Isotopes of thorium

The homogeneous reactor experiment-2 (HRE-2) was tested as a power-breeder in the late 1950s. The core contained highly enriched uranyl sulfate in heavy water and the reflector contained a slurry of thorium oxide [1314-20-1J, Th02, in D2O. The reactor thus produced fissile uranium-233 by absorption of neutrons in thorium-232 [7440-29-1J, the essentially stable single isotope of thorium. Local deposits of uranium caused reactivity excursions and intense sources of heat that melted holes in the container (18), and the project was terrninated. 

Cheng H, Edwards RL, Mttrrell MT, Benjamin TM (1998) Uranium-thorittm-protactinium dating systematics. Geochim Cosmochim Acta 62 3437-3452 Cherdyntsev W, Kazachevskii IV, Kttz mina YA (1965) Age of carbonate determined from the isotopes of thorium and uranium. Geochem Int 2 749-756... 

Natural lead, a metallic element, is a mixture of the following four isotopes lead-204, lead-206, lead-207, and lead-208. Only lead-204 is a primordial isotope of nonradiogenic origin all the others are radiogenic, each isotope being the end product of one of the radioactive decay series of isotopes of thorium or uranium, namely, uranium-238, uranium-235, and thorium-232 the decay series of the uranium isotopes are listed in Figure 12 ... 

The four isotopes, as those of any element, have the same chemical properties. The four are not, however, uniformly distributed in the earth s crust the occurrence of three of them, in minerals and rocks, is associated with the radioactive decay of isotopes of thorium and uranium. In most minerals and rocks the relative amounts (or the isotopic ratios) of the isotopes of lead (often expressed relative to the amount of stable lead-204) are generally within well-known ranges, which are independent of the composition of the mineral or rock they are, however, directly related to the amounts of radioactive thorium and uranium isotope impurities in them. 

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. 

FIGURE 3-1. Uranium and Thorium Isotope Decay Series Showing the Sources and Decay Products of the Two Naturally-Occurring Isotopes of Thorium... 

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 air concentrations of thorium and other airborne radioactivity near a former thorium and rare-earth extraction facility in the United States were measured. The maximum radioactivity due to all three isotopes of thorium at a site about 450 feet from the primary waste pile was 0.66 fCi/m. Although the background thorium radioactivity was not reported, the total radioactivity at a site about 4000 feet south of the waste pile was about 3.5 times lower than a site 450 feet from the pile (Jensen et al. 1984). 

Since other projectiles, such as neutrons, protons, and deuterons, have also been used to produce artificial radioactivity, the number of active elements thus created already exceeds by far the number of naturally occurring radio-elements (129, 130, 131). By January, 1940, three hundred and thirty artificial radioactivities had been described these include isotopes of every known element in the range of atomic numbers 1 to 85 inclusive, as well as isotopes of thorium (atomic number 90) and of uranium (atomic number 92) (132). Thus the work of M. and Mme. Joliot-Curie opened up vast avenues of research on the physical, chemical, and radioactive properties of these isotopes and on their therapeutic uses. In 1935 they were awarded the Nobel Prize in chemistry (133). 

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. 

Uranium-238 emits an alpha particle to become an isotope of thorium. This unstable element emits a beta particle to become the element now known as Protactinium (Pa), which then emits another beta particle to become an isotope of uranium. This chain proceeds through another isotope of thorium, through radium, radon, polonium, bismuth, thallium and lead. The final product is lead-206. The series that starts with thorium-232 ends with lead-208. Soddy was able to isolate the different lead isotopes in high enough purity to demonstrate using chemical techniques that the atomic weights of two samples of lead with identical chemical and spectroscopic properties had different atomic weights. The final picture of these elements reveals that there are several isotopes for each of them. 

The isotopes of thorium include mass numbers 223-234. 232Th has a half-life of 1.39 x 1010 years, See also Radioactivity. It emits an alpha-particle and forms meso-thorium 1 (radium-228), which is also radioactive, having a half-life of 6.7 years, emitting a beta-particle. Since 2 2Th captures slow neutions to form, by a series of nuclear reactions, >>U which is fissionable, thorium can be used as a fuel for nuclear reactors of the breeder type. Thorium occurs in earth minerals, an average content estimated at about 12 ppm. Findings of hc Apollo 11 space flight indicated that thorium concentrations in some lunar rocks are about the same as the concentrations in terrestrial basalts. 

The four commonly used isotopes of thorium (234Th,228Th, 230Th, and 232Th) are produced from the decay of uranium and radium parents (figure 7.5). Thorium is present in highly insoluble forms and can be rapidly removed by scavenging of particulate matter. 

The Thorium Series. The third natural radioactive series begins with the long-lived naturally occurring isotope of thorium, Th, which has half-life 1.39 X years (Fig. 33-4). It leads to another stable isotope of lead, Pb- . ... 

The trick is to start with an isotope of thorium-232, which has a very iong haif iife of 14 biiiion years, if thorium-232 is bombarded with neutrons, it undergoes a series of nuciear changes, first to thorium-233, then to protactinium-233, and finaiiy to uranium-233. The whoie process takes about a month. At the end of the month, a suppiy of uranium-233 has been produced. This isotope of uranium has a fairiy iong haif iife, about 163,000 years. So once it has been made, it stays around for a iong time, it can then be used for nuciear fission. 

Twenty-nine isotopes of thorium are known. All are radioactive. The isotope with the longest half life is thorium-232. Its half life is about 14 billion years. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element s name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. 

Thorium is a radioactive element that was discovered in the 1800s. The most common isotope of thorium has a mass number of 232. What is the atomic number of thorium How many neutrons does an atom of this isotope contain ... 

The most common isotope of thorium, Th, is a radioactive alpha-particle emitter. What product results when thorium-232 decays by emitting an alpha particle Write an equation for the process. 

Table 6.2 lists the most important isotopes of thorium, together with their properties of greatest significance in nuclear technology. 

Fission cross sections are denoted by For fissionable isotopes of thorium and elements of higher atomic number, the average number of neutrons produced per fission is listed in the same row as the fission cross section, in the same column as the mass, to conserve space in the table. The average number of prompt and delayed neutrons produced by fission with a thermal neutron is denoted by V. The average number of prompt neutrons produced by fission with a thermal neutron is denoted by Vp. The average number of neutrons emitted per spontaneous fission is denoted by t jp. ... 

The lanthanoids resemble each other much more closely than do the members of a row of t/-block metals. The chemistry of the actinoids is more complicated, and in addition, only Th and U have naturally occurring isotopes. Studies of the transuranium elements (those with Z > 92) require specialized techniques. The occurrence of artihcial isotopes among the /-block elements can be seen from Appendix 5 all the actinoids are unstable with respect to radioactive decay (see Section 24.9), although the half-lives of the most abundant isotopes of thorium and uranium ( Th and t = 1.4 x lO and 4.5 x 10 yr respectively) are... 

The first product of disintegration is mesothorium I, discovered by Hahn in 1907. It is isotopic with radium and is used as a substitute for certain radium preparations. As large quantities of thorium minerals are now worked up in connection with the gas-mantle industry, and mesothorium is a by-product, it has assumed commercial importance. It is separated from thorium in monazite being precipitated along with barium as sulphate. Thorium Xy discovered by Rutherford and Soddy in 1902, is another isotope of radium. Radiothorium, RdTh, is an active isotope of thorium and cannot be separated from it directly it has to be obtained from mesothorium I by disintegration if required free from isotopes. 

---