Thorium decay products

Unstable decay products of thorium may exceed their parent in their importance both in occupational and nonoccupational environments as contributors to the radiation dose. They produce only radiological effects. In view of the lesser importance of thorium decay products as compared to the uranium decay chains, this aspects are dealt with later (see Chapter 26.2, Section 26.2.6.3). 

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

Lead occurs naturally as a mixture of four non-radioactive isotopes, and Pb, as well as the radioactive isotopes ° Pb and Pb. All but Pb arise by radioactive decay of uranium and thorium. Such decay products are known as radiogenic isotopes. 

Radon-220 [22481 8-7], Rn, a decay product of thorium, was discovered by Owens and Rutherford in 1900. The more common radon-222, a decay product of radium, was discovered later in the same year and was isolated in 1902. 

Total reserves of thorium at commercial price in 1995 was estimated to be >2 x 10 metric tons of Th02 (H)- Thorium is a potential fuel for nuclear power reactors. It has a 3—4 times higher natural abundance than U and the separation of the product from Th is both technically easier and less expensive than the enrichment of in However, side-reaction products, such as and the intense a- and y-active decay products lead to a high... 

Ernest Rutherford observed that the paths taken by energetic particles emitted by radioactive uranium and thorium responded in three ways to magnetic fields slightly bent, strongly bent, and unaffected. He gave them the designations or, fi, and y. Even though scientists soon identified the particles, they still use these names to emphasize that they are nuclear decay products. 

In 1900 Ernest Rutherford (1871-1937) detected the radon-220 isotope as a decay product of thorium. In the same year, Dorn showed the radon-222 isotope to be a decay product of radium. 

The plutonium concentration in marine samples is principally due to environmental pollution caused by fallout from nuclear explosions and is generally at very low levels [75]. Environmental samples also contain microtraces of natural a emitters (uranium, thorium, and their decay products) which complicate the plutonium determinations [76]. Methods for the determination of plutonium in marine samples must therefore be very sensitive and selective. The methods reported for the chemical separation of plutonium are based on ion exchange resins [76-80] or liquid-liquid extraction with tertiary amines [81], organophosphorus compounds [82,83], and ketones [84,85]. 

Figure 32.3 The three still-existing natural decay series. A. Uranium-238 B. Uranium-235 and C. Thorium-232. (Modified from Holtzman 1969 LWV 1985 UNSCEAR 1988 Kiefer 1990 Rose etal. 1990). Principal decay products occur within the heavy borders outlined.

Radium, thorium, and other radionuclides accumulate in uranium mill tailings. The potential environmental effects of these radionuclides has become of increasing concern to the public. In the future, it may be necessary to modify existing uranium recovery processes to accommodate removal of radium and perhaps other radioactive decay products of uranium. 

Two methods to secure very small samples of francium for examination use the decay processes of other radioactive elements. One is to bombard thorium with protons. The second is to start with radium in an accelerator, where, through a series of decay processes, the radium is converted to actinium, which in turn rapidly decays into thorium, and finally, thorium decays naturally into francium. Following is a schematic of the decay process used for the production of small amounts of Fr-223 which, in turn, after several more decay processes ends up as stable lead (Pb) ... 

Radon is the heaviest of the noble gases and is the only one that is radioactive. It is the decay product of radium, thorium, and uranium ores and rocks found underground. As it decays, it emits alpha particles (hehum nuclei) and is then transmuted to polonium and finally lead. The Earth s atmosphere is just 0.0000000000000000001% radon, but because radon is 7.5 times heavier than air, it can collect in basements and low places in buildings and homes. 

Radon s source is a step in the transmutation of several elements uranium —> thorium — radium —> radon —> polonium —> lead. (There are a number of intermediate decay products and steps involved in this process.) Radon-222 forms and collects just a few inches below the surface of the ground and is often found in trapped pockets of air. It escapes through porous soils and crevices. 

As thorium undergoes natural radioactive decay, a number of products, including gases, are emitted. These decay products are extremely dangerous radioactive poisons if inhaled or ingested. 

