Yrast

A Yrast state ( yrast : superlative from Swedish yr = roughly “dizzy”, as an allusion to fast turning) is the state with the lowest excitation energy possible for an atomic nucleus with a certain angular momentum . The nucleus in a Yrast state is “cold” because its excitation energy is (almost) completely contained in the rotational energy

${\displaystyle E_{rot}={\frac {I(I+1)\hbar ^{2}}{2J}}}$

(with the angular momentum and the moment of inertia of the core ). If one plots the possible excitation energies of the nucleus as a function of the angular momentum and connects the points of the Yrast states with one another, the Yrast line results . ${\displaystyle I}$ ${\displaystyle J}$

If a nucleus is highly excited by collective rotation, it first gives off its energy through a series of transitions that tend towards the Yrast line through nucleon emission ( Yrast cascade ). This is then no longer left; the energy and the angular momentum are then gradually emitted mainly via gamma rays. Such nuclei are therefore mostly observed with gamma spectroscopy .

Yrast states give the opportunity to study nuclear matter under extreme conditions with high centrifugal and Coriolis forces . For example, inertial forces can break the nucleon pairings so that the nucleus is deformed and its moment of inertia changes. In addition to the Yrast state itself, its excited states, including collective excitations, also provide information about the nucleus.

The name originated in the 1960s when it became possible to excite nuclei in heavy ion experiments with angular momentum, for example in the range of . The compound core with high angular momentum formed during the peripheral collisions of the heavy ions emitted its excitation energy through gamma radiation cascades, with one or two angular momentum units being dissipated in each of the individual steps via gamma quanta. But there are also other types of decay (such as alpha decay ); at high angular momentum, the deformation of the nuclei can lead to splitting . ${\displaystyle 70\hbar }$

literature

• Theo Mayer-Kuckuk Nuclear Physics - An Introduction , 7th edition Teubner Stuttgart, page 224 f.
• Sven Bjørnholm: Nuclear structure at high angular momentum , Physikalische Blätter, Volume 34, December 1978, pp. 672-680, online

Individual evidence

1. ^ Bohr, Mottelson: Nuclear Structure, Volume 2, p. 41
2. ↑ Bohr / Mottelson cite J. Robb Grover, Shell-Model Calculations of the Lowest-Energy Nuclear Excited States of Very High Angular Momentum, Phys. Rev. 157, 1967, 832, abstract