37 a ete ee reall hae tronic equipment(Figure 52). The air conditioning and dehumidification were of considerable value in maintaining the stability of the instruments in the tropical climate. The subjects were ferried out to the ship, and, before being counted, showered and donned paper coveralls and slippers in order to minimize the possibility of counting external contamination from the island environment on their bodies and clothing. The subjects were seated on a folded hospital cot and placed in a standard, fixed position under the detector, as shown in Figure 53, Figure 54 shows a Marshallese subject leaving the counting room through the pneumatically-driven sliding door. The counting geometry employedin the portable counteris identical to that used in the per- can be explained in terms of the thinnershielding in the portable counter and the omission of Pb lining for the ceiling of the counting room. To some extent the thinnershielding was compensated for by the shielding provided by the water under and aroundtheship. Since difficulties had been experiencedin identifying small photopeaks of various isotopes deposited in the Marshallese in the presenceof relatively large amounts of Cs'*’ and Zn’, the counting time was increased for a number ofsubjects over that used in previous years. In addition, a larger crystal detector was substituted for the 5-in. detector formerly used. The majority of the subjects were counted for 10 min, and a large number were counted for 30 min. An 8 4-in. NaI (Tl) crystal (Harshaw) detector was placed abovethe patient at a distance of 19 in. (see Figure 53). Pulses from three 3-in. photomultiplier tubes were fed into a Nuclear Data 256-channel transistorized pulse-height analyzer (Model 120). The analyzer fed the data directly to an IBM typewriter and simultaneously to a Tally paper punch unit, Model 420. Provision is made in this analyzerfor transferring spectra recorded on paper tapes into the memory of the analyzer so that calibration spectra can be compared with the incoming data when desired. The data recorded on the punched papertapes were transferred to IBM cards and thence to the manent whole-body counter at BNL, which makes possible the cross calibration of the two units.” The efficiency and precision of the portable counter for the various isotopes are presented in Table 16. The values obtained are very similar to those for the whole-body counter at BNL. The background observed in the Marshall Islands counting room in the range 100 kev to 2 Mev was 1796 cpm (counts per minute), a value somewhathigher than the average background level observed at BNL (1400 cpm). The increased background count in the Marshall Islands was chiefly in the very low energy range and probably Table 16 Properties of Portable Whole-Body Counter for Measuring Specific Radionuclides Photopeak energy (Mev) Energy band measured (Mev) Background (cpm) Calibration factor (C.F.) (cpm/pC) Precision (P ) of counter* (mpzC) Percent standard deviation of count rate** Cs137 Co"? Fn K 0.66 0.61-0.71 75.5 6114 0.360 0.23 1.17 1.12-1.22 29.6 4320 0.324 10.0 1.12 1.07-1.17 33.7 1733 0.866 £1.73 1.46 1.41-1.51 40.2 0.86 (cpm/kg) 3.8 (g) + 3.58 Integrated background (cpm) at 0,2 to 2 Mev 1796 *For 70-kg phantom in standard counting geometry, pV RtRei) — C.F. , where &. = Sample plus background counting rate (cpm), R,=background counting rate (cpm), ¢, = sample plus background counting time (min), and ¢, = background counting ume(min). ** Average Marshallese adult male. | i eV a gg G ( Psd ad oe U SaAARR A RT OR, EETme = ET OR mm Sap ES STR SI SAog A rg ee Oegee emg ee Se EReT eee moe ae . me