# Spontaneous fission

﻿
Spontaneous fission

Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 u (elements near ruthenium). In practice, however, spontaneous fission is only energetically feasible for atomic masses above 230 u (elements near thorium). The elements most susceptible to spontaneous fission are the high-atomic-number actinide elements, such as mendelevium and lawrencium, and the trans-actinide elements, such as rutherfordium.

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::$hbox\left\{Z\right\}^2/hbox\left\{A\right\}ge45.$Where Z is the atomic number and A is the mass number (e.g., 235 for U-235).

As the name suggests, spontaneous fission follows exactly the same process as nuclear fission, 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-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: for very short time intervals the probability of a spontaneous fission is proportional to the length of time.

The spontaneous fission of uranium-238 leaves trails of damage in uranium containing minerals as the fission fragments recoil through the crystal structure. These trails, or "fission tracks" provide the basis for the radiometric dating technique: fission track dating.

pontaneous fission rates

Spontaneous fission rates: [cite book
last = Shultis
first = J. Kenneth
coauthors = Richard E. Faw
title = Fundamentals of Nuclear Science and Engineering
publisher = Marcel Dekker, Inc.
date = 2002
pages = pp. 137 (table 6.2)
id = ISBN 0-8247-0834-2
]

In practice SimpleNuclide|Plutonium|239 will invariably contain a certain amount of SimpleNuclide|Plutonium|240 due to the tendency of SimpleNuclide|Plutonium|239 to absorb an additional neutron during production. SimpleNuclide|Plutonium|240's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% SimpleNuclide|Plutonium|240.

The rarely-used gun-type atomic bomb has a critical insertion time of about one millisecond, and the probability of a fission during this time interval should be small. Therefore only SimpleNuclide|Uranium|235 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.

Notes

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• spontaneous fission — ▪ physics       type of radioactive decay in which certain unstable nuclei of heavier elements split into two nearly equal fragments (nuclei of lighter elements) and liberate a large amount of energy. Spontaneous fission, discovered (1941) by the …   Universalium

• spontaneous fission — savaiminis dalijimasis statusas T sritis fizika atitikmenys: angl. spontaneous fission vok. spontane Spaltung, f rus. спонтанное деление, n pranc. fission spontanée, f …   Fizikos terminų žodynas

• spontaneous fission — savaiminis dalijimasis statusas T sritis chemija apibrėžtis Savaiminis sunkiųjų elementų branduolių skilimas į du artimos masės branduolius. atitikmenys: angl. spontaneous fission rus. спонтанное деление …   Chemijos terminų aiškinamasis žodynas

• 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. Uranium 238 undergoes spontaneous fission decay at a known rate. The fragments emitted …   Wikipedia

• Fission product — Fission products are the atomic fragments left after a large nucleus fissions. Typically, a large nucleus like Uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat… …   Wikipedia

• Fission Spontanée — La fission spontanée (FS) est une forme de désintégration radioactive caractéristique des isotopes lourds. Elle est théoriquement possible pour tous les noyaux atomiques dont la masse est supérieure à 100 uma, c’est à dire à peu près plus… …   Wikipédia en Français

• Fission spontanee — Fission spontanée La fission spontanée (FS) est une forme de désintégration radioactive caractéristique des isotopes lourds. Elle est théoriquement possible pour tous les noyaux atomiques dont la masse est supérieure à 100 uma, c’est à dire… …   Wikipédia en Français

• Spontaneous — means a self generated event, typically requiring no outside influence or help.The word spontaneous may also refer to:* Spontaneous abortion * Spontaneous bacterial peritonitis * Spontaneous combustion * Spontaneous emission * Spontaneous fission …   Wikipedia

• fission-track dating — Method of determining the age of a mineral that utilizes the damage done by the spontaneous fission of uranium 238, the most abundant isotope of uranium. The fission results in radiation damage, or fission tracks, that can be made visible by… …   Universalium

• Fission spontanée — La fission spontanée (FS) est une forme de désintégration radioactive caractéristique des isotopes lourds. Elle est théoriquement possible pour tous les noyaux atomiques dont la masse est supérieure à 100 uma, c’est à dire à peu près plus… …   Wikipédia en Français