the Project 2.2 instruments. The Conrad detectors were calibrated with the 200-curie Co® source and the 250-kv X-ray generator. ‘The initial-gamma instruments, the Gustaves and the photomultiplier feedback-circuit detectors, were calibrated with the 250-kv X-ray and the 2.5-Mev Van de Graaff generator. The 250-kv X-ray machine was operated at an applied potential of 250 kv, and 10-ma current. The X-ray beam was hardened with 1 mm of cadmium filtration to give an effective energy of 190 kev. The instrument response to this beam was the same as for Co® since the instrument response was flat to below 125 kev. The maximum (OM i 45V sy " iM [ IM 1M A tM 4 IM 4 “bi ~~ 82K tM LIS Phosphor 22M Figure 2.5 Schematic diagram showing the photomultiplier feedback circuit of the initial-gamma detector system. usable-exposure rate attainable with this X-ray generator (consistent with good geom- etry) was 6,400 r/hr. The Van de Graaff generator was operated at 1.0, 1.5, and 2.0 Mev, resulting in a maximum rate of 10° r/hr. The 200-curie field calibrator was specifically designed for operation under EPG weather conditions. The main components were the source container and the contral trailer. The source container was made of stainless steel] and the plug-and-rise-tube assembly of Monel metal. The source, inclosed in a double-walled Monel capsule, was raised and lowered pneumatically and was supported by three spring-loaded pins, one of which actuated a microswitch to indicate when the source was up. The Cowas in pellet form and filled a space 0.39 inch in diameter and 1.58 inches in length. The Mcnel metal shielding (capsules and rise tube) was 0.33 inch. The 24