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Spontaneous fission (SF) is a form of radioactive decay
Radioactive decay

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide....
 characteristic of very heavy isotope
Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
s, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 u
Atomic mass unit

The unified atomic mass unit , or dalton or, sometimes, universal mass unit, is a Units of measurement of mass used to express atomic weight and molecular masses....
 (elements near ruthenium
Ruthenium

Ruthenium is a chemical element that has the symbol Ru and atomic number 44. A rare transition metal of the platinum group of the periodic table, ruthenium is found associated with platinum ores and used as a catalyst in some platinum alloys....
). In practice, however, spontaneous fission is only energetically feasible for atomic mass
Atomic mass

The atomic mass is the mass of an atom, most often expressed in Atomic mass units. The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom ....
es above 230 u (elements near thorium
Thorium

Thorium is a chemical element with the symbol Th and atomic number 90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium....
). The elements most susceptible to spontaneous fission are the 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 of the actinides, mendelevium is synthesized by bombarding einsteinium with alpha particles and was named after Dmitri Mendeleev, who was responsible for the Periodic Table....
 and lawrencium
Lawrencium

Lawrencium is a radioactive synthetic element with the symbol Lr and atomic number 103.Its most stable known isotope is 262Lr, with a half-life of approximately 3.6 hours....
, and the trans-actinide elements, such as rutherfordium
Rutherfordium

Rutherfordium is a chemical element in the periodic table that has the symbol Rf and atomic number 104.This is a radioactive synthetic element whose most stable known isotope is 267Rf with a half-life of approximately 1.3 hours....
.

For uranium and thorium, the spontaneous fission mode of decay does occur, but is not seen for the majority of radioactive breakdowns and is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements.






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Spontaneous fission (SF) is a form of radioactive decay
Radioactive decay

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide....
 characteristic of very heavy isotope
Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
s, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 u
Atomic mass unit

The unified atomic mass unit , or dalton or, sometimes, universal mass unit, is a Units of measurement of mass used to express atomic weight and molecular masses....
 (elements near ruthenium
Ruthenium

Ruthenium is a chemical element that has the symbol Ru and atomic number 44. A rare transition metal of the platinum group of the periodic table, ruthenium is found associated with platinum ores and used as a catalyst in some platinum alloys....
). In practice, however, spontaneous fission is only energetically feasible for atomic mass
Atomic mass

The atomic mass is the mass of an atom, most often expressed in Atomic mass units. The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom ....
es above 230 u (elements near thorium
Thorium

Thorium is a chemical element with the symbol Th and atomic number 90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium....
). The elements most susceptible to spontaneous fission are the 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 of the actinides, mendelevium is synthesized by bombarding einsteinium with alpha particles and was named after Dmitri Mendeleev, who was responsible for the Periodic Table....
 and lawrencium
Lawrencium

Lawrencium is a radioactive synthetic element with the symbol Lr and atomic number 103.Its most stable known isotope is 262Lr, with a half-life of approximately 3.6 hours....
, and the trans-actinide elements, such as rutherfordium
Rutherfordium

Rutherfordium is a chemical element in the periodic table that has the symbol Rf and atomic number 104.This is a radioactive synthetic element whose most stable known isotope is 267Rf with a half-life of approximately 1.3 hours....
.

For uranium and thorium, the spontaneous fission mode of decay does occur, but is not seen for the majority of radioactive breakdowns and 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 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 atomic nucleus of an atom. It is conventionally represented by the symbol Z....
 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....
 (e.g., 235 for U-235).

As the name suggests, spontaneous fission follows exactly the same process as nuclear fission
Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the atomic nucleus of an atom splits into smaller parts, often producing free neutrons and lighter atomic nucleus, which may eventually produce photons ....
, except that it occurs without the atom having been struck by a neutron or other particle. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a chain reaction. Also, radioisotopes for which spontaneous fission is a nonnegligible decay mode may be used as neutron sources; californium
Californium

Californium is a metallic chemical element with the symbol Cf and atomic number 98. A Radioactive decay transuranic element, californium is used in starting nuclear reactors, optimizing coal-fired power plants and cement production facilities , medical treatment of cancer, and oil exploration via down hole well logging....
-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 Sim?on-Denis Poisson , is the stochastic process in which events occur continuously and memorylessness ....
: 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 Isotopes of uranium of uranium found in nature. When hit by a neutron, it becomes uranium-239 , an unstable isotope which radioactive decay into neptunium-239 , which then itself decays, with a half-life of 2.355 days, into plutonium-239 ....
 leaves trails of damage in uranium containing minerals as the fission fragments recoil through the crystal structure
Crystal structure

In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice....
. These trails, or fission tracks provide the basis for the radiometric dating
Radiometric dating

Radiometric dating is a technique used to date materials, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates....
 technique: 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 Nuclear 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 7.0x10-9 % 1.86 1.0x10-5
4.47x109 years 5.4x10-5 % 2.07 0.0136
2.41x104 years 4.4x10-10 % 2.16 2.2x10-2
6569 years 5.0x10-6 % 2.21 920
2.638 years 3.09 % 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 nuclear fission-based nuclear weapons whose nuclear weapon design assembles their fissile material into a Critical 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 critical mass....
 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
Implosion

Implosion is a process in which objects are destroyed by collapsing on themselves. The opposite of explosion, implosion concentrates matter and energy....
 method.

Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes superdeformation
Superdeformation

In nuclear physics a superdeformed Atomic nucleus is a nucleus that is very far from sphere, forming an ellipsoid with axes in ratios of approximately 2:1:1....
.