Hypernucleus

(also called hyperfragment), an atomic nucleus made up of a hyperon in addition to nucleons. A hypernucleus is formed during the interaction of high-energy particles with nucleons of the nucleus or when a slow K^- meson is captured by the nucleus. As a result of this a slow Λ⁰ hyperon that forms a bound system with the nucleus appears. The lifetime of a hypernucleus is determined by the lifetime of the Λ⁰ hyperon (~10^-10 sec).

The first hypernucleus was detected in 1952 by the Polish physicists M. Danysz and J. Pniewski, who used nuclear emulsions exposed to a cosmic-ray flux. All known hyper-nuclei are lambda hypernuclei, that is, nuclei containing a Λ⁰ hyperon. This is because all other hyperons enter into fast reactions with nuclear particles, but for the Λ⁰ hyperon such reactions are forbidden by the selection rules. A hypernucleus is designated by the chemical symbol of the element with the subscript A at left. For example, the nucleus of hyperhelium, whose structure is 2p + 2n + Λ⁰, is designated by the symbol _Λ⁵ He.

Hypernuclei are studied by means of nuclear photoemulsions and bubble chambers. The characteristics of more than ten types of light hypernuclei are known. Upon the interaction of high-energy particles with the heavy nuclei of the photoemulsion, the formation of heavy hypernuclei with mass number A of up to 100 is observed. The existence of hypernuclei attests to the fact that an attractive force exists between hyperons and nucleons. The first double hypernucleus, _ΛΛ¹⁰Be (4p + 4n + 2Λ°) was detected in 1963, and in 1966_ΛΛ⁶He (2p + 2n + 2Λ°) was observed. The study of the properties of double hypernuclei makes it possible to determine the nature of the forces acting between the two hyperons.