2.2.9 Co® Field Calibration. Exposures were made with @ well calibrated 4a, 200-curie Co® source that had an effective energy of 1.2 Mev. The source consisted of two cylindrical Co® pellets with a total height of 1.58 inches and a diameter of 0.39 inch. The pellets were gold plated and sealed in two concentric monel capsules. The source capsule was stored in a lead pig and was forced up a monel metal tube by compressed air for use. nel capsules and tube was 0.33 inch. The total thickness of the mo- Instruments were exposed on a horizontal wooden platform 3 inches belowthe level of the raised source and 2 feet above the lead pig. The source was calibrated on site using Victoreen r-chambers that had 5~mm lucite caps. These chambers were calibrated at NBS for use at 22C and 760 mm of pressure. Corrections for pressure and temperature differences were applied to chamber readingsat the timeof calibration. Corrections for decay of the source were applied to calibration curves after the calibration was completed. A 200-curie Co™ calibration curve for exposure rate versus distance is shown in Figure 2.6. Calibrations for Shots Lacrosse and Cherokee were made using an 80-curie Co™ source of simiTABLE 2.4 Station Series STATION CONSTRUCTION Material ena Diameter Wall Thickness Height Above Surface inch inch ft ramos) Depth Below Surface ft 210.0 Aluminum 3 0.25 3 _ 210.27! 10.30! a . Steel 8 0.45 2.5 6 210.23! 210.34! Steel 8 0.45 2 1 213.0 Steel 4 0.30 4 4 113.0 Steel 3 0.25 5 —_ 212.0 lar design used as a collimated source (Reference 3). Cherokee because of capsule rupture. This source was discarded after Shot 2.3 DATA REQUIREMENTS To accomplish Project 2.1 objectives, gamma-radiation measurements were required at surveyed distances from ground zero for each of six high-yield thermonuclear devices detonated at Bikini Atoll. It was necessary that these measurements should permit discrimination between initial- and residual-gammaradiation so that a true measure of initial-gamma radiation could be made. Measurements of the residual-gamma exposure rate and decay rate were required at known times for stations instrumented in a contaminated field, and after all shots to allow extrapolation of residual-exposure measurements to times other than recovery time. For those stations at which initial-gamma data were recorded, residual-field gamma exposure rate measurements were required to allow for correction of the initial data to account for the effect of the residual- gammafield. The initial exposure values are accurate to within 30 percent. This figure is based on an overall 20-percent accuracy of the NBS dosimeter for initial-gamma measurements in the energy range from 115 kev to 10 Mev and in the exposure range from 1 to 50,000 r (Reference 3). The variations in wall thickness and other possible station-shielding errors in shielding corrections amounted to about 15 percent. The error in mutual shielding effects among the instruments as 21