be identified and the required sample was not collected within the flight time available, Needed for planning purposes was an "optimum time" at which an acceptable radiation exposure would not necessarily or accidently be exceeded but at which it would always be possible to collect the required sample, Experience from a number of sampling missions established that these conditions were well fulfilled at two hours after burst for both jet fighter and bomber-type aircraft. Probable relative errors arose from uncertainties in the operational data (the chief uncertainty being in the radiation exposure used which is determined by film dosimetry) and the approximations used in its calculation, Although the above values were equal within the probable relative errors, the observed small difference between them was due to the location of structures with better radiation scattering properties closer to the pilot in the fighter-type compared with structures located near the average crew position in the bomber-type aircraft, With known constants, it was possible to obtain good approximate evaluations for any type aircraft carrying any type of filter papers and to find values in advance of operational use, For particular aircraft and associated filter devices it was sometimes convenient to express equations ‘ in the form of the curves shown in which the amount of explosion debris collected was a function of cloud penetration time for the two types of aircraft discussed. The curves were based upon altitudes and indicated airspeeds at which the aircraft were frequently used (25,000 feet and 200 miles per hour for B-29, and 35,000 feet and 300 miles per hour for the fighter-type) as well as upon a reference in-cloud exposure of 1.0 roentgen, 209 AFWL/HO SWEH-2~003), -s