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Relationships between Dose Rates measured 1m above Ground Level and the 137CsDepth Distribution in the Chernobyl Grounds

R. Sakamoto1, K. Saito1, M. Tsutsumi1, T. Nagaoka1

I. Stolyarevsky2, S. Glebkin2, V. Tepikin2, N. Arkhipov2

V. Ramzaev3, A. Mishine3, A. Barkovski3

1Japan Atomic Energy Research Institute, Tokai, 319-1195, Japan2Chernobyl Scientific and Technical Center for International Research,

Chernobyl, 255620, Ukraine3St.-Petersburg Institute of Radiation Hygiene, St.-Petersburg, 197101, Russia

1. IntroductionJAERI carried out a mobile survey to establish a dose rate distribution map of the Chernobyl area

aimed at an analysis of the environmental radiological consequences. This was implemented by a method of“Wide Area Surveying”, using the newly developed mobile survey unit consisting of a dose rate meter,GPS(Global Positioning System) and a computer. It became clear that dose rates thus obtained (inferred) neededto be converted to represent actual dose rates in the field, taking into account variations as presented in a separatepaper(1). The inferred dose rates needed to be converted to the activity per unit of surface area of the field. Thisstudy was necessary not only for comparison with other data(2) collected by other scientists but also fordeveloping a method to measure activity remotely when a source distribution is presumed. The methods(3,4,5,6)to estimate activity per unit of surface area or to determine depth distribution have been developed by somescientists. But these were not applicable to this study. From this point of view, it is important to understand therelationship between dose rate at 1m above ground level and the activity profile as being exponential (or anotherformat of depth on radioactivity levels). If this relationship becomes clear, it is possible that the activity per unitof surface area is inferred from the dose rate obtained by the Wide Area Survey as described above.

The dose rates on the ground depend on the activity per unit of surface area and the depth distributionprofile of the soil. To examine this relationship, soil sampling and measurements of gamma dose rates werecarried out as shown in Fig.1. We estimated the dose rates at 1m above ground level from the activity distributedby depth, using the dose rate conversion factors(7,8) obtained by the environmental gamma-ray transportcalculation code(YURI3) as simulated by the Monte Carlo method. The dose rates thus inferred were comparedwith dose rates actually measured.

2. Soil core sampling and dose rate measurementsDuring 1997 - 1999, soil samples were taken from both sides of the road where remote measurements

were taken, and converted to the “dose rates of activity per unit of surface area” nearby. Soil samples were takenwith pipes made from aluminum, or steel, being approximately 30 cm in length and having cross sectional areasof 19cm2, 21cm2 and 38cm2. The samples were cut into 6-7 sections, taken from the lower parts of the sample tolimit contamination. The small diameters of the pipes used were insufficient to collect enough soil to accurately

Fig.1 Gamma ray measurement andsoil sampling in the field

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represent the activity in each area , thus we took 3-5 soil samples in the same area. The samples cut were thengathered in a vinyl bag and analyzed in the laboratory. As a result, we got data of W.E. depth(g/cm2) (W.E. depthmeans Water Equivalent depth; grams in wet soil, including litter per unit of surface area), wet density(g/cm3),137Cs activity per unit of surface area(Bq/cm2) and per unit mass(Bq/g) as shown in Fig.2.

The total W.E. depth of soil samples, including organic material and roots of plants, ranged from 25g/cm2 to 35 g/cm2. The wet density of layers, including surface litter, ranged from 0.3 g/cm3 to 2 g/cm3

approximately. The radioactivity of 137Cs per unit of surface area ranged from 5 Bq/cm2 to a few hundredsBq/cm2.

Remote dose rate measurements were taken from the roadside, aimed at an area 20 meters away, asdescribed above. We used a dose rate meter with 1 inchφx 1 inch NaI(Tl) and the “spectrum weightingfunction” method(9,10). Dose rates thus obtained ranged from 100 nGy/h to a few thousands nGy/h.

The correlation between gamma dose rates and activities per unit of surface area are shown in Fig.3.This correlation shows that the levels of dose rates, on both sides of the roads, are similar, however, the activitiesper unit of surface area aren’t thus questioning an hypothesis that activities must be similar on both sides.

3. Depth distribution in soilUntil now, only two methods, i.e. soil core sampling and remote gamma ray measurements, were

available to arrive at depth distribution profiles. Remote measurement methods conveniently scan large areas butdo not supply detailed information about depth distribution while soil sampling figures are accurate but can onlybe representative in the small area where the sample was taken. It would be useful therefore to be able toestimate the activity in soil from remote measurements (even-though a source distribution has to be presumed).

The fallout from the Chernobyl accident (April 1986) leached, over time, through the surface into thedeeper soil layers. Most activity is still evident, even after 10 years, in the top few cm of a core sample.

Fig.3 Correlation between dose rates measured and activities.

