r process

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The r-process (r for English rapid , fast ') is one of the processes of nucleosynthesis .

It is a neutron capture process that, in contrast to the slow s process, takes place at high neutron densities and temperatures . A high neutron flux creates unstable neutron-rich atomic nuclei , which quickly decay into stable neutron-rich nuclei of the heavy elements from iron to lead and the unstable, long-lived isotopes of bismuth , thorium , uranium and plutonium .

procedure

The location of the r-process is not yet known with certainty. Mainly discussed at the moment are:

A detailed spectral analysis of the Kilonova AT2017gfo was able to provide clear evidence that strontium was formed in larger quantities via the r-process. However, it cannot be ruled out that the r-process also takes place during supernovae at the end of a star's life cycle . The shock wave , which originates from the incompressible degenerate neutron nucleus in the center of the star (see neutron star ), entrains neutron-rich material from its outer area and hurls it into space.

However, the relatively low frequency of elements synthesized in the r-process presupposes that either only a small proportion of supernovae releases them into space, or that each supernova releases only a small amount of them.

Due to the very high neutron flux (in the order of magnitude of more than 10 trillion = 10 22 neutrons per square centimeter per second), a large number of neutron accumulations can take place on one and the same atomic nucleus in fractions of a second, especially on short-lived intermediate products before radioactive β - decay occurs . The process is only slowed down by three factors:

  1. through closed neutron shells for isotopes with neutron  numbers N around 50, 82 and 126, corresponding to mass numbers  A of around 70–90, 130–138 and 195–208, at which the probability of further neutron deposition decreases and therefore increases the time required for this. In fact, the abundance of these isotopes is somewhat increased, which can be seen as confirmation of the theory of the r-process;
  2. at a limit at which the binding energy of newly deposited neutrons becomes zero ( ), so that no further neutrons can be captured and the nucleus first has to "wait" for a beta decay ;
  3. by the decrease in core stability with increasing mass number. Therefore, the r-process ends in kernels from which short half-life , the spontaneous fission occurs, so that break down naturally into two lighter nuclei. This is the case with mass numbers  A around 260, for example in the area of ​​the elements curium to rutherfordium .

With every neutron deposition, energy is released in the form of gamma quanta . Neutron number  N and mass number  A each increase by 1, a new isotope of the same element is created.

During the subsequent β - decays of the unstable isotopes, one neutron is converted into a proton by emitting an electron  e - and an electron antineutrino  . This creates an atomic nucleus of another element with the same mass number, but 1 higher atomic number Z (proton number) and 1 lower neutron number  N ; the atom “wanders” in the periodic table .  

See also

literature

  • Klaus Blaum, Hendrik Schatz: Core masses and the origin of the elements. How the world came into being and what precision measurements on short-lived radionuclides tell us about it. Physik-Journal 5 (2006), No. 2, p. 35
  • Margaret Burbidge , Geoffrey Burbidge , William Alfred Fowler , Fred Hoyle : Synthesis of the Elements in Stars , Rev. Mod. Phys. 29 (1957) 547
  • CE Rolfs, WS Rodney: Cauldrons in the Cosmos , Univ. of Chicago Press, 1988
  • Heinz Oberhummer : Cores and Stars , Barth, 1993

Individual evidence

  1. ↑ Darach Watson, Camilla J. Hansen, Jonatan Selsing, Andreas Koch, Daniele B. Malesani: Identification of strontium in the merger of two neutron stars . In: Nature . tape 574 , no. 7779 , October 2019, ISSN  0028-0836 , p. 497-500 , doi : 10.1038 / s41586-019-1676-3 ( nature.com [accessed November 15, 2019]).
  2. ↑ https://www.pro-physik.de/nachrichten/die-schnelle-quelle-schwerer-elemente