- Email: [email protected]
Method and device to measure 137Cs soil contamination in-situ
Nuclear Instruments and Methods in Physics Research A 420 (1999) 336—344
Method and device to measure Cs soil contamination in-situ A.V. Chesnokov*, A.P. Govorun, V.N. Fedin, O.P. Ivanov, V.I. Liksonov, V.N. Potapov, S.B. Shcherbak, S.V. Smirnov, L.I. Urutskoev RECOM LTD, Russian research center **Kurchatov Institute++ Schukinskaya St., 12-1, Moscow 123182, Russia Received 24 February 1998; received in revised form 29 May 1998
Abstract A method to measure the Cs soil contamination in situ is described. Based upon the measurements of a c-ray spectrum of the soil surface, this method allows to determine the soil deposition with an accuracy of 20% and to evaluate the thickness of the soil layer containing more than 80% of the contamination. The article contains the description of a portable device CORAD based on this method and of the software used to process the obtained data. The exposition time of CORAD does not exceed 10 min if the Cs deposition is more than 20 kBq/m. The results obtained by use of this method and those of the soil sampling are compared. The developed Cs contamination maps are presented. These maps can be used for rehabilitation simulation. Effective dose rate distributions may be calculated on the basis of the obtained data. 1999 Elsevier Science B.V. All rights reserved. Keywords: Cs soil contamination; Gamma radiation; Collimated detector
1. Introduction The need for detailed radioactive contamination measurements had arisen in connection with the first nuclear tests [1]. The same problem should be solved for areas contaminated as a result of the Chernobyl NPP accident [2], as vast territories are contaminated very heterogeneously. The scale of this heterogeneity varies from several meters to several hundreds meters and depends on many factors. There exists a set of traditional methods used to measure the soil radioactive contamination. The air
* Corresponding author: Tel: 007 095 1967665; 007 095 1961635; e-mail: [email protected].
Fax:
gamma survey is one of them [3,4]. This method allows to measure the contamination with spatial resolution near 100 m. Another type is the sampling method. This technique is known to be the most reliable now [5,6], yet the most laborious one. The spatial resolution of this technique is near 0.1 m and a large number of measurements is required in order to map the contamination in detail. Yet another method of field gamma spectrometry [7—9] has been proposed to reach the productivity of the air gamma survey and the reliability of the sampling method. The complex contamination of gamma emitting radionuclides is measured by the traditional methods. But now the main gamma emitting isotope at vast territories contaminated as a result of the Chernobyl and the South Ural accidents and
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 6 1 - X
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
337
release of nuclear facilities is Cs. The suggested simple and highly efficient technique allows measurement and mapping of Cs soil deposition at these contaminated areas and evaluation of the soil penetration depth of radioactive contamination [10]. An additional advantage of the proposed method is the spatial resolution of &2 m which permits the detailed mapping of heterogeneous soil contamination [11]. The paper describes the proposed method and the software used to process the data obtained. The results obtained by the use of this method and those of the soil sampling, were compared by German experts and a good coincidence in terms of method accuracy was established [12]. The soil Cs contamination was mapped in detail in Bryansk and Chelyabinsk regions in Russia.
2. Description of measuring method The proposed method is based on COllimated RADiometer (CORAD) measurements of a c-ray spectrum. The CORAD is a collimated scintillation gamma detector (see Fig. 1) which is placed in a lead shielding (thickness&30 mm) and measures a photon flux emitted by the soil surface restricted by solid angle of a collimator (&4.5 s, a collimated aperture is &90°). It is placed over the contaminated soil and measures the count rates in three energy ranges: E 400—560 keV (main measuring channel of the spectrometer); E 600—720 keV (the channel corresponding to the photo absorption of Cs radiation); E 720—900 keV (to determine a background radiation of U and Th chains, K and Cs). The collimated radiometer placed over the contaminated soil at a distance h, measures the photon flux emitted by a circular soil surface of r radius. Suppose that q is the homogeneous surface activity of soil contamination. It is known that the count rate N of the detector in a photopeak (a full absorption c-ray peak) is determined by the detection of direct (unscattered) Cs radiation only. The photopeak reading of the detector is connected to
Fig. 1. The view of the CORAD device at the measuring position.
surface activity q by a simple equation N"Seqn/4 ln(1#(r/h))#N , (1) where N is the number of the gamma photons passed through the lead shielding; n is the outcome of photons per one decay; S is the cross-section area of the scintillated crystal; and e is the detector efficiency for the detected radiation. Eq. (1) may be rewritten in the following form: *N"N!N "AqX, (2) where A is the radiometer sensitivity, which is fixed for standard measuring conditions and is determined at the calibration of the radiometer by use of a special Cs flat source (the source area should correspond to the solid collimator angle) and X is the solid angle of the collimator.
