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Topic: Spontaneous fission

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	Spontaneous fission - Wikipedia, the free encyclopedia
		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.
	en.wikipedia.org /wiki/Spontaneous_fission (261 words)

	Spontaneous fission (Site not responding. Last check: 2007-11-07)
		Spontaneous fission is a form of radioactive decay characteristic of very heavy isotopes and is theoretically possible for uranium and thorium or indeed for any atomic nucleus mass is greater than or equal to amu (ruthenium).
		For uranium and thorium the spontaneous fission of decay is not seen for the of radioactive breakdowns and is usually neglected for the exact considerations of branching ratios determining the activity of a sample containing elements.
		As the name suggests spontaneous fission follows exact same process as nuclear fission only it is not self-sustaining and not generate the neutron flux necessary to critical" and continue such fissions.
	www.freeglossary.com /Spontaneous_fission (503 words)

	Nuclear fission - Wikipedia, the free encyclopedia
		In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom.
		Fission can be induced by several methods, including bombarding the nucleus of a fissile atom with another particle of the correct energy.
		Fission products are usually highly radioactive since these other nuclei are not stable isotopes.
	en.wikipedia.org /wiki/Nuclear_fission (3007 words)

	Spontaneous fission (Site not responding. Last check: 2007-11-07)
		Spontaneous fission is a form of radioactive decay characteristic of very heavy isotope s, and is theoretically possible for uranium and thorium, or indeed for any atomic nucleus whose mass is greater than or equal to 100 amu (ruthenium).
		In practice, however, spontaneous fission is only energetically feasible for atomic mass es above 230 amu (thorium).
		Essentials of Spontaneous Prose Kerouac's explanation of the spontaneous prose method he invented.
	www.serebella.com /encyclopedia/article-Spontaneous_fission.html (353 words)

	spontaneous fission -- Encyclopædia Britannica
		Spontaneous fission, discovered (1941) by the Russian physicists G.N. Flerov and K.A. Petrzhak in uranium-238, is observable in many nuclear species of mass number 230 or more.
		The fission process results in the release of several hundred million electron volts of energy and produces a large amount of radiation damage before its energy is fully absorbed.
		A fissionable system (uranium-238, for example) in its ground state (i.e., at its lowest excitation energy and with an elongation small enough that it is confined inside the fission barrier) has a small but...
	www.britannica.com /eb/article?tocId=9069207&query=spontaneous&ct= (853 words)

	Manhattan Project:Physics Project (Site not responding. Last check: 2007-11-07)
		The minimum mass of a fissionable material that will just maintain a fission chain reaction under precisely specified conditions, such as the nature of the material and it's purity, the nature and thickness of the tamper (or neutron reflector), the density, and the physical shape.
		The spontaneous fission rate is the probability per second that a given atom will fission spontaneously - that is, without any external intervention.
		The probability that a fission neutron with an initial energy of about 1 MeV will induce fission is rather low, but can be increased by a factor of hundreds when the neutron is slowed down through a series of elastic collisions with light nuclei such as hydrogen, deuterium, or carbon.
	www.bergen.org /AAST/manhattan_proj/projects.mp/phy_trjc.html (1141 words)

	Spontaneous fission - Encyclopedia.WorldSearch (Site not responding. Last check: 2007-11-07)
		In practice, however, spontaneous fission is only energetically feasible for atomic masses above 230 amu (thorium).
		Mass distributions of fission fragments for the compound nucleus Pu[superscript 240];: Comparison between spontaneous fission and fission induced by neutrons of several energies below 1 eV
		Calibration of a spontaneous fission neutron source (Atomic Energy Research Establishment)
	encyclopedia.worldsearch.com /spontaneous_fission.htm (222 words)

	March 16 (Site not responding. Last check: 2007-11-07)
		Fission track analysis is a technique for geological dating and thermal history reconstruction.
		These and other applications all flow from observations made on etched fission tracks in minerals --- minute trails of damage to the crystal structure that are formed when uranium atoms inside the mineral undergo spontaneous fission.
		Fission tracks in a mineral can be represented mathematically by a stochastic process of randomly oriented line segments in three dimensions.
	www.stats.gla.ac.uk /Seminars/16-03-05.htm (272 words)

	Dating Exhibit
		Fission tracks are created at a constant rate throughout time so that from the number of tracks present it is possible to determine the amount of time that has past since the track accumulation began.
		Spontaneous fission is accomplished by causing the nucleus of one parent uranium atom to split in two with such a force that it leaves a trail (track) of damage on the crystal in a mineral.
		Fission is induced through controlled irradiation with thermal neutrons of Uranium 235 (0.7 x 10 9 half-life) in a nuclear reactor.
	www.mnsu.edu /emuseum/archaeology/dating/dat_fission.html (658 words)

