Rapid neutron capture

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

Beyond iron lies a first population of so-called s-process nuclei, which includes among others barium and lead. This population has an abundance distribution with peaks around mass numbers 87, 138 and 208. These nuclei are produced by slow neutron capture, referred to as the s process. A second population, slightly shifted from the first, including gold, platinum and uranium, is imputed to the process of rapid neutron capture, referred to as the r process. 

Heavy nucleus abundances in ancient stars are determined by rapid neutron capture, very probably associated with type II supernovas. 

The heavier isotopes of the element may result from rapid neutron capture process caused by intense neutron fluxes from thermonuclear explosions, followed by a series of p decay (Cunningham, B.D. 1968. Curium. In Encyclopedia of Chemical Elements, ed. C. A. Hampel, pp. 173-177. New York Reinhold Book Corp.)... 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

Fig. 4. Nucleosynthesis of superheavy nuclei in the r-process rapid neutron capture alternating with IT-transilions during a supernova explosion. Shown in the Z-N plane is the r-process path of very neutron-rich nuclei extending to ZA100, from where p -decay chains directed towards the belt of p-stable nuclei would lead to Z 114, N=184 nuclei (dots). From D.N. Schramm and W.A. Fowler [28], reprinted with permission from Nature, Copyright (2002) Macmillan Magazines Limited.

Neutron absorption processes occur at different times and places in the course of the evolution of massive stars. The S-process (slow neutron capture) occurs in the He burning region (state of a red giant). The R-process (rapid neutron capture) occurs during the super nova explosion, either within a short distance of the forming neutron star or in the shell where He burning took place prior to the blast when the shock wave hits this area. 

The r process is the rapid neutron capture process. In this process, the neutron density is so high that neutron captures occur much more rapidly than beta decays. The result is that the isotopes of an element come into equilibrium under exchange of neutrons. Flow to the next element occurs as these isotopes then beta decay. Since the nuclei present are more neutron rich than the stable isotopes, the nuclear flow can reach up to the actinides. The limit on the extent of the flow may be that the heavy nuclei produced simply fission when they get too large. 

Hillebrandt, W. The rapid neutron-capture process and the synthesis of heavy and neutron-rich elements. Space Sci. Rev. 21, 639-702 (1978)... 

Figure 1 A schematic curve of atomic aboundance (reiative to Si = 10 ) versus mass number A for the Sun and simiiar main sequence stars. The symbois s, rand p stand for the slow and rapid neutron capture processes and the proton capture process, respectively.

H = high L = low m = mass p = proton capture process r = rapid neutron capture process s = slow neutron capture process T = time period from big bang to isolation of the solar nebula = the age of the Solar System 6 = deviation from terrestrial com-position(%o) A = time period from isolation of solar nebula to solidification of Solar System bodies Aw = difference in mass e = 1 part of 10 . ... 

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