338
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
The thickness of the lead shielding is defined by condition *N N . (3) The value of N is measured when the collimator is directed into the top direction. Thus, making two measurements at each point, the average surface activity q of the soil, restricted by the solid collimator angle, could be determined from the photopeak reading of the collimated radiometer. However, this method of surface activity determination by photopeak count rate is only correct when the radionuclides are present in the top thin soil layer. In a real situation, radionuclides penetrate into the soil, the soil scatters and absorbs the gamma radiation and makes this approach to a cesium deposition measurements uninterpretable without knowledge about the vertical distribution of the deposition in the soil. Let’s suppose that the Cs contamination spreads homogeneously inside some soil layer of the thickness Z. A numerical Monte-Carlo simulation and measurement at the volumetric calibration source were performed for the purpose of comparing the apparatus spectra of the radiometer. A set of flat surface Cs sources of the sizes 500;500 mm has been produced. The Russian State Standard Organization has independently certified these sources. The homogeneity of their surface activity is less than $6%. The volumetric source used in laboratory experiments is a flaky set of these flat sources. The spaces between them have been filled by iron sheets of different thickness. The otal square of this volumetric source was near 1500;1500 mm. The interaction of Cs radiation in the iron is the same as in the soil. The main process in the first Cs radiation interaction is the Compton photon scattering by electrons of atoms in both cases. As a consequence, the only important factor for the interaction is the substance density. The different substance densities are easy to take into account. Thus this volumetric source corresponds to the soil completely if the thickness of the source and the thickness of the contaminated soil are measured in terms of the mean free path (mfp) of Cs gamma radiation. The experimental c-ray spectrum of a flat source covered by an iron layer of 20 mm thickness which
Fig. 2. The collimated detector spectra for Cs flat source covered by iron layer of 20 mm thickness. 1 - experimental data; 2 - calculation.
was measured by the CORAD is shown in Fig. 2. The numerical Monte-Carlo simulation of the apparatus spectrum which took into account the repeated interactions of photon in the soil, air and scintillated crystal of the detector were performed. The absorption of direct radiation by the lead of shielding and collimator was also taken into account. The results are shown in Fig. 2. The experimental results are in good agreement with the Monte-Carlo simulation. Numerical Monte-Carlo simulation of spectra for homogeneous depth distribution of Cs contamination up to the penetration depth of 3k (k is the Cs radiation mfp, the depth corresponds near 25—30 cm of the soil) was performed. The results are shown in Fig. 3. The count rate of photopeak range decreases rapidly with the increasing of penetration depth, but it changes very slowly in the energy range of 400—560 keV. Thus, to measure the Cs deposition taking into account its penetration depth in the soil it is necessary to make use of the count rate in the Compton part of the spectrum (N — in the 400— 560 keV range) in addition to count rate in the photopeak (N ). The count rate N is proportional to the value of the cesium deposition and weakly
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
Fig. 3. The Monte-Carlo simulation of collimated detector spectra for Cs contamination distributed homogeneously in the soil. The depth of contaminated layer Z is: 1—Z"0; 2—Z"1; 3—Z"2; 4—Z"3 mfp.
depends on the value of penetration depth. The curve 1 in Fig. 3 corresponds to the spectrum of the flat thin surface source, and curve 4 to the homogeneous source spread up to 3k into the soil. The curves 1 and 2 in Fig. 4 correspond to the same situation, but they show only the direct radiation contribution to the spectrum. The weak influence of radionuclide penetration depth on N is connec ted with the nature of the dependence of the two main components of radiation contributing to the detector count rate in the 400—560 keV energy range as a function of the penetration depth. The direct (unscattered) radiation (first component) decreases with increasing penetration depth as can be seen from Fig. 4 (curves 1 and 2). The number of small angle, singly scattered c-ray quanta in soil (second component) increases with an increasing penetration depth. The first photon interaction in soil is the mainly Compton scattering, since the part of quanta completely absorbed is very small. The quanta scattered in soil at the small angle contribute to the count rate in the 400—560 keV energy range. Curve 3 in Fig. 4 corresponds to the apparatus spectrum of radiation interacted in the
339
Fig. 4. The calculated radiation spectra for the collimated detector from the surface plane Cs source (curve 1). The direct (curve 2) and the scattering (curve 3) calculated radiation spectra from Cs contamination distributed homogeneously in the soil layer with the thickness of Z"2 mfp.
soil. Thus these two processes practically compensate each other, resulting in a weak dependence of N on the value of radionuclide penetration depth. The total count rate in the 400—560 keV energy range is the sum of the radiation interacted in the soil (curve 3) and the direct radiation (curve 2). It almost coincides with the count rate in this energy range for the flat thin surface source (curve 1 in Fig. 3). The weak dependence of N on the value of penetration depth is observed only where the depth does not exceed the value of 3k. It should be noted that the count rate in the indicated Compton part of the spectrum weakly depends not only on the cesium penetration depth into the soil but also on its vertical distribution. The dependence of count rate in the 400—560 keV range on the penetration depth for different characters of vertical distribution is presented in Fig. 5. It shows the ratio of N for the source with the homo geneous, triangular, exponential depth distribution in the soil to N for the thin flat surface source versus the depth in mfp. In reality, the Cs distribution in the soil is a combination of these kinds of
340
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
the penetration depth and on the vertical distribution of Cs in the soil. Nevertheless, the dependence of N /N on penetration depth for homogeneous Cs soil depth distribution may be selected as universal function for other kinds of distribution (for example exponential, triangular or their combinations). Therefore, the thickness of the soil layer containing 80—100% of the Cs contamination may be estimated by using this dependence. This information is sufficient to take the right decision for decontamination actions. The estimation of Cs penetration depth is of important value for calculating the effective dose rate distribution over contaminated areas.
