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Encyclopedia > Spontaneous fission
Nuclear processes

• Neutron Capture
• The R-process
• The S-process
• Proton capture:
• The P-process
• The Rp-process
• Spallation

Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 amu (elements near ruthenium). In practice, however, spontaneous fission is only energetically feasible for atomic masses above 230 amu (elements near thorium). The elements most susceptible to spontaneous fission are the trans-actinide elements, such as rutherfordium. The R process (R for rapid) is a neutron capture process for radioactive elements which occurs in high neutron density, high temperature conditions. ... The S process (S for slow) is a neutron capture process in the decay of radioactive elements that occurs in lower neutron density, lower temperature conditions. ... The p process was believed to be a proton capture process which occurrs during supernovae explosions. ... The rp process (rapid proton capture process) consists of consecutive proton captures onto seed nuclei to produce heavier elements. ... In nuclear physics, spallation is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy proton, thus greatly reducing its atomic weight. ... Radioactive decay is the set of various processes by which unstable atomic nuclei emit subatomic particles (radiation). ... Isotopes are forms of an element, therefore their nuclei have the same atomic number â€” the number of protons in the nucleus â€” but different mass numbers because they contain different numbers of neutrons. ... The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. ... General Name, Symbol, Number Ruthenium, Ru, 44 Chemical series transition metals Group, Period, Block 8, 5, d Appearance silvery white metallic Atomic mass 101. ... The atomic mass of a chemical element is the mass of an atom at rest, most often expressed in unified atomic mass units. ... General Name, Symbol, Number thorium, Th, 90 Chemical series Actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass 232. ... General Name, Symbol, Number rutherfordium, Rf, 104 Chemical series transition metals Group, Period, Block 4, 7, d Appearance unknown, probably silvery white or metallic gray Atomic mass (261) g/mol Electron configuration probably [Rn] 5f14 6d2 7s2 Electrons per shell 2, 8, 18, 32, 32, 10, 2 Phase presumably a...

For uranium and thorium, the spontaneous fission mode of decay does occur but is not seen for the majority of radioactive breakdowns and is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements. Mathematically, the criterion for whether spontaneous fission can occur is approximately:

$hbox{Z}^2/hbox{A}ge45.$

Where Z is the atomic number and A is the mass number (e.g., 235 for U-235). In chemistry and physics, the atomic number (Z) is the number of protons found in the nucleus of an atom. ... The mass number (A), also called atomic mass number or nucleon number, is the number of protons plus the number of neutrons in an atomic nucleus. ...

As the name suggests, spontaneous fission follows the exact same process as nuclear fission, except that it occurs without the atom having been struck by a neutron or other particle. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a chain reaction. Also, radioisotopes for which spontaneous fission is a nonnegligible decay mode may be used as neutron sources; californium-252 (half-life 2.645 years, SF branch ratio 3.09%) is often used for this purpose. The neutrons may then be used to inspect airline luggage for hidden explosives, to gauge the moisture content of soil in the road construction and building industries, to measure the moisture of materials stored in silos, and in other applications. An induced nuclear fission event. ... General Name, Symbol, Number californium, Cf, 98 Chemical series actinides Group, Period, Block n/a, 7, f Appearance unknown, probably silvery white or metallic gray Atomic mass (251) g/mol Electron configuration [Rn] 5f10 7s2 Electrons per shell 2, 8, 18, 32, 28, 8, 2 Physical properties Phase solid Density...

As long as the fissions give a negligible reduction of the amount of nuclei that can spontaneously fission, this is a Poisson process: for very short time intervals the probability of a spontaneous fission is proportional to the length of time. A Poisson process, named after the French mathematician SimÃ©on-Denis Poisson (1781 - 1840), is a stochastic process which is defined in terms of the occurrences of events. ...

The spontaneous fission of uranium-238 leaves trails of damage in uranium containing minerals as the fission fragments recoil through the crystal structure. These trails, or fission tracks provide the basis for the radiometric dating technique: fission track dating. There are two objects with this name: Unterseeboot 238 Uranium-238, the most common isotope of uranium This is a disambiguation page &#8212; a navigational aid which lists other pages that might otherwise share the same title. ... Rose des Sables (Sand Rose), formed of gypsum crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Radiometric dating is a technique used to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances. ... Fission track dating is a radiometric dating technique based on analyses of the damage trails or tracks left by fission fragments in certain uranium bearing minerals and glass. ...

Spontaneous fission rates:

• U-235: 5.60E-03 fissions/s-kg
• U-238: 6.93 fissions/s-kg
• Pu-239: 7.01 fissions/s-kg
• Pu-240: 489,000 fission/s-kg (ca. 1,000,000 neutrons/s-kg)

In practice Pu-239 will invariably contain a certain amount of Pu-240 due to the tendency of Pu-239 to absorb an additional neutron during production. Pu-240's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7% Pu-240.

The gun-type fission weapon has a critical insertion time of ca. 1ms, and the probability of a fission during this time interval should be small. Therefore only U-235 is suitable. Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the gun method: shooting one piece of sub-critical material into another. ... The term insertion time is used to describe the length of time which is required to rearrange a subcritical mass of fissile material into a critical mass. ...

Results from FactBites:

 Spontaneous fission - Wikipedia, the free encyclopedia (261 words) Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 amu (elements near ruthenium). For uranium and thorium, the spontaneous fission mode of decay does occur but is not seen for the majority of radioactive breakdowns and is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements. However, spontaneous fissions release neutrons as all fissions do, so radioisotopes for which spontaneous fission is a nonnegligible decay mode may be used as neutron sources; californium-252 (half-life 2.645 years, SF branch ratio 3.09%) is often used for this purpose.
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