of the first half-hour of readings, since after that time, the instrument went off scale at 100 millirads per hour. Later Surveys One known factor concerning radioactive materials is the rate at which the radioactivity '"decays'’, or decreases. This rate is directly proportional to time. As an illustration, it is known that the activity will decrease by a factor of ten after a sevenfold increase in time. be one-tenth of one~hundredth, the original, after 24 days, Thus after seven hours, the radioactivity will after two days (7 x 7 = 49 hours), one-one-thousandth, and so on. it will be one- Consequently, when it was relatively safe, RadSafe crews returned to Rongelap and other islands and took measurements of the radioactivity there several days after the people had been evacuated. From the readings on their instruments, combined with the approximate known time when fallout ended, scientists calculated roughly how intense the radioactivity had been during the time the people were exposed. In other words, since normal decay rates are known, the present level of radioactivity known, and the rough time of when the fallout stopped was known, what scientists did was to calculate backwards, so to speak, from the period several days later, when the people were still on the island. to the time One discovery by this later survey was that the readings taken during the time of the evacuation were apparently too low by one-half, or 50 percent. used at that used. This was apparently due to the fact that the instrument time had not been calibrated (checked for accuracy) before it had been (1944, p-5) Another discovery they made concerned the nature of "mixed fission products.’ 1 The fallout was composed of many kinds of radioactive isotopes giving off gamma and beta rays of different strengths. ae The relative strength of these particles is 34