3. Collimated radiometer CORAD Fig. 5. The relative count rate in 400—560 keV energy interval dependence on Cs penetration depth in the soil: 1 - homogeneous distribution; 2 - triangular distribution; 3 - exponential distribution; 4 - thin flat source placed at the fixed depth; 5 - experimental data (䊐).
distributions. It means that the developed method may be used practically in all the cases where the penetration depth does not exceed 3k. It is possible to determine the deposition even in the case where the thin contaminated layer is covered by soil layer of the thickness up to 1.5k. To verify the Monte-Carlo simulation comparison of the results for thin flat source which is covered by soil layer with thickness up to 1.7k with laboratory experiments was performed (see Fig. 5, curves 4 and 5). This figure shows that quantitative and qualitative agreement of the experimental and calculation results is observed within the limits of the experimental error. The comparison shows that the systematic error of Cs contamination definition versus the penetration depth does not exceed 30%. As mentioned above, the count rate in the photo absorption range N strongly depends on the pene tration depth (see Fig. 3). N and N are propor tional to cesium deposition in the soil; therefore, the ratio of these values N /N characterizes the Cs penetration depth. The results of the theoretical analysis show that the ratio of N /N depends on
To realize the technique described above, a new portable RKG-09N device, called CORAD, and an appropriate software were developed [13]. CORAD is mainly intended to perform a radiation survey of vast contaminated areas and has been made in a weatherproof version. Simplicity of application of this device is reached by use of a microprocessor and by complete automation of measurements. The CORAD consists of the collimated scintillation detector with the NaI(Tl) crystals of sizes H50;50 mm, control unit, preamplifier, unit of high-voltage and low-voltage supply, battery, spectrometric amplifier with programmed amplification, 256-channel digital converter, 80C31 microprocessor. RAM enables to store 44 spectra and 1000 measuring results, as well as the memory for processing software. A communication interface RS-232 is used for transferring the data and spectra obtained to computer for subsequent processing. The portable device CORAD weighing about 15 kg allows to make several thousands of measurements during the summer field season. The threshold of the device determination is 20 kBq/m. The exposed time of measurement does not exceed 10 min. The software of the device permits to accumulate spectrum, to process it and to determine the surface activity in-situ. The modes of measurement and processing results are completely automated. The collimated detector is placed at a height of 80 cm
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
over the ground surface and measures gamma radiation from the area of 2 m. The principle and order of measurement incorporated in the radiometer software correspond to the measuring technique mentioned above [14]. According to this method, the three energy ranges are selected: 400—560 keV — the main energy range; 600—720 keV — the photo absorption range; 740—900 keV — the auxiliary one. Count rates N , N and N in these ranges (the background stipulated by radiation passing through the shield of the detector is subtracted) are defined. Cesium deposition in soil G (G "bN ) is calculated from the count rate in the main energy range. The factor of proportionality b is determined at the certification of the device by the use of the flat Cs source. N corresponds to the photo absorp tion range and defines effective surface density of activity A"cN . The factor c is determined at the calibration from condition G"A for the flat certified surface source. The count rate N determines a contribution of background components stipulated by Cs radiation, natural radionuclides of U, U and Th chains, and K in main energy and photo absorption ranges. The ratio G/A evaluates the thickness of the soil layer containing more than 80% of the total cesium deposition. In the latest versions of the algorithm the correction of the deposition G* "K (G/A);G with the view of the existing weak dependence G on cesium penetration depth is incorporated. The functional dependencies of cesium penetration depth and correction factor K on G/A are deter mined by preliminary Monte-Carlo simulation and by the results of laboratory experiments with a volumetric source.
4. Software ACTIV The software ACTIV is used to process all the data obtained and to represent them in the form of a digitized map. The software ACTIV is an applied system developed for personal computers which works with measurements data of the surface activity and penetration depth, maps the Cs contamination and calculates the Effective Dose Rate (EDR). The measurement data and the coordinates
341
of the points of measuring should be input into the computer as initial data for the ACTIV. The ACTIV has three groups of executable programs. The first group maps the surveyed area and automatically inputs data to the digitized map from the device. The second group of programs processes the data and represents them. It begins with Delaunay triangulation for a set of initial data. The Delaunay triangulation proceeds data obtained from local representation to their continuous distribution. A continuous function, accepting the value of the data obtained at the points of measuring, is under construction on a triangulation grid. Pass from local to continuous measurements data distribution is used to perform various evident presentations of measurements data. The program calculates the isolines of given surface activity and shades, or colors, the intervals between them. The continuous representation of data allows to perform simulation of removing the top soil layer and evaluation of decontamination efficiency. The last group of programs calculates a space EDR distribution over a real soil surface when the Cs contamination distribution is known [15]. The accumulated experimental data, obtained in results of the long-term measurements in Bryansk and Chelyabinsk regions of Russia, show that an error of EDR calculations performed is not more than 20%. 5. Results of CS contamination measurements The developed method and the device CORAD were used to perform the radiation survey of contaminated areas in different regions of the former SU [16]. The larger part of measurements was made in Bryansk region (Russia). The results of the measurements carried out in village Zaborie near the river Oleshnia at Krasnogorskiy district (220 km to the northeast from Chernobyl NPP), are presented in Fig. 6. Fig. 6 shows three maps of southeast part of the village which characterize the Cs contamination: the deposition distribution (Fig. 6A), the penetration depth of contamination in the soil
342
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
Fig. 6. Cs contamination distribution in south-east part of village Zaborie: (A) - deposition; (B) - contamination depth; (C) - effective dose rate.
