630 ADOLF F. V O I G T 



a sample of insufficient weight is obtained, a correction can be made 

 by constmcting a cur^'e as in Figure 11 of obsen-ed acti^ntj^ vs. 

 sample weight (specific activity constant) ^^'ith sample thicknesses 

 less than "infinitely thick."' 



A word of caution should be inserted \\-ith regard to any counting 

 method that makes use of powdered or flaky material in such a wa^- 

 that the active powder could flake off near the counter. This is 

 one of the easiest ways to raise the background of the counter and use 

 of such a method should be avoided where possible; if the method is 

 unavoidable, extreme care should be exercised. If at all possible the 

 material should be cemented or covered completely Avith paper. As 

 cement Zapon lacquer is good, and a dilute solution of Celluloid in 

 ethyl acetate-acetone or similar organic solvent is probably adequate. 



4. Radioactivity Standards 



For many research problems, absolute knowledge of the intensity 

 of the radioactive sources is unnecessarv". However in some cases it 

 is desirable to know the total number of disintegrations coming from 

 the source. Although calibration methods using natural radioactive 

 materials, such as a weighed uranium oxide sample. haA'e been stand- 

 ardized and can be used AAithout insurmountable difficulties, a prefer- 

 able method is to obtain standardized sources from the National 

 Bureau of Standards (4-3) • This bureau has available at a nominal 

 cost standards for /3-ray counting as well as 7-ray standards, which 

 can be used for calibrating sur^-ey and monitoring instruments. 

 Radium and Co^ are both used for the latter purpose. In the meas- 

 urement of absolute activities, it is probable that the investigator -^-ill 

 need to determine the absorption characteristics of the element. Pro- 

 cedure for doing this vdW be found in texts on radioactivity and will 

 not be discussed further here. 



F. UNIQUE PROBLEMS AND TECHNIQUES IN IL\NDLING 



RADIOACTIVE TR_\CERS 



1. Radiocheuiical Problems 



Though the subject of this chapter is the biological application of 

 radioactive tracers, there are certain topics in the chemistiy of these 

 materials that should be mentioned. In handling radioactive materi- 



