Since the radium atom in breaking up gives rise to one atom of radon and one atom of helium, its atomic weight should be 226-4 = 222. Radon appears to have no definite chemical prop erties, and in this respect belongs to the group of inert monatomic gases of which helium and argon are the best known examples. It is partially soluble in water, and readily absorbed by charcoal.
Radon has proved of great service not only in radioactive re searches but also in therapeutic work. The radium salt is dis solved in an acid solution and the emanation is pumped off with the large quantity of hydrogen and oxygen liberated by the action of the radiations on water. After sparking the mixture, the radon can be purified by condensation with liquid air. A very intense source of 0- and 7-radiation can be obtained by introducing the purified radon into fine capillary tubes. Such radon needles have been widely used for therapeutic purposes, while the use of very thin-walled tubes provides a powerful line source of a-rays. The 0- and 7-activity of such tubes rises to a maximum about four hours after introduction of the radon, and then decays with the period of radon, viz., 3.83 days. The quantity of radon liberated from one gram of radium is called a curie and from one milligram a millicurie. The quantity of radon in a tube can be accurately determined by comparison of its 7 -ray activity with that of a radium standard, since the penetrating 7-rays, both from the radium and the radon in equilibrium, arise mainly from the same product radium C.
Transformations of Thorium.—The first product observed in thorium was the emanation of period 54 sec., and this gives rise to the active deposit, which has been shown to consist of at least four successive products called thorium A, B, C, C". The emanation, after the emission of an a-particle, changes into a product of very short life emitting a-rays. Its period was found by Geiger and Moseley to be second. The succeeding product, thorium B, emits only weak 0- and y-rays with a period of io.6 hours, changing into thorium C of period one hour. We have seen that thorium C breaks up in a complex way, emitting three distinct groups of particles. Thorium C" is readily separated from C by the method of recoil. It emits penetrating 0- and 7-rays with a half period of three minutes. The active deposit as a whole decays ultimately with the period of thorium B, viz.,
io.6 hours.
A special interest attaches to the product thorium X, first separated by Rutherford and Soddy, since experiments with it laid the foundation of the general theory of radioactive transforma tions.
A close analysis of thorium has led to the discovery by Hahn of a number of other important products. When the thorium X is separated from a thorium, mineral or old thorium preparation, there appears with it another product called mesothorium I, of period 6.7 years, which is transformed with the emission of weak a-rays into mesothorium 2, of period six hours, which emits swift 0-particles and penetrating y-rays. This changes into an a-ray product, radiothorium, of period two years, which is transformed into thorium X.
Radiothorium is an isotope of thorium, while mesothorium is an isotope of radium. The radiothorium can readily be sepa rated from a solution of mesothorium and obtained in a concen trated form. Mesothorium when first separated would show a very weak activity, but in consequence of the growth of its sub sequent product radiothorium, its activity would increase for sev eral years. After reaching a maximum it would ultimately decay with the period of mesothorium, viz., 6.7 years.
Large quantities of mesothorium have been obtained as a by product in the separation of thorium from monazite sands. This substance emits only j3-rays but soon grows radiothorium and subsequent products which emit a-rays and penetrating j3- and y-rays. As a source of powerful f3- and 7-radiation, this substance is very analogous to radium and can be obtained in about the same concentration. Since and mesothorium are isotopic ele ments, they are always separated together. Most commercial sources of thorium contain also uranium and radium, and con sequently radium is always separated with the mesothorium and in relative amount depending upon the proportion of uranium to thorium in the mineral. Since mesothorium has a radioactive life short compared with radium, it commands a smaller price. The amount of mesothorium is standardised by comparison of its 7-ray effect with a radium standard. Radiothorium, separated from preparations of mesothorium, is widely used instead of radium as sources of 0- and y-rays for therapeutic purposes.