(Fig. 6B) and EDR calculated at height of 1 m over the soil surface (Fig. 6C). The total number of measuring points is more than 4000 for the whole village and about 400 for the presented map. The measurements were performed along the net with the distance of 10—20 m between the points. At the places where the gradients of contamination are very high, the distance between the points of measuring was 2 m. The average measured Cs deposition for presented map is 3.2 MBq/m. The local measured value varies from several dozens of kBq/m at decontaminated roads and some private farmsteads up to 34 MBq/m at two hot spots. One can see the larger in size spot in the northern part of the presented map. It is located on a small elevation. The reason of high-deposition value here is unknown. The possible explanation of this fact could be a lightning, which may had happened at the moment of the fallout. The small spot is between the river and the pond (see the southern part of the map) at one of the deepest places. It has been forming during several years as a result of horizontal radionuclide migration with water streams after
rains and snow melting. The highest surface activities are frequently observed at the banks of ponds. The Cs deposition changes from average up to the maximum value at a distance of less then 5 m for both places. The average contamination for the village is 2.4 MBq/m. Any possible data processing is available. One may calculate the square of the area contaminated within the established levels, simulate the dose rate change when the top contaminated soil layer is removed and the area is covered by the clean sand or ground layer of certain thickness. Fig. 6B shows the distribution of the penetration depth. The thickness of contaminated layer is expressed in mfp of 662 keV radiation in the soil (to determine the thickness in cm one should use the formula ¸ (cm)"12.9;¸ (mfp)/o, where o(g/cm) is the soil density). The measured values change from 0.5 to 2.5 mfp. The EDR values calculated for measured contamination (see Fig. 6C) vary from 0.5 up to 6 lSv/h. The same measurements in contaminated flood plain of the Techa river (during 1949—1954 a facility called “Mayak” released nuclear wastes in water
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
343
6. Conclusions The method and device allowing to measure the gamma-radiated nuclides soil contamination in situ were developed. The method determines the Cs soil deposition with spatial resolution of 2 m and evaluates the thickness of soil layer containing more than 80% of total deposition. The efficiency of the method described above is higher than that of the traditional techniques. This method is very useful for the detailed mapping of vast Cs-contaminated areas and settlements. The results obtained are in good agreement with those of the sampling method. The contamination digitized maps of vast areas contaminated as a result of Chernobyl accident and “Mayak” facility release of nuclear wastes are developed.
Fig. 7. The comparison of sampling (G ) and CORAD 1 +.*',% (G ) measuring data obtained in settlement Brodokalmak !-0 " (Chelaybinsk region, South Ural in Russia).
system of this river [17]) were performed in 1994— 1996. Fig. 7 shows a comparison of the sampling method and CORAD measurements in settlement Brodokalmak (Chelybinsk region in Ural of Russia). The detailed study of the data shows a divergence between values measured by the laboratory sample method and field radiometry in some points. The main reason for this divergence is an essential difference between analyzed areas. The CORAD device integrates a photon flux from 2.0 m of the soil and the sample has a total square near 0.01 m. Secondly, the CORAD reliably detects quanta from the soil layer to a depth of not less than 30 cm, and the samples were taken up to 20 cm in depth (most of the points in Fig. 6 lay above the solid line corresponding to equaled means). This divergence can be reduced if the area and the depth of samples are increased. The significant divergence of Cs deposition determination is observed for levels less than the device threshold of 20 kBq/m (see in Fig. 6 points lay below mean of 1). Nevertheless, a good coincidence of data is observed for three decades of soil deposition means.
References [1] L.I. Boltneva, U.A. Izrael, V.A. Ionov, I.M. Nazarov, Atomnaya energia 42 (5) (1977) 355 (in Russian). [2] Sh.D. Fridman, A.N. Pegoev, Method and some results of air gamma survey of radioactive contamination of European part of Russia, Gidrometeoizdat, Sankt-Peterburg, 1994 (in Russian). [3] K.P. Mahan’ko, A.N. Silant’ev, I.G. Shkuratova, The control on radioactive contamination of environment around NPP, Gidrometioizdat, Leningrad, 1985 (in Russian). [4] I. Winkelmann, M. Thomas, G. Carloff, Kerntechnik 62 (2—3) (1997) 118. [5] Yu.F. Rodionov, N.P. Yashin, VANT, ser. Nuclear-Physical Investigations (theory and experiment) 12 (2) (1989) 50 (in Russian). [6] S.K. Lisin, Yu.F. Rodionov, N.D. Slonov et al., VANT, Ser. Nuclear-Physical investigations (theory and experiment) 13 (5) (1990) 142 (in Russian). [7] H.L. Beck, J.A. DeCampo, C.V. Gogolak, In situ Ge(Li) and NaI(Tl) c-ray spectrometry, HASL Report No.HASL-258, 1972. [8] C.V. Gogolak, K.M. Millier, New development in field gamma-ray spectrometry, USDOE Report No.EML-332, 1977. [9] G.I. Borisov, L.I. Govor, V.A. Kurkin, VANT, ser. Nuclear-Physical investigations (theory and experiment) 12 (2) (1989) 53 (in Russian). [10] A.G. Volkovich, V.I. Liksonov, D.A. Lobanovskii et al., Atomnaya Energiya 69 (4) (1990) 259 (in Russian). [11] A.V. Chesnokov, A.P. Govorun, O.P. Ivanov et al., IEEE Trans, Nucl. Sci. NS- 44 (3) (1997) 769. [12] K. Heinemann, R. Hille, Kerntechnik 62 (2—3) (1997) 113.
344
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
[13] A.P. Govorun, V.I. Liksonov, V.P. Romashko et al., Pribory i technika experimenta 5 (1994) 207 (in Russian). [14] A.P. Govorun, V.I. Liksonov, V.N. Potapov et al., Atomnaya Energiya 78 (3) (1995) 199 (in Russian). [15] O.P. Ivanov, V.N. Potapov, S.B. Shcherbak, Atomnaya Energiya 79 (2) (1995) 130 (in Russian).
[16] D.A. Lobanovskii, S.V. Smirnov, V.E. Stepanov et al., Distribution of the contamination density in cesium radionuclides at the Partizan settlement, the Tula region, Proc. Germany—Russian Conf. on Results of Joint Measuring Program in Russia, Moscow, October 1991, pp. 45. [17] V.A. Yachmenev, L.W. Isageva, Health Phy. 71 (1) (1996) 61.
