three isotopes of biological importance. The teats of megaton yield weapons during the CASTLE series revealed that’ These relative fission and fusion values will change with weapon design, so that in the future more of the total energy may be derived from the fusion process, resulting in less radioactive bomb debris. As an approximation valid for current U.S. thermonuclear weapons, it is estimated that 3.0 x 10% curies of gamma radioactive material will be available from a~ burst thermonuclear weapon at one hour after time for some mechanfam of deposition, either as early or as late fall-out. In the case of thermonuclear weapons|_ the general shape of the contow pattern involved in fall-out will depend upon the total energy yield, but levels of radiation will depend upon the mixed fission product yield. The radioactive decay of mixed fission products has been found to follow the forn I= 1,t7'**, in which I = radiation reading at time t in roentgens per hour; aT = reading at one howr in roentgens per hour; t = time in hours after initial reading. All radioactive isotopes de- cay at a specific rate determined by the half-life of the isotope, and the total fission product decay law is the weighted mean of all half- lives found in the mixture. Thus, of the total number of isotopes formed, half-lives are found to vary from a fraction of a second to several years. The I = r,t7°, or t+? aecay rate as it is frequent- ly stated, holds rather well for times up to 120 days; then, gradually, the half-life decay of specific long-lived isotopes begins to predominate, particularly after one. year. Because of the change with time in specific isotope concentration, a change in the over-all energy spectrim of the mixture would be ex-= pected. This in fact does occur, and the gemma radiation is found to 8