fractionation can occur in a number of ways and that environment has considerable effect upon the concentration of various radio-elements of biological importance. The specific importance of fractionation in certain hazard calculations will be discussed in a later section. G. Areas Involved and Material Available. The area involved in fall-out resulting from a surface or near surface detonation depentls upon the energy yield of the detonation, the height of burst, velocity and direction of winds at all altitudes up to where the mushroom cloud atabilizes, and the type of bomb. As was in- dicated in a previous section, the yield of fission products, irrespective of the total energy of the weapon, determines the quantity of available radioactive materials which can be deposited as fall-out. Thus - As compared to a weapon with 5 MI fission yield and 5 MI fusion yield, the physical mechanism involved in each case would be approximately the same but the relative intensities of radiation in the area would be considerably higher, possibly by as much as two orders of magnitude, for the 5 M? fission plus 5 MI fusion yield weapon. After fall-out of radioactive debris occurs, the radiological situation may be ascertained by use of radiac instruments. These may be used on the ground, taking the readings at about 3 feet above the surface, or from a low flying plane at 200 to 500 feet above the surface and applying suitable correction factors which depend on the altitude of measurement and the instrument used. In any event, readings are taken at specific points and plotted on a map at various times following the end of the fall-out. The final situation cannot be known dur- ing the period that fall-out is occurring. During fall-out the final radiological situation can only be estimated when the height of burst, the fission yield, the total yield of the weapon, and the wind history since burst time at all altitudes where the cloud has gone, are known. The true situation can be known after fall-out is completed and all readings are made and intensities eat specific times after detonations 34