Method and device to measure Cs soil contamination in-situ A.V. Chesnokov*, A.P. Govorun, V.N. Fedin, O.P. Ivanov, V.I. Liksonov, V.N. Potapov, S.B. Shcherbak, S.V. Smirnov, L.I. Urutskoev RECOM LTD, Russian research center **Kurchatov Institute++ Schukinskaya St., 12-1, Moscow 123182, Russia Received 24 February 1998; received in revised form 29 May 1998
Abstract A method to measure the Cs soil contamination in situ is described. Based upon the measurements of a c-ray spectrum of the soil surface, this method allows to determine the soil deposition with an accuracy of 20% and to evaluate the thickness of the soil layer containing more than 80% of the contamination. The article contains the description of a portable device CORAD based on this method and of the software used to process the obtained data. The exposition time of CORAD does not exceed 10 min if the Cs deposition is more than 20 kBq/m. The results obtained by use of this method and those of the soil sampling are compared. The developed Cs contamination maps are presented. These maps can be used for rehabilitation simulation. Effective dose rate distributions may be calculated on the basis of the obtained data. 1999 Elsevier Science B.V. All rights reserved. Keywords: Cs soil contamination; Gamma radiation; Collimated detector
1. Introduction The need for detailed radioactive contamination measurements had arisen in connection with the first nuclear tests [1]. The same problem should be solved for areas contaminated as a result of the Chernobyl NPP accident [2], as vast territories are contaminated very heterogeneously. The scale of this heterogeneity varies from several meters to several hundreds meters and depends on many factors. There exists a set of traditional methods used to measure the soil radioactive contamination. The air
* Corresponding author: Tel: 007 095 1967665; 007 095 1961635; e-mail: [email protected].
Fax:
gamma survey is one of them [3,4]. This method allows to measure the contamination with spatial resolution near 100 m. Another type is the sampling method. This technique is known to be the most reliable now [5,6], yet the most laborious one. The spatial resolution of this technique is near 0.1 m and a large number of measurements is required in order to map the contamination in detail. Yet another method of field gamma spectrometry [7—9] has been proposed to reach the productivity of the air gamma survey and the reliability of the sampling method. The complex contamination of gamma emitting radionuclides is measured by the traditional methods. But now the main gamma emitting isotope at vast territories contaminated as a result of the Chernobyl and the South Ural accidents and
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 6 1 - X
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
337
release of nuclear facilities is Cs. The suggested simple and highly efficient technique allows measurement and mapping of Cs soil deposition at these contaminated areas and evaluation of the soil penetration depth of radioactive contamination [10]. An additional advantage of the proposed method is the spatial resolution of &2 m which permits the detailed mapping of heterogeneous soil contamination [11]. The paper describes the proposed method and the software used to process the data obtained. The results obtained by the use of this method and those of the soil sampling, were compared by German experts and a good coincidence in terms of method accuracy was established [12]. The soil Cs contamination was mapped in detail in Bryansk and Chelyabinsk regions in Russia.
2. Description of measuring method The proposed method is based on COllimated RADiometer (CORAD) measurements of a c-ray spectrum. The CORAD is a collimated scintillation gamma detector (see Fig. 1) which is placed in a lead shielding (thickness&30 mm) and measures a photon flux emitted by the soil surface restricted by solid angle of a collimator (&4.5 s, a collimated aperture is &90°). It is placed over the contaminated soil and measures the count rates in three energy ranges: E 400—560 keV (main measuring channel of the spectrometer); E 600—720 keV (the channel corresponding to the photo absorption of Cs radiation); E 720—900 keV (to determine a background radiation of U and Th chains, K and Cs). The collimated radiometer placed over the contaminated soil at a distance h, measures the photon flux emitted by a circular soil surface of r radius. Suppose that q is the homogeneous surface activity of soil contamination. It is known that the count rate N of the detector in a photopeak (a full absorption c-ray peak) is determined by the detection of direct (unscattered) Cs radiation only. The photopeak reading of the detector is connected to
Fig. 1. The view of the CORAD device at the measuring position.
surface activity q by a simple equation N"Seqn/4 ln(1#(r/h))#N , (1) where N is the number of the gamma photons passed through the lead shielding; n is the outcome of photons per one decay; S is the cross-section area of the scintillated crystal; and e is the detector efficiency for the detected radiation. Eq. (1) may be rewritten in the following form: *N"N!N "AqX, (2) where A is the radiometer sensitivity, which is fixed for standard measuring conditions and is determined at the calibration of the radiometer by use of a special Cs flat source (the source area should correspond to the solid collimator angle) and X is the solid angle of the collimator.
338
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
The thickness of the lead shielding is defined by condition *N N . (3) The value of N is measured when the collimator is directed into the top direction. Thus, making two measurements at each point, the average surface activity q of the soil, restricted by the solid collimator angle, could be determined from the photopeak reading of the collimated radiometer. However, this method of surface activity determination by photopeak count rate is only correct when the radionuclides are present in the top thin soil layer. In a real situation, radionuclides penetrate into the soil, the soil scatters and absorbs the gamma radiation and makes this approach to a cesium deposition measurements uninterpretable without knowledge about the vertical distribution of the deposition in the soil. Let’s suppose that the Cs contamination spreads homogeneously inside some soil layer of the thickness Z. A numerical Monte-Carlo simulation and measurement at the volumetric calibration source were performed for the purpose of comparing the apparatus spectra of the radiometer. A set of flat surface Cs sources of the sizes 500;500 mm has been produced. The Russian State Standard Organization has independently certified these sources. The homogeneity of their surface activity is less than $6%. The volumetric source used in laboratory experiments is a flaky set of these flat sources. The spaces between them have been filled by iron sheets of different thickness. The otal square of this volumetric source was near 1500;1500 mm. The interaction of Cs radiation in the iron is the same as in the soil. The main process in the first Cs radiation interaction is the Compton photon scattering by electrons of atoms in both cases. As a consequence, the only important factor for the interaction is the substance density. The different substance densities are easy to take into account. Thus this volumetric source corresponds to the soil completely if the thickness of the source and the thickness of the contaminated soil are measured in terms of the mean free path (mfp) of Cs gamma radiation. The experimental c-ray spectrum of a flat source covered by an iron layer of 20 mm thickness which
Fig. 2. The collimated detector spectra for Cs flat source covered by iron layer of 20 mm thickness. 1 - experimental data; 2 - calculation.
