Design of the Brookhaven experiment on the effects of Ionzing radiation on a terrestrial ecosystem

Radiation Botany, 1963, Vol. 3, pp. 125 to 133. Pergamon Press Ltd. Printed in Great Britain.

D E S I G N OF T H E B R O O K H A V E N E X P E R I M E N T O N THE EFFECTS OF I O N I Z I N G R A D I A T I O N O N A T E R R E S T R I A L ECOSYSTEM* GEORGE

M . WOODWEI.,]L

Biology Department, Brookhaven National Laboratory, Upton, N.Y. (Received 28 February 1963) Abstx~ct--A forest ecosystem on Long Island is being irradiated experimentally with chronic

g a m m a radiation and will bc studied over a period of years to appraise the potential effects of ionizing radiation at ecological levels of organization. The source of radiation is Cesium 137, arranged for safety and simplicity of dosimctry in a centrally located tower and capable of being shielded by operation of a winch in a remote building. Exposure rate decreases around this tower approximately with the square of the distance from the source and m a y vary by a factor of 2 along any arc around it duc to shielding by the vegetation. The steep decline in exposure rate with increasing distance from the source produces steep gradients of clTccts and the amount of replication, especially at the species level and during the firstyear of the study when principal effects arc close to the source, m a y bc small. T w o types of control data arc available from (I) measurements of plant and animal populations in the experimental area prior to irradiation and (2) other rncasurcmcnts from nonirradiatcd stands nearby. Within the experiment, data can bc referred to a radial survey which defines location with reference to the source and hence exposure rate. Results of the first year's exposure of a stand of the Long Island oak-pine forest show that although plant populations wcrc generally more sensitive to damage than had bccn anticipated, the firstyear's effects on plants can bc predicted within broad limits from characteristics of the cell nucleus. The ability to make such predictions is of great value in planning this type of experiment. R~smm~---Un dcosyst6me forestier de Long-lsland est irradid expdrimentalement par les rayons y chroniques ct scra dtudid pendant uric pdriodc dc plusicurs armdes cn vuc d'estimcr les cffcts potcntiels des radiations ionisantes tL diffdrcnts nivcaux d'organisation dcologiquc. La source dc rayons est Ic Cesium 137 disposd pour la sdcuritd ct la simplicitd dc la dosimdtric dans unc tour localisdc au centre ct susceptible d'etre protdgdc cn actionnant unc manivcllc situdc dans un b~timcnt dloignd. Lc ddbit autour dc ccttc tour ddcrolt approximativemcnt avcc Ic carrd dc la distance dc la source ct pcut varicr par un factcur 2 pour chaquc arc dc ccrclc cn raison de la protection par la vdgdtation. La ddcroissancc rapidc de la dose cn fonction dc la distance croissantc dc la source produit unc diminution rapide des cffcts ct dc l'importancc de la rdponse aux rayons spdcialemcnt au niveau des esp~ces et pendant la premiere annde de l'dtudc quand les cffcts principaux sont proches dc la source. Dcux types dc donndes tdmoins sont disponibles: (I) Lcs dvaluations des populations vdgdtales ct animales dc l'airc expdrimcntalc avant irradiation. (2) Des dvaluations d'autres pcuplcmcnts voisins. Dans l'airc cxpdrimcntalc, Ics donndcs pcuvcnt ~trc rapportdes ~ un rclcvd radial ddfinissant la position par rapport A la source ct, dc IA, Ic ddbit. Les rdsultats dc l'cxposition pendant une premiere anndc, d'unc for~t dc pin-chfinc dc Long-lsland montrcnt quc los cffcts des rayons darts la premiere annde sur les plantes pcuvcnt etrc prddits dans dc largcs limites ttpartir des caract&res du noyau ccllulairc, bicn quc les populations vdgdtales soicnt gdndralcmcnt plus scnsibles quc cc qui dtait prdvu. La possibilitd dc fairc dc tclles prdvisions est d'unc grandc valeur pour la "programmation" dc cc type d'cxpdricncc. *Research carried out at Brookhaven National Laboratory under the auspices of the U.S. Atomic Energy Commission.

