Uranium spontaneous fission

Gr. technetos, artificial) Element 43 was predicted on the basis of the periodic table, and was erroneously reported as having been discovered in 1925, at which time it was named masurium. The element was actually discovered by Perrier and Segre in Italy in 1937. It was found in a sample of molybdenum, which was bombarded by deuterons in the Berkeley cyclotron, and which E. Eawrence sent to these investigators. Technetium was the first element to be produced artificially. Since its discovery, searches for the element in terrestrial material have been made. Finally in 1962, technetium-99 was isolated and identified in African pitchblende (a uranium rich ore) in extremely minute quantities as a spontaneous fission product of uranium-238 by B.T. Kenna and P.K. Kuroda. If it does exist, the concentration must be very small. Technetium has been found in the spectrum of S-, M-, and N-type stars, and its presence in stellar matter is leading to new theories of the production of heavy elements in the stars. 

Holden NE (1989) Total and spontaneous fission half-lives for uranium, plutonium, americium and curium nuclides. Pure Appl Chem 61(8) 1483-1504... 

Fission of the nucleus, whereby it splits into two roughly equal halves, is accompanied by a huge release of energy. It was first observed by Hahn and Strassman (1939), who were bombarding uranium with neutrons. Many heavy elements are susceptible to induced fission, but spontaneous fission can occur in some of the heaviest elements, and is thought to be the principal mode of decay for the transuranic elements. 

The most stable isotope of plutonium is Pu-244, with a half-life of S.OOxlO+ years (about 82,000,000 years). Being radioactive, Pu-244 can decay in two different ways. One way involves alpha decay, resulting in the formation of the isotope uranium-240, and the other is through spontaneous fission. 

The principal decay mode of 238U is -decay, but a small fraction of the decays are by spontaneous fission. The emission of an a-particle initiates a series of decays known as the uranium series (Fig. 8.14), which ends at 206Pb. The uranium series can be summarized as... 

Radioisotopes that decay by spontaneous fission with the direct accompanying release of neutrons are usually associated with the natural elements of uranium and thorium and the manmade element plutonium. However, the rate of decay of these elements by fission is so slow that it is only by incorporating them into large nuclear piles or chain reactors that they can be utilized as intense neutron sources. In the US Dept of Energy National Transplutonium Program, small quantities of elements heavier than plutonium are produced for basic research studies and to discover new elements with useful properties. One of these new elements, californium-252 (2S2Cf), is unique in that it emits neutrons in copious quantities over a period of years by spontaneous fission... 

Ragettli, R. A., Hebeda, E. H., Signer, P., Wieler, R. (1994) Uranium-xenon chronology Precise determination of V136Ysf for spontaneous fission of 238U. Earth Planet. Sci. Lett., 128, 653-70. 

Whetherill, G. W. (1953) Spontaneous fission yields from uranium and thorium. Phvs. Rev., 92, 907-12. 

Since spontaneous fission is extremely rare in Nature, detection of fission events in natural samples would give a strong hint. Alpha-particle spectra would be less specific, because the energies predicted for superheavy nuclei fell into the range covered by the natural decay series deriving from uranium... 

Fig. 5. Neutron counting as detection method for spontanous fission events of superheavy nuclei. The recorded neutron rates (points) were found to follow the relative cross sections of cosmic-ray induced spallation reactions (curve) and were, thus, due to background events. The numbers are rates for natural uranium and thorium. From W. Grimm, G. Herrmann and H.-D. Schiissler [40].

Promethium occurs in tiny amounts in uranium ores, thus a sample of Congolese pitchblende was found to contain (4 1) x 10 g of " Pm per kg of ore it was formed mainly by spontaneous fission of It is also one of the fission products of uranium-235 and can be obtained from a mixture of lanthanides by ion exchange. The longest-lived isotope is... 

The half-lives of the longest-lived isotopes of transuranium elements (Fig. 14.8) show a continuous exponential decrease with increasing atomic number Z. Whereas up to element 103 the half-life is mainly determined by a decay, the influence of spontaneous fission seems to become predominant for elements with Z > 106. The drop model of nuclei predicts a continuous decrease of the fission barrier from about 6 MeV for uranium to about zero for element 110. That means that according to the drop model, elements with Z > 110 are not expected to exist, because normal vibrations of the nuclei should lead to fission. 

Because C1 stays predominantly in the aqueous phase, it is mainly applied for hydrological studies, e.g. on the time of transport of water within deep layers, the rate of erosion processes and the age of deep groundwaters. In the case of ground-waters without access of cosmogenic C1, the production of C1 by the reaction Cl(n, ) C1 induced by neutrons from spontaneous fission of uranium contained in granite has to be taken into account. 

In practice, only one natural radionuclide, produces observable in situ spontaneous-fission components in terrestrial (and most lunar) samples, and even then the fission components are observable only in samples unusually deficient in noble gases and/or rich in uranium. The spectra of fission components, and the yields at xenon and krypton, are well known (e.g., see Ozima and Podosek, 2002), so that radiogenic xenon and krypton from fission of are readily identi-... 

Iodine-129 has major sources in both surface and subsurface environments. It is produced in the atmosphere by cosmic-ray spallation of xenon and in the subsurface by the spontaneous fission of uranium. Iodine-129 of subsurface origin is released to the surface environment through volcanic emissions, groundwater discharge, and other fluxes. Due to its very long half-life, 15.7 Ma, these sources are well mixed in the oceans and surface environment, producing a specific activity of 5 xBq (g I) (equivalent to a I/I ratio of 10 ). Due to its low activity, the preferred detection method of I is AMS (Elmore et al., 1980). 

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