Nuclear physics |
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Nucleus·Nucleons (p, n) ·Nuclear matter·Nuclear force·Nuclear structure·Nuclear reaction |
Liquid drop·Nuclear shell model·Interacting boson model·Ab initio |
Isotopes – equal Z Isobars – equal A Isotones – equal N Isodiaphers – equal N − Z Isomers – equal all the above Mirror nuclei – Z ↔ N Stable·Magic·Even/odd·Halo (Borromean) |
Binding energy·p–n ratio·Drip line·Island of stability·Valley of stability |
Alpha α·Beta β (2β, β+) ·K/L capture·Isomeric (Gamma γ·Internal conversion) ·Spontaneous fission·Cluster decay·Neutron emission·Proton emission Decay energy·Decay chain·Decay product·Radiogenic nuclide |
Spontaneous·Products (pair breaking) ·Photofission |
electron (2×) ·neutron (s·r) ·proton (p·rp) |
Spallation (by cosmic ray) ·Photodisintegration |
Nuclear fusion Processes:Stellar·Big Bang·Supernova Nuclides: Primordial·Cosmogenic·Artificial |
Quark–gluon plasma·RHIC·LHC |
Alvarez·Becquerel·Bethe·A. Bohr·N. Bohr·Chadwick·Cockcroft·Ir. Curie·Fr. Curie·Pi. Curie·Skłodowska-Curie·Davisson·Fermi·Hahn·Jensen·Lawrence·Mayer·Meitner·Oliphant·Oppenheimer·Proca·Purcell·Rabi·Rutherford·Soddy·Strassmann·Świątecki·Szilárd·Teller·Thomson·Walton·Wigner |
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; spontaneous breakdown into smaller nuclei and a few isolated nuclear particles becomes possible at greater atomic mass numbers.
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- For this nuclide it has been found that the number of prompt fission photons is 8.13 ± 0.35 photons per fission over the energy range 0.1 to 10.5 MeV, and the energy carried by this number of photons is 7.25 ± 0.26 MeV per fission.
History[edit]
By 1908, the process of alpha decay was known to consist of the ejection of helium nuclei from the decaying atom;[1] however, as with cluster decay, alpha decay is not typically categorized as a process of fission.[2]
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[3][4] by their observations of uranium in the Moscow MetroDinamo station, 60 metres (200 ft) underground.[5]
Cluster decay was shown to be a superasymmetric spontaneous fission process.[6]
Feasibility[edit]
Elemental[edit]
Spontaneous fission is feasible over practical observation times only for atomic masses of 232 amu or more. These are elements 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 numbers from 100 onwards.
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.
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
- [7]
where Z is the atomic number and A is the mass number (e.g., Z2/A = 36 for uranium-235). However, all known nuclides which undergo spontaneous fission as their main decay mode do not reach this value of 47, 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.
Nu- clide | Half-life (yrs) | Fission prob. per decay (%) | Neutrons per | Spontaneous half-life (yrs) | Z2/A | |
---|---|---|---|---|---|---|
Fission | Gram-sec | |||||
235 U | 7.04·108 | 2.0·10−7 | 1.86 | 000.0003 | 3.5·1017 | 36.0 |
238 U | 4.47·109 | 5.4·10−5 | 2.07 | 000.0136 | 8.4·1015 | 35.6 |
239 Pu | 24100 | 4.4·10−10 | 2.16 | 000.022 | 5.5·1015 | 37.0 |
240 Pu | 06569 | 5.0·10−6 | 2.21 | 920 | 1.16·1011 | 36.8 |
250 Cm | 08300 [9] | ~74 | 3.31 | 01.6·1010 | 1.12·104 | 36.9 |
252 Cf | 02.6468[10] | 3.09 | 3.73 | 02.3·1012 | 85.7 | 38.1 |
In practice, 239
Pu
will invariably contain a certain amount of 240
Pu
due to the tendency of 239
Pu
to absorb an additional neutron during production. 240
Pu
's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% 240
Pu
.