125

126

BROOKHAVEN EXPERIMENT ON THE EFFECTS OF IONIZING RADIATION Zus-mmemfasstmg--Ein forstliches Oekosystem auf Long Island wlrd gegenwtirtig experimentell einer chronischen Gammastrahlung ausgesetzt. Es soU fiber eine Periode yon Jahren hinaus untersucht werden, um die rn6gliche Wirkung yon ionisierenden Strahlen auf oekologischer Ebene abzusch~itzen. Die Strahlenquelle besteht aus Caesiumls7, das aus Sicherheitsgrfinden und urn eine einfache Dosimetrie zu erm6glichen in einem zentral gelegenen Turrn angeordnet ist. Es kann yon einem entfernt gelegenen Geb~iude aus durch Bettitigung einer Winde abgeschirmt werden. Die Expositionsrate rund urn den Turin nimmt ab mit dern Quadrat der Entfernung von der Strahlenquelle. Sic kann wegen Abschirrnung durch die Vegetation entlang eines jeden Kreisbogens urn den Faktor 2 variieren. Der steile AbfaU der Expositionsrate mit zunehmendern Abstand yon der Q uelle erzeugt steile Gradienten der Wirkung. Die Vermehrung, besonders auf der Ebene der Arten und wtihrend des ersten Jahres der Untersuchung, in dem die wichtigsten Efl'ekte dicht an der Q uelle auftreten, kann klein sein. Zwei Gruppen yon Kontrolldaten sind verfiigbar: Erstens aus Messungen der pflanzlichen und tierischen Populationen im Gebiet vor der Bestrahlung, und zweitens aus anderen Messungen yon unbestrahlten Nachbararealen. Die Daten der Experimente selbst k6nnen auf eine kreisl~6rrnige Verrnessung bezogen werden, die den Abstand zur Quelle und damit die Expositionsrate angibt. Die Ergebnisse nach der ersten einj~ihrigen Bestrahlung eines Areals aus dem Eichen-Kiefern Forst auf Long Island zeigen folgendes: Obwohl pflanzliche Populationen im allgemeinen strahlenempfindlicher sind, als verrnutet wurde, kann die Wirkung eines ersten Bestrahlungsjahres auf Pflanzen innerhalb welter Grenzen aus bestimmten Charakteristika des Zellkernes vorausgesagt werden. Die M6glichkeit, solche Vorhersagen zu machen, ist fiir die Planung der Experimente yon grossem Wert.

INTRODUCTION attributable to biological interactions. As with RESEARCH on the effects of ionizing radiation on D D T and other general poisons, differential organisms has been heavily concentrated on sensitivity among organisms can be expected animals and in general at cellular levels or to alter the trophic structure of the ecosystem, below. A growing body of evidence from possibly changing its physiognomy as well as its ?lantsC10,1s-le, ls) shows that there are important species composition. Measurement of such effects at exposures approximating those which changes is part of these objectives. cause effects in mammals, emphasizing that S E L E C T I O N OF T H E E C O S Y S T E M ionizing radiation of sufficient intensity would have gross effects on the plants and animals of A forest ecosystem was selected as representaman's environment as well as on m a n himself.(19) tive of the mosaic of such structurally and To appraise the type of ecological changes functionally complex and near stable ecosystems which might be induced by various intensities of which are the principal ecological units of m u c h ionizing radiation, a forest ecosystem on Long of the biosphere, especially that part of it which Island is being irradiated experimentally with supports man. T h e vegetation is an oak-pine g a m m a radiation (Fig. 1). The purpose of this wood which is simple in species composition paper is to describe those aspects of the design and comparatively homogeneous over a large of this experiment which might be of general area of central Long Island. application in other ecological research. One summer's work was devoted to the study Three objectives dominated the planning of of this ecosystem prior to commencement of the this experiment: first, the measurement of long irradiation. This s-miner's data served as one term effects of chronic ionizing radiation on an type of control data for the experiment; a ecosystem and its components. Second, appraisal second type of control data is available from of the potential effects of nuclear catastrophics untreated stands nearby. on ecosystems; and third, the need for basic research on the structure and function of S E L E C T I O N OF T H E P A T T E R N OF T H E R A D I A T I O N FIELD ecosystems. Potential effects of ionizing radiation include not only direct genetic and somatic Virtually an infinite variety of patterns of effects of exposure but also indirect effects g a m m a radiation fields is possible ranging from

FIe. 1.

