182 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



for self-absorption are required if a windowless counter is used (42). This 

 is primarily because of the window and air-space attenuation in the case 

 of the end-window counter. It may be noted in Fig. 5-8 that, if the 

 values for the internal counters are calculated on the basis of an A^'o value 

 of 5.9 mg/cm-, which represents the air and window thickness, then the 

 self-absorption correction is the same for both types of counters. 



It must he emphasized that this type of curve cannot be used as the mass 

 increases to the poi7it where there is little change of N/No with sample mass, 

 because, as the sample gets thicker and thicker, the radiation from the 

 lowest layer is essentially absorbed completely by the upper layers. Such 

 a sample is called infinitely thick. The curve decreases from that point 

 on simply because of dilution of the activity ])y the mass and not at all 

 because of self-absorption. 



Infinitely Thick Samples. If the sample measured is sufficiently thick 

 so that the radiation from the lower layer is completely absorbed by the 

 upper layers, then self-absorption corrections are not necessary. Further- 

 more the counting rate depends only upon the concentration of activity in 

 the sample and not at all upon the amount of sample measured. This 

 may simplify the procedure, since it will not be necessary to know how 

 much sample is actually being measured. In this case, however, it is 

 necessary to construct a standard curve in which the counts are plotted 

 against the known concentration of radioactivity, all measurements being 

 made at infinite thickness. As a rule of thumb, infinite thickness, so far 

 as activity measurements are concerned, approximates 0.75 times the 

 range, which is 0.75 X 64 = ^ 50 mg/cm- for Ca*^. 



It is important to remember that, although under these conditions the 

 counting rate is unaffected by the thickness, the surface of the sample 

 should always be about the same distance from the window of the counter; 

 this becomes increasingly critical at shorter distances. Disadvantages of 

 this method are that the over-all sensitivity is decreased, because there 

 are portions of each sample which are being counted inefficiently or not 

 at all, and there are often difficulties in handling the large amounts of 

 sample necessary to give infinite thickness. It will be noted that most 

 solution measurements actually fall in the category of infinitely thick 

 samples. 



In summary, the primary considerations in the choice of chemical treat- 

 ment of the biological sample are as follows: (a) ratio of activity to mass 

 of sample as measured, (b) physical uniformity of sample, and (c) amount 

 of activity in sample. Table 5-4 illustrates some calcium/mass ratios for 

 typical biological materials. It is clear that there would be a considerable 

 gain in sensitivity by separation of calcium oxalate from samples such as 

 urine, soft animal tissues, feces, and plant material, whereas there would 

 be no particular advantage in making such separation from bone. Obvi- 



