"THORIUM B" IN BLOOD VOLUME STUDIES 593 



3. The procedure for calculating blood volume is described in detail. In prin- 

 ciple it consists in comparing the radioactivity of original whole blood samples 

 with that of a sample removed from the patient after dilution. It requires no calcu- 

 lations of cell — plasma ratios; ot;ntrifugat ion is unnecessary. 



APPENDIX 



Radiation Protection and Radiation Exposure 



Among the components of the active deposit of thorium, produced through 

 decay of thorium emanation, is found thorium C ' which emits hard y-rays. 

 In view of the penetrating rays emitted by the radiothorium sample, the operator 

 must be protected from the effect of this radiation. Placing the glass tube contain- 

 ing the sample in the center of a lead block 4.5 by 4.5 by 4.5 cm., reduces the 

 intensity of the y-rays emitted to about one-eighth, as 1.5 cm of lead cuts down 

 the radiation by half. 



A still more effective precaution is to increase the distance between the sample 

 and the operator. While the y-radiation of radiothorium having the activity 

 of 1 mgm of radium produces, at 1 cm distance, a dose of 8.6 roentgen equivalent 

 physicals (rep) per hour, at a distance of 100 cm, only 1/10,000 of that dose is 

 produced. 



When passed through the blood sample, the oxygen still contains some thoron. 

 Though, owing to the short hfe-time of thoron, the activity released into the atmos- 

 phere is rather restricted, it is advisable to lead the oxygen stream which has 

 already left the blood through an aggregate of wash bottles containing olive oil 

 or some other vegetable oil, before releasing it into the atmosphere. While the 

 distribution coefficient of thoron between water and air at 20° C amounts only 

 to 0.26, the corresponding figure for ohve oil and air is as high as 28. 



Radiation dosage is always an important consideration when applying radio- 

 active indicators. Owing to the short half-life of ThB, the patient is exposed 

 to radiation for a much shorter time than in administering ^^F, for example. 

 This difference is partly offset by the fact that the disintegration products of 

 ThB emit a-rays, which are, in the mammalian tissue, more effective in producing 

 radiation effects than the less densely ionizing /9- and y-rays. The maximum 

 number of rep produced in the body of a human subject weighing 70 kgm by 

 ThB and its disintegration products having the activity of 1 mgm of radium is the 

 following : 



The emits a-particles having an energy of 9.42 X lO"^ erg. A dose of 1 roentgen 

 equivalent physical (rep) corresponds to the absorption of 93 ergs/gm tissue. 

 This dose is thus produced by 0.94x10^ a-particles; 1 microcurie emits 3.2x109 

 a-particles per day, producing 3.4x102 rep. Assuming the weight of the human 

 subject to be 70 kgm, 4.8 X 10-^ rep per gram are produced. 



We stated above the number of a-particles emitted by 1 microcurie in the cours 

 of a day. The effective half-hfe of ThB is, however, only 0.31 day. Furthermore, 

 35 per cent of the ThC atoms disintegrate only under emission of a-particles. 

 The number of reps, produced by the a-particles of ThC in equihbrium with 1 micro- 

 curie of ThB thus works out to 0.52 x lO^^ rep./gm. 



One microcurie of ThC, 65 per cent of which disintegrates under emission of 

 a-particles has an energy of 1.46 x 10-^ erg, produces during its hfe-time 1.4 X 10"' 

 rep/gm. 



The biologic effect of the densely ionizing a-radiation is appreciably larger 

 than that of ^- or y-radiation producing the same number of ions. To account 



38 Hevesy 



