Radioactivity of thorium

Nuclear dating has been most helpful in establishing the history of the earth and of the moon and of the meteorites. The fact is, there is no other way of measuring their ages. Prior to the discovery of natural radioactivity in the late 19th century, indirect methods were used to estimate the age of the earth, but there were no real answers until the radioactivity of thorium, uranium, and potassium were discovered and we began to understand atomic structure and to realize that nuclear transformation was essentially independent of the chemical form. 

McCoy. H. N. and Ross, W. H. The specific radioactivity of thorium and the variation of the activity with chemical treatment and with time. J. Am. Chem. Soc. 29, 1709-1718 (1907). 

Professor Curie continued his researches on the growth of crystals, and his young wife prepared for her examinations. Many chemists consider her dissertation (55) to be the most remarkable thesis ever presented for the doctorate. She continued the work begun by Becquerel, and tested most of the known elements, including a number of rare ones loaned by E.-A. Demarpay and Georges Urbain, with Prof. Curie s piezoelectric quartz electrometer, and found that thorium and uranium were the only ones whose compounds produced appreciable ionization (26, 54, 55). The radioactivity of thorium was discovered independently by Gerhardt Carl Schmidt, professor of physics at the University of Munster (25). 

Since the radioactivity of thorium salts is smaller than that of the minerals, B. B. Boltwood (93) thought that some of the radiothorium must have been lost during the purification process. On the assumption that radiothorium was formed directly from thorium, he computed that the half-life period of the former ought to be at least six years, whereas... 

Mme. Curie and G. C. Schmidt independently discover the radioactivity of thorium. 

RADIOACTIVITY. The spontaneous disintegration of the nucleus of an atom with the emission of radiation. This phenomenon was discovered by Becquerel in 1896 by the exposure-producing effect on a photographic plate by pitchblende (uranium-containing mineral) while wrapped m black paper in the dark, Soon after this, it was found that uranium minerals and uranium chemicals showed more radioactivity than could be accounted for by the uranium content. About the same dmc. radioactivity of thorium minerals and thorium chemicals was also discovered. 

The relation between the amount, x, of substance formed by two consecutive unimolecular reactions and the time t or the intensity of the excited radioactivity of thorium or radium emanations at the time t, is given by the... 

Similar results were obtained by E. Rutherford and F. Soddy when investigating the radioactivity of thorium. Later Rutherford and F. E. Dorn found that radioactive gases (emanation) could be separated from salts of uranium and thorium. After separation of die gas from the salt, the radioactivity of the gas decreased with time, while new radioactivity grew in the salt in a manner similar to that shown in Fig. 1.1. The rate of increase with time of the radioactivity in the salt was found to be completely independent of chemical processes, temperature, etc. Rutherford and Soddy concluded from these observations that radioactivity was due to changes within the atoms themselves. They proposed that, when radioactive decay occurred, the atoms of the original elements (e.g. of U or of Th) were transformed into atoms of new elements. 

Very soon after the radioactivity of thorium and uranium had been discovered it was found that pure samples of both of these elements were only very weakly radioactive. However, such pure samples became more and more radioactive with time until they reached a steady level identical to that in the original samples before pmification. This suggested that the uranium or thorium atoms were transforming or decaying into other radioactive daughter elements and that hitherto undiscovered series of such elements might exist. The search for the radioactive products of uranium by Marie and Pierre Curie led to the characterisation of two new elements, which were named polonium, Po, and radium, Ra. Both elements are far more radioactive than uranium and decay so rapidly that no ore deposits are formed. They exist only because they are formed constantly from naturally occurring uranium. 

Soon after Becquerel s discovery of uranium s radioactivity, Marie Sklodowska Curie (1867—1934), also working in France, studied the radioactivity of thorium (Th) and began to search systematically for new radioactive elements. She showed that the radioactivity of uranium was an atomic property— that is, its radioactivity was proportional to the amount of the element present and was not related to any particular compound. Her experiments indicated that other radioactive elements were probably also present in certain uranium samples. With painstaking technique, she and her husband Pierre Curie (1859-1906) separated the element radium (Ra) from uranium ore and found that it is more than one million times more radioactive than uranium. In 1903, Marie and Pierre Curie shared the Nobel Prize in physics with Henri Becquerel for their discovery of radioactivity. After Pierre died. 

As early as 1900, Rutherford was able to show that the erratic electrometer readings he observed when measuring the radioactivity of thorium compounds were due to the emanation of a radioactive gas into the ionization chamber (Rutherford 1900). The observation that a sample of the radioactive gas faded away within a few minutes according to a characteristic... 

Sus F, Krtil J, Malek Z, Beran M (1998) Determination of trace radioactivity of thorium, uranium, neptunium, plutonium, americium, and curium in swipes and environmental samples. Nucleon, vol 1, pp 13-22... 

The important raw material for cerium, monazite, is also the most important mineral source for thorium. As this metal was desirable it initially supported and accelerated the extraction of rare earth elements from monazite. TTie radioactivity of thorium has, however, created environmental problems for monazite. Nowadays, minerals free from radioactive components are favored for the extraction of REMs. 

Cerium is a component of misch metal, which is extensively used in the manufacture of pyrophoric alloys for cigarette lighters. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. The oxide is an important constituent of incandescent gas mantles and is emerging as a hydrocarbon catalyst in self cleaning ovens. In this application it can be incorporated into oven walls to prevent the collection of cooking residues. 

Rutherford E (1900a) A radioactive substance emitted from thorium compounds. Philos Mag 49 1-14 Rutherford E (1900b) Radioactivity produced in substances by the action of thorium compounds. Philos Mag 49 161-192... 

Attree RW, Cabell MJ, Cushing RL, Pieroni JJ (1962) A calorimetric determination of the half-life of thorium-230 and a consequent revision to its neutron capture cross section. Can J Phys 40 194-201 Bateman H (1910) Solution of a system of differential equations occurring in the theory of radioactive transformations. Proc Cambridge Phil Soc 15 423-427 Beattie PD (1993) The generation of uranium series disequilibria by partial melting of spinel peridotite ... 

The normal or constant radioactivity possessed by thorium is an equilibrium value, where the rate of increase of radioactivity due to the production offresh active material is balanced by the rate of decay of radioactivity of that already formed (Rutherford and Soddy 1902). 

---