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tubes used as proportional counters have a slightly better signal-to-noise 

 ratio than most scintillation counters. 



The methods described were concerned with precise measurements of 

 the activity of the radioisotope present. This activity is determined by 

 counting. However, for some problems it is not so important to know 

 the exact rates of disintegration, but rather to distinguish in which 

 portion of a cell the radioisotope is located. A similar problem occurs in 

 paper chromatography in which various compounds are spread out over a 

 large piece of filter paper. Then one wants to find the radioactive spots 

 on the paper. To discover this, one may place a tissue section (or filter 

 paper) against a piece of photographic paper. The latter will be exposed 

 by the radiation and when developed shows the location of the radio- 

 active isotopes. Such a film is called an autoradio graph. 



Any of the three detection methods described (as well as others) can 

 be used with any radioisotope availabfe. Although there are radio- 

 active isotopes known for all elements, only a limited number have been 

 widely used in biological research. Among the most popular of these 

 have been C 14 , I 131 , and P 32 . Each will be discussed briefly in the 

 following sections. 



3. C' 4 



Naturally occurring carbon consists almost entirely of the stable isotope 

 C 12 . In addition, there is a small' amount (about 1 per cent) of the 

 stable isotope C 13 and a trace of the radioactive element C 14 . Using 

 artificial means, it is possible to produce C 14 and also the radioactive 

 isotopes C 10 and C 11 . (The isotope C 15 has been detected but is not 

 useful for tracer studies.) The isotopes C 10 and C 11 have half-lives of 

 19 sec and 20.5 minutes respectively, both emitting positive beta particles 

 and decaying to the corresponding Be isotopes. These half-lifes are so 

 short that C 14 is usually the only radioactive carbon isotope employed 

 in biological tracer studies. Because carbon is an essential part of all 

 biological compounds, C 14 is a very widely used isotope. 



The isotope C 14 can be prepared by a number of reactions. If 

 C 13 -enriched carbon is bombarded by neutrons, some of it is converted 

 to C 14 according to the scheme 



C 13 + n ->- C 14 + y° 



Similarly, if the C 13 -enriched carbon is bombarded by deuterons, C 14 is 

 formed, this time by the reaction 



Ci3 + D 2 ->C 14 + H' 



Neither of these is desirable because a mass spectrometer must be used 

 to concentrate first C 13 and then C 14 . 



