18 WORLDWIDE EFFECTS OF ATOMIC WEAPONS ORIGIN AND NATURE OF RADIOACTIVE DEBRIS Table 1 particles as Rb’ or Sr°® until after the solid particles have been formed and cooled. These facts have many ramifications in the consideration of the effects CONTRIBUTION OF ELEMENTS AS PERCENTAGE OF TOTAL ACTIVITY AT VARIOUS TIMES* Element Kr 10 sec 20 sec 1 min t hr 1 day 1 week 45 — _—— 18 15 10 Xe 14 14 13 3.5 18 15 I 20 15 9 € 19 i? Rb 13 12 . 12 5 — —_—— Br 8 8 6 1 —— —. Cs 7 12 17 G —— — Sb 5 6 7 ——~ —_— — Te 4.5 6 6 12 4 8 12 1.5 9 La 3.5 5 7 Sr 2.0 3.5 6 4.5 6 2 Mo 1 1.5 2.5 3.5 4 9 Nd 1 1 — 1 — 4 Y —- 1 2 13 19 Ba --—— — — 10 1 2 —— Pr — — —_— 6 3 8 Ce —— —— —_—— 5 6 8 Zr — — —- — 9 3 Nb — — —_—. — 9 Rh — —— — —— — 3 Ru —— —- —— —- — 2 ——_ “Those elements contributing less than 1 per cent of the total are indicated by dashes. 19 of Sr°". Particularly important are the following: 1. A large fraction of the mass-90 chain is condensed minutes after detonation, so that the solid particles formed from the fireball have become very dilute and are mixed with inactive solid particles of the air. Thus Sr°° may be fairly well separated from most of the other fission products and its detection by counting total activity may be more difficult. 2. The Sr° produced in this manner ts much more available in the biosphere, since it will be in very soluble form on the surface of the particles or actually dissolved in the water droplets of the cloud. Asindicated by the large percentage of the total fission products present as rare gases or other volatile elements at the time of initial condensation, the mass-90 chain is only one example of substances that will condense late in the history of an explosion. Other mass-number chains of possible importance are the 89, 91, 137, 139, and 140 chains. The time scale and condensation times discussed above refer to 20-KT bombs. When larger or smaller bombs are involved, the time to reach a given temperature can be scaled approximately by the factor (E/20)%* to obtain the time for a bombofyield E.* Thus if we take 2500°K as the condensation temperature, the time of condensation for a 1-MT bombis about 25 sec. Therate of cooling is also much slower for a higher-yield bomb, so that condensation can occur under the mass-90 chain. Most of the material that eventually ends up as Sr°° is produced in the primary fission as Kr°° (lesser amounts of Br°* dnd Rb* are formed and a very little Sr’) and must go through the decay chain: short 33 Sec 2.74 min’ 19.9 yr 61 hr (Bre) > Kr? 5 Rb? -» S90 _, Yor 5 Zr", Most of the material, therefore, spends the first few seconds after explo- sion as Kr°° and hence has no chance to enter into the growth of solid circumstances corresponding more nearly to equilibrium conditions tending to give larger particles. On the other hand, since the density of condensable material is inversely proportional to E, large values of E tend to . . Haalealltel on the particle size. The experimental data on particle size are not suffi- ciently reliable to determine how the particle size varies with yield. One *Vacious scaling laws have been suggested, but this law seems to correspond most nearly with the observed facts.