202 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



sible. For example, at S.75 per mc one could plan to use a considerable 

 amount of V^\ whereas experimental work will be limited with Cl*^ at 

 $5 per /xc. Chemical contaminants are listed where known or suspected, 

 and radiochemical contaminants are given in terms of the amount present 

 in the irradiated unit. The half-lives of the radiocontaminants are listed 

 as well as an indication of daughter relationships. (Note added in proof: 

 A revised summary will be found at the end of the book.) 



Next are listed the minimal detectable activities for a thin-mica-end- 

 window counter, an internal counter, and a scintillation counter as deter- 

 mined experimentally in this laboratory and in terms of the microcuries of 

 activity required to give a count equivalent to the background of the 

 instrument (see Chap. 5). These are arbitrary values for ordinary lab- 

 oratory operating conditions as described below but should be helpful in 

 estimating the amounts of activity required for adequate measurement 

 of a sample. For these measurements all samples were prepared with 

 essentially zero mass in a stainless-steel cup of 4.75-cm2 area, with the cup 

 edges 6 mm in height. The end-window tube was halogen-filled, with a 

 28-mm-diameter mica window of 1.4 mg/cm^. It was operated at 

 750 volts in a standard lead shield and had a background of 20 counts/- 

 min. The sample was placed 20 mm from the surface of the window. 

 The internal counter was a commercial, windowless restricted-atmosphere 

 proportional counter employing a gas mixture of 90 per cent argon and 

 10 per cent methane. It was operated at about 1750 volts. The count- 

 ing chamber, which was 55 mm in diameter, was not shielded. The 

 background averaged 100 counts/min. It should be pointed out that 

 internal counters with a smaller counting chamber or a shielded chamber 

 would have a lower background which might give sensitivities two- to 

 threefold better than those reported here. The scintillation counter 

 utilized a commercial Nal(Tl) crystal of 1-in. diameter which was encased 

 in metal as furnished. A piece of thin aluminum foil was placed over it 

 to prevent contamination, and for measurement the bottom of the metal 

 cup was centered over the crystal. The unit was shielded with 2 in. of 

 steel and had a background of 100 counts/min. In comparing sensitiv- 

 ities it should always be remembered that the beta counts will be reduced 

 by self-absorption, whereas the gamma counts will be relatively unaffected. 



The Rf/mc value represents the milliroentgens per hour at 1 ft from 

 1 mc of the radioisotope, which can be used for estimation of shielding 

 requirements, as explained in Chap. 3. These were calculated from the 

 published disintegration schemes and are therefore subject to any uncer- 

 tainties therein. To give an idea of the magnitude of self-absorption, 

 values are listed for the sample mass in mg per cm-, which gives a 50 per 

 cent reduction in counts, as measured with a thin-mica-end-window 

 counter. As indicated, some of the values were actually determined 



