NUCLEAR PHYSICS — DUBRIDGE 235 



The other notations are similar, the symbols p, d, n, a, y standing 

 respectively for proton, deuteron, neutron, alpha-particle (He nucleus) 

 and gamma-ray. In general the bombarding particle must have an 

 energy of from 1 to 5 million electron volts. To produce reasonable 

 amounts of radioactive material intense beams of such particles must 

 be available. This requires a cyclotron or some form of high-voltage 

 equipment. 



The possibilities of using these materials in particular biological 

 problems are so numerous that we can mention only a few by way of 

 illustration. The more obvious possibilities (others are certain to 

 come) may be grouped in two classes. 



1. Therapeutic uses. — Radioactive isotopes of suitable elements 

 may be injected into the body or directly into the organ to be treat- 

 ed. If it is found (and this requires much careful study) that the 

 radiations they emit have definite therapeutic effects, then it is pos- 

 sible that induced radioactive isotopes may be more conveniently and 

 effectively used than the naturally radioactive materials. Of par- 

 ticular interest is the possibility of selective irradiation of various 

 organs of the body through the use of radioactive isotopes of ele- 

 ments which happen to be concentrated in those organs either under 

 normal or diseased conditions. Thus one would use radioactive 

 calcium or phosphorus for bone treatment, radio-iodine for the 

 thyroid, etc., the active material going automatically to the spot 

 where it is needed. 



The clinical applications of these materials are thus both important 

 and possibly spectacular. For tins reason they have been perhaps 

 prematurely hailed as a new boon to medicine. They certainly open 

 up new possibilities, but their practical value is yet to be demonstrated 

 and this will require a long period of careful research. 



2. Tracing. — From the viewpoint of fundamental biology the use 

 of radioactive materials as tracers is of far more interest than the 

 therapeutic uses. In this field there are almost unbelievable possibil- 

 ities which represent the most biologically promising of all aspects of 

 nuclear physics. A prominent physiologist is quoted as stating his 

 belief that the technique of using radioactive materials as tracers in 

 biology may open a new era in that subject in as fundamental a way 

 as did the invention of the microscope. The microscope made it 

 possible to follow individual cells — the radioactive isotopes make it 

 possible to follow individual atoms. 



The uses to which this tool could be put are so many that it is as 

 yet difficult even to classify them. Almost any chemical or biological 

 problem in which it is desirable to follow the course of particular 

 elements or compounds through a given system should be open to 

 attack by this method. I will mention a few such problems which 

 happen to have come to my attention for purpose of illustration. 



