By 1908, physicists understood that alpha decay involved ejection of helium nuclei from a decaying atom.[2] Like cluster decay, alpha decay is not typically categorized as a process of fission.[3]
The first nuclear fission process discovered was fission induced by neutrons. Because cosmic rays produce some neutrons, it was difficult to distinguish between induced and spontaneous events. Cosmic rays can be reliably shielded by a thick layer of rock or water. Spontaneous fission was identified in 1940 by Soviet physicists Georgy Flyorov and Konstantin Petrzhak[4][5] by their observations of uranium in the Moscow MetroDinamo station, 60 metres (200 ft) underground.[6]
Feasibility
Elemental
Spontaneous fission occurs over practical observation times only for atomic masses of 232 atomic mass units or more. These are nuclei at least as heavy as thorium-232—which has a half-life somewhat longer than the age of the universe. 232Th, 235U, and 238U are primordial nuclides and have left evidence of undergoing spontaneous fission in their minerals.
The known elements most susceptible to spontaneous fission are the synthetic high-atomic-number actinides and transactinides with atomic number 100 onward.
For naturally occurring thorium-232, uranium-235, and uranium-238, spontaneous fission does occur rarely, but in the vast majority of the radioactive decay of these atoms, alpha decay or beta decay occurs instead. Hence, the spontaneous fission of these isotopes is usually negligible, except in using the exact branching ratios when finding the radioactivity of a sample of these elements, or in applications that are very sensitive to even minuscule numbers of fission neutrons (such as nuclear weapon design).
Mathematical
The liquid drop model predicts approximately that spontaneous fission can occur in a time short enough to be observed by present methods when
where Z is the atomic number and A is the mass number (e.g., Z2/A = 36 for uranium-235). However, no known radioactive isotope except oganesson-294 reaches a value of 47 (approximately 47.36), as the liquid drop model is not very accurate for the heaviest known nuclei due to strong shell effects.
Spontaneous fission rates
Spontaneous fission half-life of various nuclides depending on their Z2/A ratio. Nuclides of the same element are linked with a red line. The green line shows the upper limit of half-life. Data taken from French Wikipedia.
In practice, 239Pu invariably contains 240Pu due to the tendency of 239Pu to absorb an additional neutron during production. 240Pu's high rate of spontaneous fission makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% 240Pu.
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 235U is suitable. Almost all nuclear bombs use some kind of implosion method.
Spontaneous fission can occur much faster when a nucleus undergoes superdeformation.
Poisson process
Spontaneous fission gives much the same result as induced nuclear fission. However, like other forms of radioactive decay, it occurs due to quantum tunneling, without the atom having been struck by a neutron or other particle as in induced nuclear fission. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a self-sustaining chain reaction. Radioisotopes for which spontaneous fission is not negligible can be used as neutron sources. For example, californium-252 (half-life 2.645 years; SF branch ratio 3.1%) can be used for this purpose. The neutrons released can be used to inspect airline luggage for hidden explosives, to gauge the moisture content of soil in highway and building construction, or to measure the moisture of materials stored in silos, for example.
As long as the spontaneous fission gives a negligible reduction of the number of nuclei that can undergo such fission, this process can be approximated closely as a Poisson process. In this situation, for short time intervals the probability of a fission is directly proportional to the length of time.
The spontaneous fission of uranium-238 and uranium-235 leaves trails of damage in the crystal structure of uranium-containing minerals when the fission fragments recoil through them. These trails, or fission tracks, are the foundation of the radiometric dating method called fission track dating.
^Sadhukhan, Jhilam (29 October 2020). "Microscopic Theory for Spontaneous Fission". Frontiers in Physics. 8: 567171. Bibcode:2020FrP.....8..418S. doi:10.3389/fphy.2020.567171.
^Rutherford, E.; Royds, T. (1908). "XXIV.Spectrum of the radium emanation". Philosophical Magazine. series 6. 16 (92): 313–317. doi:10.1080/14786440808636511.
^Santhosh, K P; Biju, R K (1 January 2009). "Alpha decay, cluster decay and spontaneous fission in (294–326)122 isotopes". Journal of Physics G: Nuclear and Particle Physics. 36 (1): 015107. Bibcode:2009JPhG...36a5107S. doi:10.1088/0954-3899/36/1/015107.
^Scharff-Goldhaber, G.; Klaiber, G. S. (1946). "Spontaneous Emission of Neutrons from Uranium". Phys. Rev. 70 (3–4): 229. Bibcode:1946PhRv...70..229S. doi:10.1103/PhysRev.70.229.2.
^"Igor Sutyagin: The role of nuclear weapons and its possible future missions". Nato.int. Retrieved 7 July 2022.
