186 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



Soft -beta Counting. In order to avoid loss of sensitivity by absorption 

 of soft-beta particles in the window of the counter tube, it is essential that 

 the window have a thickness not greater than 2 to 4 mg/cm-. Thus for 

 radioisotopes such as C^'* or S^*, the thin-mica-end-window counter is 

 used. Commercial counters of this sort are now relatively rugged. 

 However, it must be remembered that the thinner the window, the more 

 fragile it will be. 



Increased sensitivity with the soft-beta emitters can be obtained by 

 the use of windowless gas-flow counters operating in either the Geiger or 

 proportional region. Essentially these are shielded counters into which 

 solid samples are inserted directly and through which there is a gas flow 

 to prevent air contamination. Comparative sensitivities for the various 

 radioisotopes are listed in Chap. 6. However, in general, the windowless 

 counter is several times more sensitive for C^^ and S'^^ than the end- 

 window counter is. These units may be operated with the regular scalers, 

 although some are supplied complete with electronic circuits, timer, and 

 register. The increased sensitivity results from the fact that there is no 

 loss due to air and window absorption and also that the geometry factor 

 is high. In purchasing these units one should inquire as to the size of 

 sample that can be accommodated, since this is restricted in many cases. 

 Measurements with the internal counter are not so simple as with the ordi- 

 nary Geiger counter. The sample must be thoroughly dry and must not 

 contain any pockets of entrapped air or solvent which may poison the 

 counting atmosphere. In many cases a static charge tends to build up 

 which decreases the counting rate and may even cause particles of sample 

 to jump onto the walls and electrode. This becomes especially trouble- 

 some if the mount is a nonconductor such as porcelain, and it is therefore 

 recommended that metal cups be used. In making measurements with 

 these instruments it is wise to make a mental note of the count at 

 the end of each minute so that any decrease in counting rate due to 

 the factors just mentioned can be detected. This type of instrument is 

 also difficult to clean in the event of contamination of the counting 

 chamber. 



If high sensitivity is required for the counting of C^^, then the vibrating- 

 reed electrometer (a type of dynamic condenser electrometer) in combina- 

 tion with an ionization chamber for gas counting is perhaps the method 

 of choice. This ecjuipment is expensive and should not be considered 

 unless large numbers of low-activity samples will have to be measured 

 (see Chap. 6 under C^"*). Au.xiliary apparatus for handling the C^*02 

 will also be required. A more inexpensive gas-counting apparatus, after 

 that of Bernstein and Ballentine (44), is now in commercial production. 

 Geiger counting tubes employing the C^^02 as part of the covmting gas 

 have been devised but have not found widespread use. 



