188 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



losses during the measurements. With organic solvents this may become 

 a problem. The effect of solids content and density of the liquid may 

 have to be taken into account, although in our hands the counting rate 

 of P^'- was little affected by these variables. In any event, the standards 

 should be measured under identical conditions in typical sample solutions, 

 which will eliminate these variables unless there is a great difference 

 among samples. Care must be taken that there is no significant phase 

 separation during the measurement, e.g., the settling of red blood cells or 

 the layering of fat globules. Samples should not be allowed to stand for 

 long periods in the measurement container before being counted. Serial 

 counts should be made on typical samples to ensure that there is no signif- 

 icant change with time. 



Any of the counter tubes mentioned may be used for counting solid 

 samples of hard-beta emitters. 



Gamma Counting. It will be recalled from Chap. 3 that gamma rays 

 are very penetrating. This gives a real advantage in gamma counting 

 because the counting rate is essentially independent of the sample mass, 

 and therefore the fresh tissue can often be counted as is without the need 

 of any pretreatment. A word of caution — the sensitivity of the scintil- 

 lation counter to low-energy, scattered radiation may introduce errors 

 with some isotopes if the samples and standards are not similar in mass 

 and geometry. On account of the high penetrating power and low 

 specific ionization, the ordinary Geiger counter tubes are relatively insen- 

 sitive to gamma rays. Special gamma counters are available, however, 

 which give a 6- to 10-fold increase in sensitivity over the ordinary Geiger 

 counters. 



The development of the scintillation counter has provided a most useful 

 instrument for efficient measurement of gamma rays, and this detector 

 is now considered the instrument of choice (46 to 49). In principle, the 

 radiation produces light in a suitable crystal which in turn is collected by 

 photomultiplier tubes to create a pulse that can be recorded. Commercial 

 units now available can be used with most of the modern scalers and have 

 proved satisfactory in practice. The sample can be presented to the flat 

 surface of the crystal just as to a counter tube. A very sensitive unit 

 utilizes a crystal containing a well which can accommodate a small test 

 tube holding about 2 or 3 ml of solution. In this case the sample is sur- 

 rounded by crystal surface except for the top; this results in very efficient 

 collection of the gamma rays emitted. Unlike a Geiger counter; the 

 response as a function of voltage is dependent upon the gamma-ray 

 energy and will differ among isotopes. The operating characteristics of 

 the instrument should therefore be studied for each activity used. 



Before the advent of the scintillation counter it was necessary, with 

 many isotopes, to measure the beta particles emitted in order to attain 



