ASSAY OF RADIOACTIVITY 



(Nq), since it can be shown that the relationship between these quantities is 



llNo=llN-T 



(if a table of reciprocals is available, this is probably the most convenient 

 form of the relationship). However, complication may arise through varia- 

 tion in T, since for the Geiger tube itself the dead time varies with operating 

 voltage, being perhaps twice as great near the starting potential as it is at the 

 upper end of the plateau. This difficulty can be overcome by determining r 

 under the standard operating conditions, by counting two samples, one of 

 them a very active one, whose relative activities are known precisely before- 

 hand (e.g. by a known dilution): but this determination will have to be 

 repeated for each individual Geiger tube, and it may be preferable to work 

 with a longer, but strictly reproducible, dead time determined by a quench 

 probe. In this case the value of t can be measured with two samples as before, 

 measured with a multiple pulse generator, or (which is most convenient of 

 all) pre-set by the makers of the quench probe. 



Whether or not a quench probe is employed it is advisable not to work at 

 such high counting rates that large dead-time corrections are called for. 

 Counting rates can usually be reduced to manageable proportions by divid- 

 ing up the samples or by placing them further from the Geiger tube. 



For the sake of completeness, other obvious sources of error in radio- 

 activity measurements are as follows : 



Decay of the isotope — It often happens that measurements are spread over 

 a period of the same order as the half-life of the isotope. In this case it is 

 necessary to note the time at which each count is made, and subsequently to 

 correct all results to the same zero time, using the value for the decay constant 

 of the isotope. It is generally advisable to check for each isotope sample 

 that the rate of decay is what it should be; this provides a guard against 

 contamination by unwanted isotopes. It may also be noted that differences 

 in half-life can be used as a means of distinguishing between the components 

 of mixtures of isotopes in multiple labelling experiments. 



Positioning errors — Radioactive samples emit their radiation unselectively 

 in all directions, but it is only the radiation which penetrates into the sensitive 

 volume of the Geiger tube that is recorded. The counting rate obtained for 

 a given sample is therefore roughly proportional to the solid angle subtended 

 by the counter at the sample, and may be considerably altered by small 

 movements of the one relative to the other. In making absolute measure- 

 ments of the radioactivity of a sample, determination of the effective solid 

 angle within which counts are recorded is a matter of some difficulty, but 

 this problem does not arise in most tracer experiments since the measure- 

 ments usually made are relative ones, the activity of an unknown being 

 compared with that of a standard sample. It will be clear, however, that 

 positioning errors will arise unless care is taken to see that all samples and 

 standards are counted in exactly the same position. Alternatively, if the 

 total solid angle for counting can be increased to 4?? or near it (e.g. by 

 placing the sample within the sensitive volume of the counter tube or by 

 surrounding it with a ring of tubes), positioning errors will become less 

 important. 



434 



