‘ Later Surveys One known factor concerning radioactive materials is the rate at which the radioactivity "decays", or decreases. As an illustration, This rate is directly proportional to time. it is known that the activity will decrease by a factor of ten after a sevenfold increase in time. Thus after seven hours, the radioactivity will be one-tenth of the original, after two days (7 x 7 = 49 hours), one-hundredth, after 24 days, one-one-thousandth, and so on. ona scale at 100 millirads per hour. ~~ * since after that time, the instrument went off ooh of the first half-hour of readings, d fy it will be one- 44 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 4 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. since normal decay rates are known, In other words, 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, to the time q i 4 | i { j One discovery by this later survey was | that the readings taken during the time of the evacuation were apparently too low i when the people were still on the island. by one-half, or 50 percent. used at used. that 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.” L The fallout was composed of many kinds of radioactive isotopes giving off gamm: r The relative strength of these particles moe and beta rays of different strengths. 94 J :