Actinium decay series

X 10 yr) and ends with stable ° Pb, after emission of eight alpha (a) and six beta (jS) particles. The thorium decay series begins with Th (ti/2 = 1.41 X 10 °yr) and ends with stable ° Pb, after emission of six alpha and four beta particles. Two isotopes of radium and Th are important tracer isotopes in the thorium decay chain. The actinium decay series begins with (ti/2 = 7.04 X 10 yr) and ends with stable Pb after emission of seven alpha and four beta particles. The actinium decay series includes important isotopes of actinium and protactinium. These primordial radionuclides, as products of continental weathering, enter the ocean primarily by the discharge of rivers. However, as we shall see, there are notable exceptions to this generality. 

Table 4.3. Actinium decay series (actinium family) . 4 = 4n + 3. ...

The actinium decay series consists of a group of nuclides whose mass number divided by 4 leaves a remainder of 3 (the 4n + 3 series). This series begins with the uranium isotope which has a half-life of 7.04 X 10 y and a specific activity of 8 X 10 MBq/kg. The stable end product of the series is ° Pb, which is formed after 7 a- and 4 /3-decays. The actinium series includes the most important isotopes of the elements protactinium, actinium, ftancium, and astatine. Inasmuch as U is a conqx>nmt of natural uranium, these elem ts can be isolated in the processing of uranium minerals. The longest-lived protactinium isotope, Pa (ti 3.28 X 10 y) has been isolated on the 100 g scale, and is the main isotope for the study of protactinium chemistry. Ac (t 21.8 y) is the longest-lived actinium isotope. 

Figure 1. The three decay series from uranium, thorium, and actinium as published by Soddy in 1913 (Soddy 1913b).

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

ISOTOPES There are 41 isotopes of polonium. They range from Po-188 to Po-219. All of them are radioactive with half-lives ranging from a few milliseconds to 102 years, the latter for its most stable isotope Po-209. Polonium is involved with several radioactive decay series, including the actinium series, Po-211 and Po-215 the thorium series, Po-212 and Po-216 and the uranium decay series, Po-210, Po-214, and Po-218. 

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

Actinium decays via a series of short-lived isotopes, eventually ending with stable lead. The presence of these radioactive daughters, particularly Th (which is a strong y-emitter), necessitates the use of lead-lined gloved boxes and remote control manipulators. Consequently, the metallurgy of actinium has been little studied and, due to the great expense and trouble involved, probably will not be studied extensively in the future. 

Pa, protactinium, was first identified in 1913 in the decay products of U-238 as the Pa-234 isotope (6.7 h) by Kasimir Fajans and Otto H. Gohring. In 1916, two groups, Otto Hahn and Lisa Meitner, and Frederick Soddy and John A. Cranston, found Pa-231 (10 years) as a decay product of U-235. This isotope is the parent of Ac-227 in the U-235 decay series, hence it was named protactinium (before actinium). Isolation from U extraction sludges yielded over 100 g in 1960. 

One of the most important observations of atoms is the set of relationships between elements that belong to one of the series of radioactive decays. The parent elements of uranium, thorium and actinium decay through many intermediates to the stable element lead. The Nobel Prize in Chemistry for 1921 was awarded in 1922 to Frederick Soddy for his complete characterization of these processes. The story is beautifully told in his Nobel Lecture entitled The origins of the conception of isotopes (25). 

Portion of the chart of the nuclides illustrating the decay of 235U to 207Pb. This decay series is often called the actinium series. 

PROTACTINIUM. [CAS 7440-13-13], Chemical element, symbol Pa, at. no. 91, at. wt, 231.036, radioactive metal of the Actinide Series, mp is estimated at less than 1600°C, All isotopes arc radioactive. The most stable isotope is 23IPa with a half-life of 3,43 v 104 years, The latter is a second-generation daughter of a5U and a member of the actinium (2n + 3) decay series, See also Radioactivity, Electronic configuration... 

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 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),... 

The final members of the decay series are stable nuclides ° Pb at the end of the thorium family, Pb at the end of the uranium-radium family, Pb at the end of the actinium family, and Bi at the end of the neptunium family. In all four decay series one or more branchings are observed. For instance, Bi decays with a certain probabihty by emission of an a particle into Tl, and with another probability by emission of an electron into Po. os-pj decays by emission of an electron into Pb, and Po by emission of an a particle into the same nuclide (Table 4.1), thus closing the branching. In both branches the sequence of decay alternates either a decay is followed by P decay or p decay is followed by a decay. 

