Further Contributions on Gross Beta Radioactivity of Biological and Related Samples at the Eniwetok Proving Ground, 1952-1958

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Thie report wae prepared 86 an account of Government sponeored work. Neither UZI United Stater, nor the Commiseion. nor any person acting on khalf of the Commterion: A. Makes any warranty or representation. expreered or implied, with reepect to the accuracy, completeness. or ueefulnees of the Information contained in thir report, or that the u6e of any Information. apparatus, method, or process diecloeed In thle report may not infringe privately owned rlghte; or B. Assumer any liabilitier wttb respect to the uee of, or for damages reeulting from the uw of any information, apparatus method, or procere diecloeed in thle report. Ae used in the above, “pereon acting on behalf of the Commission” includer any employee or contractor of the Commiaaion. or employee,of euch contractor, to the extent tit such employee or contractor of the Commission. or employee of such contractor prepare6 diseeminater, or provide8 acceeo to, any information purruant to hir employment or contract with the Commireion, or his employment with such contractor. This report has been reproduced directly from the best Printed in USA. Price $1.25. Available from the Office of Technical Services, Department of Commerce, Washington

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FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF BIOLOGICAL AND RELATED SAMPLES AT THE ENIWETOK PHYSICAL DECAY OF SAMPLES FROM ENIWETOK ATOLL IN 1852. FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF PLANKTON AND BOTTOM SAMPLES AT RONGELAP FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF FISHES AT RONGELAP ATOLL, 1854 THROUGH Operated by the University of Washington under Contract No. AT(45-1)540 with the United States Atomic Energy Commission Laboratory of Radiation Biology University of Washington

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Three sections of subject matter are combined in this paper, Section I shows the pattern of radioactive decay over periods of about six years for 102 samples from Eniwetok Atoll, plus 2 sand samples from Nevada, Logarithmic plots commonly had slopes of from -1.2 to -1.8. The slopes for the last three years tended to be steeper and to show more variability than for the first three years. An exceptionally slow rate of decay, -0.06, prevailed for the last three years in a sample of rat muscle where Csl8’ was almost the only radioisotope. Slow decays occurred in muscle, liver, bone, and kidney of various animals. In Section II, available data on plankton from Rongelap Atoll for 1954-1958 are brought together, including not only values based on wet weight as reported earlier, but also new values computed on the basis of ash weight of sample, which gave more consistent results. Mesh size did not influence activity levels of the plankton samplea. For August 1958, the amount of activity in 10 pumped plankton samples was shown not to be correlated with the activity of samples of lagoon bottom material taken at the same stations.

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In Section III, the trends in average activity of liver, muscle, and bone of reef fish at Rongelap and Kabelle Islands, and in lagoonpelagic fish of Rongelap Atoll are given for the period from 1954 through August 1958. Ultimate decline was to levels of about 0.005 microcuries per kilogram of wet tissue.

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Gross beta is a useful criterion of the amount of persisting radioactivity. Plotting the logarithm of this value against the logarithm of the time after detonation depicts the trends of both physical decay and of decline in radioactivity due to a combination of physical decay and various biological and environmental factors. The term decline as used here indicates the trend with time in the amount of radioactivity in successive samples of a particular type of substance at a particular locality. The term is appropriate because the amount usually decreases with time. However, the amount may increase or remain constant for a while if there is an influx of radioactivity into the environment, so that minor fluctuations in decline are to be expected. In this respect decline differs from phyeical decay, which can not remain constant or increase but can only decrease. The three sections of this paper deal respectively with decay data of early samples from Eniwetok Atoll, and with decline of plankton and fish samples from Rongelap Atoll. All have the feature in common that their radioactivity was derived primarily from a single, though not the same, detonation rather than from two or more successive detonations.

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PHYSIC-AL DECAY OF SAMPLES FROM ENIWETOK ATOLL IN 1952 Previous decay studies (UWFL-53) of the biota and other materials from the Marshall Islands have covered periods up to nearly three years following the detonation of atomic testing devices. The earliest curves (AECD-3446:27) were for oyster, XE-19, and ovary of damselfish, XE-40, whose slopes from 20 to 800 days were -1.6. Decay of 83 samples from Rongelap Atoll (UWFL-42:46) over a period of from 38 to 500 days following March 1, 1954 averaged -1.4. Coconut milk was most unusual among these, having a decay rate of Decay data for samples collected in 1952 offer the most comprehensive picture of long-term decay trends that may now be contributed from studies by the Laboratory of Radiation Biology. A period of approximately seven years is involved. These samples number 102, of which 96 were from Eniwetok Atoll collected within 10 days after the Mike test, 6 were within 10 days pre-Mike and would be referred to the Greenhouse series. In addition there were 2 Nevada sand samples collected on November 29, 1951, related to the Uncle-Jangle atomic tests, making a total of 104 decay curves.

