Thoron emanations

Twenty isotopes are known. Radon-22, from radium, has a half-life of 3.823 days and is an alpha emitter Radon-220, emanating naturally from thorium and called thoron, has a half-life of 55.6 s and is also an alpha emitter. Radon-219 emanates from actinium and is called actinon. It has a half-life of 3.96 s and is also an alpha emitter. It is estimated that every square mile of soil to a depth of 6 inches contains about 1 g of radium, which releases radon in tiny amounts into the atmosphere. Radon is present in some spring waters, such as those at Hot Springs, Arkansas. 

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). 

The calibration of this method was performed with Rn-222 and Rn-220 emanating from standard Ra-226 and Th-228 solutions, respectively. It is defined that the calibration factor is the ratio of experimental counts to one expected from used radon and/or thoron gases by using Equation (7) The calibration factors of radon and thoron were evaluated to be 0.81 and 0.31, respectively. 

R. B. Owens. He observed in 1899 that the ionization current through a confined volume of air exposed to the rays from thorium compounds decreased to a minimum when air was drawn through his apparatus. Rutherford showed that this effect is caused by the emission of thorium emanation, now known as thoron. This photograph was taken in about 1910 when Professor Owens was at McGill University. 

Rn is formed by the alpha disintegration of 226Ra. Actinon, its isotope of mass number 219, is produced by alpha disintegration of 223 Ra (AcX) and is a member of the Actinium Series. Similarly, thoron, its isotope of mass number 220. is a member of the thorium senes. Since the name radon may be considered to be specific for the isotope of mass number 222 (from the radium series), the term emanation is sometimes used for element number 86 in general. Other isotopes of radon include those of mass numbers 209-218 and 221. 

All soil gases contain a mixture of radon, thoron and actinon. Whilst the half-life of the last (5.4 sec.) is too short to interfere with Rn determinations, Tn is always present in in-situ determinations. To relate Rn and Tn emanations quantitatively to U and Th concentrations is difficult, if not impossible. Too many factors, such as radioactive disequilibrium of the series in the soil as a result of weathering, changes in the emanation efficiency due to soil density and grain size, moisture content, and distribution of radioactive particles, cause erroneous extrapolations. However, precise, quantitative results are not of prime importance in prospecting. The important point is that there be correspondence between Rn in soil emanations and U in the area nearby. That this correspondence exists has been demonstrated numerous times and is discussed in more detail below. 

Radon was discovered in 1899 by the McGill University professors Ernest Rutherford and Robert Owens, who found that radioactive thorium produced radioactive gas. They named this gaseous substance thorium emanation, later to become thoron. It was found that radium gave off a similar emanation (radon), as did actinium (actinon), in 1900 and 1904, respectively. Once the structure of the atom and the elemental transmutation process became better understood, it was determined that thoron, radon, and actinon were different isotopes of the same element (radon)— °Rn, Rn, and Rn, respectively. 

Rutherford sought for a similar gaseous emanation from radium compounds, but the quantity of these at his disposal was too small. Within a few months, however, Dorn detected the presence of the gas and three years later Debiernet found that actinium behaved likewise. The three emanations are now known as thoron of half-life 54 secs., radon 3 825 days, and actinon. 3 9 secs. 

Among the methods for radioactivity measurements several others should be at least mentioned. The ecotoxicollogically important radon ( Rn), thoron ( Rn) and acti-non ( Rn) and their direct parents (e.g. Ra) can be reliably determined by emanation analysis (Toelgyessy and Kyrs, 1989). It demands the quantitative removal of the inert gas from the sample before its activity is measured. The method is easily applicable for waters but solids have to be melted either directly or by fusion. 

The natural radioactivity in air is due mainly to radioactive emanations (radon, thoron, actinon) and to those of their deposits that produce radioactive... 

Figure 4 Emanation equipmerrt for the determination of thoron 1, purge system 2, fritted glass disk 3, filter with glass wool 4, ionization chamber and 5, electrometer.

Rutherford and Owens decided that thorium continuously emitted a gaseous radioactive substance, which they called the emanation (from the Latin to flow) of thorium, or thoron. 

By way of analogy, it was suggested that other radioactive elements could also evolve emanations. In 1900 the German physicist E. Dorn discovered the emanation of radium and three years later Debierne observed the emanation of actinium. Thus, two new radioactive elements were found, namely, radon and actinon. An important observation was that all the three emanations differed only in their half-lives—51.5 s for thoron, 3.8 days for radon, and 3.02 s for actinon. The longest-lived element is radon and therefore... 

Friedrich Ernst Dom in 1900. He used the same apparatus as Rutherford with thorium emanation and confirmed this. Using radium bromide he found a similar emanation, which did not penetrate aluminium as Rutherford (incorrectly) said thorium emanation did. Some doubts about the existence of radium emanation were removed by Rutherford and Soddy, who liquefied it by cooling in liquid air, and by Ramsay and Collie, who determined its spectrum. R. W. Gray and Ramsay determined its density by weighing on a microbalance and proposed for it the name m/on, shining . Actinium emanation was discovered by Debieme and by Giesel. The modem names thoron, radon, and actinon were proposed by W. Schmidt. ... 

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