Yrast

"Yrast" (Swedish pronunciation: IPA| [y:ɾɐst] , English: IPA| [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. [cite journal
last = Grover
first = J. Robb
year = 1967
month = May
title = Shell Model Calculations of the Lowest-Energy Nuclear Excited States of Very High Angular Momentum
journal = Phys. Rev.
volume = 157
issue = 4
pages = 832–847
doi = 10.1103/PhysRev.157.832
url = http://link.aps.org/abstract/PR/v157/p832
format = subscription required ]

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

Background

An unstable nucleus may 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 with frequencies in that order. Theoretically, a nucleus has a very small probability of emitting a gamma ray even if it could 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; that is, nuclei appear to have more than enough energy to eject neutrons, but decay by gamma emission instead. This discrepancy is found by the energy of a nuclear angular momentum, [cite journal
last = Grover
first = J. Robb
year = 1967
month = July
title = Angular Momentum Effects in the Gamma-Ray De-Excitation of Fission Fragments
journal = Phys. Rev.
volume = 159
issue = 4
pages = 980–984
doi = 10.1103/PhysRev.159.980
url = http://link.aps.org/abstract/PR/v159/p980
format = subscription required ] 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 nucleons, because they can carry away angular momentum more effectively. [cite journal
last = Grover
first = J. Robb
year = 1967
month = May
title = Emission of Alpha Particles from Nuclei Having Large Angular Momenta
journal = Phys. Rev.
volume = 157
issue = 4
pages = 823–831
doi = 10.1103/PhysRev.157.823
url = http://link.aps.org/abstract/PR/v157/p823
format = subscription required ]

Yrast isomers

Sometimes there is a large gap between two yrast states. For example, the nucleus ⁹⁵Pd 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 strong 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. [cite journal
author = N. Marginean et al.
year = 2003
month = June
title = Yrast isomers in ⁹⁵Ag, ⁹⁵Pd, and ⁹⁴Pd
journal = Phys. Rev. C
volume = 67
doi = 10.1103/PhysRevC.67.061301
url = http://link.aps.org/abstract/PRC/v67/e061301
pages = 061301
format = subscription required ]

References