" yoentgens, when an strerartlth a speed of at least 0.8 mec.ls used, and when. gampling is performed later than two hours after shot time. c. : Collection Times As mentioned in caragraph 1, the amount of radivactive material collected for a fixed pilot exposure (3.5R total) increases with how much radio- active decay has taken place by the time sampling is performed. Opposing delay ‘’ dn samoling is the risk of failure which increases as cloud dircersal takes place - according to the effects of the vertical velocity gradient and angular shear of the wince structure in the region of the desired altitudes. J'or Mike Shot on IVY a slow rate of cloud dispersal due to a favorable wind structure permitted suc> effective three hours after shot time. Later samples had to be collected at lower altitudes where the wird structure did not disperee the cloud so rapidly. Although dispersal effects can be estimated from the wind structure predicted for shot time, a wind structure which is predicted to be unfavorable may not offer a very powerful argument for a change in shot schedule. In order to achieve maximm sanple sizes, operational planning for CASTILE cloud sampling should be based on actual cloud penstrations conducted in three phases over the following approoriate tine intervals: lst phase: H/ 2:00 to HY 3:00 Hours 2nd phases Hf 3:00 to BY 4:00 Hours 3rd phase: Hf 4:00 to Hf 5:30 Bours These times sre chosen on the basis of favorable cloud dispersal and radiation intensities as the minimum times to reach an in-cloud rediation exposure equivalent to a total exsosure of 3.5R on landing. In the event of unfevorable wind structure, the first flight will certainly collect the desired - samples, the second will orobably do so, and if necessary the third can be flown at other, vossibly lower, altitudes where the cloud persists longer, On the basis that the above minimm time for the third flight may frequently be insufficient under operational conditions, a longer flight tine would be desirable. Since cloud diszersal appears to be lees rapid at 50,000 feet or above, a higher altitude would also be desirable for this flight, if this were possible. An alternative to the above schedule, in the event of an extremely unfavorable wind condition, would be to start to obtain all samples at not later than two hours after shot time and to sacrifice a possible increase in sample size. d. Collection Altitudes The degree to which a sansle is representative of the total bomb detris is normally found to depend on how far below the main cloud mass it is collected. For this reason, it is desirable to collect samples either in the main cloud mass or as close to it as possible, For bombs with yields up to 500 kilotons, and perhaps to 1000 kilotons, satisfactory sampling of the main cloud mass can be achieved at altitudes up to about 15,000 feet true (usually not lower than 35,000 feet except for very small yields) on tests conducted at the Eniwetok Proving Ground where the meen tropopause height is about 55,000 feet. For the higher yields of the really "super" devices the desirable sampling altitude should be at least 55,000 feet to permit collection of material more representative of the main cloud mass. “Kos wD cessful sampling as late as six hours at about 17,000 feet true altitude, while for King Shot that portion of the'main body of the cloud which lay between 38,000 and 18,000 feet dispersed so rapidly that samoling at these altitudes was in- - '