External Radiation Levels on Bikini Atoll, May 1967

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Thin repxt was prepared as an account of Government sponeored work. Neither the United Staten, nor the Comm18eion, nor any per Bon acting on behalf of the Commieelon: A. Makes any warranty or representation, expreesed or implied, with respect to the accuracy, completeneea, or uaefulneaa of the information contained in this report, or that the uoe of any information, apparatus, method, or process dinclosed in this report may not infringe privately owned rights; or B. Aesumee 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 ueed in the ahove, “per~on acting on behalf of the Commission” includes enY employee or contractor of the Commission, or employee of euch contractor, to the extent that such empioyee or contractor of the Comminaion, or employee of ouch contractor preperea, diaaemlnatea, or providee acceae to, any information purmiant to bis employment or contract with the Commi8eion, or him employment with such contractor. This report has been reproduced directlyfrom the best Printed in USA. Price $3.00. Availablefrom the Clearinghouse for Federal Scientificand TechnicalInformation,Na-

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

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An imtensi.ve 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 system.j a high pressure ionization chamber> scintillation and G.M. survey meters, and thermoluminescent dosimeters. A large number of soil samples were taken for laboratory Nal(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 ~r/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.

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TABLE OF CONTENTS II. INSTRUMENTATION, DATA COLLECTION AND ANALYSIS. ...... 4 Error in Total Gamma-Ray Exposure Rates ..........13 Error in Partial Exposure Rates for Major III. EXTERNAL 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

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TABLE OF CONTENTS (Cent’d) 2. HASL RADIOCHEMICAL AND GAM1-SPECTROMETRIC ANALYSES OF BIKINI ATOLL SOILS (pc/gm OF 2. Typical NaI(Tl) field spectrum. Bikini Island, O Transect., Location 7, 10 minute

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

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8 TABLE OF CONTENTS (Cent’d) of soil sample taken from of soil sample taken of soil sample taken Figure 10d. Ge(Li) spectrum of soil sample taken Figure ha. Ge(Li) spectrum of soil sample taken Figure llb. Ge(Li) spectrum of soil sample taken Figure llc. Ge(Li) spectrum of soil sample taken Figure lld. Ge(Li) spectrum of soil sample taken 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

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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

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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 san;.piesof rats, birds, soils, and marine life for laboratory analysis, Thus, the gathering of additional samples of these types on this survey was not a primary requirement. Howeverj 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 deterrtined by laboratory gamma ray spectroscopy, radiochem,istry, and lithkum drifted germanium. spectroscopy. The exterrial radiaticm survey techniques utilized were largely those developed and used by the Health and Safety Laboratory for the past several years in conducting detailed investigations of’ the properties of the external radiation environment in the United Statesl ‘2’3. In addition to the survey measu~ements 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 inclu,ded Edward Held, U1iiversitY of Washington Marine Rad~obiclcgist, Arnold Joseph of the Divisioil of Biology and Medicine; James Hiyane, Trust Territory D~strick Agriculturist; Jack Tobin, former Tri~st Territory District Anthropologist; and Francis !l’omr,o~:ekand Edward Jones of the U. S. Naval Radiological Defense Laboratory @SNRDL) who conducted most of the TIJ3 studies and supplied and serviced the G-M The survey team spent a total, of 16 working days on the as a base of ope~ations. 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

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{ [ 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 analysis, and present environm.ental radiation results for each island. Tables containing data pertinent to external radiation levels on Bikini Atoll conclude the report.

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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 chamber$ and related power suppliesj and the resultant difficulty in transporting the equipment from the ship via small boats to the shore and thence in many cases through hea~zy brush to a survey sites 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~ntillation 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 locatiori~ their use enabled us to extend our measurements over a fairly large area conveniently and consequently obtain a more 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 activity. All of these samples were returned to the United States for laboratory analysis, however~ several were also spectrally analyzed on our 4 in. x 4 in. NaI(Tl) detector aboard ship to obtain identification of the major ,}/-rayemitters in the sample. At several locatlons a complete depth profile set of soil

