Source of the Rare-Earth Element Peak inr-Process Nucleosynthesis

Abstract

We use network calculations of r-process nucleosynthesis to pin down the origin of the peak in the solar r-process abundance distribution near nuclear mass number A ഠ 160. The peak is due to a subtle interplay of nuclear deformation and b decay, and forms not in the steady phase of the r process, but only just prior to freeze-out, as the free neutrons rapidly disappear. Its existence should therefore help constrain the conditions under which the r process freezes out. [S0031-9007(97)04032-5] PACS numbers: 26.30. + k The r process is responsible for synthesizing roughly half the heavy nuclei in the solar system [1]. It is widely believed to occur in type II supernovae, beginning at a time when the density of free neutrons is so high that neutron capture by nuclei occurs much more rapidly than nuclear b decay. Under these conditions equilibrium between neutron capture and photodisintegration [called ͑n, g͒-͑g, n͒ equilibrium] establishes itself so that before long very neutron-rich isotopes of each element are populated with their relative abundances set only by their neutron separation energies and partition functions, and the temperature and density of the environment. Towards the end of this "steady" phase of the r process-so called because creation and destruction reactions balance and abundances change slowly-the neutrons begin to disappear, making ͑n, g͒-͑g, n͒ equilibrium more difficult to maintain and causing nuclear abundances to change rapidly. Eventually the neutron capture and photodisintegration reactions "freeze out," and the nuclei simply b decay back to the stability line to give the final r-process abundances.The dominant features in the solar r-process abundance distribution are large peaks at nuclear mass numbers A ഠ 80, 130, and 195. These form as the nuclear flow passes through neutron-closed-shell nuclei, which capture neutrons reluctantly and have long beta-decay lifetimes, acting as bottlenecks at which abundances build up. This mechanism for peak formation has been well understood for many years [2]. The second most pronounced feature in the abundance distribution is a smaller but still very distinct peak at A ഠ 160, the region of the rare-earth elements (REEs). In contrast to the major peaks, its origin has remained something of a mystery since its presence was noted in the first detailed compilations of solar-system abundances [3,4]. In this paper we show that the REE peak is due to deformation in nuclei created after the steady phase of the r process ends. Previous r-process studies have usually limited themselves to the steady phase. Thus, although deformation has long been suspected to play a role in the formation of the REE peak [2], no compelling connection has been made until now.Nuclei deform when deformation increases stability. Up to a point, increasingly neutron-rich isotopes of an element can thus gain relative stability by increasing their deformation. At some isotope, however, deformation can increase no more; the next isotope is then less stable. Because of the sudden drop i...

Neutrino-driven wind simulations and nucleosynthesis of heavy elements

“…Neutron capture rates [87] have been shown to have a non-negligible impact on the final abundances as they shift the position of peaks and affect the rare-earth peak (A ≈ 160) [88,89,47,90]. Reactions involving neutrinos have been discussed extensively within the neutrino-driven wind models.…”

The Rare Earth Peak and the Astrophysical Location of the r Process

“…A prime candidate for such a study is the rare earth peak (REP), which is believed to be formed during the freeze-out or last stage of the r process when nuclei decay back to stability. The formation of the REP has been shown to be sensitive to both astrophysical conditions and nuclear physics inputs [2][3][4][5]. Recently, Monte Carlo studies of nuclear masses have been used in the region to explore the trends required to reproduce the solar REP [6,7].…”

Bands and Isomers in Neutron-Rich Rare-Earth Nuclei in PHF Model

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