Neutron capture processes

Dust as we are, the immortal spirit grows Like harmony in music there is a dark Inscrutable workmanship that reconciles Discordant elements. 

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Neutron capture processes give rise to the so-called magic-number peaks in the abundance curve, corresponding to closed shells with 50, 82 or 126 neutrons (see Chapter 2). In the case of the s-process, the closed shells lead to low neutron-capture cross-sections and hence to abundance peaks in the neighbourhood of Sr, Ba and Pb (see Fig. 1.4), since such nuclei will predominate after exposure to a chain of neutron captures. In the r-process, radioactive progenitors with closed shells are more stable and hence more abundant than their neighbours and their subsequent decay leads to the peaks around Ge, Xe and Pt on the low-A side of the corresponding s-process peak. 

A separate neutron capture process is needed for neutron-rich nuclides by-passed by the s-process and for species above 209Bi. A possible path for this rapid or r-process is shown in Fig. 6.9. 

In addition to all these fusion and neutron capture processes, there is a further type of nuclear reaction, called spallation. Rather than fusing together, nuclei are smashed up or chipped to produce smaller species. This process is thought to be the origin of most of the lithium, beryllium and boron in the Universe. 

The term s process is an abbreviation for slow neutron capture process . Here, capture is slow relative to the characteristic time for internal transformation of the neutron into a proton (radioactive decay). Between two neutron captures, there is ample time for () decay to occur. The r process represents quite the opposite situation. Neutron capture is not interrupted by () decay. 

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.)... 

For the slow neutron capture process, there is an equilibrium between the production and loss of adjacent nuclei. Stable nuclei are only destroyed by neutron capture. For such nuclei, we can write for the rate of change of a nucleus with mass number A ... 

For process 3, only slow neutrons of defined energy (25 eV) were effective, making it a typical resonance neutron capture process. Thus it appeared that all three processes were neutron capture, with the effective isotope in all three cases being 238U. 

Although it is quite efficient when it occurs, the p process acts on only a small amount of mass in stars. This causes the yields of the p process to be much less than those for the neutron-capture processes, which are restricted to lower temperatures. But those neutron irradiations at lower temperatures either occur in a much larger... 

The third means of radionuclide production involves target irradiation by ions accelerated in a cyclotron. One example of this approach is provided by the production of Ge, which decays with a 280 day half-life to the positron emitter Ga. Proton irradiation of Ga produces Ge in a (p,2n) reaction. After dissolution of the target material a solution of the Ge product in concentrated HCl is prepared and adsorbed on an alumina column which has been pre-equilibrated with 0.005 M EDTA (ethylenediaminetetraacetate) solution. The Ga daughter may then be eluted using an EDTA solution in a system which provides the basis of a Ga generator. Cyclotron production of radionuclides is expensive compared with reactor irradiations, but higher specific activities are possible than with the neutron capture process. Also, radionuclides with particularly useful properties, and which cannot be obtained from a reactor, may be prepared by cyclotron irradiation. In one example, cyclotron produced Fe, a positron emitter, may be used for bone marrow imaging while reactor produced Fe, a /3-emitter, is unsuitable. " ... 

Neutron capture reactions can occur at low temperatures but stars can activate neutron sources only at high temperatures. It was apparent very early in the search for the origins of the chemical elements that two different neutron capture processes are at work in the Universe - see the classical papers by Cameron (1957) and Burbidge, Burbidge, Fowler Hoyle (1957). The processes are distinguished by the neutron density achieved at the... 

For heavy elements with atomic masses greater than ss 56 their proton numbers are so large that electrostatic repulsion inhibits charged particle reactions (e.g., proton and a captures) except under very specific conditions. Most heavy nuclei are instead formed by neutron addition onto abundant Fe-peak elements. For this reason, neutron-capture processes are secondary. That is, they require that some Fe-peak nuclei (e.g., 56Fe) is already present in the star. The solar abundance distribution is characterized by peaks that can be explained by... 

Neutron-capture processes require a source of free neutrons, given that neutrons are unstable and decay in 10 minutes. There are two important neutron sources available during He-shell burning in AGB stars ... 

One of the most basic questions in nuclear astrophysics is How do the nuclei heavier than iron get produced This question was first answered by Burbidge, Burbidge, Fowler and Hoyle in 1957 [35]. They proposed that these elements are produced through the slow (s) and rapid (r) neutron-capture processes. The words rapid/slow refer to the rate of neutron capture compared to the rate of /3-decay in the astrophysical conditions. Figure 13 shows the nuclei involved in the r- and s-processes. The s-process path stays close to the valley of stability whereas the r-process path moves staying close to the drip line. The figure also shows the nuclei involved in the rp-process these are proton rich nuclei where capture of protons are involved and that the rate is compared to the / + rates. 

The book has been organized into three parts to address the major issues in cosmochemistry. Part I of the book deals with stellar structure, nucleosynthesis and evolution of low and intermediate-mass stars. The lectures by Simon Jeffery outline stellar evolution with discussion on the basic equations, elementary solutions and numerical methods. Amanda Karakas s lectures discuss nucleosynthesis of low and intermediate-mass stars covering nucleosynthesis prior to the Asymptotic Giant Branch (AGB) phase, evolution during the AGB, nucleosynthesis during the AGB phase, evolution after the AGB and massive AGB stars. The slow neutron-capture process and yields from AGB stars are also discussed in detail by Karakas. The lectures by S Giridhar provide some necessary background on stellar classification. 

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