Pu
will invariably contain a certain amount of 240
Pu
due to the tendency of 239
Pu
to absorb an additional neutron during production. 240
Pu
's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% 240
Pu
.
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 235
U
is suitable. Almost all nuclear bombs use some kind of implosion method.
U
is suitable. Almost all nuclear bombs use some kind of implosion method.
Fission 2 5 0 44
Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes superdeformation.
Poisson process[edit]
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 about 3.1 percent) 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 spontaneous fission is directly proportional to the length of time. Download graphicconverter 10 5 5.
The spontaneous fission of uranium-238 and uranium-235 does leave 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[edit]
Notes[edit]
- ^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. doi:10.1088/0954-3899/36/1/015107.
- ^G. Scharff-Goldhaber and G. S. Klaiber (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
- ^Petrzhak, Konstantin. 'How the spontaneous fission was discovered' (in Russian).
- ^Dorin N Poenaru; et al. (1984). 'Spontaneous emission of heavy clusters'. Journal of Physics G: Nuclear Physics. 10 (8): L183–L189. Bibcode:1984JPhG..10L.183P. doi:10.1088/0305-4616/10/8/004.
- ^Krane, Kenneth S. (1988). Introductory Nuclear Physics. John Wiley & Sons. pp. 483–484 (Equation 13.3). ISBN978-0-471-80553-3.
- ^Shultis, J. Kenneth; Richard E. Faw (2008). Fundamentals of Nuclear Science and Engineering. CRC Press. pp. 141 (table 6.2). ISBN978-1-4200-5135-3.
- ^Entry at periodictable.com
- ^Entry at periodictable.com
External 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&oldid=952019021'
Discussion
Heavy nuclei split into two fragments of roughly equal mass. Energy is released in the process. Fission powers nuclear reactors and 'small' nuclear weapons.
spontaneous
neutron induced
23592U | + | 10n | → | fission fragments | + | 2.4 neutrons | + | 192.9 MeV |
23994Pu | + | 10n | → | fission fragments | + | 2.9 neutrons | + | 198.5 MeV |
For example
10n | + | 23592U | → | ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩ | 8735Br | + | 14657La | + | 310n |
9236Kr | + | 14156Ba | + | 310n | |||||
9037Rb | + | 14455Cs | + | 210n | |||||
9038Sr | + | 14354Xe | + | 310n | |||||
10n | + | 23994Pu | → | 9436Kr | + | 14458Ce | + | 210n |
chain reaction: subcritical, critical, supercritical
Cartoon. Alternating series of parents and daughters and parents and daughters. At the end its nothing but fission fragments and free neutrons. Watch out.
history
chain reaction timeline
- Szilard fled to London to escape Nazi persecution. While in London, he read an article written by Ernest Rutherford in the London Times, after which he conceived the idea of a nuclear chain reaction.
- Filed a patent on the nuclear chain reaction. He first attempted to create a chain reaction using Beryllium and Indium, but neither yielded the reaction he deliberated.
- Assigned the chain-reaction patent to the British Admiralty to ensure secrecy of the patent.
- Moved to New York
- Concluded that uranium would be the element capable of the chain reaction. Composes Einstein's first letter to President Franklin Delano Roosevelt.
- On December 2, 1942, Szilard and Enrico Fermi were successful in creating the first controlled nuclear chain reaction.
Leo Szilard recalls the day
variant 1
As the light changed to green and I crossed the street, it… suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction…. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.
As the light changed to green and I crossed the street, it… suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction…. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.
variant 2
I found myself in London about the time of the British Association meeting in [12] September 1933. I read in the newspapers a speech by Lord Rutherford, who was quoted as saying that he who talks about the liberation of atomic energy on an industrial basis is talking moonshine. This set me pondering as I was walking the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row [at Russell Square]. As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. Soon thereafter, when the discovery of artificial radioactivity by Joliot and Mme. Joliot was announced, I suddenly saw that tools were at hand to explore the possibility of such a chain reaction. I talked to a number of people about this…. [I]in the spring of 1934 I had applied for a patent which described the laws governing such a chain reaction. It was the first time, I think, that the concept of critical mass was developed and that a chain reaction was seriously discussed. Knowing what this would mean - and I knew it because I had read H.G. Wells - I did not want this patent to become public. The only way to keep it from becoming public was to assign it to the government. So I assigned this patent to the British Admiralty.
