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Spontaneous fission

Spontaneous fission

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Spontaneous fission is a form of radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 characteristic of very heavy isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s. Because the nuclear binding energy
Nuclear binding energy
Nuclear binding energy is the energy required to split a nucleus of an atom into its component parts. The component parts are neutrons and protons, which are collectively called nucleons...

 reaches a maximum at a nuclear mass greater than about 60 atomic mass unit
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...

s (u), spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses. Because of constraints in constructing the daughter fission product nuclei, spontaneous fission into known nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....

s becomes theoretically possible (that is, energetically possible) for many atomic nuclei (nuclide) with a mass greater than or equal to 93 atomic mass unit
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...

s (u), with the possibility increasing as mass number grows above this boundary. The lightest nuclides found naturally which are in theory subject to spontaneous fission, are niobium-93 and molybdenum-94 (elements 41 and 42 respectively). Spontaneous fission has never been observered in naturally-occurring isotopes of these ligher elements, however. Operationally they are stable isotope
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...

s.

In practice, spontaneous fission is feasible over practical observation times, only for atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....

es above 232 u. These are elements heavier than thorium-232, which has a half life approximately equal to the age of the universe, but is the lightest primordial nuclide
Primordial nuclide
In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...

 that has been observed to undergo spontaneous fission. The elements most susceptible to spontaneous fission are the artificially-produced high-atomic-number actinide elements, such as mendelevium
Mendelevium
Mendelevium is a synthetic element with the symbol Md and the atomic number 101. A metallic radioactive transuranic element in the actinide series, mendelevium is usually synthesized by bombarding einsteinium with alpha particles. It was named after Dmitri Ivanovich Mendeleev, who created the...

 and lawrencium
Lawrencium
Lawrencium is a radioactive synthetic chemical element with the symbol Lr and atomic number 103. In the periodic table of the elements, it is a period 7 d-block element and the last element of actinide series...

, and the trans-actinide elements, such as rutherfordium
Rutherfordium
Rutherfordium is a chemical element with symbol Rf and atomic number 104, named in honor of New Zealand physicist Ernest Rutherford. It is a synthetic element and radioactive; the most stable known isotope, 267Rf, has a half-life of approximately 1.3 hours.In the periodic table of the elements,...

.

For uranium and thorium, spontaneous fission mode of decay does occur, as noted, but it is not seen for the majority of radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 which is by alpha decay
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...

, and so it is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements. Mathematically, the criterion for whether spontaneous fission can occur in a time short enough to be observed by present methods, is approximately:

where Z is the atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

 and A is the mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...

 (e.g., 235 for U-235).

As the name suggests, spontaneous fission gives much the same result as induced nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

. However, like other forms of radioactive decay, it occurs due to quantum tunneling, without the atom having been struck by a neutron or other particle as in induced nuclear fission. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a self-sustaining chain reaction. Also, radioisotopes for which spontaneous fission is a nonnegligible decay mode may be used as neutron sources; californium
Californium
Californium is a radioactive metallic chemical element with the symbol Cf and atomic number 98. The element was first made in the laboratory in 1950 by bombarding curium with alpha particles at the University of California, Berkeley. It is the ninth member of the actinide series and was the...

-252 (half-life 2.645 years, SF branch ratio 3.09%) is often used for this purpose. The neutrons may then be used to inspect airline luggage for hidden explosives, to gauge the moisture content of soil in the road construction and building industries, to measure the moisture of materials stored in silos, and in other applications.

As long as the fissions give a negligible reduction of the amount of nuclei that can spontaneously fission, this is a Poisson process
Poisson process
A Poisson process, named after the French mathematician Siméon-Denis Poisson , is a stochastic process in which events occur continuously and independently of one another...

: for very short time intervals the probability of a spontaneous fission is proportional to the length of time.

The spontaneous fission of uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

 leaves trails of damage in uranium-bearing minerals as the fission fragments recoil through the crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

. These trails, or fission tracks, provide the basis for the radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...

 technique known as fission track dating
Fission track dating
Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses...

.

Spontaneous fission rates


Spontaneous fission rates:
Nuclide Half-life Fission prob. per decay Neutrons per fission Neutrons per (g.s)
7.04x108 years 7.0x10−11 1.86 1.0x10−5
4.47x109 years 5.4x10−7 2.07 0.0136
2.41x104 years 4.4x10−12 2.16 2.2x10−2
6569 years 5.0x10−8 2.21 920
8300 years 0.80 ? ?
2.638 years 3.09x10−2 3.73 2.3x1012


In practice will invariably contain a certain amount of due to the tendency of to absorb an additional neutron during production. 's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% .

The rarely-used gun-type atomic bomb
Gun-type fission weapon
Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of sub-critical material into another...

 has a critical insertion time
Insertion time
The term insertion time is used to describe the length of time which is required to rearrange a subcritical mass of fissile material into a prompt critical mass. This is one of the three main requirements in a nuclear weapon design to create a working fission atomic bomb...

 of about one millisecond, and the probability of a fission during this time interval should be small.
Therefore only is suitable. Almost all nuclear bombs use some kind of implosion method.

Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes superdeformation
Superdeformation
In nuclear physics a superdeformed nucleus is a nucleus that is very far from spherical, forming an ellipsoid with axes in ratios of approximately 2:1:1. Normal deformation is approximately 1.3:1:1. Only some nuclei can exist in superdeformed states....

.

History


The first nuclear fission process discovered was the fission induced by neutron
Neutron
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...

s. Because cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s produce some neutrons, it was difficult to distinguish between induced and spontaneous fission events. Cosmic rays can be reliably shielded by a thick layer of rock or water. The spontaneous fission was identified in 1940 by Soviet
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 physicists Georgy Flyorov
Georgy Flyorov
Georgy Nikolayevich Flyorov was a prominent Soviet nuclear physicist.-Biography:Flyorov was born in Rostov-on-Don and attended the Leningrad Polytechnic Institute Georgy Nikolayevich Flyorov (March 2, 1913 – November 19, 1990) was a prominent Soviet nuclear physicist.-Biography:Flyorov was born...

 and Konstantin Petrzhak by their observations of uranium in the Moscow Metro
Moscow Metro
The Moscow Metro is a rapid transit system serving Moscow and the neighbouring town of Krasnogorsk. Opened in 1935 with one line and 13 stations, it was the first underground railway system in the Soviet Union. As of 2011, the Moscow Metro has 182 stations and its route length is . The system is...

 Dinamo station, 60 metres (196.9 ft) deep underground.

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