Infi'a-red photograph of the irradiated ecosystem experiment.

FIG. 5. The winch. The perforated locking bar is fixed to the frame of the winch and is hollow. Another perforated rod which is keyed to the winch barrel rotates freely within the locking bar when all padlocks have been removed.

GEORGE M. WOODWELL that attained by the distribution of short-lived isotopes through the ecosystem, an arrangement which would preclude safe entry into the research area until the isotope had decayed, through an array of several sources, to a single source arranged to move on a track through the ecosystem or, as adopted, to reside in a stationary tower. A mixed gamma-neutron flux can be obtained from reactors and the ecological effects of such fluxes and of g a m m a exposures alone are being investigated by a group at E m o r y University under th.e direction of Dr. ROBERT PLATr.(n) A reactor capable of rapid pulses as well as sustained operation has been installed at O a k Ridge National Laboratory specifically for research in biology.C9) In the Brookhaven irradiated ecosystem experiment a single source of g a m m a radiation has been suspended in a tower over a leadshielded container which completely shields the source when appropriate. A similar mechanism has been in use at Brookhaven( 12,17~ and elsewhere(3, e) for experimental irradiation of cultivated plants in the field. The simplicity of operation and of dosimetry associated with a stationary source prevailed over any advantages of either an array of sources or a mobile source. With the more complex systems, the possibility of breakdown of the handling mechanism presented serious safety problems as well as a difficult dosimetry problem. These two factors, safety and dosimetry, were the primary considerations involved in selection of a single, centrally located, manually operated source which could be shielded at will from a safe distance. PATTERN O F T H E LAND SIYRVEY T h e commitment to a single, central source of ionizing radiation defined the geometric pattern of the facility and the general arrangement of study areas. Since accuracy in interpretation of dose-response relationships requires precise knowledge of distance from the source for every observation, a professional surveyor was employed to lay out 16 radii which were marked at 20 m intervals by stakes. Radii were numbered and the distance from the source indicated on each stake. Where obstructions prevented correct positioning of the stake, the offset was

127

always made along the radius and the distance of the offset marked on the stake. For detailed studies additional markers were placed, dividing each sector into plots and subplots as shown in Fig. 2. Each tier of plots was given a Ring Zone number as outlined by WOODW~.LL and HnJviMOND.(20) This use of small plots proved convenient for machine data handling as well as for orientation in recording data in the field. ESTIMATION

O F T H E STT.E O F T H E S O U R C E

Since the primary objectives of this research lie at the ecosystem level, the source had to be large enough to cause measurable effects at this level. However, the degree and type of homogeneity existing within the ecosystem in addition to the experimental objectives are factors to be considered in determining the size of the source. A tropical rainforest, for instance, m a y have as m a n y as 100 species per hectare but be homogeneous from a physiognomic standpoint. In such an ecosystem it might be necessary to forgo studies of populations of tree species and concentrate on physiognomy and trophic structure. In less complex ecosystems such as the Long Island oak-pine forest, it is possible to expose a large enough area to near lethal levels for trees to obtain detailed information about the behavior of the principal plant species taken individually. So it is clear that the greater the degree of uniformity of species composition, the greater the replication at the species level and the greater the precision possible at this level. Small abundant organisms such as sessile insects are therefore more amenable to study at the populations level in such an experiment than are the trees. Such considerations as these must determine the practical limits of the objectives of the study. Once the objectives at various levels of ecosystem organization have been defined, it is possible to estimate the effects of exposure of plants to any intensity of ionizing radiation on the basis of chromosome n u m b e r and sizeClSJ and on this basis to estimate the size of the source required. WOODW~-LL and SPARROW(~1) have shown that this technique was useful in predicting the results of one year's exposure of the Long Island oak-pine forest to chronic g a m m a irradiation and that it can be used now to

128

BROOKHAVEN EXPERIMENT ON THE EFFECTS OF IONIZING RADIATION

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predict the sensitivity of the principal species of any vegetation within broad limits. Refinements of the technique seem possible and will probably be available soon. T H E S O U R C E A N D ITS H A N D L I N G

MECHANISM The source The use of large sources of g a m m a radiation in botanical research has been summarized by SPARROW. (12) Materials available for such sources include reactor fucl rods, cobalt s°, and cesium lay. Most field installations have used cobalt or cesium. These two isotopes differ in half-life and in the energies of the photons emitted. The half-life of cobalt a° is 5-27 years, while that of cesium 187 is 30 years. Cobalt 6° emits g a m m a photons with energy peaks at 1"17 and 1.33 M c V and cesium lay, at 0.66 M c V .