was measured by the CORAD is shown in Fig. 2. The numerical Monte-Carlo simulation of the apparatus spectrum which took into account the repeated interactions of photon in the soil, air and scintillated crystal of the detector were performed. The absorption of direct radiation by the lead of shielding and collimator was also taken into account. The results are shown in Fig. 2. The experimental results are in good agreement with the Monte-Carlo simulation. Numerical Monte-Carlo simulation of spectra for homogeneous depth distribution of Cs contamination up to the penetration depth of 3k (k is the Cs radiation mfp, the depth corresponds near 25—30 cm of the soil) was performed. The results are shown in Fig. 3. The count rate of photopeak range decreases rapidly with the increasing of penetration depth, but it changes very slowly in the energy range of 400—560 keV. Thus, to measure the Cs deposition taking into account its penetration depth in the soil it is necessary to make use of the count rate in the Compton part of the spectrum (N — in the 400— 560 keV range) in addition to count rate in the photopeak (N ). The count rate N is proportional to the value of the cesium deposition and weakly
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
Fig. 3. The Monte-Carlo simulation of collimated detector spectra for Cs contamination distributed homogeneously in the soil. The depth of contaminated layer Z is: 1—Z"0; 2—Z"1; 3—Z"2; 4—Z"3 mfp.
depends on the value of penetration depth. The curve 1 in Fig. 3 corresponds to the spectrum of the flat thin surface source, and curve 4 to the homogeneous source spread up to 3k into the soil. The curves 1 and 2 in Fig. 4 correspond to the same situation, but they show only the direct radiation contribution to the spectrum. The weak influence of radionuclide penetration depth on N is connec ted with the nature of the dependence of the two main components of radiation contributing to the detector count rate in the 400—560 keV energy range as a function of the penetration depth. The direct (unscattered) radiation (first component) decreases with increasing penetration depth as can be seen from Fig. 4 (curves 1 and 2). The number of small angle, singly scattered c-ray quanta in soil (second component) increases with an increasing penetration depth. The first photon interaction in soil is the mainly Compton scattering, since the part of quanta completely absorbed is very small. The quanta scattered in soil at the small angle contribute to the count rate in the 400—560 keV energy range. Curve 3 in Fig. 4 corresponds to the apparatus spectrum of radiation interacted in the
339
Fig. 4. The calculated radiation spectra for the collimated detector from the surface plane Cs source (curve 1). The direct (curve 2) and the scattering (curve 3) calculated radiation spectra from Cs contamination distributed homogeneously in the soil layer with the thickness of Z"2 mfp.
soil. Thus these two processes practically compensate each other, resulting in a weak dependence of N on the value of radionuclide penetration depth. The total count rate in the 400—560 keV energy range is the sum of the radiation interacted in the soil (curve 3) and the direct radiation (curve 2). It almost coincides with the count rate in this energy range for the flat thin surface source (curve 1 in Fig. 3). The weak dependence of N on the value of penetration depth is observed only where the depth does not exceed the value of 3k. It should be noted that the count rate in the indicated Compton part of the spectrum weakly depends not only on the cesium penetration depth into the soil but also on its vertical distribution. The dependence of count rate in the 400—560 keV range on the penetration depth for different characters of vertical distribution is presented in Fig. 5. It shows the ratio of N for the source with the homo geneous, triangular, exponential depth distribution in the soil to N for the thin flat surface source versus the depth in mfp. In reality, the Cs distribution in the soil is a combination of these kinds of
340
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
the penetration depth and on the vertical distribution of Cs in the soil. Nevertheless, the dependence of N /N on penetration depth for homogeneous Cs soil depth distribution may be selected as universal function for other kinds of distribution (for example exponential, triangular or their combinations). Therefore, the thickness of the soil layer containing 80—100% of the Cs contamination may be estimated by using this dependence. This information is sufficient to take the right decision for decontamination actions. The estimation of Cs penetration depth is of important value for calculating the effective dose rate distribution over contaminated areas.
3. Collimated radiometer CORAD Fig. 5. The relative count rate in 400—560 keV energy interval dependence on Cs penetration depth in the soil: 1 - homogeneous distribution; 2 - triangular distribution; 3 - exponential distribution; 4 - thin flat source placed at the fixed depth; 5 - experimental data (䊐).