The LIVEChart of Nuclides - IAEA with filter on spontaneous fission decay
August 23, 2023
spontaneous, fission, form, radioactive, decay, that, found, only, very, heavy, chemical, elements, nuclear, binding, energy, elements, reaches, maximum, atomic, mass, number, about, iron, spontaneous, breakdown, into, smaller, nuclei, isolated, nuclear, parti. Spontaneous fission SF is a form of radioactive decay that is found only in very heavy chemical elements The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 e g iron 56 spontaneous breakdown into smaller nuclei and a few isolated nuclear particles becomes possible at greater atomic mass numbers 1 Contents 1 History 2 Feasibility 2 1 Elemental 2 2 Mathematical 3 Spontaneous fission rates 4 Poisson process 5 See also 6 Notes 7 External linksHistory EditBy 1908 physicists understood that alpha decay involved ejection of helium nuclei from a decaying atom 2 Like cluster decay alpha decay is not typically categorized as a process of fission 3 The first nuclear fission process discovered was fission induced by neutrons Because cosmic rays produce some neutrons it was difficult to distinguish between induced and spontaneous events Cosmic rays can be reliably shielded by a thick layer of rock or water Spontaneous fission was identified in 1940 by Soviet physicists Georgy Flyorov and Konstantin Petrzhak 4 5 by their observations of uranium in the Moscow Metro Dinamo station 60 metres 200 ft underground 6 Feasibility EditElemental Edit Spontaneous fission occurs over practical observation times only for atomic masses of 232 atomic mass units or more These are nuclei at least as heavy as thorium 232 which has a half life somewhat longer than the age of the universe 232Th 235U and 238U are primordial nuclides and have left evidence of undergoing spontaneous fission in their minerals The known elements most susceptible to spontaneous fission are the synthetic high atomic number actinides and transactinides with atomic number 100 onward For naturally occurring thorium 232 uranium 235 and uranium 238 spontaneous fission does occur rarely but in the vast majority of the radioactive decay of these atoms alpha decay or beta decay occurs instead Hence the spontaneous fission of these isotopes is usually negligible except in using the exact branching ratios when finding the radioactivity of a sample of these elements or in applications that are very sensitive to even minuscule numbers of fission neutrons such as nuclear weapon design Mathematical Edit The liquid drop model predicts approximately that spontaneous fission can occur in a time short enough to be observed by present methods when Z 2 A 47 displaystyle hbox Z 2 hbox A approx 47 where Z is the atomic number and A is the mass number e g Z2 A 36 for uranium 235 However no known radioactive isotope except oganesson 294 reaches a value of 47 approximately 47 36 as the liquid drop model is not very accurate for the heaviest known nuclei due to strong shell effects Spontaneous fission rates Edit Spontaneous fission half life of various nuclides depending on their Z2 A ratio Nuclides of the same element are linked with a red line The green line shows the upper limit of half life Data taken from French Wikipedia Spontaneous fission rates 7 Nuclide Half life yrs Fission rate of decays Neutrons per Spontaneoushalf life yrs Z2 AFission Gram sec235 U 7 04 108 2 0 10 7 1 86 0 0 0 0003 3 5 1017 36 0238 U 4 47 109 5 4 10 5 2 07 0 0 0 0136 8 4 1015 35 6239 Pu 24100 4 4 10 10 2 16 0 0 0 022 5 5 1015 37 0240 Pu 0 6569 5 0 10 6 2 21 920 1 16 1011 36 8250 Cm 0 8300 8 74 3 31 0 1 6 1010 1 12 104 36 9252 Cf 0 2 6468 9 3 09 3 73 0 2 3 1012 85 7 38 1In practice 239Pu invariably contains 240Pu due to the tendency of 239Pu to absorb an additional neutron during production 240Pu s high rate of spontaneous fission makes it an undesirable contaminant Weapons grade plutonium contains no more than 7 0 240Pu 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 235U is suitable Almost all nuclear bombs use some kind of implosion method Spontaneous fission can occur much faster when a nucleus undergoes superdeformation Poisson process EditSpontaneous fission gives much the same result as induced nuclear fission However like other forms of radioactive decay it occurs due to quantum tunneling without the atom having been struck by a neutron or other particle as in induced nuclear fission Spontaneous fissions release neutrons as all fissions do so if a critical mass is present a spontaneous fission can initiate a self sustaining chain reaction Radioisotopes for which spontaneous fission is not negligible can be used as neutron sources For example californium 252 half life 2 645 years SF branch ratio 3 1 can be used for this purpose The neutrons released can be used to inspect airline luggage for hidden explosives to gauge the moisture content of soil in highway and building construction or to measure the moisture of materials stored in silos for example As long as the spontaneous fission gives a negligible reduction of the number of nuclei that can undergo such fission this process can be approximated closely as a Poisson process In this situation for short time intervals the probability of a fission is directly proportional to the length of time The spontaneous fission of uranium 238 and uranium 235 leaves trails of damage in the crystal structure of uranium containing minerals when the fission fragments recoil through them These trails or fission tracks are the foundation of the radiometric dating method called fission track dating See also EditNatural nuclear fission reactorNotes Edit Sadhukhan Jhilam 29 October 2020 Microscopic Theory for Spontaneous Fission Frontiers in Physics 8 567171 Bibcode 2020FrP 8 418S doi 10 3389 fphy 2020 567171 Rutherford E Royds T 1908 XXIV Spectrum of the radium emanation Philosophical Magazine series 6 16 92 313 317 doi 10 1080 14786440808636511 Santhosh K P Biju R K 1 January 2009 Alpha decay cluster decay and spontaneous fission in 294 326 122 isotopes Journal of Physics G Nuclear and Particle Physics 36 1 015107 Bibcode 2009JPhG 36a5107S doi 10 1088 0954 3899 36 1 015107 Scharff Goldhaber G Klaiber G S 1946 Spontaneous Emission of Neutrons from Uranium Phys Rev 70 3 4 229 Bibcode 1946PhRv 70 229S doi 10 1103 PhysRev 70 229 2 Igor Sutyagin The role of nuclear weapons and its possible future missions Nato int Retrieved 7 July 2022 Petrzhak Konstantin How the spontaneous fission was discovered in Russian Shultis J Kenneth Faw Richard E 2008 Fundamentals of Nuclear Science and Engineering CRC Press pp 141 table 6 2 ISBN 978 1 4200 5135 3 Entry at periodictable com Entry at periodictable comExternal links Edit The LIVEChart of Nuclides IAEA with filter on spontaneous fission decay Retrieved from https en wikipedia org w index php title Spontaneous fission amp oldid 1126541678, wikipedia, wiki, book, books, library,
