THE r-PROCESS
Chemical elements heavier than those of the iron peak are produced in nature owing largely to neutroncapture reactions followed by beta decay. An analysis of the curve of element abundances in the solar system reveals [1] that the neutron-induced synthesis of elements heavier than iron proceeds in two processes characterized by different conditions. The first of them is slow neutron capture (so-called s-process); it proceeds in the case where the rates of the beta decay of product unstable nuclei are substantially higher than the (n, γ)-reaction rates—that is, λβ λnγ (at neutron densities satisfying the condition nn ≤ 1016 cm−3 )—with the results that the nucleosynthesis path goes along the boundary of long-lived nuclei. The s-process mechanism is rather well understood, since either stable or long-lived nuclei well studied experimentally are involved in this process. The second process is rapid (r-process); it proceeds under conditions characterized by high neutron-density values such that λβ λnγ , nuclei involved in such a nucleosynthesis process having a high neutron excess and a short lifetime. Numerous studies of this process over the past 50 years determined quite well conditions necessary for the synthesis of heavy nuclei (see, for example, [2] and references therein). However, the site of the r-process and the dynamics of changes in its conditions (that is, the astrophysical scenario of the emergence of conditions for the occurrence of the rapid-nucleosynthesis process) have yet to be clarified conclusively.
After the appearance of the article by G. R. Burbidge and his coauthors [3], who classified various nucleosynthesis processes in nature, different authors proposed different r-process models (both steadystate [4] and dynamical [5]) leading to the emergence of conditions under which heavy-element synthesis is possible. More than 10 scenarios were put forth throughout the period of studying the r-process (see, for example, [6–8]). They include an explosion at the neutron-star surface [9], a c
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