564 Tracer Techniques /30 : 3 



The most efficient reaction for forming C 14 consists in bombarding 

 N 14 with neutrons. 2 This reacts as 



N 14 + n'->C 14 + H' 



The compounds NH 4 N0 3 and BeN0 2 are often used as nitrogen sources. 

 The C 14 can then be removed by standard electrochemical techniques. 

 A certain amount of C 14 is formed in this fashion by cosmic rays acting 

 on the N 14 in the atmosphere. This results in about 10 counts per 

 minute per gram of carbon in equilibrium with the C0 2 in the atmos- 

 phere. 



The half-life of C 14 is about 5,760 years. Thus, comparatively high 

 concentrations are necessary to obtain a measurable counting rate. In 

 disintegrating, C 14 emits only a negative beta particle and no gamma 

 rays ; it becomes N 14 . The maximum energy of the emitted beta particles 

 (electrons) is about 0.154 Kev. This is comparable to the energy of a 

 clinical X-ray tube and is much lower than particles from many other 

 radioactive isotopes. It is difficult to detect these low-energy beta 

 particles. Sometimes the C 14 -containing sample is made into a BaC0 3 

 disk and pressed against the end of a Geiger tube (or inside it). Certain 

 Geiger tubes are designed to admit the C 14 in the form of C0 2 gas. 

 Various liquid scintillators are also used. In any case, the sample 

 cannot be very thick because all the emitted beta particles are absorbed 

 in passing through a few millimeters of solid or liquid sample. 



In other words, C 14 is not an ideal tracer element. Its half-life is 

 too long, and its emitted beta particle has too low an energy. In spite 

 of this, C 14 has been very important in research. Two specific examples, 

 chosen from a multitude of applications, are protoporphyrin synthesis 

 and carbon dating. 



Protoporphyrin is part of the heme group, which is the prosthetic 

 group of hemoglobin and several enzymes. Its structural formula is 

 shown in Chapter 18. Several experiments indicated that the amino 

 acid glycine is used in the formation of protoporphyrin. Accordingly, 

 two forms of glycine were prepared 



NH 2 O NH 2 O 



\ // I // 



H— C*— C H— C— C* 



H OH H OH 



where the * indicates the tracer atom, in this case C 14 . It was found with 

 the first of these that radioactivity was incorporated into the porphyrin 



2 The manufacture of radioactive isotopes by neutron bombardment often 

 employs neutrons from a pile (atomic reactor). Neutrons from such a source are 

 plentiful and relatively inexpensive. 



