‘ soften with time. The change is rather small. effective gamma energy is a little over 1 Mev. Initially, the mean After 10 days the energy of the major portion of the isotopes present shows two peaks, one at about 0.5 to 0.7 mev and the other (and smaller) at about 1.3 mev, with very little change at later times. It is sometimes useful to speak of quantities of specific isotopes formed and distributed because of differences in physical, chemical or biological properties which might be of interest for a particular problem. The average weight of any isotope formed can be found by utiliz- ing the percentage yield curve. The yield in curies per gram can be estimated to an approximation by the formula ———————~ AT, 7-66 x 1077 1/2 ,,, where A = atomic weight; qTJe = radioactive half-life of the isotope in days. Thus, to use the three biologically important isotopes previously cited as exemples, there will be 1.24 x 10° curies of iodine-131, 2.62 x 10! curies of strontium-89, and 2 x 10° curies of strontium-90 formed per KT of fission yield.: From the number of curies of activity calculated for the three examples used, it can be seen that the half-life of an isotope is a very important factor in the calculation. As a consequence, the bulk of the gamma radiation emitted early in the fall-out field is due to the lerge number of curies of energetic gamma emitters from a relatively small percentage yield of short half-lived isotopes. The fusion process does not contribute to the quantity of radio- active materials which are available for fall-out except those formed from induced activity, which will be discussed later. It does make a measurable contribution, however, to the low level of tritium which is normally found in the atmosphere resulting from cosmic radiation. This quantity of tritium is approximately’ a per Mf of fusion energy re- lease (1.24 x 10° curies per MT). The possibility of dangerous concen- trations of tritium in the atmosphere is very remote because of the great dispersion effect of the winds, the tendency toward upward diffusion of light gases, and the slow reaction rate of tritium with oxygen to form water.