Radioelements thorium

The natural radioelements are listed in Table 14.1. Isotopes of these elements are members of the uranium, actinium and thorium families (Table 1.2, and Tables 4.1 to 4.3). In the ores of U and Th the concentrations of natural radioelements are relatively high and proportional to the half-life. The average concentration of U in the earth s crust is about 2.9 mg/kg (ppm) and that of Th about 11 mg/kg (ppm). The... 

Literature on the radionuclide content of phosphates is rather extensive. Radium, uranium, thorium and members of their decay series are the principal radioelements present in fertilizers. Radium content in fertilizers is extensively discussed by some authors (Guimond, 1990 Roessler, 1990). 

The radioactive elements were called radioelements. Lacking names for these radioelements, letters such as X, Y, Z, A, B, etc., were added to the symbol for the primary (i.e. parent) element. Thus, UX was produced from the radioactive decay of uranium, ThX from that of thorium, etc. These new radioelements (UX, ThX, etc.) had chemical properties that were different from the original elements, and could be separated from them through chemical processes such as precipitation, volatilization, electrolytic deposition, etc. The radioactive daughter elements decayed further to form still other elements, symbolized as UY, ThA, etc. A typical decay chain could be written Ra - Rn RaA - RaB - , etc. Fig. 1.2. 

By 1910 proximately 40 differrait chemical species had been id tified through their chemical nature, the properties of their radiation, and their characteristic half-lives. The study of the generic relationships in the decay of the radioactive species showed that the radioelements could be divided into three distinct series. Two of these originated in uranium and the third in thorium. B. Boltwood found that all three of the series ded in the same... 

A major difficulty obvious to scientists at that time involved the fact that while it was known from the Periodic Table (Appendix I) that there was space for only 11 elements between lead and uranium, approximately 40 radioelements were known in the decay series from uranium to lead. To add to the confusion was the fact that it was found that in many cases it was not possible to separate some of the radioelements from each other by normal chemical means. For exanq)le, the radioelement RaD was found to be chemically identical to lead. In a similar manner, spectrographic investigations of the radioelem t ionium showed exactly the same spectral lines that had been found previously to be due to the element thorium. 

Now it is clear that radioelements are just isotopes of natural radioactive elements. The three emanations are the isotopes of the radioactive element radon, the number 86 in the periodic system. The radioactive families consist of the isotopes of uranium, thorium, polonium, and actinium. Later many stable elements were found to have isotopes. An interesting observation may be made here. When a stable element was discovered this meant simultaneous discovery of all its isotopes. But in the cases of natural radioactive elements individual isotopes were discovered first. The discovery of radioelements was the discovery of isotopes. This was a significant difference between stable and radioactive elements in connection with the search for them in nature. No wonder that the periodic system was badly strained when accommodation had to be found for the multitude of radioelements,—it was a classification of elements, after all, not isotopes. The discovery of the displacement law and isotopy greatly clarified the situation and paved the way for future advances. 

This was done in mid-March 1913 by K. Fajans and his young assistant 0. Goring who detected a new beta-emitting radioelement with a half-life of 1.17 min and chemical properties similar to those of tantalum. In October of the same year they clearly stated that UXj was a new radioactive element located between thorium and uranium and suggested to name it brevium (from the Greek for short-lived ). 

A decisive role in further developments was played by the radioelement UY, a thorium isotope discovered in 1911 by the Russian radiochemist G. Antonov who worked in Rutherford s laboratory. The radioelement UXi (also a thorium isotope) in the uranium family emits beta particles and gives rise to brevium (UXg). 

The natural isotopes of uranium and thorium in long series of successive radioactive transformations give rise to secondary chemical elements. In the first decade of the 20th century scientists had in their disposal about forty radioactive isotopes of the elements at the end of the periodic system, that is, from bismuth to uranium. These radioelements comprised three radioactive families headed by thorium-232, uranium-235 and uranium-238. Each radioactive element sent, its representatives to these families with the only exception of eka-iodine and eka-cesium. None of the three series had links that would correspond to the isotopes of element 85 or 87. This suggested an unexpected idea that eka-iodine and eka-cesium were not radioactive. But why Nobody dared to answer this question. Under this assumption it was meaningless to look for these elements in the ores of uranium and thorium which contained all the radioactive elements without exception. 

In 1913 the British scientist A. Cranston worked with the radioelement MsTh-II (an isotope of actinium-228). This isotope emits beta particles and converts into thorium-228. But Cranston thought that he detected a very weak alpha decay, too. If that was true the product of the decay had to be the long-expected eka-cesium. Indeed, the process is described by... 

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. 

The need to exclude low levels of natural radionuclides from unnecessary legislation was recognised at an early stage in the UK. The Radioactive Substances Act (RSA, 1993) defined eight radioelements, all within the natural uranium and thorium decay chains, that constituted radioactive material only if present above a certain threshold. Problems arose with anthropogenic radionuclides, not considered above, and owing to various, established practices that required a large number of bespoke and somewhat ad hoc exemption orders the most important of which in the current context was the Phosphatic Substances and Rare Earths etc. Exemption Order (PSRE). The need for elemental activity concentrations meant that, polonium for example was calculated from ... 

Of the radioelements studied, plutonium and uranium(IV) are most sensitive to the presence of organic materials. Thorium is least sensitive thorium sorption onto some rock types is completely unaffected by organic material (e.g. fracture infill material shown in Fig. 9). 

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