distributions. It means that the developed method may be used practically in all the cases where the penetration depth does not exceed 3k. It is possible to determine the deposition even in the case where the thin contaminated layer is covered by soil layer of the thickness up to 1.5k. To verify the Monte-Carlo simulation comparison of the results for thin flat source which is covered by soil layer with thickness up to 1.7k with laboratory experiments was performed (see Fig. 5, curves 4 and 5). This figure shows that quantitative and qualitative agreement of the experimental and calculation results is observed within the limits of the experimental error. The comparison shows that the systematic error of Cs contamination definition versus the penetration depth does not exceed 30%. As mentioned above, the count rate in the photo absorption range N strongly depends on the pene tration depth (see Fig. 3). N and N are propor tional to cesium deposition in the soil; therefore, the ratio of these values N /N characterizes the Cs penetration depth. The results of the theoretical analysis show that the ratio of N /N depends on
To realize the technique described above, a new portable RKG-09N device, called CORAD, and an appropriate software were developed [13]. CORAD is mainly intended to perform a radiation survey of vast contaminated areas and has been made in a weatherproof version. Simplicity of application of this device is reached by use of a microprocessor and by complete automation of measurements. The CORAD consists of the collimated scintillation detector with the NaI(Tl) crystals of sizes H50;50 mm, control unit, preamplifier, unit of high-voltage and low-voltage supply, battery, spectrometric amplifier with programmed amplification, 256-channel digital converter, 80C31 microprocessor. RAM enables to store 44 spectra and 1000 measuring results, as well as the memory for processing software. A communication interface RS-232 is used for transferring the data and spectra obtained to computer for subsequent processing. The portable device CORAD weighing about 15 kg allows to make several thousands of measurements during the summer field season. The threshold of the device determination is 20 kBq/m. The exposed time of measurement does not exceed 10 min. The software of the device permits to accumulate spectrum, to process it and to determine the surface activity in-situ. The modes of measurement and processing results are completely automated. The collimated detector is placed at a height of 80 cm
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
over the ground surface and measures gamma radiation from the area of 2 m. The principle and order of measurement incorporated in the radiometer software correspond to the measuring technique mentioned above [14]. According to this method, the three energy ranges are selected: 400—560 keV — the main energy range; 600—720 keV — the photo absorption range; 740—900 keV — the auxiliary one. Count rates N , N and N in these ranges (the background stipulated by radiation passing through the shield of the detector is subtracted) are defined. Cesium deposition in soil G (G "bN ) is calculated from the count rate in the main energy range. The factor of proportionality b is determined at the certification of the device by the use of the flat Cs source. N corresponds to the photo absorp tion range and defines effective surface density of activity A"cN . The factor c is determined at the calibration from condition G"A for the flat certified surface source. The count rate N determines a contribution of background components stipulated by Cs radiation, natural radionuclides of U, U and Th chains, and K in main energy and photo absorption ranges. The ratio G/A evaluates the thickness of the soil layer containing more than 80% of the total cesium deposition. In the latest versions of the algorithm the correction of the deposition G* "K (G/A);G with the view of the existing weak dependence G on cesium penetration depth is incorporated. The functional dependencies of cesium penetration depth and correction factor K on G/A are deter mined by preliminary Monte-Carlo simulation and by the results of laboratory experiments with a volumetric source.
4. Software ACTIV The software ACTIV is used to process all the data obtained and to represent them in the form of a digitized map. The software ACTIV is an applied system developed for personal computers which works with measurements data of the surface activity and penetration depth, maps the Cs contamination and calculates the Effective Dose Rate (EDR). The measurement data and the coordinates
341
of the points of measuring should be input into the computer as initial data for the ACTIV. The ACTIV has three groups of executable programs. The first group maps the surveyed area and automatically inputs data to the digitized map from the device. The second group of programs processes the data and represents them. It begins with Delaunay triangulation for a set of initial data. The Delaunay triangulation proceeds data obtained from local representation to their continuous distribution. A continuous function, accepting the value of the data obtained at the points of measuring, is under construction on a triangulation grid. Pass from local to continuous measurements data distribution is used to perform various evident presentations of measurements data. The program calculates the isolines of given surface activity and shades, or colors, the intervals between them. The continuous representation of data allows to perform simulation of removing the top soil layer and evaluation of decontamination efficiency. The last group of programs calculates a space EDR distribution over a real soil surface when the Cs contamination distribution is known [15]. The accumulated experimental data, obtained in results of the long-term measurements in Bryansk and Chelyabinsk regions of Russia, show that an error of EDR calculations performed is not more than 20%. 5. Results of CS contamination measurements The developed method and the device CORAD were used to perform the radiation survey of contaminated areas in different regions of the former SU [16]. The larger part of measurements was made in Bryansk region (Russia). The results of the measurements carried out in village Zaborie near the river Oleshnia at Krasnogorskiy district (220 km to the northeast from Chernobyl NPP), are presented in Fig. 6. Fig. 6 shows three maps of southeast part of the village which characterize the Cs contamination: the deposition distribution (Fig. 6A), the penetration depth of contamination in the soil
342
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
Fig. 6. Cs contamination distribution in south-east part of village Zaborie: (A) - deposition; (B) - contamination depth; (C) - effective dose rate.
