Long 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. 

A few techniques exist that do not provide for direct dating but rather give information as to whether the object is of modem manufacture. One of these is dating (53). In the decay series of uranium, the first long-Hved member after Ra, with its 1622 yr half-life, is which has a 22 yr... 

If we consider this pair of radioactive isotopes for time scales greater than six half-lives of N2, Equation (3b) can be simplified. Because each decay series starts with a long-lived parent, it is commonly the case that A,2. In this case, after six half lives, e approaches zero and can be removed from the equation. For time scales such that 6T2 [Pg.6]

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. 

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

For the Th decay series to ° Pb, the slowest step is the decay of h with decay constant of 4.948 x 10 (half-life 14.01 B) ). The longest-lived intermediate is Ra with a half-life of only 5.75 3. That is, there are basically no intermediates with long enough half-lives of geologic interest. At secular equilibrium, the total number of atoms of intermediate species is 0.55 ppb of that of Th. Hence, there is no need to account for the concentrations of the intermediate species when appl3dng the h- ° Pb geochronometer. 

Dating using intermediate nuclides in decay series requires an understanding of the evolution of the concentrations of intermediate nuclides after disturbance. The full evolution becomes increasingly more complicated for an intermediate nuclide that requires more steps from the long-lived parent. Pa is the third... 

For the nuclides in the decay series, and °Th are of particular interest because of their long half-lives. is the fourth nuclide in the decay series (Table 2-2a) with a half-life of 244,000 years, and °Th is the fifth nuclide with a half-life of 75,400 years. Denote as nuclide 1, Th as 2, " Pa as 3, as 4, and °Th as 5. For because (decay constant of is the smallest, and X4 (decay constant of is the second smallest and far smaller than Xz and X4, the following may be easily derived ... 

An unstable parent nucleus may decay into either a stable or an unstable daughter. When the daughter is unstable, which is often the case, the daughter will decay. Often the journey from an unstable nucleus to a stable nucleus involves a long series of steps referred to as a radioactive decay series. One example is the decay series for radium (Figure 17.2). 

Unlike its lower homologues, sulfur, selenium, and tellurium, polonium has no long-lived or stable isotopes. It has, in fact, one of the most unstable nuclei of naturally occurring elements, the only readily accessible isotope being that of mass 210 this decays by alpha emission with a half-life of 138.4 days and occurs in nature as the penultimate member of the radium decay series, the last three stages being... 

Because the half-life of 2g U is very long compared with those of the other members of the decay series (Table 21.3) to reach fPb, the number of nuclides in intermediate stages of decay is negligible. That is, once a 2g U nuclide starts to decay, it reaches relatively fast. 

The great variety of radionuclides present in thorium and uranium ores are listed in Tables 4.1, 4.2 and 4.3. Whereas thorium has only one isotope with a very long half-life (- Th), uranium has two and giving ri.se to one decay scries for Th and two for U. In order to distinguish the two decay series of U, they were named after long-lived members of practical importance the uranium-radium series and the actinium series. The uranium-radium series includes the most important radium isotope ( Ra) and the actinium scries the most important actinium isotope ( Ac),... 

Table 11.3. equilibrium. Long-lived members of the and decay series in secular radioactive... 

Radionuclides of major importance in the geosphere and the biosphere are listed in Table 21.1. Not taken into account are radionuclides with half-lives h/2 < 1 d (in the case of activation products of materials used in nuclear reactors, i/2 < 1 y) and with half-lives ti/2 > lO y, radionuclides with fission yields <0.01%, radioisotopes of elements that are not members of the natural decay series,and radionuclides produced solely for medical or technical applications. The radionuclides are arranged according to their position in the Periodic Table of the elements, in order to facilitate the discussion of their chemical behaviour. Radionuclides with half-lives >10y are underlined, because their behaviour over long periods of time is of special importance. 

No such long decay component of 7.4 ns is observed for para-substituted dendritic structures p-CnPn. The absence of the long decay component is therefore due to the different position of substitution leading to a better spatial separation of the individual chromophores. This is also supported by a comparison of the molecular structures of the para- and meta-substituted dendrimers obtained from molecular modeling, since the average center-to-center distance among the chromophores is 2.9 nm for the para series but only 2.6 nm for the meta series in the first generation series. 

Because of its long half-life, potassium-40 can be used to date objects up to 1 million years old by determination of the ratio of to jgAr in the sample. The uranium-lead method is based on the natural uranium-238 decay series, which ends with the production of stable lead-206. This method is used for dating uranium-containing minerals several billion years old because this series has an even longer half-life. All the 2 6pb in such minerals is assumed to have come from Because of the very long half-life of 4.5 billion years, the amounts of intermediate nuclei can be neglected. A meteorite that was 4.6 billion years old fell in Mexico in 1969. Results of studies on such materials of extrater-... 

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