Spontaneous fissions

In the startup of a reactor, it is necessary to have a source of neutrons other than those from fission. Otherwise, it might be possible for the critical condition to be reached without any visual or audible signal. Two types of sources are used to supply neutrons. The first, appHcable when fuel is fresh, is califomium-252 [13981-174-Jwhich undergoes fission spontaneously, emitting on average three neutrons, and has a half-life of 2.6 yr. The second, which is effective during operation, is a capsule of antimony and beryUium. Antimony-123 [14119-16-5] is continually made radioactive by neutron... 

Some heavy nuclei will fission spontaneously. Others can be induced to fission through interaction with a neutron. In both spontaneous nuclear fission and induced nuclear fission the pool of neutrons and protons is conseiwed. For example, the nucleus "" Cf (Californium) fissions spontaneously. The 98 protons and 154 neutrons in the nucleus of Cf are reconfigured into other nuclei. Usually a few neu-... 

The nucleus """U (uranium) does not fission spontaneously, but It can be induced to fission through interaction with a neutron. Pictorially, a typical neutron-induced fission of " U producing two nuclei and three neutrons is depicted in Figure 2. 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

Because lawrencium does not exist in nature, it had to be produced artificially. This was done in 1961 by the team of scientists at Berkeley, using an ion accelerator to bombard three different isotopes of the element californium with heavy ions of the elements boron and "boron along with some neutrons that produced the isotope jj,jLr-258. The resulting product weighed only about two millionths of a gram and had a half-life of only 4.1 seconds, fissioning spontaneously. 

Minor amounts of 4He and 3He are produced in ternary fission. Spontaneous fission of 2 i2Th has also been observed (Whetherill, 1953), as has induced fission of 23,Pu in the Oklo natural chain reactor (Drozd et al., 1974). b Except as noted, data from review/compilation by Hyde (1974). 

Which isotopes of which elements undergo fission Experiments with particle accelerators have shown that every element with an atomic number of 80 or more has one or more isotopes capable of undergoing fission, provided they are bombarded at the right energy. Nuclei with atomic numbers between 89 and 98 fission spontaneously with long half-lives of 10 to 10 years. Nuclei with atomic numbers of 98 or more fission spontaneously with shorter half-lives of a few milliseconds to 60.5 days. One of the natural decay modes of the transuranium elements is via spontaneous fission. In fact, all known nuclides... 

Many of the nuclides in the actinide family—U, Np, Pu, etc.—fission spontaneously as one of the modes of radioactive decay. Usually, for a nuclide with multiple modes of radioactive decay, the half-life of the nuclide is determined from the total decay rate, representing all the decay processes for that nuclide. However, in the case of spontaneous fission, a separate half-life for that process alone is used. Examples of nuclides that undergo spontaneous fission are given in Table 2.5. 

Spontaneous fission Spontaneous fission dose rate, mrem/h Surface flux of spontaneous fission neutrons, n/(cm s)... 

Chain reaction—A process that initiates its own repetition in a fission chain reaction, a fissile nucleus absorbs a neutron and fissions spontaneously releasing additional neutrons. 

In 1940, Georgi Flerov and Konstantin Petrzhak showed that natural uranium fissions spontaneously, albeit with an extremely low rate (Flerov and Petrzhak 1940 Petrzhak and Flerov 1940). As spontaneous fission was subsequently studied in other nuclei, it was found that its rate increases steeply with atomic number. This decay mode may thus finally terminate the periodic table of elements. 

Most nuclides with mass numbers between 225 and 250 do not undergo fission spontaneously (except for a few with extremely long half-lives). They can be induced to undergo fission when bombarded with particles of relatively low kinetic energies. Particles that can supply the required activation energy include neutrons, protons, alpha particles, and fast electrons. For nuclei lighter than mass 225, the activation energy required to induce fission rises very rapidly. 

In addition to the decay modes discussed, another decay mechanism, fission decay, should be briefly discussed for a completion of radioactive decay introduction. The products of fission are found in a number of sites as the sources of radioactive contaminants. Fission decay involved two types of process spontaneous fission and neutron-induced fission. Spontaneous fission is a naturally occurring decay process in which a nucleus breaks into two fragments, along with the emission of two to three neutrons. An example of a spontaneous fission decay process is ... 

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