Spontaneous fission 409529 224557447 2008-07-09T11:57:40Z 78.53.145.101 /* Spontaneous fission rates */ fixed SF branching ratio for U-235 with value from Nudat 2 and recalculated neutron rate for U-235 '''Spontaneous fission''' (SF) is a form of [[radioactive decay]] characteristic of very heavy [[isotope]]s, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 [[atomic mass unit|u]] (elements near [[ruthenium]]). In practice, however, spontaneous fission is only energetically feasible for [[atomic mass]]es 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: :<math>\hbox{Z}^2/\hbox{A}\ge45.</math> 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]]. ==Spontaneous fission rates== Spontaneous fission rates:<ref>{{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 }}</ref> {| class="wikitable" |- ! Nuclide ! Half-life ! Fission prob. per decay (%) ! Neutrons per fission ! Neutrons per (g.s) |- | {{SimpleNuclide|Link|Uranium|235}} | 7.04x10<sup>8</sup> years | 7.0x10<sup>-9</sup> % | 1.86 | 1.0x10<sup>-5</sup> |- | {{SimpleNuclide|Link|Uranium|238}} | 4.47x10<sup>9</sup> years | 5.4x10<sup>-5</sup> % | 2.07 | 0.0136 |- | {{SimpleNuclide|Link|Plutonium|239}} | 2.41x10<sup>4</sup> years | 4.4x10<sup>-10</sup> % | 2.16 | 2.2x10<sup>-2</sup> |- | {{SimpleNuclide|Link|Plutonium|240}} | 6,569 years | 5.0x10<sup>-6</sup> % | 2.21 | 920 |- | {{SimpleNuclide|Link|Californium|252}} | 2.638 years | 3.09 % | 3.73 | 2.3x10<sup>12</sup> |} 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% {{SimpleNuclide|Plutonium|240}}. The [[gun-type fission weapon]] has a [[insertion time|critical insertion time]] of about 1ms, and the probability of a fission during this time interval should be small. Therefore only {{SimpleNuclide|Uranium|235}} is suitable. Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes [[Superdeformation]]. == Notes == <references/> {{Nuclear_processes}} <!--Interwiki--> <!--Categories--> [[Category:Radioactivity]] [[ar:انشطار تلقائي]] [[de:Spontane Spaltung]] [[fr:Fission spontanée]] [[ko:자발 핵분열]] [[nl:Spontane splijting]] [[ja:自発核分裂]] [[pt:Fissão nuclear espontânea]] [[sr:Спонтани распад]] [[sh:Spontani raspad]] [[sv:Spontan fission]] [[fi:Spontaani fissio]] [[ur:مختاری انشقاق]]