50 MILLER AND SARTOR the relative worldwide disposition of all the radionuclides produced in a nuclear detonation. The ratio of the roentgens per hourat 1 hr to kilotons per square mile, where the kilotons are taken as being a measure Of the radioactivity carried by the fallout particles, is used in the integration of fallout-pattern contours to estimate the fraction of the device accounted for in the pattern. Other more detailed uses of the ratio include the effects of terrain roughness and radionuclide fractionation on gamma-radiation intensities. Some mathematical fallout models utilize this ratio in estimating standard intensities (i.e., the roentgens per hour at 1 hr values at 3 ft above a uniformly contaminated plane) in computing fallout patterns. It is generally known that the intensity—activity ratio can be defined for the intensity as observed at a given location or, as an average, for a whole fallout area, where it is sometimes called the intensity—area integral per unit fission yield. It is convenient in data analyses to express the surface density of the radioactivity in fissions per square foot rather than in kilotons per square mile. Both representations of the surface density of radioactivity are independentof time after detonation. Since the relation between the number of fissions and the energy released in fission is about the same for most common fissile materials, namely, (1.45 + 0.03) x 10”3 fissions per kiloton of fission yield, the two representations of the surface density of radio- activity are related by A,= 5.20 x 10°A,, (1) where A; is in fissionS per square foot and A, is in kilotons per square mile. The intensity —activity ratio for a given location in a fallout region is defined by I, =z (2) where I, is an observed or a measured value of the (standard) intensity in roentgens per hour at 1 hr at 3 ft above an extended open flat area uniformly covered with fallout particles carrying the appropriate amount of radionuclides to result in the surface density, A;, in fissions per square foot or the equivalent surface density, A,,, in kilotons per Square mile, as given by Eq. 1. The standardintensity, I,, is usually determined from a measurement of the roentgens per hourat the location after all the fallout has been deposited and a decay correction of the observed intensity to the standard time of 1 hr after detonation has been made.