ISOTOPIC TRACERS 169 



tion, and then another sample can be counted. Several automatic sample 

 changers are available. These devices place one sample under the tube, 

 count to a predetermined number of counts, print the time required, and 

 then place another sample under the G-M tube. Thirty-six or so samples 

 can be counted in sequence over a period of several hours without any 

 attention from the operator. 



Radioactive materials on paper-strip chromatograms are counted by 

 strip scanners which automatically feed the strip of paper under the 

 counting tube and then print or record the radioactivity as a function 

 of distance along the paper strip. 



A G-M tube usually counts all pulses, whether they were produced by 

 a, P, or y radiation and regardless of the energy of the radiation. A scin- 

 tillation counter— or a G-M tube operating in another voltage range- 

 produces a response related to the energy of the particles or quanta. A 

 discriminating circuit can be set to allow only pulses above a certain 

 size to be counted. Since the setting is adjustable, a whole spectrum of 

 radiation could be determined by a series of counting operations. Gamma 

 quanta vary in energy, depending on the emitting isotope, so a mixture 

 of two or more y-emitters could be counted, yielding information about 

 amounts of the different isotopes in the mixture. 



Detection of Stable Isotopes: As stable isotopes differ from each other 

 only in mass, any measurement of these isotopes must depend on this 

 property. A mass spectrometer is an instrument which separates mole- 

 cules on the basis of mass and determines the amounts of each kind of 

 molecule. The mass spectrometer consists of an evacuated tube across 

 which a high voltage is applied. The gas to be analyzed is admitted at 

 one end of the tube, ionized by a stream of electrons, and accelerated 

 toward the opposite end of the tube. Figure 12-3 shows one such instru- 

 ment, and Fig. 12-4 shows another which separates materials on a differ- 

 ent principle. 



In the "time-of-flight" mass spectrometer (Fig. 12-3), ions traverse the 

 length of the tube, but the greater the mass, the longer this passage will 

 take. As the ions arrive at the negative electrode they set up a momentary 

 current whose magnitude is dependent upon the number of such ions. 

 If the tube is long enough, ions of different masses arrive at distinctly 

 different times. Once all the desired masses have been measured, a new 

 quantity of gas can be admitted and the operation repeated. The repeti- 

 tions can be extremely rapid (up to several thousand cycles per second), 

 so that the "mass spectrum" can be displayed on a cathode-ray oscillo- 

 scope. 



