External Radiation Levels on Bikini Atoll - May 1967

Page 2

This report was prepared as an account of Government sponsored work. Neither the United States, nor the Cormnission,nor any person acting on behalf of the Commission: A. hlakesany warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus> method, or process disclosed in this report. As used in the above, “person acting on behalf of the Commission” includes any employee or contractor of the Commission, or employee of such contractor, to the -tent that such ewloyor contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor. Printed in the United States of America Clearinghouse for Federal Scientific and Technical Information National Bureau of Standards, U. S. Department of Cotomerce Price: Printed Copy $3.00; Microfiche $0.65

Page 3

EXTERNAL RADIATION LEVELS ON BIK-INI ATOLL - MAY, 1967 Health and Safety Laboratory New York, New York Division of Operational Safety

Page 4

An intensive radiological survey of the islands of Bikini Atoll was conducted in April-May 1967 for the purpose of determining the levels and components of the external gamma radiation fields in this former weapons testing area. Fourteen islands and the two island complexes of the atoll were surveyed with instrumentation which included a field gamma spectrometer systemj a high pressure ionization chamberj scintillation and G.M. survey meters, and thermoluminescent dosimeters. A large number of soil samples were taken for laboratory Na:l(Tl) and Ge(Li] gamma spectral analysis. Total exposure rates were found to varY considerably from site to site and island to island. Levels measured over soil ranged from, less than 10 &r/hr to over 500 Ur/hr. Major contributors to the radiation fields number of other isotopes present. The large amount and consistency of the data indicate that a reliable and comprehensive picture has been obtained of the external gamma radiation environment of the atoll.

Page 5

TABLE OF CONTENTS II. INSTRUMENTATION, DATA COLLECTION AND ANALYSIS. . ..... 4 Error in Partial Exposure Rates for Major 111. EXTE~AL ENVIRONMENTAL RADIATION ON BIKINI ATOLL. ...15 Total and Individual Isotope Exposure Rates Total and Individual Isotope Exposure Rates Total Exposure Rates Along the 1 Transect .......’23 Total Exposure Rates Along the South and North Total and Individual Isotope Exposure Rates at Summary of Exposure Rates on Bikini Island ... .. ..28

Page 6

TABLE OF CONTENTS (Cent’d) Figure 3. Bikini Island. Approximate locations of survey transects are indicated. Numbered circles are locations of field spectrometric Figures 4 and 5. Exposure rate profiles of O and 1 Transects. Points are scintillation counter measurements and stars are Figure 8a. Ge(Li) spectrum of soil sample taken from Bikini Island, Pit 6, 0 Transect, Ge(Li) spectrum of soil sample taken from Bikini Island, Pit 6, 0 Transect, ) Figure 8c. Ge(Li) spectrum of soil sample taken from Bikini Island, Pit 6, 0 Transect, Figure 8d. Ge(Li) spectrum of soil sample taken from Bikini Island, Pit 6, 0 Transect, Figure 9a. Ge(Li) spectrum of soil sample taken from Figure 9b. Ge(Li) spectrum of soil sample taken from

Page 7

TABLE OF CONTENTS (Cent’d) Figure 9d. Ge(Li) spectrum of soil sample taken from Figure lOa. Ge(Li) spectrum of soil sample taken from Figure 10b. Ge(Li) spectrum of soil sample taken from Figure 10d. Ge(Li) spectrum of soil sample taken from Figure ha. Ge(Li) spectrum of soil sample taken from Figure llb. Ge(Li) spectrum of soil sample taken from Figure llc. Ge(Li) spectrum of soil sample taken from Figure lld. Ge(Li) spectrum of soil sample taken from Figure 12a. Ge(Li) spectrum of soil sample taken from Figure 12b. Ge(Li) spectrum of soil sample taken from Figure 12c. Ge(Li) spectrum of soil sample taken from Figure 12d. Ge(Li) spectrum of soil sample taken from

