RADIOACTIVITY METHODS 



995 



where N is the total number of events recorded. We have an even chance 

 that a measurement will differ from the average of a large number of 



Fig. 607. — Probable error (in per cent) as a function of total number 

 of counts recorded in one measurement. 



measurements by an amount smaller than p. In other words, if we take a 

 large number of repeated measurements with a perfect instrument, half of 

 the readings will differ from the average by an amount smaller than p 

 and half will differ by more than p. It is important to realize that p is a. 

 function of A^ only, and it does not depend on the time it took to make the 

 measurement. The probable error therefore can be kept constant in a series 

 of radioactivity measurements by measuring the time recjuired to rack up 

 a given number of counts rather than the number of counts per unit of 

 time. This procedure is commonly used 

 in routine testing of radioactive 

 samples. 



In some applications of radiation 

 measurement, especially drill-hole log- 

 ging as described in Chapter XI, it is 

 convenient to use integrating instru- 

 ments. These devices measure the flux 

 of radiation through an ionization 

 chamber or the rate at which impulses 

 are received from a counter. The 

 statistical error of these measurements 

 will show itself as an irregular fluctu- 

 ation of the observed quantity (see 

 Figure 608. ) Again, the magnitude of 

 the fluctuation will depend only on the 

 total number of ionizing events re- 

 corded in the inherent interval given 



, ^, > J . . (-,1 , ,'1 Fig. 608. — Counting-rate meter records of 



by the characteristics Ot that particular constant rates of 570 counts/min.deft) and 



:~,(,i_,-_„p„i 8,400 counts/min. (right). Note the difler- 



msirumeni. ence in statistical fluctuation. 