	A Theory of Population, deduced from the General Law of Animal Fertility
		Whether it be by spontaneous fission, by gemmation, or by gemmules; whether the detached products be bulbels, spores or seeds, ovisacs, ova or spermatozoa; or however the process of multiplication be modified, it essentially consists in the throwing off of parts of adult organisms for the purpose of making new organisms.
		And in fact, this gemmation may be regarded as the form which spontaneous fission must assume in ceasing to be fatal; seeing that gemmation essentially consists in the separation, not into halves, but into a larger part and a smaller part; the larger part continuing to represent the original individual.
		The length of the body is still increased after birth ''by gemmation from (or partial fission of) the penultimate segment.'' The lower members of the class are distinguished from the higher by the greater extent to which this gemmation is carried.
	www.victorianweb.org /science/science_texts/spencer2.html (11227 words)

	nuclear energy: Nuclear Fission
		The process of nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann and was explained in early 1939 by Lise Meitner and Otto Frisch.
		The average energy released by the fission of uranium-235 is 200 million electron volts, and that released by uranium-233 and plutonium-239 is comparable.
		Fission can also occur spontaneously, but the time required for a heavy nucleus to decay spontaneously by fission (10 million billion years in the case of uranium-238) is so long that induced fission by thermal neutrons is the only practical application of nuclear fission.
	www.factmonster.com /ce6/sci/A0860068.html (413 words)

	Nuclear Fission
		If a massive nucleus like uranium-235 breaks apart (fissions), then there will be a net yield of energy because the sum of the masses of the fragments will be less than the mass of the uranium nucleus.
		Other fissionable isotopes which can be induced to fission by slow neutrons are plutonium-239, uranium-233, and thorium-232.
		U-238 has a small probability for spontaneous fission and also a small probability of fission when bombarded with fast neutrons, but it is not useful as a nuclear fuel source.
	hyperphysics.phy-astr.gsu.edu /hbase/nucene/fission.html (592 words)

	Spontaneous fission (from nuclear fission) -- Encyclopædia Britannica (Site not responding. Last check: 2007-11-07)
		A fissionable system (uranium-238, for example) in its ground state (i.e., at its lowest excitation energy and with an elongation small enough that it is confined inside the fission barrier) has a small but finite…
		Binary fission represents the major reproductive procedure of unicellular organisms, but it also occurs in the process of embryonic development and tissue growth and repair of higher plants...
		The fission fragments are highly unstable because of their abnormally large number of neutrons compared with protons; consequently they...
	www.britannica.com /eb/article-48307?tocId=48307 (954 words)

	Micronukes (Carey Sublette)
		Due to its prodigious spontaneous fission rate, Cf-252 is useful as a compact, intense, fission-spectrum neutron source.
		A general rule is that highly fissile isotopes have odd-numbered masses (and also have low spontaneous fission rates), while isotopes with even numbered masses are much less fissile, and have high spontaneous fission rates.
		Pu-239 and Pu-241 are highly fissile, with very low spontaneous fission rates, while Pu-238 and Pu-240 are much less fissile, with very high rates of spontaneous fission.
	www.yarchive.net /nuke/micronuke.html (683 words)

	Search Results for spontaneous - Encyclopædia Britannica
		Spontaneous combustion may occur when combustible matter, such as hay or coal, is stored in bulk.
		The lighter actinides such as uranium rarely decay by spontaneous fission, but at californium (element 98) spontaneous fission becomes more common (as a result of changes in energy balances) and...
		Spontaneous expulsion of an embryo or fetus from the uterus before it...
	www.britannica.com /search?query=spontaneous&submit=Find&source=MWTEXT (457 words)

	Michael Swanwick's Periodic Table of Science Fiction
		With its prodigious spontaneous fission rate, Cf-252 serves well as a compact, intense, fission-spectrum neutron source, and is thus the only isotope of californium of any significant industrial importance.
		Here's why: As a rule, isotopes with even-numbered masses have high spontaneous fission rates and are less fissile.
		Cf-251 is extremely fissile, with a comparatively low spontaneous fission rate, and an 800-year half-life.
	www.scifi.com /scifiction/elements/californium.html (224 words)

	Reactors (B. Alan Guthrie)
		Small wires of Cf-252 are inserted into selected fuel assemblies (the spontaneous fission rate is so high that only gram amounts of Cf-252 are needed).
		The mass numbers of the fission products are centered around the mass numbers of about 90 to about 100 and from about 134 to about 144, although mass numbers of fission products with other mass numbers are possible.
		For fission induced by a thermal neutron, a fission product with a mass number of say 95 is about 500 times more likely than one with a mass number of 115.
	yarchive.net /nuke/reactors.html (874 words)

	CERN Courier - Second postcard from the isl - IOP Publishing - article
		Each consisted of two sequential alpha decays that were terminated by spontaneous fission characterized by a large energy release in the detectors (figure 1a).
		This corresponds to a mean fission fragment kinetic energy of 230 MeV (taking into account the energy loss in the dead layer of the detector), which is characteristic of the fission of a rather heavy nucleus.
		Comparing spontaneous fission and alpha-decay half-lives for nuclei with 110 and 112 protons with earlier data for the lighter isotopes of these elements shows a significant increase in the stability of heavy nuclei with increasing neutron number.
	www.cerncourier.com /main/article/41/8/17 (1550 words)

