ISOTOPIC TRACERS 



167 



Detection methods 



Tracers, of course, are useless unless they can be detected after the 

 experiment is complete. Since very small amounts of tracer materials are 

 used, special methods are necessary to recover the tracer. A number of 

 methods have been used to prepare materials for examination. If a physical 

 movement from place to place is being investigated, nothing more than a 

 dissection of the animal or plant may be required. Chemical experiments 

 offer more difficulty. The suspected product of a reaction must be sep- 

 arated from other compounds in the 

 cells or reaction mixtures; nowadays 

 chromatography is used for this separa- 

 tion. 



One simple method of detecting 

 radioactive materials is to place the plant 

 or animal parts, or the chromatogram, 

 against a sheet of photographic film. 

 The radioisotopes "take their own pic- 

 ture" or produce a "radioautograph" be- 

 cause irradiated areas of the film show 

 up on development. 



The G-M Tube: A number of electri- 

 cal instruments are employed for quan- 

 titative determination of radioactivity. 

 The Geiger-M tiller (G-M) tube is prob- 

 ably still most commonly used. It con- 

 sists of a hollow metal tube, filled with a 



gas mixture, with a wire extending along its center for most of its 

 length. The circuitry is diagrammed in Fig. 12-2. The behavior of the 

 tube depends on the applied voltage. Over the commonly used range 

 of voltages, the so-called G-M region, the tube responds to incoming 

 radiation in a characteristic way. Beta particles enter through the win- 

 dow, gamma quanta through the window or walls, and produce ions in 

 the gas in the tube. At this voltage, negatively charged ions migrate to- 

 ward the center wire, producing other ions as they travel. Eventually 

 a "cloud" of ions strikes the center wire as a pulse of charged particles, 

 setting up a momentary electric current in the wire. Positively charged 

 ions move in the opposite direction, producing the same effect. Elec- 



Window 



Fig. 12-2. A Geiger-Miiller Tube. 

 The window is frequently of 

 mica. 



