174 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



The usefulness of rigorous statistical treatment of counting procedures 

 in most biological studies is open to (luestion because (a) the errors in 

 other parts of the study usually exceed those of counting, (6) there is gen- 

 erally time to count the samples to give reasonable total counts, and (c) 

 very often the experiment can be performed so as to have relatively high 

 counting rates in the samples. Where the samples have low counts, less 

 than background, one must always bear in mind the possibility of con- 

 tamination before assigning significance to the results, even though this 

 may be warranted by the counting statistics. 



Comparison of Counter Sensitivities. In comparing counter tubes, 

 attention may be given to such operating characteristics as the change of 

 counting rate vs. voltage, the resolution time, and the number of spurious 

 pulses. However, most commercial counters are satisfactory in this 

 regard, and perhaps a more important factor is the sensitivity of the tube 

 for the radiation to be measured. Thomas (36) has presented equations 

 for the calculation of counter sensitivities and has discussed some of the 

 factors that must be taken into account for a rigorous comparison, such as 

 the counting rates involved and the optimum time allocation to sample 

 and background measurement. The important point is that the over-all 

 sensitivity of the counter is largely dependent upon its sensitivity to the 

 radiation of the source as compared with the background. As already 

 indicated, with weak samples the time required for reduction of back- 

 ground error may be considerable. 



In this text comparisons will be simply expressed in terms of the micro- 

 curies of a given radioisotope reciuired to give a count eciual to the back- 

 ground, which we shall designate as the minimal detectable activity. For 

 example, if the MDA of a given counter with a background of 20 counts/ 

 min is hsted as 1.8 X 10"^ nc for Co^^, this means that 1.8 X 10"^ ficCo^^ 

 would give a net count of 20 counts/min when measured. The micro- 

 curie values are usually derived from the supplier of the isotope, and 

 although they may be shghtly in error, the comparisons between the 

 instruments listed in Chap. 6 will still be valid, because the same shipment 

 of isotope was used to make the comparison, and any errors would can- 

 cel. Expression of sensitivities in this manner, as is done for the individ- 

 ual isotopes in Chap. 6, gives a fairly good picture of the abihties of differ- 

 ent counters and also allows estimation of the amount of activity that 

 will be required in the samples. It should be noted that with this method 

 of expression the higher the MDA value, the less sensitive the instrument. 



The term geometry is often used in connection with counting-tube 

 arrangements and refers to the percentage of rays leaving a sample which 

 reach the sensitive volume of the counter. Thus a gas sample contained 

 within a counter would have 100 per cent geometry, whereas a solid sam- 

 ple in the usual internal counter might approximate 50 per cent geometry 



