172 RADIOISOTOPES IN BIOLOGY AND AGKICULTURE 



10 mg of the stable element is added to serve as a carrier. This can be 

 added to any stage. However, if it is added to the original sample and 

 the final plate is weighed, it becomes possible to estimate the recovery of 

 the over-all procedure. Specific methods will be given in Chap. 6 for 

 some of the radioisotopes. The primary disadvantage of electroplating 

 as compared with some of the direct methods is the time and labor 

 required. 



Briquet Formation. Dry powders may be pressed into shallow dishes 

 with a spatula or may be formed into briciuets with a laboratory press and 

 piston-cylinder apparatus. The latter method has been used successfully 

 for the convenient measurement of P^'- in plant material (34). These 

 procedures, however, would not be satisfactory where thin samples are 

 required, as in the assay of soft-beta emitters. 



SOME CONSIDERATIONS IN COUNTING PROCEDURES 



The term count is used to designate the response of a measuring device 

 to an ionizing event. Thus the counting rate, that is to say, the counts 

 measured in a given time by the Geiger counter, is directly proportional 

 to the amount of radioisotope in the sample. The counting rate is here 

 expressed as counts per minute (counts/min; cpm). The Geiger-Miiller 

 counter tube, which contains the sensitive volume responding to the 

 radiation, is variously referred to by the following terms: Geiger counter 

 tube, Geiger counter, Geiger tube, counter tube, or counter. The last two 

 terms may also be used for other types of detectors, such as the propor- 

 tional counter and the scintillation counter. Discussion of auxiliary 

 electronic equipment and particularly of different types of counter tubes 

 is deferred to page 183, since it seems most logical to describe the detec- 

 tion devices in terms of the measurement for which they are best suited. 



Statistical Evaluation of Counting Errors. There will always be some 

 degree of error in the observed counting rate on account of the statistical 

 fluctuations inherent in a random process such as radioactive decay. 

 References (6, 8, 9, 11, 35) may be consulted for a detailed discussion of 

 counting statistics. The investigator interested in extremely low-level 

 counting will want to study this matter at some length. The primary 

 interest here is to establish the order of magnitude of the errors in terms 

 of the counting rates usually encountered. It has been shown that the 

 accuracy, assuming no abnormalities in the instrument, depends only 

 upon the total number of counts taken and that for practical purposes 

 the standard deviation will eqiml the square root of the number of counts. 

 Thus, in samples that have total counts of 100, 1000, and 10,000, the 

 standard deviations are 10, 32, and 100 counts, respectively. It requires 

 about 1000 total counts to yield a standard deviation of about 3 per cent. 



