°” y or south-westerly direction, so the heavy fallout areas were island complexes, located on a coral rim surrounding 6 lagoon twenty-two miles long and thirteen miles wide primarily the islands of the south-western reef. There were exceptions to the normal wind pattern, however, (Fig. 1). Total land area of the atoll is 2-32 square miles. More than half the area is included in the three largest islands, Bikini. Eneu and Nam. The largest island, most notably for shot Bravo in 1954 when unexpected high altitude winds carried fallout eastward over Bikin: Island and on to the Marshallese natives of Rongelap Atoll. Bikini. is 2-5 miles long and 0-5 mile wide. The survey techniques utilized were largely those developed by the Health and Safety Laboratory for detailed investigations of the properties of the external Thus non-blast, low fallout areas, which include the islands Eneman Complex, experienced lesser but not insignificant amounts of local fallout. Eneu on the south-eastern mm of the atoll was the most favourably situated to avoid local fallout and exhibited some of the lowest exposure rates measured (3—7 ur./h). radiation environment in the United States’. The instrumentation included a high-pressure ionization chamber, an Nal(TI) field spectrometer system for in situ y-ray spectrometry, and a number of hand-held survey instruments (Geiger-Miller counters and scintillation detectors). In addition to this general pattern of radiation levels around the atoll, we found considerable variation on Because of the number of islands to be sur- individual islands, similar to that shown in Fig. 2 of a typical radiation profile across the middle of Bikini veyed andthe difficult logistical problems and environ- mental conditions (difficult access, dense vegetation, high temperatures, humidity and so on), we restricted spectrometer and ionization chamber measurements to representa- Island. The lowest levels were measured near the shores where the vegetation was sparse and the soil very sandy, conducive to weathering and deeper penetration of tive locations on the main islands of Bikini, Eneu and Nam. 120 - 110 a /3\ 100 + ; MS 90 Py S wl rs 70 - / & 2 et we“ee + & 5 5 4 ie _7 . \ i vO 7 < €° ° x s 50 l + so L17 a“ 7, eo" x ra e 7 7 “ - I 1 e \ a“ \LeT7 2° a” eoo \ }| e . Xx \ e XN x® e x e-—~~ ao, xf lee’ / A 1 o/ ~ \ ¢ ~ x ~~ wee * ee 7A ! XN ‘ ~“ \x! v 0 ’ fLoc 6 L0c.3 LOCs Ane 0 “ “. ° \"e. e ‘ve & gloc.2 ioc 1 ~. ° ‘ t- yo 20,2» 10 § . oN e “XN 300 Loc 8 L 600 1,200 x ~~ \ «*\ \ \ x,° ~ _4 LOC 7 1 900 e@ \ 1,500 1 1 ve 1,800 2,100 2,400 2,700 Distance along transect (feet) Fig. 2. Ex ure rate profile of a survey transect across central Bikini Island from the lagoon shore to the ocean beach. The dotted lines enclose the various measurementa with the scintillation detector (@), the Getger-MQUer counter ( x ) and the ionization chamber(). The hand-held survey meters were used to extend the survey throughout the atoll so that variations in radiation levels from island to island and on individual islands could be studied in some detail. Pattern of Radiation Distribution The external y-radiation levels were found to vary considerably from island to island around the atoll. Wecan, however, roughly classify most islands into three general areas, characterized by the relative exposure rates and also the composition of the radiation fields: blast areas immediately surrounding the ground zeros of testa where the highest exposure rates were measured, heavy fallout areas down-wind from the blast areas with intermediate exposure rates, and non-blast, low fallout areas with the lowest exposure rates. The location, code name and year of each announced nuclear weapons test‘ are indicated on the map in Fig. 1. It can be seen that the blast areas include the western tip of Eneman, Lomilik near the centre of the Aomen-Iroij Complex, and the north-western reef near Nam. The prevailing winds in the area are generally in a westerly fallout. Other significantly low exposure rate i-vels could be associated with weathered areas, such as {1 mer roadways. Higher levels were recorded in the central parts of the islands, where the vegetation was much more dense and where increased amounts of organic matt--r in the soil apparently influenced the retention of fallout near the surface of the ground. Exposure Rates and Isotopic Contributors Representative exposure rates in air 1 m abave the ground from y-ray emitters in the soil obtained from the analysis of the field spectra and ionization chamberiieas- urementa on Bikini, Eneu and Nam are given (Table 1). Exposure rates from cosmic radiation (3-4 «ar./h) are not included. More detailed data on the measurement «1 the radiation fields on Bikini Atoll are given in ref. 5. The exposure rates for Bikini Island were in general 20-40 ur./h near the share, 50-80 ur./h in the interior. and up to 120 ur./h at scattered hot spots. The field spectrometer measurements showed the exposure rate levels on Bikini Island to be due primarily to three radionuclides, with about 75 per cent of the exposure rate at a viven tn. me an of Bikini, Eneu and the eastern half of the Aeroko)j-