SDR abe ea ale anil a alatidcts fhnthe 106 TABLE 45. Batch No. | Ant | Code || Dura- || | L2878 | ! \ Man | O02 ! Man; 08 Sample | 40d | | ; 49d Ra 22Th — ' Rat ! Man: | Man: [Rat Rat i Rat | Rat 72 8&3 85 0.51: ' Vertebra® Rib™ Vertebra™ 0.430: 0.38 1.2 | Long bones | Femur | Skull 6.1 1.7 — TH2 | 18m | Long bones) Man 66 — Ly | Man 166 DOW | | SS eae ee i | Rib Femur | Vertebra®) ( | — — — 1.1 — — 9 »1 >1 1 | 1.3 0.8 — 0.53 O45 — | | we ee 2.7 — — Ra /eTh| 28TH/28Ro —— | — »>1 TH2 ' 22m © Long bones THA | 12m ‘© Long bones; TH5 » 22m | Long bones ee os Weighted| 38d , 26d , 48d 2284¢-/228Ra ne Lib! Po | ; 00940 means Feuruinrium Activity Ratios or Trortum Davestern Propucts 1x Bone | — | | 1.16 1.03 | | 1.8 1.16 1.06 | | | — L4 | | | | 1.0 | 1.4 -— -— 1.0 — — 2.2 — — 1.3 1.6 1.6 0.9 ~1.0 — 0.8 1.1 1.3 1.3 — 0.9 | 212 Bi /: | | — — — — 22 Ph /24Ra — 1.7 — 1.8 ~ — — 0.8 | 0.7 0.7 1 | — ——} ~1 ~I] ~ — - — 0.9 ———_-- 1 2) Containing bone marrow. Caleulation based on an assumed 2%Th/2%Ra activity ratio of 1.00. would be expected that manyof them, and particularly “SRa, would be translocated to other organs or exereted. Since all the decay products are ultimately derived from 7°*Th or **°Th located within the particulate phase, there is no problem in explaming the fact that some of these decay-produet atoms are physically confined «within the particles. What does require an explanation, however, is the inference from the biological data that not all the decay-product atoms remain bound with the “8“Th in this way. It is probably significant in this connection that in experiments with Thorotrast samples fron: the same hatch but in different animals and for different durations of the burden, the 7*8Ra/*8°Th ratios reported for the RES cover rather a narrow range of values (see for example Tables 42 and 43, batch 13098). The “retention” of 7*8Ra therefore appears to be a property of the Thorotrast itself rather than the animal. A plausible mechanismfor the escape of radioactive daughter products from the Thorotrast particles might be thought to be diffusion. However, the physical halfhfe of a radionuchde would then be one of the most important parameters determining its fractional retention. Consideration of the available biological data for cach thorium daughter product in relation to its physical half-life, renders this possibility unlikely (see Table 50). Particularly this is evident in the case of the two radium isotopes. *“*Ra (half-life 5.8 years; fractional retention at late times in the particles of the RES ~0.45) and *7*Ra (half-life 3.6 davs; fractional retention in the particles of the RES ~0.7), since a diffusion mechanism would require the fraction tention to depend on the inverse square root | half-life. A more plausible mechanism predicts that dai products can escape from the particles by virtue recoil energy which they gain at the momento1 creation.'**) In the case of those nuclides whic formed concurrently with the emission of an @ ra\ the parent nucleus, this recoil] energy is consid. (Table 47). By the application of standard 1 energy relationships for heavy nuclei‘?3% it ¢ calculated that a **8Ra atom, for example, has coil range of approximately 60 A in ThOs. Th portant consideration now is whether this 1s shlong compared with the size of Thorotrast pai From measurements of eleetron micrographsit ay that the particle size is not closely controlled, for been variously reported as 30 to 100 A with a mi 70 A, 80 to 200 A, and 55 + 25 A.*) Nevertheless, these are of the same order of n tude as the calculated recoil ranges of several : thorium decay products, and it is, therefore, to | pected that a substantial proportion of these would be able to escape from the particulate ph the Thorotrast. In the case of atoms formed by transitions, the recoil range is very much lower. ertheless, the recoil energy commonly exceedrequired for breaking chemical bonds (1-3 eV would thus be sufficient to allow some of the atc surface locations on the particles to break free, ¢ the change in chemical state alone were not suf