I found myself in London about the time of the British Association meeting in [12] September 1933. I read in the newspapers a speech by Lord Rutherford, who was quoted as saying that he who talks about the liberation of atomic energy on an industrial basis is talking moonshine. This set me pondering as I was walking the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row [at Russell Square]. As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. Soon thereafter, when the discovery of artificial radioactivity by Joliot and Mme. Joliot was announced, I suddenly saw that tools were at hand to explore the possibility of such a chain reaction. I talked to a number of people about this…. [I]in the spring of 1934 I had applied for a patent which described the laws governing such a chain reaction. It was the first time, I think, that the concept of critical mass was developed and that a chain reaction was seriously discussed. Knowing what this would mean - and I knew it because I had read H.G. Wells - I did not want this patent to become public. The only way to keep it from becoming public was to assign it to the government. So I assigned this patent to the British Admiralty.
Fission 2 5 0 48
variant3
On Tuesday, September 12, 1933, while waiting at the lights to cross the road to the British Museum in Bloomsbury, Leo Szilard, a Hungarian theoretical physicist, had the flash of insight which was to result in the Little Boy and Fat Man bombs being dropped on Hiroshima and Nagasaki less than 12 years later. 'As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons, and which would emit two neutrons when it absorbs one, such an element could sustain a nuclear chain reaction.'
On Tuesday, September 12, 1933, while waiting at the lights to cross the road to the British Museum in Bloomsbury, Leo Szilard, a Hungarian theoretical physicist, had the flash of insight which was to result in the Little Boy and Fat Man bombs being dropped on Hiroshima and Nagasaki less than 12 years later. 'As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons, and which would emit two neutrons when it absorbs one, such an element could sustain a nuclear chain reaction.'
Just take excerpts from this letter by Leo Szilard
I feel that I ought to let you know of a very sensational new development in nuclear physics. In a paper in the Naturwissenschaften Hahn reports that he finds when bombarding uranium with neutrons the uranium breaking up into two halves giving elements of about half the atomic weight of uranium. This is entirely unexpected and exciting news for the average physicist. The Department of Physics at Princeton, where I spent the last few days, was like a stirred-up ant heap.
Apart from the purely scientific interest there may be another aspect of this discovery, which so far does not seem to have caught the attention of those to whom I spoke. First of all it is obvious that the energy released in this new reaction must be very much higher than in all previously known cases. It may be 200 million (electron-) volts instead of the usual 3-10 mil-lion volts. This in itself might make it possible to produce power by means of nuclear energy, but I do not think that this possibility is very exciting, for if the energy output is only two or three times the energy input, the cost of investment would probably be too high to make the process worthwhile.
Unfortunately, most of the energy is released in the form of heat and not in the form of radioactivity.
I see, however, in connection with this new discovery potential possibilities in another direction. These might lead to a large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs. This new discovery revives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good as abandoned in the course of the last two years. At present I am running a high temperature and am therefore confined to my four walls, but perhaps I can tell you more about these new developments some other time. Meanwhile you may look out for a paper in 'Nature' by Frisch and Meitner which will soon appear and which might give you some information about this new discovery.
Thorium reactor?
23290Th + 10n → 23392U + 20−1e + 200ν
and U233 fissions into junk and neutrons. Neutrons hit thoriums making more U233s and life goes on.
Fission 2 5 0 42
- Talk about burying the past. The US plans to dump an unused stash of uranium-233 – created in the 1960s and 70s – at an underground facility in Nevada. A report by the Institute for Policy Studies estimates the government spent about $5.5 billion to make 1.5 tonnes of the isotope, but it turned out to be more expensive and less useful than natural uranium. Read more: https://www.newscientist.com/article/mg21528843-100-60-seconds/