Halo core

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Halo nuclei are atomic nuclei in which individual nucleons ( i.e. protons and neutrons ) have a relatively large distance from the rest of the nucleus. Depending on whether protons or neutrons are far from the rest of the nucleus, halo nuclei are divided into proton halos and neutron halos. The number of neutron halos outweighs the number of proton halos among the halo nuclei discovered so far. Halo nuclei are unstable nuclei close to the drip line (break-off edge) for decay through proton or neutron emission.

Halo nuclei were discovered in 1986 by Isaho Tanihata and colleagues at the Lawrence Berkeley National Laboratory (LBNL) at one of the first accelerator facilities for radioactive nuclei in the scattering of nuclei as nuclei with abnormally large dimensions. The interpretation as a halo phenomenon came from Björn Jonson and P. Gregers Hansen in 1987 . For example, they were examined in more detail in the ISOLDE system at CERN and at GSI Darmstadt . They were predicted in 1972 by Arkady Beinussowitsch Migdal .

The name is reminiscent of the ring-shaped light effects ( halo ) of the same name .

Properties of the halo nuclei

Due to the great distance to the rest of the nucleus, the halonucleons have a significantly lower binding energy than normally bound nucleons, which have a binding energy of around 5 MeV in the lithium range. The strong nuclear force , which concentrates the nucleons in the nucleus, has a range of around 2 to 3 femtometers (fm), whereas, for example, the mean distance of the halo neutron of the 1-neutron, which was examined more closely in 2009 by Wilfried Nörtershäuser (Mainz) and colleagues using laser spectroscopy Halo core 11 Be is 7 fm with a core radius of 2.5 fm. According to the laws of classical physics, there would therefore be no bond between the core and the halonucleons. The nonetheless existing binding energy can be explained by the blurring of the halonucleons. The probability of presence of halonucleons is spatially extensive, so that there is a sufficient probability that the nucleons are close enough to the core to experience the strong interaction. With the 2 neutron halo nucleus lithium 11, which was examined more closely, the wave function extends almost to the radius of the heavy lead nucleus (lead 208 with 7 fm radius) and the separation energy of one of the outer neutrons is only around 0.3 MeV.

Two-neutron halo nuclei are also called Borromean (after Borromean rings , no two of which are connected without the third). In general, Borromean nuclei are bound three-body systems in which the two-body subsystems are unbound. Although not all halo nuclei are Borromean (the 1-neutron halos and 1-proton halos, for example, are not) the Borromean rings are often used as symbols for halo nuclei. Two-neutron halo nuclei can be understood as dineutrons stabilized by the nucleus (or as a highly diluted nuclear matter cloud around the nucleus) and the neutron-neutron nuclear force and the quantum mechanical three-body problem can be studied on them.

Helium 8 can probably best be described as a 4-neutron halo around the alpha particle core, although its radius is not that large. Here, the halo neutrons close to the core rather form a kind of neutron skin (tanihata). Boron 19 and beryllium 14 are also discussed as 4 neutron halo nuclei. It is noticeable that boron 19 and helium 8 are the only known nuclei in which the removal of 1 and 3 neutrons results in unbound states. Beryllium 14 has at least one 2-neutron halo. Marques and colleagues from the GANIL accelerator carried out scattering experiments on beryllium 14 in 2002, in which, in their opinion, the outer neutrons separated as tetraneutrons , but this was criticized.

Even with heavier elements such as carbon 19, good candidates for halo nuclei were found, in this case a 1-neutron halo. Its neutron separation energy is as low as that of beryllium 11 and more detailed investigations at the GSI Darmstadt supported its classification as a halo nucleus.

In some representations, the deuterium is also included as the simplest halo nucleus.