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Preparation of plates was the 8ame ae deecribed in WT-616 (UWFL-33) : 19. Briefly, the wet or motet sample8 were placed on weighed 1 I/2-inch ( 1 518 inch over-all) stainless steel plates, weighed, dried overnight at 97” -99’C, reweighed, ashed overnight 550’C, and finally reweighed. The ash was slurried with alcohol cover the plate smoothly, dried and affixed with a few drops of 0 per cent Formvar in ethylene dichloride. bearing the plates stored on 2 l/2 x 3 l/4-inch cards perforated hole into which the plate8 fitted. The cards were etored in wooden boxes with tall, narrow, carde bearing plates could be inplatea to the box. Firm fixation slotted compartments, so that 25 serted in each tier, totalling 100 of aeh to the plate, care in handling, and occasional inspection of the cards for evidence of epil guarded against loss or contamination of ash. However two decay curves had to be excluded, and one more curve wab discontinued, on account of lose or contamination. In addition ~XJthe description of counting methods given in WT-616 it may be said that a special 1 l/2-inch-thick lead shield

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each methane-flow counting chamber used in conthe Nucleometers to give a background of only about per minute (c/m). The same counting equipment was used throughout the seven years covered by this report. Counting time followed the schedule of WT-616:20 during the C/m, sample Counting time, plus background minutes For later counts, as more machine time became longer counts of from 30 minutee to several hours were sible for the low -counting samples. The absolute level of radioactivity as indicated in the column of Table 1 headed “Plate count, 1000 days, ” serves as an index to the reliability of the points. Graphing and Reproduction Values from the decay data-sheets were plotted as circles on thtn logarithmic graph paper. Even where circles overlapped, almost all of the values were graphed. Free, outstanding circles pertain to the nearest curve, and lines connect those points whose identity was

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considered to be uncertain, or where the gap between points was 60 great as to cause difficulty in perceiving the continuity of the curve. From the original graphs the curves were copied over a light-table onto tracing paper. A somewhat natural sequence of subject matter was arranged, progressing from soil through kingdom 8. Each graph was oriented by means the plant and animal Thus, the position of the curve in the series is no index of the amount of radioactivity involved. Absolute levels may be computed by referring to the column of Table 1 headed “Plate count, 1000 days. ” For example, if for curve 25, the value of the plate count at 2300 days were required, then reference to Table 1 would show a plate count at 1000 days of 700, while the ordinates as scaled from Figure 1 on days 1000 and 2300 were 18 and 2.4 respectively, The ratio, 2.4118, is multiplied by 700 to give a result of about 93 c/m, whtch is reasonably close to the actual value, 90 c/m (original data, not shown here), Slopes of curves were scaled from the finished drawings of Figures l-6, using a transparent slope-scaler calibrated in tenths from slope -0. 1 to -2.0 and by greater increments from -2.0 to -5. For each curve, the portions of markedly different slope were measured separately, thus necessitating subjective interpretations as to what constituted a change

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Figures 1 - 3. Logarithmic plots showing radioactive decay of samples listed in Table 1. Absolute levels of curves may be calculated bi referring to the column of Table 1 headed “Plate count, 1000 days’: Dates of detonations:

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Figures l-6 show graphically the decay rates of 104 samples collected in 1951 and 1952. Logarithm of radioactivity is related to log of time after detonation. Detonation date is stated in the legend of the figures. The graphs do not show absolute levels of radioactivity, (although this may be computed by reference to Table 1) but rather permit a comparison of decay rates between samples. Table 1 liste, in the left-hand portion, reference data for the samples whose decay curves appear in Figures l-6, while the five regions in the right- hand portion give meaeurements of slope as scaled from the graph8 in such a manner as to consider most of the major inflections of the curves where observations were sufficiently frequent . Inflections of curvature might of course also be expected during the period8 when obeervatione were not made. The slopes of the straight portions of the curves were tabulated in the region that seemed chronologically most appropriate, except that ff the entire curve was straight, the data were entered only in the first region. Thus, from the first region under “Negative slope----” in Table 1 the nearly straight-line decays are Been to be numbers 38, 41, 58, and 81. Similarly, the last region containe, among others, those entries for curves that were linear from the period of the first region through 2300 days after detonation.

