Nucleosynthesis

Reading Assignment

Synthesis of the elements in stars: forty years of progress, George Wallerstein, Icko Iben, , Jr., Peter Parker, Ann Merchant Boesgaard, Gerald M. Hale, Arthur E. Champagne, Charles A. Barnes, Franz Käppeler, Verne V. Smith, Robert D. Hoffman, Frank X. Timmes, Chris Sneden, Richard N. Boyd, Bradley S. Meyer, and David L. Lambert Rev. Mod. Phys. 69, 995 – Published 1 October 1997

https://www.sciencenews.org/article/rarest-nucleus-reluctant-decay

http://adsabs.harvard.edu/abs/1957RvMP...29..547B

http://adsabs.harvard.edu/abs/1965ApJS...11..121S

http://adsabs.harvard.edu/abs/1999PrPNP..43..419K

http://adsabs.harvard.edu/abs/2007PhR...450...97A

s-process

The main location for the s-process is AGB stars. There is also a second component in massive stars that go supernovae, called the weak s-process. The s-process generates neutrons through the following reactions during helium burning,

$^{13}\mathrm {C} (\alpha ,\mathrm {n} )^{16}\mathrm {O} ~\mathrm {and} ~^{22}\mathrm {Ne} (\alpha ,\mathrm {n} )^{25}\mathrm {Mg} ,$

where the former occurs mainly in AGB stars and the latter in massive stars. The neutrons are captured onto large nuclei in the main process such as Sr and Y and goes to Pb. The weak process starts with Fe and goes to Sr and Y.

The s-process terminates at Pb and Bismuth because of

The s-process is a secondary nucleosynthesis process because it requires that there already be iron present in the star to build up even heavier elements. In the main s-process this means that you have to build a star from material previously in a supernovae and wait until this star becomes an AGB star (one billion years or so) for the nucleosynthesis to occur. Even the weak s-process requires a previous generation but less time because stars can reach the supernovae in just a few million years.

r-process

p-process

There are several nuclei that cannot be produced by adding neutrons to stable nuclei. Many of these are known as p-nuclei and whatever makes them is known generically as the p-process.

Some possibilities are the γ-process, the ν-process and the rp-process.

γ-process

rp-process

Cosmic-Ray Spallation

Where stuff gets made

Assignment

For elements with atomic numbers less than that of calcium, the most abundant isotope of each element with an even number of protons has Z=N, e.g. ⁴He, ¹²C, ¹⁶O, ⁴⁰Ca, ... and those with odd proton numbers, N, Na, Al ... have Z nearly equal to N. When one goes to heavier nuclei however there is a surplus of neutrons in the most abundant isotopes; iron-56 has 26 protons and 30 neutrons. Explain both these trends why light nuclei have Z about equal to N while heavy nuclei have Z less than N.
Why is combination of a single neutron and a single proton stable but two protons is not?
Calculate the energy released in erg/g when a composition of pure helium burns to pure carbon-12 and to 50/50 carbon-12 and oxygen 16. What is the energy released when each of these mixtures is burned to Nickel-56? In both cases how much nickel has to be made to produce 10⁵¹ erg?
The neutron capture cross sections at 30 keV for the stable isotopes of barium are ¹³⁰Ba, 715 mb ¹³²Ba, 447mb, ¹³⁴Ba, 221 mb ¹³⁵Ba, 457 mb, ¹³⁶Ba, 69 mb, ¹³⁷Ba, 57 mb, and ¹³⁸Ba, 3.9 mb. The s-only isotopes of barium are 134 and 136 and the nuclear charge is 56. a) Why is the cross section of ¹³⁵Ba greater than that of ¹³⁴Ba or ¹³⁶Ba? Why is the cross section of ¹³⁸Ba so small? What do you expect for the solar ratio of the abundance of ¹³⁴Ba to that of ¹³⁶Ba? Your discussion should at least mention why reactions with large releases of energy have large cross-sections.