Spontaneous fission

Spontaneous fission ( spontaneous fission, and spontaneous decay, spontaneous division; spontaneous fission English ) is a nuclear fission that occurs as radioactive decay. A heavy atomic nucleus (atomic number 90 or higher) divides this without external action - even without neutron irradiation - in two (rarely more ) mostly medium heavy nuclei.

The alpha decay, ie the emission of a helium-4 nucleus, does not count for spontaneous fission.

Examples

For all naturally occurring thorium and uranium isotopes, 232Th, 234U, 235U and 238U, and the spontaneous fission of the relevant atomic nucleus into two nuclei with the emission of usually two or three neutrons, in addition to the predominant alpha decay are observed as additional decay channel, for example, according to

Or

Sf = spontaneous fission

Even with very many of the even heavier nuclides, the transuranic elements occurs next alpha or beta decay, spontaneous fission.

Declaration and properties

The spontaneous fission is basically explained as the alpha decay and induced fission by the tunnel effect. However, it must be expected with a Coulomb barrier of complicated shape for fission fragments compared to alpha particles.

As the induced cleavage is also the spontaneous fission preferred asymmetric, ie the two fission fragment nuclei are usually of different size. The mass distribution of the resulting nuclides is therefore a curve with two " knuckles " at the mass numbers 90 and 140, similar to the cleavage by thermal neutrons. And the energy spectrum of the neutrons is released very similar to that of the induced fission.

The partial decay constant ( probability per unit time ) for the spontaneous fission is usually smaller than that for the alpha decay of the same nuclide. If you want to express it by the somewhat more descriptive, fictitious partial half-life, this is correspondingly long.

Discovery history

The possibility of spontaneous fission of uranium has been suggested for the first time in 1939 by Niels Bohr and John Archibald Wheeler.

A year later succeeded Georgi Fljorow and Konstantin Petrschak to demonstrate this phenomenon in natural uranium. They applied this developed by Otto Frisch Ionisationskammermethode on (see discovery of nuclear fission ). However, they had the chamber volume significantly larger to accommodate a sample volume of approximately 15 g of uranium oxide U3O8 it. This sample is about six pulses per hour have been registered by the apparatus; the ionization chamber was empty ( without U3O8 filling), so not a single pulse was measured in five hours. Due to this measurement and numerous control experiments, the authors came to the conclusion that the observed pulses could only have come from very high-energy, emitted by the U3O8 surface fragments of uranium. Since the participation of neutrons could be excluded, the experimental results could be explained only by the assumption of a spontaneous fission.

The experiment was, however, no information as to which of the three had with natural uranium isotopes, 238U, 235U and / or 234U, the spontaneous fissions occurred. Today three isotopes are known their nonzero partial decay probabilities of spontaneous fission for all. That of 238U is greatest.

Branching ratios between alpha decay and spontaneous fission

The following table under "Frequency " branching ratios, ie percentages of the decay channels for some nuclides of atomic numbers 90-106. A total of about 100 nuclides with the two decay channels alpha decay and spontaneous fission are known.

In the third column of the table is for each nuclide whose " fission parameters " (also gap parameter, fissility; engl: fissionability parameters. ) Z2 / A ( Z = atomic number, A = mass number ) specified. It increases with increasing atomic number. Calculations based on the droplet model revealed that nuclei with a value of

Are not viable because they would disintegrate immediately after their formation by spontaneous fission. In the previously experimentally shown transuranic elements ( elements 93-118 ) is Z2 / A is at most 47.4. Only at Z > 130, the limit should be achieved 49. According to recent findings is also uncertain whether really the immediate spontaneous fission occurs in each case.

Specifying the half-life in years ( a), hours ( h), milliseconds (ms). Origin of the table values ​​: Advanced nuclear chart of Korean Atomic Energy Research Institute

As you can see, the percentage of spontaneous fission of total decays very small for the elements with atomic numbers up to about 95. The same applies to atomic numbers of 107 and above ( see list of isotopes).

Data collections

Whether or not, in a spontaneous fission nuclide has been observed, one finds, for example, in the Karlsruhe Nuclide Chart. Accurate branching ratios can be found in data collections such as the Table of Isotopes.

Application as neutron sources

Since the spontaneous fission of a nucleus about two to four neutrons are released spontaneously fissioning nuclides can be used as neutron sources. They are used for the neutron activation analysis of inaccessible material ( rocks on Mars, manganese nodules on the ocean floor). Since the neutron spectrum of the induced nuclear fission is very similar, they are also involved in experimental studies of reactor physics and as " Anfahrquelle " in nuclear reactors a role. Most is used 252Californium.