In the early stages of dating by nuclear methods, the measurement of He formed by a decay in the natural decay series (9, 6 and 7 He atoms in the uranium series, the thorium series and the actinium series, respectively) has been applied. The preferred method was the U/He method which allows dating of samples with very low concentrations of U of the order of 1 mg/kg. Helium produced by a decay is driven out by heating and measured by sensitive methods, e.g. by MS. However, it is difficult to ensure the prerequisites of dating by the U/He method neither " He nor a-emitting members of the decay series must be lost and no " He atoms must be produced by other processes such as decay of Th and spallation processes in meteorites. 

In the first steps of its radioactive decay series, thorium-232 decays to radium-228, which then decays to actinium-228. What are the balanced nuclear equations describing these first two decay steps ... 

The terrestrial occurrence of Ac, Pa, U, and Th is due to the half-lives of the isotopes 235U, 238U and 232Th which are sufficiently long to have enabled the species to persist since genesis. They are the sources of actinium and protactinium formed in the decay series and found in uranium and thorium ores. The half-lives of the most stable isotopes of the trans-uranium elements are such that any primordial amounts of these elements appear to have disappeared long ago. However, neptunium and plutonium have been isolated in traces from uranium13 minerals in which they are formed continuously by neutron reactions such as... 

FlC- 1-2, The three naturally occurring radioactive decay series and the man-made neptunium series. Although (which is the parent to the actinium series) and (which is the parent to the thorium series) have been discovered in nature, die decay series shown here begin with the most abundant Icmg-Uved nuclides. 

In the case of the decay series shown by Figs. 7.4-7.7, another type of branching decay occurs a decay competes with decay. For instance, Ac, the eponymous member of the actinium (4n + 3) series (see Fig. 7.7), undergoes the following disintegration ... 

The actinium (4n + 3) series is the third one of the naturaiiy occurring decay series on Earth. The haif-iife of the primary parent (T,/2 = 7.04 x 10 a - note, for comparison, that the Cambrian Period started some 5.7 x 10 years ago) is approximateiy one sixth of the estimated age of the Earth. Thus, nearly 2 part (or, more exactly, just about 1%) of the atoms that once existed are still potentially available as nuclear fuel... 

The exponential laws of radioactive-series decay and growth of radionuclides were first formulated by Rutherford and Soddy in 1902, to explain their results (Rutherford and Soddy 1902,1903) on the thorium series of radionuclides. In 1910, Bateman (Bateman 1910) derived generalized mathematical expressions that were used to describe the decay and growth of the naturally occurring actinium, uranium, and thorium series until the discovery of nuclear fission and other new radioactive decay series were found in the 1940s. For the description of half-lives and decay constants, activities and number of radionuclides involved in the decay of two radionuclides, Friedlander et al. (1981) have given a representative overview (see also O Chap. 5 in Vol. 1). 

The radionuclide Rn (Ty2 = 56 s) is an intermediate of the thorium decay series (from Th to Pb) this radionuclide (another isotope of the element radon besides its most common isotope Rn) has the special name of thoron. Due to the short half-life, the exhalation rate of Th from minerals to the atmosphere is small, and the concentration in air is about 10 times less than that of Rn, at equilibrium. Therefore, the dose contribution is negligible, except in the atmosphere where the upper soils are rich in thorium content. The actinium series (from to Pb) involves the intermediate of Rn (Ti/2 - 4 s). Due to the very short half-life of Rn, its exhalation from soil is very small, as is the dose contribution. 

Once it became clear that radioactive elements were decaying to new elements, which were themselves radioactive, a great deal of effort was expended in working out the decay sequences. In most cases the new elements were at first obtained in quantities too small to be weighed and were distinguished from each other only by the type of decay they exhibited and the rate at which the decay occurred. Three decay series were elucidated the uranium series proceeded through radium and terminated with the stable radium G the thorium series ended in the stable thorium D and the actinium series ended in actinium D. Between them, these series contained around 25 new radioelements. 

---