Fig.2 Soil core cutting and data preparation for calculation

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Especially cesium is active since it is bound to the organic parts of the soil, in case of a forested terrain.Obviously, terrain characteristics effect the depth distribution patterns. These differences are shown in

Fig.4. Bq/cm2 and Bq/g are the units of measurement used. Our aim was to show a clear relationship between137Cs activity distributed in depth and dose rates on the ground. The unit Bq/g is often used as it is helpful tounderstand the character of a depth distribution profile and the unit Bq/cm2 is convenient for the calculationsnecessary to infer dose rate on the ground. However, Bq/g relies on the soil being dry, thus we used the “W.E.depth in wet soil” unit of measurement. The depth profile, as shown in figure 4 as Bq/g, shows comparableactivity in the soil sections, in fact, the activity was nearly the same near the surface compared to the deepersections taken from samples in cultivated terrain. However, distribution appeared exponential or similar format,in undisturbed, forested and natural terrain.

4. Calculations of dose rate from depth distribution in soilThe data set of dose rate conversion factors(7,8), show a contribution from monoenergetic infinite

plane sources in soil to the height above ground, as shown in Fig.5. This data set was used as for the calculationof dose rates contribution from the volume sources inferred by the assumption that the form of depth distributionin the soil is a form of histogram as shown in Fig.4.

The dose rates where soil samples were collected were calculated using the data of W.E. depth andactivity per unit of surface area. The total dose rate from the volume source activity distributed was calculated asthe integral contribution by the infinite plane sources continuously placed from the surface plane to the deepestplane (about 30g/cm2 in depth). We used a form of histogram as the depth profile, though the distribution form isoften expressed as being exponential, or by using any functions to fit. But it is just complicated, it’s also difficultto clearly express depth distribution profiles.

Fig.4 Typical depth distribution in soil; Forested, Natural and Cultivated field in Novozybkov, Russia

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5. Result and Discussion

The results of this experiment are shown in Fig.6, as a correlation between actual dose rate data(measurement) and inferred dose rate data(calculation). The result showed a positive correlation, i.e. the doserates inferred, from a form of histogram distribution, are nearly equal to the dose rates measured. This supportsthe contention that dose rates remotely measured can be representative of the activity per unit of surface area inthe soil, though a source distribution has to be presumed.

However, the result showed a tendency for the calculated dose rates to be slightly lower (10-20%)compared to the dose rates actually measured.

The main reasons for this are as follows;◯1 An error relating to the histogram model,◯2 an error in dose rate measurement, or an◯3 error in the soil analysis process. Also, the◯4 effects on dose contribution from other radio-nuclides (except 137Cs),◯5 not having sufficient data to form a representative sample, and the◯6 variability in deposition.

Fig.6 Correlation between results of measurementsand calculations

Fig.5 Contribution from plane sources in soil as atotal activity of 1 Bq/m2

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Systematic errors, relating to the histogram model as described above, have been examined on theassumption of there being an exponential profile. The results are shown in Fig.7. Thus, if layers of a soil corewere cut into sections of 2 g/cm2, data would show that the calculated dose rate will be about 10% lowercompared to the inferred dose rate using an exponential model (and the relaxation mass being 1 g/cm2).

We measured dose rates using a dose rate meter (Aloka TCS-166) but this unit could only measure atotal dose rate at each given point. Another reason for the variations is the contribution made by the 134Cs nuclide.We estimated a contribution of a few percents from 137Cs (in 1997 at Chernobyl area). Although other reasonsmust be listed as described.

References

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2. The international Chernobyl Project Surface Contamination Maps,ISBN 92-0-129291-0, IAEA, (1991).3. J. Roed, C. Lange, K. G. Andersson, H. Prip, S. Olsen, V. P. Ramzaev, A. V. Ponamarjov, A. N.

Barkovsky, A. S. Mishine, B. F. Vorobiev, A. V. Chesnokov, V. N. Potapov, S. B. Shcherbak :Decontamination in a Russian settlement, Riso-R-870(EN), (1996).

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5. U. Hillmann, W. Schimmack, P. Jacob, K. Bunzl, In situ γ-spectrometry several years after deposition ofradiocesium Part I.Approximation of depth ditributions by the Lorentz function, Radiat. Environ.Biophys. 35(1996).

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7. K. Saito, and S. Moriuchi, Development of a Monte Carlo Code for the Calculation of Gamma RayTransport in the Natural Environment, Radiat.Prot. Dosim.12(1985).

8. K. Saito, and P. Jacob, Fundamental Data on Environmental Gamma-ray Fields in the Air due toSources in the Ground. JAERI-Data/Code 98-001(1998).

9. S. Moriuchi, and I. Miyanaga, A spectrometric method for measurement of low-level gamma exposuredose, Health Phys.12(1966).

10. S. Moriuchi, A new method of dose evaluation by spectrum-dose conversion operator anddetermination of

the operator,JAERI-1209(1971).

Fig.7 Error estimation by using histogram model

Thicknessof layer