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A hypernucleus is a nucleus which contains at least one hyperon in addition to nucleons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952.

Since the strangeness quantum number is conserved by the strong and electromagnetic interactions, at least hypernuclei containing the lightest hyperon, the Lambda, live long enough to have sharp nuclear energy levels. Therefore they offer opportunities for nuclear spectroscopy, as well as reaction mechanism study and other types of nuclear physics (hypernuclear physics). Their physics is different from that of normal nuclei because a hyperon, having a different value of the strangeness quantum number, can share space and momentum coordinates with the usual four nucleons that can differ from each other in spin and isospin. The ground state of helium-5-Lambda, for example, must resemble helium-4 more than it does helium-5 or lithium-5 and must be stable, except for the weak decay of the Lambda. Sigma hypernuclei have been sought with apparent success.

Hypernuclei can be made by a nucleus capturing a Lambda or K meson and boiling off neutrons in a compound nuclear reaction, or, perhaps most easily, by the direct strangeness exchange reaction

Hypernuclei were first observed by their energetic but delayed decay, but have also been studied in their production by measuring the momenta of the K and pi mesons in the above strangeness exchange reaction.

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