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In an effort to detect a pattern, the five regions of Table 1 pertaining to slope were assigned the following approximate time-spans in days, respectively: <lOO, 100-300, 300-700, 700-1200, and 1200-2300. Because the points of inflection of the various curves do not coincide, these limits for the five periods had to be arbitrary. Omitting the first eight curves, and, therefore, considering only the post-Mike material (curves 9-104 inclusive), the slope for a curve was entered in the blank space so that a slope was available for each of the five time-periods involved. Computation of mean and standard deviation ( dm)), not standard error, of the 92-96 slopes for each of the five time-periods - 1. 25fO. 37, -1.78fO. 79. The mean slope of - 1.35 for the period up to 100 days is steeper than for the three following periods, -1. 26, -1. 25, and -1.25, but is less steep than for the last period -1.78. Likewise, the standard deviation of slopes was greater for the first than for the succeeding three periods, but less than for the fifth, and this same ability is also discernible from the graphs. There is thus evidence general increase in rate of decay of post-Mike samples during the last three years, over the first three years, with the exception of certain samples to be discussed later. The present decay rates may be compared to the theoretical gross beta decay curve for slow-neutron-induced fission products

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of U235 as depicted by Hunter and Ballou (1951). From their Figure 1 the following approximate slopes were obtained graphically: 20-100 Thus, the mean slope for the Eniwetok samples is sLightly steeper than that of Hunter and Ballou up to 100 days; then, in the period from 2 to 6 years, the Eniwetok samples tend to be at first less steep, and toward the end of the period, steeper, Future counting may show whether these curves will level slightly to -0.7 as does Hunter and Ballou’s curve at about 10 years. For the first six curves (Table 1) of pre-Mike samples involving products of the Greenhouse tests the curves tend to be steeper than for corresponding periods for post-Mike samples. Contamination of postMike samples by the older products from Greenhouse would tend to produce an effect in this direction, but so marked an effect is rather surIn contrast to the general tendency as noted above for the average of the slopes to become steeper after 3 years, certain curves showing a slower decay than average may be of special importance. Curve 49 for rat muscle decayed most slowly of all those studied, with a half life during the last 3 years, of more than 30 years. In gamma spectroscopy of this plate only Csl37 was detected. A listing of those

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curves which tended to level out, and the flattest slopes that they attained, appear in Table 2. The slow decays thus include almost all of the decays available for bird and rat as well as curves for muscle and kidney of most anim al8 . The slow rate for rats could be due to residual activity from earlier detonations in the vicinity of Biijiri I eland. Muscle with its high K4* content and kidney with (in the tridacnid clam) an affinity for strontium would be expected to exhibit slow radioactive decay, Table 2. Summary of the nineteen curves from Table 1 and Figures l-6, showing the most decided tendency toward leveling and the minimum steepness attained. Curve Organ- Tissue Latest no. ism 8 lope Curve Organ- Tissue Lateet no. ism slope

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FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF PLANKTON AND BOTTOM SAMPLES AT RONGELAP ATOLL Plankton in Rongelap lagoon took up large amounts of radioactivity from fallout following the Bravo detonation at Bikini Atoll on March 1, 1954. Since then,the radioactivity has declined with only relatively slight additions from the two succeeding series of tests, Redwing in 1956 and Hardtack in 1958. Since the last summary report on the Rongelap surveys (UWFL-43) was written, plankton has four times, July 1956 and 1957, and March and August, bottom samples once, in August 1958. The present report gives available data on plankton including a reevaluation of 1954-1955 counts which were reported in UWFL-42:43; however, ash weight as well as wet weight is used, with a modification of results, and a comparison is made of activity yielded by fine-meshed as contrasted with coarse-meshed plankton nets. The rate of physical decay of early samples is compared with the rate of change, herein termed decline, of successive samplings at later dates, up to more than four years after the original fallout.

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Methods of collecting and processing of the …