T h e longer half-life of the cesium simplifies the dosirnetry problem, eliminating the need for annual replacement of the source. Cesium xs7 was obtained for the Brookhaven installation in powder form as cesium chloride and was installed in watertight stainless steel tubes arranged in a cylindrical bracket as shown in Fig. 3. Handling mechanism Figure 4 is a cutaway drawing of the mechanism for remote handling of a g a m m a source. T h e bracket holding the stainless steel containers is secured by bolts to the bottom of the plug which is the top of the lead shielded container. T h e container with the tower attached is in a cemented pit in the center of the area to be irradiated. T h e source is arranged to be raised to the top of the tower, 12 ft above the ground,

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129

by operating a winch in the building 125 m from the source. To facilitate shielding if the winch or cable fails and as an added safe W precaution, the source and shielding plug are supported by an electromagnet powered through a switchboard in the control building. This arrangement allows interlocks with the gates and the winch to prevent entry to the zone of high radiation intensity while the source is exposed. The source is controlled by a manuallyoperated winch in the control building. While a completely mechanized source is possible and might be installed more simply, manual operation was chosen for safety to assure the presence of an observer when the source was exposed. The simplicity of the mechanism also appeared to be a substantial advantage, contributing to safety and to trouble-free operation. S A F E T Y PRECAUTIONS A series of overlapping safety devices was designed to prevent accidental exposure of personnel. The experimental area is fenced with two 8 ft fences, one enclosing a rectangular zone around the source, the other paralleling the

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130

BROOKH.AVEN EXPERIMENT ON THE EFFECTS OF IONIZING RADIATION

inner fence and at least 150 m outside it. All gates and doors as well as the winch controlling the source are locked and keys are issued only to those familiar with the hazards and the safety precautions. Each entrant to the forest places his own padlock on a lockingbar on the winch (Fig. 5) and retains the key until he leaves, when he removes the lock. To remind entrants to lock the winch, the key to the padlock opens the only door to the inner zone. This door is spring-loaded to close automatically. When the winch is operated to expose the source a bar moves across this door. A switch in the circuit which energizes the electromagnet supporting the source opens when this door opens. A further precaution involves the maintenance of a record of entrants, kept at the outer gate, where a log shows the area where the entrant expects to work, designated by coordinates, estimated time of departure and the actual time of departure. The source operator uses this log to assure that personnel have left prior to the time the source is to be exposed.

DOSIMETRY

Accurate measurement of radiation exposure is one of the most difficult problems involved in the establishment of such a facility. Close to the source in the Brookhaven facility the radiation is essentially a line spectrum with an energy of 661 keV. In this region large trees produce sharply defined shadows in which the exposure may be as little as 1/2 the unshielded exposure. At distances in excess of 100 m shadows are more diffuse with the radiation energy spread into a broad spectrum, the radiation having been scattered by the aii" and vegetation.(~) Thus the difficulties arise from variations in radiation exposure due to shielding, and from changes in the energy spectrum attributable to scattering, important in particular at distances greater than 100 m. Several techniques are available for measurement of exposure. Ionization chambers were the most accurate dosimeters available for most purposes, but measurements are expensive in time and the chambers are impractical for use in large quantities. Film badges are cheap,

