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Yrast (Swedish pronunciation: [y:ɾɐst], English: [ˈɪrast]) is a technical term in nuclear physics that refers to a state of a nucleus with a minimum of energy (when it is least excited) for a given angular momentum. Yr is a Swedish adjective derived from the Old Norse hvirfla, the same root as the English whirl. Yrast is the superlative of yr and can be translated whirlingest, although it literally means "dizziest" or "most bewildered". The Yrast levels are vital to understanding reactions, such as off-center heavy ion collisions, that result in high-spin states.^[1]

Yrare is the comparative of yr and is used to refer to the second least energetic state of a given angular momentum.

An unstable nucleus can decay in several different ways: it can eject a neutron, proton, alpha particle, or other fragment; it can emit a gamma ray; it can undergo beta decay. Because of the relative strengths of the fundamental interactions associated with those processes (the strong interaction, electromagnetism, and the weak interaction respectively), they usually occur in that order, the order of increasing energy. Theoretically, a nucleus has a very small probability of emitting a gamma ray if it has enough energy to eject a neutron, and beta decay rarely occurs unless both of the other two pathways are highly unlikely.

In some instances, however, predictions based on this model underestimate the total amount of energy released in the form of gamma rays, ie. nuclei appear to have more than enough energy to eject neutrons, but decayed by gamma ray emission instead. This discrepancy is found by the energy of a nucleus' angular momentum,^[2] and documentation and calculation of yrast levels for a given system may be used for analyzing such a situation.

The energy stored in the angular momentum of an atomic nucleus can also be responsible for the emission of larger-than-expected particles, such as alpha particles over single fermions, because they can carry away angular momentum more effectively.^[3]

Sometimes there is a large gap between two yrast states. For example, the nucleus ⁹⁴Pd has a 21/2 state that lies below the lowest 19/2, 17/2, and 15/2 states. This state does not have enough energy to undergo particle decay, and because of the large spin difference, gamma decay from the 21/2 state to the 13/2 state below is very unlikely. The more likely decay option is beta decay, which forms an isomer with an unusually long half-life of 14 seconds.^[4]

1. ^ Grover, J. Robb (May 1967). "Shell Model Calculations of the Lowest-Energy Nuclear Excited States of Very High Angular Momentum". Phys. Rev. 157 (4): 832–847. DOI:10.1103/PhysRev.157.832. 
2. ^ Grover, J. Robb (July 1967). "Angular Momentum Effects in the Gamma-Ray De-Excitation of Fission Fragments". Phys. Rev. 159 (4): 980–984. DOI:10.1103/PhysRev.159.980. 
3. ^ Grover, J. Robb (May 1967). "Emission of Alpha Particles from Nuclei Having Large Angular Momenta". Phys. Rev. 157 (4): 823–831. DOI:10.1103/PhysRev.157.823. 
4. ^ N. Marginean et al. (June 2003). "Yrast isomers in ⁹⁵Ag, ⁹⁵Pd, and ⁹⁴Pd". Phys. Rev. C 67. DOI:10.1103/PhysRevC.67.061301.