sepuege vewedewse tut wok me, The values presented on Figures 3 thru 17 and used in curputing cumilatiwe and peak radiation are net radtaticn valves, 1.66 background haa been subtracted from the cserved atoli intensity. Tt sheuid be noted that low intensity values my be considerably in error where background levels are cf the same ordar az the measured atoll intensity. measurement with a portable gamma instrument rogistera activity emitted from an area of a few square yarda while the SCINTAMETER * at an altitude of 200 feet or more sees an are of rerhaps 10,000 to 15,000 square yards. Tt 1g well known that measurezents on the grount will show con- siderable variation over a relatively small area. This was parti- cularly evident on Eniwetok (Parry) after the late fallout carried back by tre low level trades after ROMEO. Gamme intensities in the open over horizontal surfaces were up tc two tines greater than intensities in the lee of large obstructions. Similarly, measurements near the windward side of vertical surfaces were greater than neasurements over open horizontal surfaces. a ' as ae f&fter BRAVO, survey parties reported substantial variations in outside radiation measurements on all of the islands surveyed. Generally, one aerial measurement should approximate the average of many individual outside ground measurements taken over the same general area, however, the factor of instrumentation mst be recognized as a yariable. The energy response characteristics of portable instruments commonly used during CASTLE differ from each other somewhat and differ from the SCINTAMETER rather markedly. The response of the TIS, for instance, is nearly flat above 0.1 Mev. The characteristics of the AN/POR 27C are somewhat lees uniform but above 0.3 Mev are reasonably flat. The SCINTAMETER, on the other hart, peaks at about 0,25 Mev and has a uniformly decreasing re- “ sponse from the peak as the gamma energy increases. The charactcr~ 4atice of the three instruments are pigtted in Figure 30. If the or red.um source, as in inatrumenta are all calibrated with C -48- m+ 5 io ee te _ Be oe In the like manner, aerial measurements can bedistorted vy uneven terrain, scanning the lee or windward side of a mountainous island, and perhapa other factors. t ’ 4 single ground level ' tee eal’. a : aes. rigstow snenct —_ — areas scanned by the two aethods of survey. taeeqcts ef atone waren) °° C,. Relation of Aerial Mesmrenents to Ground Level Intensities. Certainly one intrinaic factor Limits the agreenent which may be achisved between any particular pair of corresponding aerial and ground measurements. ‘This is the vast difference in the effectivs . 5 SURVEY INSTRUMENTS | roe codes wee gee . - % sate wae dere de ec abes aoeed' 8g, 30