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. 

Uranium and Thorium Isotope Decay Series Showing the Sources and Decay Products of... 

The thorium isotope-232 is not stable. It breaks down into two parts. This process of breaking down is called decay. The decay of thorium-232 produces a small part called "alpha" radiation and a large part called the decay product. The decay product of thorium-232 also is not stable. Like thorium-232,... 

Due to the extremely slow rate of decay, the total amount of natural thorium in the earth remains almost the same, but it can be moved from place to place by nature and people. For example, when rocks are broken up by wind and water, thorium or its compounds becomes a part of the soil. When it rains, the thorium-containing soil can be washed into rivers and lakes. Also, activities such as burning coal that contains small amounts of thorium, mining or milling thorium, or making products that contain thorium also release thorium into the environment. Smaller amounts of other isotopes of thorium are produced usually as decay products of uranium-238, uranium-235, and thorium-232, and as unwanted products of nuclear reactions. 

Thorium occurs in nature in four isotopic forms, thorium-228, thorium-230, thorium-232, and thorium-234. Of these, thorium-228 is the decay product of naturally-occurring thorium-232, and both thorium-234 and thorium-230 are decay products of natural uranium-238. To assess the environmental fate of thorium, these isotopes of thorium with the exception of thorium-234 which has short half- life (24.1 days), should be considered. 

The dry deposition velocity of lead-212, a thoron (thoron or radon-220 itself originating from thorium-232) decay product has been reported to be in the range 0.03-0.6 cm/sec (Bigu 1985 Rangarajan et al. 1986). These low deposition velocities indicate that the thoron daughter, stable lead, may have a long residence time in the atmosphere with respect to dry deposition. 

In the environment, thorium and its compounds do not degrade or mineralize like many organic compounds, but instead speciate into different chemical compounds and form radioactive decay products. Analytical methods for the quantification of radioactive decay products, such as radium, radon, polonium and lead are available. However, the decay products of thorium are rarely analyzed in environmental samples. Since radon-220 (thoron, a decay product of thorium-232) is a gas, determination of thoron decay products in some environmental samples may be simpler, and their concentrations may be used as an indirect measure of the parent compound in the environment if a secular equilibrium is reached between thorium-232 and all its decay products. There are few analytical methods that will allow quantification of the speciation products formed as a result of environmental interactions of thorium (e.g., formation of complex). A knowledge of the environmental transformation processes of thorium and the compounds formed as a result is important in the understanding of their transport in environmental media. For example, in aquatic media, formation of soluble complexes will increase thorium mobility, whereas formation of insoluble species will enhance its incorporation into the sediment and limit its mobility. 

Francium-223 is produced from the decay of actinium-227. While the chief decay product is thorium-227 resulting from beta emission, actinium-227 also undergoes alpha emission to an extent of one percent giving francium-223 ... 

All thorium isotopes are radioactive. Also all its intermediate decay products including radon-220 are radioactive and present radiation hazard. Exposure can cause cancer. 

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. 

Professor Hans Geiger and E. Marsden noticed that the alpha particles from thoron are expelled at such very short intervals that they seem to be double. They found, as Rutherford suggested, that this strange behavior is caused by the presence of a very short-lived decay product of thoron, which they named thorium A (80). Prof. Geiger was bom in... 

The actual discovery was made by Mile. Marguerite Perey at the Curie Institute in Paris. In 1939 she purified an actinium preparation by removing all the known decay products of this element. In her preparation she observed a rapid rise in beta activity which could not be due to any known substance. She was able to show that, while most of the actinium formed radioactinium, an isotope of thorium, by beta emission, 1.2 0.1 per cent of the disintegration of actinium occurred by alpha emission and gave rise to a new element, which she provisionally called actinium K, symbol AcK (35, 36). This decayed rapidly by beta emission to produce AcX, an isotope of radium, which was also formed by alpha emission from radioactinium. Thus AcK, with its short half-life, had been missed previously because its disintegration gave the same product as that from the more plentiful radioactinium. 

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