Page 9

A radiological survey of the islands of Bikini Atoll in the mid-Pacific was conducted in late April and early May of 1967, nine years after cessation of extensive testing of nuclear devices in the area. The survey, sponsored by the Division of Biology and Medicine of the U. S. Atomic EnergY Commission, included general observations of the prevailing environmental conditions and a detailed investigation of external environmental radiation levels. Exposure rates due primarily to penetrat~ng gamma radiation were measured, and the principal radio~sotopes contributing to the total exposure rate on each of the major islands of the atoll Bikini Atoll is located in the northern Marshall Islands. The atoll consists of a number of small coral islands surrounding a lagoon 22 miles long and 13 miles wide. Total land area of the atoll is 2.32 square milesj of which 1.25 square miles comprises the three largest islands, Bikinij Eneuj and Nam. Figure 1 is a map of the atoll. The names of the islands differ on the various hydrographic charts, being usually variations of Japanese renditions of the original Marshallese names. On the map in Figure 1 and throughout this report we have used the Marshallese names of the islands. The testing of nuclear devices at Bikini Atoll occurred during 1946, 1954, 1956, and 1958 and included the detonation of some 23 devices of both fission and thermonuclear types. The locations of the tests and the code name and year of each event are indicated on the map in Figure 1. Most of the shots were detonated on barges anchored in the lagoon or on the atoll reef. TWO shots were air drops, Able and Cherokeej two were underwater, Baker and Maplej and three were surface bursts, Bravo,, Zuni, and Koon. A1l of the islands received in varying degrees the resultant radioactive fission and activation products which were spread about the area. Although prevailing winds generally carried the local fallout westward, there were exceptions - notably shot Bravo, when unexpected winds carried the fallout toward the east. .,

Page 10

.; An extensive survey of the atoll was last carried out in 1964, when the emphasis was on examining the radioactivity ] of flora and fauna and obtaining large numbers of samples of rats, birds, soiis, and marine life for laboratory analysis. Thus, the gathering of additional samples of these types on this survey was not a primary requirement. However, a ,/’ fairly large number of soil samples were taken and brought back for analysis so that the in situ measurements could be supplemented by calculations based on the isotopic concentrations determined by laboratory gamma ray spectroscopy, . radiochemistry., and lithium drifted germanium spectroscopy. The external radiation survey techniques utilized were largely those developed and used by the Health and Safety Laboratory for the past several years in conducting detailedt investigations of the properties of the external radiation In addition to the survey measurements an experiment of the radiological effects of clearing a particularly heavily vegetated area -was carried out on Bikini Island near the beginning of the survey trip. Besides the authors, who were primarily responsible for the external radiation measurements the survey team. included Edward Held, University of Washington Marine Radiobiolog~st, the survey leader; his assistant, Robert Erickson; Arnold Joseph of the Division of Biology and Medicine; James Hiyanej Trust Territory District Agriculturist; Jack Tobin, former Trust Territory District Anthropologist; and Francis Tomnovek and Edward Jones of the U. S. Naval Radiological Defense Laboratory (USNRDL) who conducted most of the TLD studies and supplied and serviced the G-M The survey team spent a total of 16 working days on the as a base of operations. Fourteen islands and the two island complexes were surveyed. Only the very small island of Adrikan in the southwest corner of the atoll was bypassed. About ten days were spent on the three large islands, particularly Bikini Island (seven days). All of ~,. ,.

Page 11

the members of the team participated in conducting the experiments and gathering the data on external radiation. The data on the marine, plant, rat samples, and agricultural and anthropological observations will be published elsewhere. In the following sections of this report we discuss in detail the radiation instrumentation, data collection and analysisj and present environmental radiation results for each island. Tables containing data pertinent to external radiation levels on Bikini Atoll conclude the report.

Page 12

II. INSTRUMENTATION, DATA COLLECTION AND ANALYSIS A high pressure ionization chamber and a y-ray spectrometer system were used to obtain in situ exposure rates and—— spectra. The spectra were then analyzed to determine the individual exposure rates contributed by each major y-ray emitting isotope in the soil. Because of’ the bulk and weight of the analyzer system~ ionization chamberj and related power suppliess and the resultant difficulty in transporting the equipment from the ship via small boats to the shore and thence in many cases through heavy brush to a survey site, these types of measurements were limited to 16 sites on the three major islands. In all some 29 field spectra were obtained. A small sc~ntill.ation counter survey meter and a number of rugged G-M counter survey meters were used to extend the total exposure rate measurements over these islands and to survey the smaller islands. Although the data obtained with these instruments is less accurate for a particular location, their use enabled us to extend our measurements over a fairly large area conveniently and consequently obtain a mo~e complete picture of the variation of radiation levels across the major islands and from island to island. In addition to these measurements, thermoluminescent dosimeters were placed at a large number of locations on Bikini and Eneu Islands at the beginning of the survey and collected about ten days later and returned to the United States for readout- These passive dosimeters were employed to provide an independent check on the data obtained with the other instrumentation. Soil samples were also taken at various locations exhibiting unusually high or low aczivity. All of these samples were returned to the United States for laboratory our 4 in. x 4 in, NaI(Tl) detector abcard ship to obtain identification of the major y-ray emitters in the sample. At several locations a complete depth profile set of soil

