Appendix B PROCEDURE USED IN CHAPTER 3 FOR ESTABLISHING THE CALIBRATION OF THE TOWED INSTRUMENTS FOR MEASURING GAMMA RAYS UNDERWATER B.1 CALIBRATION OF INSTRUMENTSIN AIR Initial plans called for calibrating the instruments repeatedly during the cruise against a large gamma ray source; however, no suitable source was found available at sailing date. In the absence of absolute standardization the measurements would still have been of value as interpolations of the measurements made by means of water samplesat fixed stations. Nevertheless, serious efforts were continued toward establishing the calibration of the instruments without reliance upon water analyses in the laboratory. The instruments were intercompared whenever possible; they were towed in pairs in contaminated sea water and exposed in pairs in the field of radiations which existed in the air above the ship’s deck due to the contaminated materials nearby. During the intercomparisons of instruments in the air over the deck the ship’s radiac instruments were read also at the same locations. The two radiac instruments (Type AN/PDR-27C) agreed well with one another, appeared to be in good condition, and might have supported some sort of an independent calibration schemehad it not been found impossible to obtain accidental fields of activity strong enough and geometrically uniform enough to intercalibrate accurately except at a few isolated intensities. Much pains were taken to keep the instruments in good order so that a calculation made after the cruise might be significant; the Mark II and Mark II instruments appeared to be in perfect order at the end of the trip; however, the Mark I instrument had to have a G. M. tube replaced during the cruise and therefore its calibrations pertaining to the cruise are quite different before and after this change. Immediately after the ship returned to Parry Island the three towed instruments were taken to the openair calibrating area which was available for standardizing radiac sets, and these were calibrated against a distant point source of radium of known activity. The results of these measurements are shown in Figures 2.1, 2.2, and 2.3 plotted in solid lines. The instruments were then boxed and shipped to the U. S. Bureau of Standards where calibration in air and against radium was repeated. Mishap during shipping caused a delay of about one month; during this time Mark I and II apparently suffered serious battery aging or other damageso that erratic behavior was exhibited during part of their calibrations, but the Mark I instrumentsatisfactorily reproduced the general character and magnitude of its calibration at Site Elmer. The dashed curve in Figure 2.2 illustrates, for example, the radium calibration made at the Bureau cor- responding to the instrument scale-range A. The 20 percent discrepancy between the Elmer curve (solid) and the Bureau curve (dashed) can be attributed to the known dropin battery voltage during the intervening time. “wee Thus, Mark I maintained reasonable constancy of calibration (against radium) after a month of rough treatment and therefore probably was well within 10 or 20 percent of truth of its Elmer calibration during the cruise. Mark II instrument was therefore chosen as cruise standard; the Mark I, Mark Il and Pot instruments’ readings also may be given absolute evaluation by means of the intercalibrations against Mark Il made during the cruise and at Site Elmer. Thus far, calibration of a limited sort only has been described. The measurements summarized by the curves in Figures 2.1 to 2.4, inclusive, relate only to the hard gamma rays of radium andarestrictly accurate only when the rays strike the instruments at normal incidence; that is, when the rays arrive normal to the axes of their cylindrical cases. Other information now must be introduced so that the effect of rays arriving at other angles at the surface of a submerged instrument can be predicted; and theory must be re~ sorted to before an estimate of the activity density in the sea can be predicted from the reading coming from 81