Sec. 13.5] STANDARDIZATION OF RADIOACTIVE SAMPLES 



363 



account a number of corrections. Without going into detail these are as 

 follows: (1) Subtract the background from the single and coincidence counting 

 rates measured. The coincidence background is due mainly to cosmic-ray 

 particles which pass through both counters. (2) Subtract the counting rate 

 caused by the gamma rays in the beta counter. The beta particles usually do 

 not get into the gamma counter. (3) Correct all counting rates for finite 



0.03 



0.006 



1.0 20 



BETA ANP GAMMA ENERGY IN MEV 

 Fig. 101. Typical counter efficiency curves obtained with a UX 2 standard and by the 

 coincidence method. The curves indicate the over-all efficiencies for the particular 

 counters and geometry used in the measurements; consequently, the curves are influenced 

 by geometry, scattering, and absorption as well as by the absolute efficiencies of the 

 counters for responding to each incident particle or photon. [Unpublished data of II. 

 Anger.] 



resolving time of each counter. (4) Correct the counting rates C for acci- 

 dental coincidences between beta and gamma counts. (5) Finally, evaluate 

 the statistical errors by the method of propagation of errors. All these 

 corrections will influence the over-all accuracy of measurement. Primary 

 standardization requires prolonged counting in order to reduce errors due to 

 statistical fluctuations. The best available determinations of absolute dis- 

 integration rates have about 1 per cent probable error. The technique of 

 coincidence counting measurements is rapidly being improved [11-13]. 

 The use of fluorescent counters appears to increase the sensitivity and 



