Spontaneous fission (more precisely: spontaneous nuclear fission ; also spontaneous fission or spontaneous division ; English spontaneous fission ) is a form of radioactive decay of very heavy atomic nuclei from an atomic number of 90 ( thorium ). It represents a form of nuclear fission in which a nucleus spontaneously, i.e. H. without external influence - especially without neutron irradiation - divides into two (rarely several) mostly medium-weight nuclei and a few neutrons.
The alpha decay and cluster decay d. H. the splitting off of light nuclei without additional neutrons do not count towards spontaneous splitting. Furthermore, the neutron-induced fission , a nuclear reaction , must be distinguished from radioactive decay through spontaneous fission.
In the case of the isotopes of the elements thorium and uranium , 232 Th , 234 U , 235 U and 238 U , which are quantitatively relevant in terms of their occurrence in nature , in addition to the prevailing alpha decay as a further decay channel , the spontaneous splitting of the atomic nucleus in question into two nuclei can also be prevented Emission of mostly two or three neutrons can be observed, e.g. B. according to
SF = Spontaneous Fission
Spontaneous fission also occurs as a competing type of decay in many of the even heavier radionuclides of the transuranic elements in addition to the decay types alpha decay , beta decay / electron capture , isomerism transition and cluster decay . In total, the Karlsruhe nuclide map (as of 2012) lists ≥ 143 radionuclides with a proportion of spontaneous fission (101 basic states; 10 explicitly shown nuclear isomers ; 32 further cases, shown only in compressed form, of one or more spontaneously splitting nuclear isomers with a half-life of <0.1 s) . Spontaneous fission can also be the dominant type of decay (e.g. at 250 Cm , 254 Cf ).
Explanation and properties
The spontaneous fission, like the alpha decay and the induced fission, is basically explained by the tunnel effect . However, a Coulomb barrier of a more complicated shape must be expected for fission fragments compared to alpha particles .
Like the induced cleavage, the spontaneous cleavage is preferably asymmetrical, i.e. H. the two fissure fragment cores are usually of different sizes. The mass distribution of the resulting nuclides is therefore a curve with two "bumps" with mass numbers around 90 and around 140, similar to the fission by thermal neutrons. The energy spectrum of the released neutrons is also very similar to that from the induced fission.
The partial decay constant (probability per unit of time) for spontaneous fission is usually smaller than that for alpha decay of the same nuclide. If you want to express it in terms of the somewhat clearer, fictitious partial half-life , this is correspondingly long.
A year later, Georgi Fljorow and Konstantin Petrschak succeeded in demonstrating this phenomenon in natural uranium. To this end, they used the ionization chamber method developed by Otto Frisch (see the discovery of nuclear fission ). However, they had to increase the chamber volume considerably in order to accommodate a sample amount of approx. 15 g uranium oxide U 3 O 8 in it. The apparatus registered about six pulses per hour with this sample; if the ionization chamber was empty ( i.e. without U 3 O 8 filling), not a single pulse was measured in five hours. Based on this measurement and numerous control experiments, the authors came to the conclusion that the observed impulses could only come from very high-energy fragments of uranium emitted from the U 3 O 8 surface. Since the participation of neutrons could be excluded, the test results could only be explained by the assumption of spontaneous fission.
In the ordinary uranium mineral, 238 U has the largest share in the spontaneous fission events due to the considerably larger proportion of this isotope.
Branching relationships between alpha decay and spontaneous splitting
The following table for some nuclides with ordinal numbers 90 to 106 gives the branching ratios under "Frequency" , i. H. percentage of the decay channels.
|Z||nuclide||Z 2 / A||half-life
|90||232 Th||34.9||1.405 x 10 10 a||≈100%||<1.0 10 −9 %|
|92||235 U||36.0||7.038 x 10 8 a||≈100%||7.0 10 −9 %|
|92||238 U||35.6||4.468 · 10 9 a||≈100%||5.45 · 10 −5 %|
|94||239 Pu||37.0||2.411 · 10 4 a||≈100%||3.0 10 −10 %|
|94||240 pu||36.8||6.56 · 10 3 a||≈100%||5.75 10 −6 %|
|98||252 Cf||38.1||2.64 a||96.908%||3.092%|
|100||254 m||39.4||3.240 h||99.9408%||0.0592%|
|106||258 Sg||43.6||2.9 ms||0%||≈100%|
As you can see, for elements with atomic numbers up to about 95, the proportion of spontaneous fission in the total decays is very small. The same applies to ordinal numbers of 107 and higher (see list of isotopes ).
The cleavage parameter Z 2 / A ( Z = ordinal number, A = mass number) is given in the third column of the table. It increases with increasing atomic number. A simple estimate based on the droplet model shows that above about Z 2 / A = 38.5 the potential barrier for spontaneous fission disappears; because of the tunnel effect, however, spontaneous splitting occurs even with smaller values. ( Thorium -232: Z 2 / A = 34.9).
Atomic nuclei with a value of Z 2 / A > 49 are non-existent, because they would have to decay through spontaneous fission immediately after their formation. For the transuranic elements (elements 93 to 118) shown experimentally so far, Z 2 / A is at most 47.4. The limit value 49 should only be reached at Z > 130. According to more recent findings, it is also uncertain whether the spontaneous split actually occurs in each case.
The indication whether spontaneous fission has been observed in a nuclide can be found e.g. B. in the Karlsruhe nuclide map. Exact branch relationships can be found in data collections such as B. can be found in the Table of Isotopes .
Application as neutron sources
Since about two to four neutrons are released during the spontaneous splitting of an atomic nucleus, spontaneously splitting nuclides can serve as neutron sources . They are used, for example, for the neutron activation analysis of inaccessible material (rocks on Mars, manganese nodules on the sea floor). Since the neutron spectrum is very similar to that of induced nuclear fission, they also play a role in experimental investigations into reactor physics and as a "start-up source" in nuclear reactors . The most commonly used is 252 Californium .
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