convenient to handle in large numbers and have a wide range of sensitivity. Phosphate glass dosimeters are less accurate than either badges or ionization chambers at low ranges, but can be used with a precision of about 5-10 per cent for total exposures of from approximately 10 r through several hundred r. Their small size makes them particularly well adapted for animal studies.( 7, s) Perhaps the most promising technique for ecological studies involves the thermo-luminescence of crystals.( 1,4) With proper equipment lithium fluoride crystals can be used with total exposures as low as 1 mr-40.25 mr and as high as 80,000 r(5) and would seem to be ideally suited for use in ecology. In addition to this wide range of sensitivity they have very low energy and dose-rate dependence, making them suitable for many field applications. To characterize the radiation field of the irradiated ecosystem a frequency distribution of radiation exposures was measured with film badges at six distances from the source. Film badges were selected on the basis of low cost, ease of handling and measuring large numbers of films, and wide range of sensitivity. To correct for changes in exposure levels due to variations in ambient temperature and pressure, as well as possible variations in film batches from day to day, a calibrated ionization chamber was placed at a reference point beside a film badge at each of several distances. Exposures obtained with film could then be corrected to an ionization chamber measuremeant at each distance. The variability in measured dose within the forest is indicated by the standard errors of the exposure curve of Fig. 6. This curve was determined by averaging abut 100film badge measurements taken randomly 1 m above the soil surface along the arc of each of six circles of radius 16, 30, 50, 80, 120 and 200 m. The dotted lines of Fig. 6 connect the limits described by ~ 1 standard error of the mean. The frequency distribution of the doses is skewed as shown in Fig. 7, the skewness varying with distance from the source. Shielding accounts for the asymmetry as well as the spread of such curves.

GEORGE M. WOODWELL

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EFFECTS OF TIlE FIRST YEAR'S EXPOSURE WOODWELL(19) has summarized early results of this experiment, showing the zonation of the vegetation after the first six months' exposure. WOODWELL a n d SPARROW(211 have shown curves of shoot elongation of various species, comparing these observations with their prediction of effects made more than a year earlier using nuclear volume and chromosome number as the principal criteria of sensitivity. Plant populations were generally more sensitive than predicted. WOODWELL and SPARROW(ga) point out that woody plants are more sensitive than herbaceous plants with similar nuclear morphology and suggest that irradiation during dormancy and the physiological stresses characteristic of natural ecosystems also contributed to the increase in sensitivity observed in this experiment over that observed in previous experiments. All of these observations show that

the zone through which effects ranged from high mortality to comparatively minor effects on growth was narrow for most species. In many instances this entire range covered a distance along a line from the source of as little as 20 m, although the difference in exposure rate might vary by a factor of 2 or more (Fig. 8). The narrowness of these zones emphasizes the importance of considering the rate of change of dose with distance from the source in selecting ecological parameters for measurement. Variation in response across a plot 10 m in diameter, if the plot diameter is taken along a line from the source, would be extreme for most purposes while variation across a plot 2 m deep might be negligible. The narrowness of these zones places strict limits on the sizes of populations available under any exposure regime and it is clear that with small sources of radiation the size of that part of

BROOKHAVEN EXPERIMENT ON THE EFFECTS OF I O N I Z I N G R A D I A T I O N

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the ecosystem w h i c h is o f interest m a y be too butions. Messrs LEE N. MILLER,JAN K. OOSTZNGand small to allow sufficient replication for g o o d GE~RD COUR~N have participated in most aspects m e a s u r e m e n t of the effects. T h e r e are clearly of the research at various times. Important contric o m p e l l i n g reasons to appraise the general butions to the design of the source holder and range of sensitivity o f the principal species of handling mechanism were made by the design group at Brookhaven under Mr. CLIVE REED, by Dr. H . J . interest prior to initiation of such an experiment. CURTIS, and by Mr. FRANK GERMAN. Dosimetry was largely under the direction of Mr. ROBERT WOODLEY with the assistance of Mr. COURTIN and with the coAcknowledgements--Numerous colleagues of the author operation of personnel of the Brookhaven Health have participated in varying degrees in the develop- Physics Department. The assistance of K A ~ z t m R. ment of this facility throughout a two-year period. WOODWSLL with the manuscript and with various The facility was originally proposed by Dr. A. H. other aspects of the research is acknowledged SPAm~ow, who has made many valuable contri- appreciatively.

