Series radioactive

All elements beyond bismuth in the Periodic Table are radioactive, most of these having several isotopes (or nuclides), each with a characteristic half-life. A small number of elements of low atomic number (K, Rb, Sm, Lu, Re, and perhaps La and H) each have one naturally occurring radioisotope also. In addition, over 700 radioisotopes have been made artificially (p. 466). 

As indicated, the above sertefe is simplified, for the so-called branches in the decay chain have been omitted. Only one mode of decay, for exam- 

Both a 4n and a 4n + 3 series of heavy radioactive elements occur naturally. The 4n series is sometimes called the thorium series, since its long-lived parent is Th232 (half-life, 14,000,000,000 years), whereas the 4n + 3 series is the actinium series. The long-lived parent of the latter is U235 (half-life 707,000,000 years), but unlike the 4n + 2 series, one of its members is actinium (Ac227). Final members of both of these series are lead isotopes, Pb208 and Pb207. 

A useful but approximate relationship exists between the half-life period associated with an a emitter and the initial energy of the a particle given off. This, the Geiger-Nuttatt rule, may be expressed in a number of ways, one of the most convenient being  

The series of transformations that take place as a radioactive element changes into successive daughter elements halts when a stable (nonradioactive) element forms. The sequence of transformations is called a radioactive series. Four different radioactive series have been described, three of which occur naturally. 

Of the atomic species taking part in this cascade, only uranium-238 has a long lifetime. Apart from the stable, nonradioactive end-product lead-206, all other species decay rapidly. These include two other radioactive lead nuclides, Pb-210 and Pb-214, which have a transitory existence. Some nuclides have two alternative ways of decay and so the path occasionally branches. For example, Bi-214 can form Po-214 by a p-decay, or Tl-210 by an a-decay. [Not all of the possible branching reactions 

Different isotopes of radon form in the radioactive series described below. 

The existence of the three naturally occurring series characterised by the formulae Ax. Ax+ 2 and 

EXAMPLE 19.7. When a nucleus disintegrates, the following series of alpha and beta particles is emitted alpha, beta, beta, alpha, alpha, alpha, alpha, alpha, beta, alpha, beta, beta, beta, alpha. (Since emission of gamma particles accompanies practically every disintegration and since gamma particles do not change the atomic number or mass number of an isotope, they are not hsted.) Show that each isotope produced has a mass number that differs from 238 by some multiple of 4. 

Each alpha particle loss lowers the mass number of the product nucleus by 4. Each beta particle loss lowers the mass 

There are four such series of naturally occurring isotopes. The series in which aU mass numbers are evenly divisible by 4 is called the 4n series. The series with mass numbers 1 more than the corresponding 4n series members is called the 4n + 1 series. Similarly, there are a 4n -f 2 series and a 4n -f 3 series. Since the mass numbers change by 4 or 0, no member of any series can produce a product in a different series. 

Some nuclei cannot gain stability by a single emission. Consequently, a series of successive emissions occurs as shown for uranium-238 in A FIGURE 21.3. Decay continues until a stable nucleus—lead-206 in this case—is formed. A series of nuclear reactions that begins with an unstable nucleus and terminates with a stable one is known as a radioactive series or a nuclear disintegration series. Three such series occur in nature uranium-238 to lead-206, uranium-235 to lead-207, and thorium-232 to lead-208. 

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Lead (13 ppm) is by far the most abundant of the heavy elements, being approached amongst these only by thallium (8.1 ppm) and uranium (2.3 ppm). This abundance is related to the fact that 3 of the 4 naturally occurring isotopes of lead (206, 207 and 208) arise primarily as the stable end products of the natural radioactive series. Only (1.4%)... 

The fugitive radioactive element astatine can hardly be said to exist in nature though the punctillious would rightly point to its temporary participation in the natural radioactive series. Thus At (t i 54 s) occurs as a... 

Element 86, the final member of the group, is a short-lived, radioactive element, formerly known as radium-emanation or niton or, depending on which radioactive series it originates in (i.e. which isotope) as radon, thoron, or actinon. It was first isolated and studied in 1902 by E. Rutherford and F. Soddy and is now universally known as radon (from radium and the termination-on adopted for the noble gases Latin radius, ray). 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

The role of radionuclides as tracer of the chemical transport in river is also reinforced by the fact that each of the U-Th-Ra elements has several isotopes of very different half-lives belonging to the U-Th radioactive series. Thus, these series permit comparison of the behavior of isotopes of the same element which are supposed to have the same chemical properties, but very different lifetimes. These comparisons should be very helpful in constraining time scales of transport in rivers. This was illustrated by Porcelli et al. (2001) who compared ( " Th/ U) and ( °Th/ U) ratios in Kalix river waters and estimated a transit time for Th of 15 10 days in this watershed. The development of such studies in the future should lead to an important progress in understanding and quantifying of transport parameters in surface waters. This information could be crucial for a correct use of U-series radioactive disequilibria measured in river waters to establish weathering budgets at the scale of a watershed. 

Decay Product, Daughter Product, Progeny—A new nuclide formed as a result of radioactive decay. A nuclide resulting from the radioactive transformation of a radionuclide, formed either directly or as the result of successive transformations in a radioactive series. A decay product (daughter product or progeny) may be either radioactive or stable. 

Equilibrium, Radioactive—In a radioactive series, the state which prevails when the ratios between the activities of two or more successive members of the series remains constant. 

Parent—A radionuclide which, upon disintegration, yields a new nuclide, either directly or as a later member of a radioactive series. 

Match the end product and the parent of each of the four radioactive series without consulting any reference tables or other data. 

Although many nuclei are naturally radioactive, there are three main radioactive series in nature, all of which are relevant to a discussion of the isotopic composition of natural lead. These start with the elements uranium and thorium (238U, 235U and 232Th) and all end in one of the three stable isotopes... 

These rules can now be applied to the three radioactive series in Figs. 1.1 and 1.2. The names of the various elemental materials are given in the figures. The atomic weights of each can be determined from the rules above, given the respective atomic weights of the parent of each series, taken here as UrI = 238.5, Th = 232.4, and... 

Two nongaseous isotopes, A and B, are involved in the same radioactive series, which (over the past eons) has established radioactive equilibrium. The half-life of A is 106yrs an

Decay Product. A nuclide that results from the radioactive disintegration of a radionuclide that is formed either directly or as tlie result of progressive transformations in a radioactive series. The nuclide thus produced is sometimes called the daughter or daughter element. 

The isotope 38U is the parent of the natural uranium 4n + 2 radioactive senes, and the isotope -3 >U is the parent of the natural actinium 4n + 3 radioactive series. 

A number of additional observations indicate that there is extra stability associated with nuclei having an even number of protons, of neutrons, or, more especially, even numbers of both. Elements of odd atomic number have no more than two stable isotopes. Of these elements, about half exist as one stable nuclide only, and two have no stable forms. Aside from the four exceptions mentioned in the preceding paragraph, each stable nucleus having an odd number of protons must have an even number of neutrons. Among the natural radioactive series, about 20 nuclides of even Z, but only 2 of odd Z, have half-lives of over one day. 

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