Yrast
Encyclopedia
Yrast (ˈyːɾɐst, ˈɪræst) 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.
Yrare is the comparative of yr and is used to refer to the second-least energetic state of a given angular momentum.
, 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, 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
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.
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.
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 stable.
Some superheavy isotopes (such as copernicium-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.
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
that refers to a state of a nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
with a minimum of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
(when it is least excited) for a given angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
. Yr is a Swedish
Swedish language
Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...
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
Heavy ion
Heavy ion refers to an ionized atom which is usually heavier than helium. Heavy-ion physics is devoted to the study of extremely hot nuclear matter and the collective effects appearing in such systems, differing from particle physics, which studies the interactions between elementary particles...
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 neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
, proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
, alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
, or other fragment; it can emit a gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
; it can undergo beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
. Because of the relative strengths of the fundamental interactions associated with those processes (the strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
, electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
, and the weak interaction
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...
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
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
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 95PdPalladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
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
Particle decay
Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
, 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
Nuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
with an unusually long half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
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 stable.
Some superheavy isotopes (such as copernicium-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.