142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



Table 2. — List of radioactive elements which are being used in biological and medical 



investigations 



[See ref. 18] 



Radioactive element: Type of radiation Half-life 



iH 3 B- 30.0 years. 



,C» B+ 21.0 minutes. 



6 C H B— 1,000.0 years. 



7 N 1S B + ,7 9.93 minutes. 



„Na ?2 B+ 3.0 years. 



n Na« B-, y 14.8hours. 



12 Mg 27 B — , 7 10.2 minutes. 



15 P 32 B- 14.3 days. 



, 6 S 35 B- 88.0 days. 



i 7 Cl 38 B-, t 37.0 minutes. 



, 9 K« B-,7 12.4 hours. 



20 Ca« B-,7 180.0 days. 



26 Mn" K, 7 310.0 days. 



28 Fe M B-,7 47.0 days. 



, 7 Co 56 K, 7, e-,B+ 270.0 days. 



o 9 Cu<* B-,B + ,K 12.8 hours. 



3oZn« B + , k, 7, e- 250.0 days. 



33 As 7 « B-,B + ,K, 7 26.8 hours. 



3 8 As« B-,B + ,7 16.0 days. 



35 Br 82 B — , 7 34.0 hours. 



37 Rb 86 - 88 B— 18.0 days. 



38 Sr 8 » B- 55.0 days. 



B3 I 131 B-,7 8.0 days. 



8 5 2 » K, a 7.5 hours. 



Tracer substances have proved of great value in studying metabolic 

 process in plants and animals, but it must not be supposed that their 

 introduction has supplanted ordinary chemical methods which still 

 have a recognized place and use. A new tool is not necessarily the 

 best for all purposes. 



An ingenious device is used at Berkeley for the preparation of 

 radioactive substances simultaneously with the production of neu- 

 trons for other purposes. The substance to be activated, e. g., red 

 phosphorus, is introduced into the vacuum chamber of the cyclotron 

 on probes so arranged as not to interfere with the bombardment of 

 the beryllium target and the emission of neutrons from the target 

 chamber (19). The cyclotron vacuum has, however, to be broken 

 when the probes are inserted or withdrawn at intervals of several 

 days. 



Various methods are used for the detection of tracer substances: 



(a) Measurement of uptake. — An animal is fed with a radioactive 

 substance and after a given time is sacrificed. An assessment is then 

 made of the radioactivity of any or all the organs of its body. The 

 process is repeated with other animals of the same species killed after 

 different intervals. The assessments of radioactive material are then 

 plotted against time on a graph, the quantity of material being given 

 as a percentage of substance originally administered found per gram 

 of tissue examined. It is, of course, not necessary to use the whole 

 organ in every case; a weighed portion may be used instead, and in 