G E O R G E M. W O O D W E L L REFERENCES I. CAMERON J. R., DANIELS F., JOHNSON M. and K'~NNEY G. (1961) Radiation dosimeter utilizing the thermolumincsccncc of lithium fluoride. Science 135, 333-334. 2. C O W A N F. P. and MEXNHOLD C. B. (1962) Radiation dosimetry for Co e° and Cs 137 g a m m a ray field irradiation facilities.Radiation Botany 2, 24b-249. 3. D'AMATO F., SCARASCIA G. T., BELLIAZZ! U., BASSANIA., CAMBIS., CEVOLLOTTOP., GIACALONE P. and TAOLIATI S. (1962) The gamma radiation field of the "Comitato Nazionale per L'Energia Nucleare", Rome. Radiation Botany 1, 243-246. 4. FRANK M. and HERFORTH L. (1962) Thermoluminescent dosimetry with CaFs: Mn. Ifernenergie 5, 173-176. NSA 16: 22469. 5. JOHNSON N. (1963) Personal communication. 6. KARAWA K. (1960) The cobalt gamma field irradiation facility. [in Japanese] Radioisotopes (oTapan) 9, 316-321. 7. KA~CES. V. (1962) Miniature glass rod dosimeters in radiation ecology. Abstr. Bull. Ecol. Soc. Am. 43, 125. 8. KAY~ S. V. (1963) Use of glass rod dosimetry in the O R N L ecology program. Abstr., Ann. Health Physics Soc. Mtg., New York, June, 1963 (to be presented). 9. LUNDXNM. I. (1962) Health physics research reactor --Hazards Summary. ORNL-3248, Oak Ridge National Laboratory, Oak Ridge, Tennessee. 98 pp. 10. MIKSCHE J. P., SPARROW A. H. and ROOERS A. (1962) The effects of chronic g a m m a irradiation on the apical mcristem and bud formation of Taxus media. Radiation Botany 2, 125-129. 11. PLATT R. B. (1963) Ecological effects of ionizing radiation on organisms, communities and ecosystems. Radioecology, Proc. 1st Natl. Syrup. on Radioecology, Fort Collins, Colorado, Sept. 1961. (Edited by SCHULTZ V. and KLEMENT A. Jr, Eds.) Reinhold, New York (in press).

133

21. WOODW'ELLG. M. and SPARROW A. H. (1963) Predicted and observed effects of chronic gamma irradiation on a near-climax forest. Unpublished manuscript. 12. SPARROW A. H. (1960) Uses of large sources of ionizing radiation in botanical research and some possible practical applications. Large Radiation Sources in Industry, Vol. II, pp. 195-219. International Atomic Energy Agency, Vienna, Austria. 13. SPARROW A. H. (1962) Predictions of radiosensitivities of plant species. Abstr. of Papers, 2nd Intern. Congr. Radiation Res., Harrogate, England. p. 178. 14. SPARROWA. H. (1963) The role of the cell nucleus in determining radiosensitivity. Brookhaven Lecture Series No. 17, BNL 766 (T-287), Brookhaven National Laboratory, Upton, New York (in press). 15. SPARROW A. H., CUANY R. L., MXKSCHE J. P. and SCHAXRZRL. A. (1961) Some factors affecting the responses of plants to acute and chronic radiation exposures. Radiation Botany 1, 10-34. 16. SPARROWA. H., SCHAmER L. A., SPARROWR. C. and CAMPBELLW. F. (1963) The radiosensitivity of gymnosperms. I. the effect of dormancy on the response of Pinus. strobus seedlings to acute gamma irradiation. Radiation Botany 3, 169-173. 17. SPARROW A. H. and SXNOLETON W. R. (1953) The use of radiocobalt as a source of gamma rays and some effects of chronic irradiation on growing plants. Am. Naturalist 88, 29--48. 18. SPARROW A. H. and WOODWSL~. G. M. (1962) Prediction of the sensitivity of plants to chronic gamma irradiation Radiation Botany 2, 9-26. 19. WOODWET.~- G. M. (1962) Effects of ionizing radiation on terrestrial ecosystems. Science 135, 572-577. 20. WOODW~LL G. M. and HAMMOND E. C. (9962)

A descriptive techniquefor study of the effects of chronic ionizing radiation on a forest ecological system. BNL 715 (T-251), Brookhaven National Laboratory, Upton, New York, 13 pp.