	Nuclear Weapon Design
		This gun-type assembly may be achieved in a tubular device in which a high explosive is used to blow one subcritical piece of fissionable material from one end of the tube into another subcritical piece held at the opposite end of the tube.
		To fission more of a given amount of fissile material, a small amount of material that can undergo fusion, deuterium and tritium (D-T) gas, can be placed inside the core of a fission device.
		For example, a standard fission weapon's total energy output would be partitioned as follows: 50% as blast; 35% as thermal energy; and 15% as nuclear radiation.
	www.fas.org /nuke/intro/nuke/design.htm (3646 words)

	Basic Principles of Fission Weapon Design (Site not responding. Last check: 2007-11-07)
		The process of assembling the supercritical mass must occur in significantly less time than the average interval between spontaneous fissions to have a reasonable chance of succeeding.
		The key to achieving objectives 1 and 2 is revealed by the fact that the critical mass (or supercritical mass) of a fissionable material is inversely proportional to the square of its density.
		By contriving a subcritical arrangement of fissionable material whose average density can be rapidly increased, we can bring about the sudden large increase in reactivity needed to create a powerful explosion.
	www.nuclearweaponarchive.org /Library/Fsswpnpr.html (518 words)

	Fission and Fusion (Site not responding. Last check: 2007-11-07)
		The attraction of nuclear fission as a source of power can be understood by comparing this value with the 50 kJ/g released when natural gas is burned.
		The amount of fissionable material necessary for the chain reaction to sustain itself is called the critical mass.
		K. In a hydrogen bomb, a fission reaction produced by a small atomic bomb is used to heat the contents to the temperature required to initiate fusion.
	chemed.chem.purdue.edu /genchem/topicreview/bp/ch23/fission.html (1691 words)

	Nuclear Fission Concepts (Site not responding. Last check: 2007-11-07)
		Models of the process suggested sustained fission reactions over a period of about a million years during a time period about two billion years ago.
		It is presumed that ground water seeping through the ore served as a natural moderator to slow down the fission neutrons.
		One of the interesting observations was that the bulk of the fission products seemed to be still in place in their geologic depository after nearly 2 billion years.
	hyperphysics.phy-astr.gsu.edu /hbase/nucene/fiscon.html (204 words)

	Multiplicity Counter - Non-Destructive Assay - CANBERRA, An AREVA Group Company (Site not responding. Last check: 2007-11-07)
		Since the mass of the material is proportional to the spontaneous fission rate, the mass can be determined.
		Both spontaneous and induced fissions contribute to the rate of the coincident neutrons from the sample.
		The induced fissions are caused by neutrons from outside the nucleus.
	www.canberra.com /products/740.asp (299 words)

	Nuclear Fusion and Fission
		Figure 21.8: Spontaneous fission of a heavy nucleus into a slightly lighter nucleus and an alpha particle occurs when the alpha particle penetrates the potential barrier illustrated by the shading and leaves the nucleus.
		Barrier penetration allows fission to occur spontaneously in the absence of the energy needed to overcome this barrier, as illustrated in figure 21.8, but is generally a slow process.
		Examples of nuclei subject to fission by slow neutron absorption are uranium 235 and plutonium 239.
	www.physics.nmt.edu /~raymond/classes/ph13xbook/node222.html (730 words)

	Howstuffworks "How Nuclear Radiation Works"
		The word "fission" means "splitting." A heavy atom like fermium-256 undergoes spontaneous fission about 97 percent of the time when it decays, and in the process, it becomes two atoms.
		These neutrons can be absorbed by other atoms and cause nuclear reactions, such as decay or fission, or they can collide with other atoms, like billiard balls, and cause gamma rays to be emitted.
		In many cases, a nucleus that has undergone alpha decay, beta decay or spontaneous fission will be highly energetic and therefore unstable.
	science.howstuffworks.com /nuclear2.htm (682 words)

	Superheavy elements (July 2004) - Physics World - PhysicsWeb
		Nuclear fission prompted Niels Bohr and John Wheeler to propose the liquid-drop model, which treats the nucleus as a drop of charged liquid without any structure.
		However, this fission barrier can be overcome if we supply enough energy to the nucleus or if the nucleus "tunnels" through the barrier.
		As a result, these elements will leave a clear experimental signature: a daughter nucleus that is lighter than its parent by two protons and two neutrons, followed by a granddaughter nucleus that is lighter by four protons and four neutrons, and so on.
	www.physicsweb.org /article/world/17/7/7 (2890 words)

	METHODOLOGY
		The number of these fission tracks reveals the time over which the tracks have been accumulating.
		U fission track densities were counted for individual apatites in the crystal and its mirror-imaged impression in the mica detector, respectively.
		To avoid a bias in the results through selection of apatite crystals, each sample was systematically scanned, and each crystal that was encountered with the correct orientation was analyzed, regardless of the track density.
	www-odp.tamu.edu /publications/161_SR/chap_21/c21_3.htm (518 words)

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