In physics, spontaneous fission is a type of radioactive decay in which an unstable atomic nucleus splits into two approximately equal smaller nuclei, releasing energy and usually one or more neutrons. Spontaneous fission only occurs in heavy nuclei with an atomic number (Z) greater than 90. While relatively rare overall, it is more common in the actinides (e.g., uranium, plutonium, americium) and heavy synthetic elements (mass numbers greater than 232) than in lighter atoms. These are isotopes at least as heavy as thorium-232.
Example
An example of spontaneous fission reaction is the splitting of californium-252 into xenon-140 and ruthenium-108, which also releases 4 neutrons:
25298Cf → 14054Xe + 10844Ru + 4 10n
Spontaneous Fission vs Induced Fission
The other type of fission is induced fission. While both types of fission give about the same result, induced fission occurs when a neutron or other particle strikes the atomic nucleus. In contrast, spontaneous fission occurs due to quantum tunneling. Because spontaneous fission usually releases neutrons, it can lead to induced fission and a chain reaction. Since spontaneous fission can lead to a chain reaction, it is a consideration in nuclear weapon design and safety, ultimately leading to the abandonment of the gun-type design using plutonium.
It can be difficult distinguishing between spontaneous and induced fission because neutron sources aren’t always obvious. For example, cosmic rays sometimes include neutrons. The discovery of spontaneous fission occurred in 1940 when Soviet physicists Georgy Flyorov and Konstantin Petrzhak examined fission in uranium 60 meters (200 feet) underground.
Spontaneous Fission vs Alpha Decay and Cluster Fission
Alpha decay, cluster decay, and spontaneous fission are related processes that are all types of radioactive decay. However, spontaneous fission splits the nucleus into approximately equal fragments, while cluster decay releases a “cluster” of protons and neutrons and alpha decay releases a helium nucleus of two protons and two neutrons. Sometimes alpha and cluster decay are considered to be separate processes, but usually alpha decay is considered to be the most common type of cluster decay. Meanwhile, spontaneous and induced fission are types of binary fission because they break the nucleus into two comparable pieces.
Some elements decay via multiple processes. For example, the uranium-238 decay scheme includes both alpha decay and spontaneous fission.
Spontaneous Fission Rates
Spontaneous fission is not a common event and its frequency varies between different isotopes. For example, uranium-238 undergoes alpha decay with a half-life around 109 years, but the half-life of its decay by spontaneous fission alone is on the order of 1016 years. The rate of spontaneous fission in plutonium-239 is around 300 times higher than its rate in uranium-235. Curium-250 and californium-253 readily undergo spontaneous fission.
| Nuclide | Half-Life (years) | Fission Rate (% of decays) | Neutrons per Fission | Spontaneous Half-Life | Z2/A |
|---|---|---|---|---|---|
| 235U | 7.04×108 | 2.0×10-7 | 1.86 | 3.5×1017 years | 36.0 |
| 238U | 4.47×109 | 5.4×10-5 | 2.07 | 8.4×1015 years | 35.6 |
| 239Pu | 24100 | 4.4×10-10 | 2.16 | 5.5×1015 years | 37.0 |
| 240Pu | 6569 | 5.0×10-6 | 2.21 | 1.16×1011 years | 36.8 |
| 250Cm | 8300 | ~74 | 3.31 | 1.12×104 years | 36.9 |
| 252Cf | 2.65 | 3.09 | 3.73 | 85.7 years | 38.1 |
Fission Tracks
When spontaneous fission occurs in uranium-235 and uranium-238, the mineral crystals show trails of damage from the impact of fission fragments. The trails are called fission tracks. Studying fission tracks helps researchers perform a type of radiometric dating called fission track dating.
References
- Krane, Kenneth S. (1988). Introductory Nuclear Physics. John Wiley & Sons. ISBN 978-0-471-80553-3.
- Scharff-Goldhaber, G.; Klaiber, G. S. (1946). “Spontaneous Emission of Neutrons from Uranium.” Phys. Rev. 70 (3–4): 229. doi:10.1103/PhysRev.70.229.2
- Shultis, J. Kenneth; Faw, Richard E. (2008). Fundamentals of Nuclear Science and Engineering. CRC Press. ISBN 978-1-4200-5135-3.