"THORIUM B" IX BLOOD VOLUME STUDIF;S 



:83 



dose, we prefer to compare the activity of dry blood samples placed in 

 an aluminium dish, and use thin window counters as when measuring 

 the activity of i^C. 



The approximate half-value thickness of the ^-rays emitted by ThB 

 and its disintegration products in aluminium is as follows: 



ThB (lead isotope) 0.10 mm 



The (bismuth isotope) 1.2 mm 



The" (thaUium isotope) 0.77 mm 



Of the disintegration products of thoron, ThA decays in a half-time 

 of 0.16 second and can thus be disregarded. ThB, which has a half-time 

 of 10.6 hours, takes about 6 hours to come into exchange equilibrium 

 with its disintegration products. This is a fact of importance in inter- 

 preting the activity figures obtained for ThB-labelled blood samples. 



After leading thoron through the blood for 20 minutes, for example, 

 the maximum ThC activity of the sample is not obtained until after 

 the lapse of 200 minutes, as shown by Table 1. In the succeeding 100 

 minutes, the activity decays by 3 per cent; later a decay with a half- 

 time of 10.6 hours sets in; thus after the lapse of 21 hours, the activity 

 is reduced to Y^, after the lapse of 42 hours to 7i6 ^^^ after 63 hours 

 to Yed- These figures indicate the maximum fraction of the injected 

 ThB still present in the organism. The actual fraction present is, however, 



Table 1. — Change or the Activity of the Blood with Time after 20 Minutes 

 Exposure to a Uniform Thoron Stream. (Only the Activity due to the 

 Rays Emitted by Thorium C and Its Disintegration Products Is Considered) 



lower than the figures stated, as some ThB is lost by excretion as well 

 as by disintegration. 



Activation of Blood 



We obtained the best results by the following procedure: Through 

 10 ml of freshly drawn heparinized blood, an oxygen stream of about 

 30 ml per minute percolates for 20 minutes after it has passed through 



