ISOTOPES — ATEN AND HEYN 225 



gi'oup how much of a substance there is, remains behind, or takes part 

 in a process (quantitative problems) ; in the third group it is particu- 

 larly a question of exchange processes. It must be said, however, that 

 often the boundaries between the groups cannot be sharply drawn. In 

 localization problems one is obviously always concerned with "mitrue" 

 indicator applications, as is also usually the case in the second group 

 (quantitative problems), while the last group contains only "true" 

 cases. 



Localization. — A very old example is the tracing of samples of 

 radium that have been lost in hospitals through carelessness or theft ; 

 the places where the sample might possibly be, for instance the refuse 

 heap, are gone over with an electron counter. A more modern case 

 is the tracing of stoppages in an oil pipe line. In the periodical 

 cleaning of the walls of the pipe a screw-shaped scraper is placed 

 in the line and pushed along by the pressure of the oil itself. If the 

 scraper gets jammed somewhere, it has to be located as quickly as 

 possible, in order to open the line at that point and remove the accu- 

 mulated deposit. With the help of a radioactive indicator this locali- 

 zation is astonishingly simple. A scraper is used that contains a little 

 radioactive material emitting y-radiation, which easily penetrates 

 through the walls of the pipe and can be detected with a somewhat 

 modified "electron counter." One rides along the line with this "count- 

 ing" apparatus until the radioactivity betrays the position where the 

 scraper has stopped. 



There is another similar application in the petroleum industry, for 

 determining the setting depth of the cement that is pumped in behind 

 the casing of an oil well. A radioactive mineral, carnotite, is mixed 

 with the cement and when a counting instrument is lowered into the 

 drill hole it indicates a strong radiation at the level of the cement. 



Extremely fine cracks in metal surfaces can be detected and local- 

 ized by applying a greasy paste containing a radioactive substance to 

 the surface of the metal under high pressure. Upon the surface be- 

 ing cleaned, the radioactive paste is left in the cracks, and by making 

 an autoradiogram of the surface the cracks can then easily be seen. 



Quantitative problems. — In order to determine the efficiency of a 

 fog, smoke, or dust filter, it is necessary to measure the very small 

 quantities of fog-forming or other substances retained by the filter. 

 As fog- forming substance, tricresyl phosphate, containing radioactive 

 phosphorus is used. When this is passed successively through several 

 filters, their efficiency can be judged by comparing the intensity of their 

 radioactivity. 



Radioactive isotopes are sometimes an excellent means of measuring 

 very small solubilities or very low vapor pressures. The vapor pres- 

 sure of thorium acetyl acetonate, for example, has been determined 



