an age to the debris. However, a method of computation has been devised by which this latter difficulty can be minimized. Methods of Computation. The adhesive-coated films, which have been exposed for 24 hr, are shipped to HASL. Thetotal beta activity of the ashed samples is measured and corrected by the 63 per cent efficiency factor. The Sr*° component of the fallout is calculated from modified Hunter and Ballou ratios. In addition, an estimate of the infinity external gamma dose in air is made from the beta activity. Fig. 5—Cumulative Sr*® deposition in United States as of June 1957, gummed film measurements. The original calculations of Sr® deposition from measurementsof total beta activity on ashed gummed film samples were performedas follows: 1. The activity measured on a given sampling day was attributed to the test immediately preceding that sampling day. 2. The measured activity on counting day was extrapolated to a fixed day by the formula A=A,+~-1.2. 3. The Sr® fraction of the total beta activity on this day was taken from modified Hunter and Ballou curves. 4. The Sr® activity values for the individual days were summed by months, and these sums were added for the desired period of accumulation. The assigning of activity on a given day to the most recent test was a reasonable approximation during the period of tropospheric fallout. The deviations between gummedfilm esti- mates and radiochemical analyses became larger as the contribution from stratospheric fallout increased. A system to improvethe estimation of Sr®*” was devised which takes stratospheric debris into account. Tests of this simplified model yielded values that are in good agreement with computations from more complex models. This method, which has been applied to data subsequent to May 1956, is as follows: 1. Estimates of the yields of total fission products and of Sr® are obtained for each weaponstest. 2. The total fission product yield for each test is added to the calculated fission product residue from previous tests. (The + —1.2 law is used for decaying total fission product activity.) 10