tight gland at the point where the cord entered the cylinder prevented water from reaching the radiac instrument. The range of the radiac instrument could be set and its batteries turned on and off by manipulating a water-tight shaft that ran through another packing gland on the cylinder. The Mark I was designed for vertical casts; it was provided with a rope line to strength~ en the meter cord. The instrument was unlike the Mark I in that its cylinder was made of standard 3-inch brass pipe; the Geiger tube had thick stainless steel walls and the Mark I circuit was provided with seven range settings. Since the Mark I maintained its calibra- tion surprisingly well and since its polished brass shell made it easy to clean, it was frequently used as a temporary standard for checking the calibration of the other instruments. A line drawing of Mark II is included in Figure 2.1. The Mark DI was encased in a copper cylinder that had a diameter of about 4 inches and a wall thickness of the inch. Its counter was a glass Geiger tube inside a brass pro- tective shell. All three of these instruments were read on microammeters located on the deck and connected to the counters by long leads. The meters were located in reasonably sheltered positions on the deck and were encased in transparent plastic bags. No equipment was available for making continuous or automatic recordings of the readings. simply read at short intervals during the survey. The meters were Figure 2.lisa line drawing giving sectional view of Mark II and showing internal con- figuration and location of important parts. The Geiger tube itself had a heavy cylindrical metal wall and the thin beta window on the end was kept capped; it was located on the axis of the pressure cylinder. This Mark I instrument was constructed from components taken mostly from a radiac device of type AN/PDR-27C. walls. The tube was Navy type 3S-1, a type having heavy metal Its responses to calibration will be discussed in detail later. 2.1.2 Calibration of Instruments. Efforts were made during the cruise and immedi- ately after it to collect all possible data needed for establishing the calibrations of the gamma instruments. Instruments were frequently intercalibrated at sea. The towed instruments were compared against the ship’s official radiac handsets at a few isolated intensity levels. The instruments were taken ashore at Site Elmer immediately following the cruise and calibrated throughout their full range of response against a point source of radium of known strength. Figures 2.2, 2.3, and 2.4 are the results of the calibration of the towed instruments at Site Elmer against radium. Figure 2.5 is the estimated response of Mark I instrument prior to the date when its Geiger tube went bad and had to be changed. This curve was derived from the more reliable calibration of Mark II and from records of intercalibrations at sea before and after the tube was changed in Mark I. The preliminary calibrations relate only to the use of the instruments under certain limited circumstances, the use of the instruments in air, and the measurementof fairly hard radiations. The derivation of a comprehensive celibration pertaining to the use under the actual field conditions required considerable additional experimental work and computation. The procedure used for establishment of this final realistic calibration is the subject of Apperdix B. The net outcome of the later study is the conclusion that the approximate gamma intensity under water and due to mixed fallout activity can be derived by applying to the 18