238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



This is an observable number, even if only 1/1 Oth of the /3-rays pro- 

 duced enter the counter. In the case of P 32 it is easy to prepare samples 

 giving more than 10 6 disintegrations per minute or about 20,000 

 per sec. Since T= 14.8 days= 1 .3 X 10 6 sec, we find 2V^3 X 10 10 radio- 

 active atoms. This is only about 10~ 6 microgram. And if 0.01 

 percent of this amount appeared in any sample taken from an organism 

 it could still be detected. 



3. While it is possible that the presence of radioactive material in 

 large quantities in certain organs might have definite biological effects 

 on them due to the radiations emitted, and thus give abnormal results 

 in metabolism experiments, it seems evident that in most cases the 

 doses given may be made sufficiently small as to preclude any 

 important effects due to the radiation itself. 



4. Finally — and this should be strongly emphasized — the tech- 

 nique of detecting and measuring the radioactivity of any material is 

 simple and capable of good precision. And when one recalls the days 

 or weeks of work required to make a quantitative chemical analysis of 

 substances containing a few micrograms of the element of interest, 

 and the utter hopelessness of detecting changes involving only 10~ 12 

 to 10~ 14 grams, one sees the radioactive technique as child's play. 

 And when one recalls that this relatively simple technique not only 

 detects these small quantities of material, but actually distinguishes 

 between the atoms already present (which were not radioactive) and 

 those introduced at a particular time (which are) the possibilities 

 become quite exciting. It is almost as though each individual atom 

 carried a red flag to herald its presence. (It is even better than this 

 since we know too little of the biological effects of red flags !) 



The most common instrument which will likely be used in such 

 investigations is the Geiger-Miiller tube counter. This device, 

 undoubtedly familiar to you, consists of a conducting cylinder aloDg 

 the axis of which is suspended a fine wire. A potential difference of 

 the order of 1,000 volts is applied between them. The whole is 

 enclosed in a gas-tight envelope filled with air or other gas at a pres- 

 sure of a few cm of Hg. A thin window must be provided for the 

 entrance of slow /3-par tides. The counter wire is connected to a 

 two- or three-stage amplifier which feeds a loud speaker or suitable 

 registering equipment. The entry of a /3-ray into the cylinder causes a 

 click in the speaker, or the pulse may be used to operate a mechanical 

 register. Counter tube, rectifier, amplifier, and registering device 

 may ail be contained in a box the size of a small radio set. 8 Observa- 

 tions consist merely in determining the number of registrations per 

 unit time. Unfortunately such counters respond also to radioactive 

 impurities which are always present in the air and in solid materials of 



• For farther discussion and references see Harnwell and Livingwood, Experimental Atomic Physics 

 p. 412, McGraw-Hill, 1933. 



