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

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FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF BIOLOGICAL AND RELATED SAMPLES AT THE EN IWETOK PROVING Laboratory of Radiation Biology University of Washington

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This report was pre~ed as an account of Government nponaored work. Neither the United States, nor the Commission, nor any pereon ecting on bahatf of the Commts810n: A. Makee any warranty or representation, expremed or impttad, wttb reopeet to the accuracy, completeness, or usefulness of the tnformatlon contelned in thts report, or that the uee of any lnformaUon, apparatus, method, or process dtecloeed in thtn re~rt may not infringe B. A#aumes my liehilitiee wltb respect to the uee of, or for damages resulting from the uae of any Information, apparatus, method, or process dtsclosed tn thtn rewrt. As used in the shove, “paraon acting on bahatf of the Commission” tncludes any employee or contractor of the Commtaaion, or employee of such contractor, to the extent that ouch employee or contractor of the Cemmiesion, or employee of such contractor preparee, diseaminates, or provides accena to, any information pursuant to MO employment or contract wItb the Commlseion, or Me 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 USAU Office of Techolcal Idumatim ExtetiOn

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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 1952. FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF PLANKTON AND BOTTOM SAMPLES AT RONGELAP FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF FISHES AT RONGELAP A TOLL, 1954 THROUGH Operated by the University of with the United States Atomic Washington under Contract No. A T(45- 1)540

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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 CS137 was almost the only radioisotope. Slow decays occurred in muscle, liver, bone, and kidney of various animals. In Section H, 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 samples. For August 1958, the amount of activity in 10 pumped plankton samples was shown not to of lagoon bottom material be correlated with the activity of samples taken at the same stations.

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I n Section 111, the trends in average activity of liver, muscle, and bone of reef fish at Rongelap and Kabelle Islands, and in lagoonpelagic fish of Rongelap Atoil are given for the period from 1954 through August 1958. Ultimate decline was to levels of about O. 00”5 microcuries per kilogram of wet tissue.

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Gross beta is a useful criterion of the amount of activity. 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 physical 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.3L 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 fish, 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 lected on November 29, 1951, related making a total of 104 decay curves. were 2 Nevada sand

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Preparation of Plates Preparation of plates was the same as described in WT-616 (UW3?L-33) :19. Brief ly, the wet or moist samples were placed on weighed 1 1/2 -inch ( 1 5/8 inch over-all) stainless steel plates) weighed, dried overnight at 97° -99 SC, 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 O per cent Formvar in ethylene dichloride. Plates were stored on 2 1/2 x 3 l/4-inch cards perforated with a 1 l/2-inch hole into which the plates fitted. The cards bearing the plates were stored in slotted compartments, so that 25 serted in each tier, totalling 100 wooden boxes wit h tall, narrow, cards bearing plates could be inplates to the box. ‘Firm fixation of ash to the plate, care in handling, and occasional inspection of the cards for evidence of spill guarded against loss or contamination of ash. However two decay curves had to be excluded, and one more curve was discontinued, on account of loss or contamination. I n addition to the description of counting methods given in WT-616 it may be said that a special 1 1 /2-inch-thick lead shieid

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surrounded each methane-flow counting chamber junction with the Nucleometers to give a background of 45-50 counts 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-6 16:20 during the C/m, sample Counting time, plus background minutes For later counts, longer counts of from as more machine time became available, 30 minutes to several hours were frequently pos - 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 thin 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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2 = 2247 alsa, bticrun mt2.N 10-22 Runit tiudef duf bioc houa ntolidm liver U-5 wia atm gut 12-7 wluil Crpraea mneta [oat 4 wtle 11-5 ntma Jenlcm. viacer. U-7 Run it Lo

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conside red to be uncertain, or where the gap between points was so 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 WI was arranged, progressing from soil through the plant and animal5 am-low kingdoms. Each graph was oriented by means of the 1000-day vertiThus, 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 11 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. 4/ 18, is multiplied by 700 to give a result of about 93 c/m, which 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 1-6, using a transparent slope-scaler calibrated in tenths from slope -O. 1 to -2.0 and by greater increments from -2.0 to -5. For each curve, the portions of markedly different slope were measured separate thus necessitating subjective interpretations as to what constituted a cha

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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 b“y referring to the column of Table 1 headed “Plate count, 1000 days’: Dates of detonations:

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Figures 1-6 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 lists, in the left-hand portion, reference data for the samples whose decay curves appear in regions in the right - hand portion give scaled from the graphs in such a manner Figures 1-6, while the five measurements of slope as 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 periods when observations were not made. The slopes of the straight portions of the curves were tabulated in the region that seemed chronologically most appropriate, except that M the entire curve was straight, the data were entered only in the first region. Thus, from the first region under “Negative slope ----” in Table I the nearly straight-line decays are seen to be numbers 38, 41, 58, and 81. Similarly, the last region contains, 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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ost of the 1n 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: <100, 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 ( {-)), not standard error, of the 92-96 slopes for each of the five time-periods -1.25 fO. 37, -1.78*0.79. 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 variability is also discernible from the graphs. There is thus evidence of a 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 corresponding periods for post-Mike curves tend to be steeper Mike samples by the older products from Greenhouse would tend to produce an effect in this di~ection, but so marked an effect is rather sur - In contrast to the general tendency ,, of the slopes to become steeper after 3 as noted above for the average 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 Cs 137 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 S1OW decays thus include almost all of the decays available for bird and rat as well as curves for muscle and kidney of most animalS. The slow rate for rats could be due to residual activity from earli er detonations in the vicinity of Biijiri I sland. Muscle with its high K40 content and kidney with (in the tridacnid clam) an affinity for strontium would be expected to exhibit slow radioactive decay, Tab le 2. Summary of the nineteen curves from Table 1 and Figures 1-6, showing the most decided tendency toward leveling and the minimum steepness attained. Curve Organ- Tissue Latest no. ism slope

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FURTHER CONTRIBUTIONS ON GROSS PLANKTON AND BOTTOM SAMPLES BETA RADIOACTIVITY OF AT RONGELAP ATOLL . . Plankton in Rongelap lagoon tookup 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 been collected four times, July 1956 and 1957, and March and August, 1958, and lagoon bottom samples once, in August 1958. The present report gives available data on plankton

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the rate of Methods of collecting and processing of the 1954-1955 plankton are …