(Fig. 6B) and EDR calculated at height of 1 m over the soil surface (Fig. 6C). The total number of measuring points is more than 4000 for the whole village and about 400 for the presented map. The measurements were performed along the net with the distance of 10—20 m between the points. At the places where the gradients of contamination are very high, the distance between the points of measuring was 2 m. The average measured Cs deposition for presented map is 3.2 MBq/m. The local measured value varies from several dozens of kBq/m at decontaminated roads and some private farmsteads up to 34 MBq/m at two hot spots. One can see the larger in size spot in the northern part of the presented map. It is located on a small elevation. The reason of high-deposition value here is unknown. The possible explanation of this fact could be a lightning, which may had happened at the moment of the fallout. The small spot is between the river and the pond (see the southern part of the map) at one of the deepest places. It has been forming during several years as a result of horizontal radionuclide migration with water streams after
rains and snow melting. The highest surface activities are frequently observed at the banks of ponds. The Cs deposition changes from average up to the maximum value at a distance of less then 5 m for both places. The average contamination for the village is 2.4 MBq/m. Any possible data processing is available. One may calculate the square of the area contaminated within the established levels, simulate the dose rate change when the top contaminated soil layer is removed and the area is covered by the clean sand or ground layer of certain thickness. Fig. 6B shows the distribution of the penetration depth. The thickness of contaminated layer is expressed in mfp of 662 keV radiation in the soil (to determine the thickness in cm one should use the formula ¸ (cm)"12.9;¸ (mfp)/o, where o(g/cm) is the soil density). The measured values change from 0.5 to 2.5 mfp. The EDR values calculated for measured contamination (see Fig. 6C) vary from 0.5 up to 6 lSv/h. The same measurements in contaminated flood plain of the Techa river (during 1949—1954 a facility called “Mayak” released nuclear wastes in water
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
343
6. Conclusions The method and device allowing to measure the gamma-radiated nuclides soil contamination in situ were developed. The method determines the Cs soil deposition with spatial resolution of 2 m and evaluates the thickness of soil layer containing more than 80% of total deposition. The efficiency of the method described above is higher than that of the traditional techniques. This method is very useful for the detailed mapping of vast Cs-contaminated areas and settlements. The results obtained are in good agreement with those of the sampling method. The contamination digitized maps of vast areas contaminated as a result of Chernobyl accident and “Mayak” facility release of nuclear wastes are developed.
Fig. 7. The comparison of sampling (G ) and CORAD 1 +.*',% (G ) measuring data obtained in settlement Brodokalmak !-0 " (Chelaybinsk region, South Ural in Russia).
system of this river [17]) were performed in 1994— 1996. Fig. 7 shows a comparison of the sampling method and CORAD measurements in settlement Brodokalmak (Chelybinsk region in Ural of Russia). The detailed study of the data shows a divergence between values measured by the laboratory sample method and field radiometry in some points. The main reason for this divergence is an essential difference between analyzed areas. The CORAD device integrates a photon flux from 2.0 m of the soil and the sample has a total square near 0.01 m. Secondly, the CORAD reliably detects quanta from the soil layer to a depth of not less than 30 cm, and the samples were taken up to 20 cm in depth (most of the points in Fig. 6 lay above the solid line corresponding to equaled means). This divergence can be reduced if the area and the depth of samples are increased. The significant divergence of Cs deposition determination is observed for levels less than the device threshold of 20 kBq/m (see in Fig. 6 points lay below mean of 1). Nevertheless, a good coincidence of data is observed for three decades of soil deposition means.
References [1] L.I. Boltneva, U.A. Izrael, V.A. Ionov, I.M. Nazarov, Atomnaya energia 42 (5) (1977) 355 (in Russian). [2] Sh.D. Fridman, A.N. Pegoev, Method and some results of air gamma survey of radioactive contamination of European part of Russia, Gidrometeoizdat, Sankt-Peterburg, 1994 (in Russian). [3] K.P. Mahan’ko, A.N. Silant’ev, I.G. Shkuratova, The control on radioactive contamination of environment around NPP, Gidrometioizdat, Leningrad, 1985 (in Russian). [4] I. Winkelmann, M. Thomas, G. Carloff, Kerntechnik 62 (2—3) (1997) 118. [5] Yu.F. Rodionov, N.P. Yashin, VANT, ser. Nuclear-Physical Investigations (theory and experiment) 12 (2) (1989) 50 (in Russian). [6] S.K. Lisin, Yu.F. Rodionov, N.D. Slonov et al., VANT, Ser. Nuclear-Physical investigations (theory and experiment) 13 (5) (1990) 142 (in Russian). [7] H.L. Beck, J.A. DeCampo, C.V. Gogolak, In situ Ge(Li) and NaI(Tl) c-ray spectrometry, HASL Report No.HASL-258, 1972. [8] C.V. Gogolak, K.M. Millier, New development in field gamma-ray spectrometry, USDOE Report No.EML-332, 1977. [9] G.I. Borisov, L.I. Govor, V.A. Kurkin, VANT, ser. Nuclear-Physical investigations (theory and experiment) 12 (2) (1989) 53 (in Russian). [10] A.G. Volkovich, V.I. Liksonov, D.A. Lobanovskii et al., Atomnaya Energiya 69 (4) (1990) 259 (in Russian). [11] A.V. Chesnokov, A.P. Govorun, O.P. Ivanov et al., IEEE Trans, Nucl. Sci. NS- 44 (3) (1997) 769. [12] K. Heinemann, R. Hille, Kerntechnik 62 (2—3) (1997) 113.
344
A.V. Chesnokov et al. /Nucl. Instr. and Meth. in Phys. Res. A 420 (1999) 336—344
[13] A.P. Govorun, V.I. Liksonov, V.P. Romashko et al., Pribory i technika experimenta 5 (1994) 207 (in Russian). [14] A.P. Govorun, V.I. Liksonov, V.N. Potapov et al., Atomnaya Energiya 78 (3) (1995) 199 (in Russian). [15] O.P. Ivanov, V.N. Potapov, S.B. Shcherbak, Atomnaya Energiya 79 (2) (1995) 130 (in Russian).
[16] D.A. Lobanovskii, S.V. Smirnov, V.E. Stepanov et al., Distribution of the contamination density in cesium radionuclides at the Partizan settlement, the Tula region, Proc. Germany—Russian Conf. on Results of Joint Measuring Program in Russia, Moscow, October 1991, pp. 45. [17] V.A. Yachmenev, L.W. Isageva, Health Phy. 71 (1) (1996) 61.