List of known halo nuclei

The following halo nuclei or good candidates for halo nuclei are known (according to Riisager 2012):

core Halotype Half-life
6 He 2 neutrons 0.801 s
8 He 4 neutrons 0.119 s
11 li 2 neutrons 8.75 ms
11 Be 1 neutron 13.8 s
14 Be 2 or 4 neutrons 4.35 ms
8 B 1 proton 0.77 s
17 B 2 neutrons 5.08 ms
19 B 4 neutrons 2.92 ms
15 C 1 neutron 2.45 s
19 C 1 neutron 49 ms
22 C 2 neutrons 6.1 ms
17 F. 1 proton 64.5 s
17 Ne 2 protons 0.109 s

Fluorine 17 is in an excited state (I = 1/2 +). Riisager (2012) also discusses Neon 31 as a 1-neutron halo candidate and Magnesium 35.

literature

  • B. Jonson, A. Richter: Halokerne: Dedicated to Professor Peter Brix on his 80th birthday. In: Physical sheets. Volume 54, No. 12, 1998, pp. 1121-1125.
  • PG Hansen, AS Jensen, B. Jonson: Nuclear Halos , Annual Review of Nuclear and Particle Science, Volume 45, 1995, pp. 591-634
  • B. Jonson: Light dripline nuclei , Physics Reports, Volume 389, 2004, pp. 1-59
  • Bethge, Walter, Wiedemann: Nuclear Physics: An Introduction . 3. Edition. Springer, 2007, ISBN 3-540-74566-1 . (The introductory textbook deals with halo nuclei on page 114)
  • Sam Austin, George F. Bertsch : Halo Nuclei, Scientific American, June 1995
  • K. Riisager, Halos and related structures, Arxiv 2012 , Nobel Symposium 152 "Physics With Radioactive Beams", Physica Scripta, Volume 152, 2013, 014001
  • K. Riisager, Nuclear Halo States, Reviews of Modern Physics, Vol. 66, 1995, 1105-1116

Web links

Individual evidence

  1. ^ I. Tanihata et al. a., Measurements of Interaction Cross Sections and Nuclear Radii in the Light p-Shell Region , Phys. Rev. Lett. 55, 1985, p. 2676, Abstract , I. Tanihata et al. a. Measurements of interaction cross sections and radii of He isotopes , Phys. Lett. B., 160, 1985, pp. 380-384
  2. ↑ Jonson, Hansen, The neutron halo of extremely neutron-rich nuclei, Europhys. Lett., 4, 1987, p. 409
  3. ^ Migdal Two interacting particles in the potential hole , Sov. J. Nucl. Phys., 16, 1972, 238. Jonson and Hansen referred to Migdal in their 1987 article
  4. ↑ W. Nörtershäuser u. a., Nuclear Charge Radii of 7,9,10 Be and the One-Neutron Halo Nucleus 11 Be, Physical Review Letters, 102: 6, February 13, 2009
  5. ↑ scienceticker.info: Atomic nucleus with satellite
  6. ↑ JS Vaagen u. a. Borromean Halo Nuclei, Physica Scripta, T 88, 2000, 209-213, pdf
  7. ↑ The term Boromean halo nuclei comes from Mikhail Zhukov et al. a. Bound state properties of Borromean Halo nuclei: 6 He and 11 Li , Physics Reports, 231, 1993, 151. Attribution according to Vaagen, Ershov, Zhukov: Lessons from two paradigmatic developments: Rutherfords nuclear atom and Halo nuclei, J. of Physics Conf. Series 381, 2012, 012049 pdf
  8. ↑ Tanihata, D. Hirata, T. Kobayashi, S. Shimoura, K. Sugimoto, H. Toki, Revelation of thick neutron skins in nuclei, Phys. Lett. B, 289, 1992, 261-266
  9. ↑ In the discussion in the review article by Riisager (2012) it is mentioned that there was increasing evidence of a 4 neutron halo interpretation
  10. ↑ Darlington: Tetraneutrons
  11. ↑ Is carbon-19 a halo nucleus? GSI News 2/99 (PDF; 82 kB)
  12. ^ K. Riisager, Halos and related structures, Arxiv 2012 , Nobel Symposium 152 "Physics With Radioactive Beams", Physica Scripta, Volume 152, 2013, 014001
  13. ↑ Jefferson Lab , Periodic Table with half-lives of isotopes