''Yrast'' ( , ) is a technical term in

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...

that refers to a state of a nucleus with a minimum of

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

(when it is least excited) for a given

angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...

. ''Yr'' is a Swedish adjective sharing 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. ''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 The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...

, or other fragment; it can emit a

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

; it can undergo

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

. Because of the relative strengths of the

fundamental interactions In physics, the fundamental interactions or fundamental forces are interactions in nature that appear not to be reducible to more basic interactions. There are four fundamental interactions known to exist: * gravity * electromagnetism * weak int ...

associated with those processes (the

strong interaction In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, n ...

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

, and the

weak interaction In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...

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, 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

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number. Until the 1960s, nucleons were thought to be ele ...

s, because they can carry away angular momentum more effectively. This is not the only reason alpha particles are preferentially emitted, though; another reason is simply that alpha particles (He-4 nuclei) are energetically very stable in and of themselves.

Yrast isomers

Sometimes there is a large gap between two yrast states. For example, the nucleus ⁹⁵ Pd 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 In particle physics, particle decay is the spontaneous process of one unstable subatomic particle transforming into multiple other particles. The particles created in this process (the ''final state'') must each be less massive than the original ...

, 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 In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

with an unusually long

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of 14 seconds. An exceptional example is the J=9 state of tantalum-180, which is a very low-lying yrast state only 77 keV above the ground state. The ground state has J=1, which is too large a gap for gamma decay to occur. Alpha and beta decay are also suppressed, so strongly that the resulting isomer, tantalum-180m, is effectively stable for all practical purposes, and has never been observed to decay. Tantalum-180m is the only currently known yrast isomer to be observationally stable. Some superheavy isotopes (such as

copernicium Copernicium is a synthetic chemical element; it has symbol Cn and atomic number 112. Its known isotopes are extremely radioactive, and have only been created in a laboratory. The most stable known isotope, copernicium-285, has a half-life of ap ...

-285) have longer-lived isomers with half-lives on the order of minutes. These may be yrasts, but the exact angular momentum and energy is often hard to determine for these nuclides.

References

{{reflist Swedish words and phrases Nuclear physics Angular momentum