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samples was taken so that the variation of isotopic concentrations with depth might later be investigated. Spectrometer System. - The HASL spectrometer system consists of a 4 in. x 4 m. “NaI detector with preamplifier and high voltage battery pack: a 400 channel m.ult~channel analyzer and parallel printer a DC–AC converter and a 12 vclt storage battery. The calibration of the detector is described in detail in previous HASL reports2’3. In brief, the exposure rate due to a partlc~lar isotope in the gro~und is inferred from the number of counts under a spectrum. total absorpt~on peak characteristic of that isotope using calibration factors determined from..a comb.inat~on of laboratory experiments and The prominent tctal absorption peaks observed in all the field spectra (see Figure 2 foz a typical field spectrum) be responsible for almost all of the total expcsure rate at the spectrometer locations. Ind~vidual exposure rates for 137CS, ‘°Co and ‘25Sb were inferred for each field location. The 165Eu exposure rate was assumed to be of minor importance due to its very low source energy. For those l~cations where the ion~zation chamber was not used or the other total exposure data were Inconsistent, the spectrcmetrlc data were Except fo~ the two Eneu, locations~ all spectra were taken in ten minute runs. Durl.ng 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 equipment had to be carried by hand throug”h heavy brush, loaded and unloaded into sm~ll boats in fairly rough surf, and operated and stored in ambient temperatures of approximately 85 - Ionization Chamber – Our high pressure ionization chamber has—._ a 5. 6 liter sens~tive volume and is filled to a pressure of about 700 psi with pure argon gasa , The chamber wall is

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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 Electrometer. 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 p+r/hr to 200 Vr/hr with a precision usually better than 2% and an accuracy of better than 5%3. A careful calibration was done in the laboratory before and after the survey and periodic checks for consistency were made in the field with a small. ‘2GRa check source. A correction for the contribution to the ionization current from cosmic rays was determined by measuring the cosmic ray component alone on top of the bridge deck of the ship in the middle of the Lagoon. The value obtained (3.4 ~r/hr) was consistent with the valu,e of the cosmic ray exposure rate for this latitude inferred from our previcus extensive cosmic ray measurements. Total exposure rates were measured with the ionization chamber at almost all the field spectrometer locations, as well as at several other sites. These total. exposure rates were later compared with the sum of the individ~al component exposure rates inferred from the field spectrometer data.” lation detector consists of a 1.5 in. x 1 in. Na’I(Tl) crys~al and 1 in. photomultiplier tube attached to a very stable count rate circuit. This instrument is relatively insensitive to beta and cosmic radiation. It is calibrated for partlc~lar field conditions by comparing readings with y-ray exposure rates determined from the ionization chamber at a number of locations. The “field calibration” was done both on Bikini and Nam with identical results. This type of calibration allowed us to use the instrument : to extend the results of a few very accurate arid precise ionization chamber and spectrometer measurements over a wide area.‘, We thus quickly obtained a large amount of data which,

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Figure 2. Typical NaI(Tl] f~el.d spectrum. Bikini Island, O Transect, Location 7, cotintmg Interval.

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although not of comparable pzecision 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 instruments were chosen for their ruggedness and dependability under severe environmental conditions. Although these survey meters were calibrated in the laboratory it was found that the calibration factor was unsuited to environexhibited a considerable energy and angular response when exposed to different energy sovrces m the laboratory: as well as a slight p-ray response even with the ~-ray shield closed. For these reasons it was felt that the laboratory calibration should be discarded in favoz of a “field calibration” against the ionization chamber and spectrometer. Because of the lack cf sens.ltivity of G-M tubes to gamma radiation at the rel.atlvely low levels usually encountered, an i-ndividual reading ta:ken with one of these instruments was imprecise. However, the mass of data taken when corrected of the instrument which was to delineate the range of exposure rates over a giver> island and the ~~ariation from~ island to Survey meter readings were made on paths which were cut through the thick brush cn many of the islands (particularly on Bikini Island) along lines which, ran lengthwise along the island, across the island or around the perimeter> 50 to 100 transects could be located and the transects frequently deviated substantially from straight lines. The meters were generally monitored constantly along the transects with were taken at about the three fGot levelj although as expected for y radiation, there was no sign~ficant variation with height. Readings were also made next tc any unusual artifacts scrap metal, bunkers, or bllllding rema~nso

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All the corrected G-M tube gamma radiation readings obtained by the s“urvey 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 …