Page 13

samples was taken so that the variation of isotcpic concentrations with depth might later be investigated. Spectrometer System - The HASL spectrometer system consists of a 4 in. x 4 in. NaI detector with preamplifier and high voltage battery packl a 400 channel mult~channel analyzer and parallel printer, a DC-AC converter and a 12 volt storage battery. The calibration of the detector is described in detail in previous HASL reports2’3. In brief, the exposure rate due to a particular isotope in the ground is inferred from the number of counts under a spectrum total absorpt~on peak characteristic of that isotope using calibration factors determined from a combination of laboratory experiments and The prominent total absorption peaks observed in all the field spectra (see Figure 2 for a typical field spectrum) be responsible for almost all of the total expcsure rate at the spectrometer locations. Individual exposure rates for 137CS, ‘°Co and ‘25Sb were inferred for each field location, The 155Eu exposure rate was assumed to be of minor importance due to its very low source energy. For those locations where the ionization chamber was net used or the other total exposure data were inconsistent, the spectrcmetr~c data were analyzed to obtain total. exposure directly3. Except for the two Eneu locations, all spectra were taken in ten minute runs. During this time there was no appreciable gain shift due to temperature changes. In almost all cases we were able to obtain excellent field spectra with only minor equipment malfunctions even though the equipr.ent had to be carried by hand through heavy brush., loaded and unloaded into small boats in fairly rough surf, and operated and stored in ambient temperatures of approximately 85 - 95°F and relative humidities of 70 – 80%. Ionization Chamber - Our high pressure ionization chamber has a 5.6 liter sensitive volume and is filled to a pressure of about 700 psi with pure argon gass, The chamber wall. is

Page 14

0.135 in. stainless steel and effectively discriminates against all beta radiation. The ionization current is read out on a Victoreen Model #475A Dynamic Capacitor Electror,eter. This chamber has been shown to have a flat energy response over all y-ray energies of importance in environmental radiation studies and to allow determination of total y-ray exposure rates from about 1 Vr/hr to 200 ~r/hr w

Page 16

although not of comparable p~ecisi. on and accuracy to the primary data, were useful in determining the range and variation of the radiation field. (IM-85/PDR-27F) , supplied by NRDL, were used to obtain a wide coverage of tctal exposure rate data, These particular bility under severe env~~onment.al conditions. Although these survey meters were calibrated in the laboratory it wa.S found that the calibration factor was unsuited to environmental radiation half-space geometry. The instruments exhibited a considers.bl.e ener~~ and angular response when exposed to different energy soxces m the l.aboratory~ as well as a slight f3-ray response even with the @-ray shield closed. For these reasons it was felt Ehat the laboratory calibration should be discard;d in favor of a “field Because of the lack of sensitivity of G–M tubes to gamma radiation at the relatively low levels us~lally encountered, an individual reading taken with one cf these instruments was imprecise. However, the mass of data taken when corrected using the “field calibration” satisfied the primary purpose of the instrument which was to del~r~eate the range of exposure rates over a given island and the variation from~ island to Survey meter readings were made on paths which were cut 1 through the thick brush cn many of the islands (particularly; on Bikini Island] along lines which ran Iengchwise along the islands across the island or around the perimeters 50 to 100 t ft. inland. The t-nic’kness of the brush dictated where transects could be iocated and the transects frequently deviated substaritially from straight lines. ‘I’hemeters were generally monitored constantly along the transects with were taken at about the three foot level: although as expected for y radiation, theze was rto sign~ficant variation with height. Readings were also made next tc any unusual artifacts scrap metal, bunkers, or building remains.

Page 17

All the corrected G-M tube gamma radiation readings obtained by the survey team are given in Table 1 for each measurement site along with all other total exposure rate data obtained at that site. Although some beta plus gamma radiation readings were taken these data were inconsistent and are not reported. Thermoluminescent Dosimeters - A large number of thermoluminescent dosimeters (TLD) were used during the survey. NRDL contributed the majority of these detectors. The predominant type used was a plastic capsule filled with LiF powder and enclosed in styrofoam inside a small plastic box. This type of dosimeter was placed every 100 ft. (measured] along each of the major transects on Bikini and …