Short decay series

A careful look at these naturally occurring long-lived nuclei reveals diat some of them appear in short decay series, e.g. Gd - Sm - Nd - Ce and Pt - Os - The heavy element series beginning with U and Th isotopes are therefore referred to as long decay series. 

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

Pyle DM (1992) The volume and residence time of magma beneath active volcanoes determined by decay-series disequilibria methods. Earth Planet Sci Lett 112 61-73 Pyle DM, Dawson JB, Ivanovich M (1991) Short-lived decay series disequilibria in the natrocarbonatite lavas of Oldoinyo Lengai, Tanzania Constraints on the timing of magma genesis. Earth Planet Sci Lett 105 378-396... 

There are four naturally occurring isotopes of Ra " " Ra (ti/2 = 5.8 a) and " Ra (3.7 d) in the Th series, (1600 a) in the series, and Ra (11.7 d) in the series (Table 1). The data for Ra are more limited, since it is generally present in low concentrations due to the low abundance of The differences in half lives and the connections across the different decay series have been used to infer a variety of groundwater and water-rock interaction features. For the short-lived Ra isotopes, the dominant input term to groundwater is recoil, rather than weathering, and steady state concentrations are often achieved (see Section 2.2). 

Vigier N, Bourdon B, Turner S, Allegre CJ (2001) Erosion timescales derived from U-decay series measurements in rivers. Earth Planet Sci Lett 193 549-563 von Gunten HR, Roessler E, Lowson RT, Reid PD, Short SA (1999) Distribution of uranium- and thorium series radionuclides in mineral phases of a weathered lateritic transect of a uranium ore body. Chem Geol 160 225-240... 

Although there are three Rji isotopes in the U- and Th-decay series, only is sufficiently long lived tm= 3.8 days) to be a useful estuarine tracer. Radioactive decay of Ra continuously produces Rn, which because of its short half-life is generally in secular equilibrium in seawater. Being chemically non-reactive except for very weak Van der Waals bonding makes this isotope a unique marine tracer in that it is not directly involved in biogeochemical cycles. 

Cochran JK (1984) The fates of U and Th decay series nuclides in the estuarine environment. In The Estuary as a Filter. Kennedy VS (ed) Academic Press, London, p 179-220 Cochran JK (1992) The oceanic chemistry of the uranium - and thorium - series nuclides. In Uranium-series Disequilibrium Applications to Earth, Marine and Environmental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 334-395 Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cochran JK, Carey AE, Sholkovitz ER, Surprenant LD (1986) The geochemistry of uranium and thorium in coastal marine-sediments and sediment pore waters. Geochim Cosmochim Acta 50 663-680 Corbett DR, Chanton J, Burnett W, Dillon K, Rutkowski C. (1999) Patterns of groundwater discharge into Florida Bay. Linrnol Oceanogr 44 1045-1055... 

Shortly thereafter, Boltwood recognized that the Pb content of minerals increases with age and it became clear that Pb was the final product of radioactivity. Boltwood was also responsible for adding another substance to the decay series through his discovery of ionium ( °Th), and therefore for linking the U and Ra decay chains (Boltwood 1907). The discovery of initially known as UrII, followed in 1912. 

The various U series were by now all but complete. Branched decays were understood, and a daughter of U-X was discovered— " Pa as it is now known, but initially named brevium to reflect it s short half-life (Fajans and Gohring 1913). By 1913, a published decay series (Fajans 1913) was remarkably close to that in use today, differing only in the absence of some of the branched decays after Ra-A ( Po), and in the precise values of some of the half-lives ... 

The element "eka-cesium" had long been suspected. Was detected as a short-lived intermediate product in the decay series of actinium. 

Rn-220 is another isotope of radon and belongs to the thorium decay series. Due to its short half life of 55.6 s, reports on its concentrations in those gases and in natural water are still scant. They are also important for a better estimate of our exposure to natural radioactivity and also for the geochemical study of the forma tion of those radon isotopes and their underground movement. 

If species with short half-lives are omitted, the first portion of the 238U decay series can be written as... 

Other conditions being equal, the intermediate species with longer half-lives in a decay series have more opportunities to be fractionated from their parents. Hence, in the decay series of two nuclides °Th and Ra have a greater chance to be fractionated. In the decay series, Pa (half-life 32.8 has the greatest chance to be fractionated. In the Th decay series, all the intermediate species have short half-lives (the longest half-life of Intermediates is 5.75 3T for Ra (A, = 0.1205 3 ) and the disturbance of this decay system does not have much utility. That is, the U-series (including U and U series) disequilibrium is much more often applied. Some examples of disturbed decay chain (i.e., fractionation of the intermediate species) are given below ... 

However, for investigating short -term recent geological processes, the intermediate isotopes in decay series cannot be ignored and in fact these short-lived intermediate isotopes are powerful in deciphering young geological processes. 

Daughters of alpha emitters The recoil method can also be useful for the separation of daughter products produced by a decay of a parent. This technique has been applied to studies of short-lived daughters In the radioactive decay series of uranium, thorium, and actinium (175) ... 

The element francium is formed in the natural radioactive decay series and in nuclear reactions. All its isotopes are radioactive with short half-lives. The ion behaves as would be expected from its position in the group. 

Holland and Gottfried (1955) studied the changes in density and unitcell dimensions as a function of the total a-particle dose D per milligram of zircon. D was estimated from the present a-activity and the Th/U ratio, together with the age T of the sample, on the assumption that equilibrium in the uranium and thorium decay series had been established in a time that was short compared with T. T was determined from the present a-activity and lead content (see Faure 1977). It was found that for )< 10, the density was approximately 4.7 g/cm. As the dose was increased, the density decreased slowly at first and then more rapidly, and finally approached asymptotically a value of approximately 3.95 g/cm. ... 

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