XA TERRESTRIAL GAMMA DOSE RATE MAPS, THEIR COMPILATION AND VERIFICATION RADIOMETRIC MAP OF THE CZECH REPUBLIC

Transcription

1 TERRESTRIAL GAMMA DOSE RATE MAPS, THEIR COMPILATION AND VERIFICATION RADIOMETRIC MAP OF THE CZECH REPUBLIC XA M. MATOLIN Faculty of Science, Charles University, Prague, Czech Republic Abstract Maps of terrestrial gamma dose rate, usually expressed in ngy.h" 1, specify the natural radiation environment. Technique of measurement, data processing and compilation of radiometric maps contribute to resultant data deviations. Substantial influence on results may have technical parameters of used instruments, calibration facilities and method of instrument calibration, geometry, density and the mode of field radiometric measurement, data processing, data levelling and their graphical presentation. If maps are used for the assessment of the natural radiation environment, reliability of reported gamma dose rate values must be acceptable and should be checked. Radiometric map of the Czech Republic 1: , published in 1995, expressed in gamma dose rate, is based on regional and detailed airborne total count ( ) and gamma ray spectrometry (from 1976 onward) measurement, completed by ground investigations. Back calibration was applied to convert the data to dose rate and to level the map. Regional terrestrial radiation in the Czech Republic, formed by magmatic, sedimentary and metamorpbic rocks, is in the range ngy.h" 1, with the mean 65.6 ± 19.0 ngy.h" 1. Preliminary verification of data reported in the radiometric map, carried out by ground gamma ray spectrometry regional traverses, showed a good map data levelling, while the mean deviation ± 13.8 ngy.h" 1 illustrates expected differences at individual sites and geological setting. 1. MAPS OF THE NATURAL RADIATION ENVIRONMENT Terrestrial radiation, causing substantial component of the natural radiation environment, varies with the geological setting at the earth surface. Potassium, uranium and thorium are fundamental sources of radioactivity of rocks. Radiometric maps of terrestrial gamma dose rate are used in geological investigations, exploration of radioactive raw materials, environmental studies, delimitations of regions of radon risk, estimation of absorbed radiation dose from terrestrial radiation and they provide a base, against which man made radiation can be determined. A series of radiometric maps has been published [1]. 2. CAUSES OF RADIOMETRIC DATA DEVIATIONS Terrestrial gamma dose rate can be well determined at particular stations by means of pressurized ionization chambers. Standard geophysical equipment, its regional survey application and methods of data processing may yield deviations from local dose rate values. There are several factors influencing resultant radiometric data. The field of gamma radiation, with natural abrupt changes, due to uneven distribution of natural radionuclides in rocks, is by routine technique determined as spatial average value, in dependence on the method of measurement. Gamma total count instruments and their response to terrestrial gamma radiation are fundamentally sensitive to type and size of used detectors and equipment energy discrimination threshold. Small size Nal(Tl) scintillation detectors and low ( kev) energy discrimination thresholds are mostly used. Response of these instruments to gamma radiation of K, U and Th in rocks need not to be proportional to the dose rate. The deviations depending on equipment parameters and on the ratios of K, U, Th in rocks, can be up to 20% relatively [2]. The range of deviations can 53

2 be estimated by equipment uranium equivalents of potassium and thorium with respect to ideal values 2.3 ppm U/l % K and 0.44 ppm U/l ppm Th [2, 3]. Range of deviations has been calculated and analyzed [4]. Calibration of total count instruments by means of a ^Ra point source generally results in an overestimation of reported dose rates. Calibration over small size calibration pads, that do not generate adequate air downward backscattered radiation, does not correspond to radiation of natural infinite plane source. Gamma ray spectrometers, determining concentrations of K, U, Th in rocks, that can be converted to gamma dose rate [2], are used in airborne and ground measurements. Calibration facilities and calibration procedures are essential for the correctness of field results. Intercomparison measurement of calibration pads in various countries showed the differencies and inaccuracy in their reported concentrations of radionuclides [5]. Further, correction for the geometry of small size calibration pads can be critical. Inconvinient material for their construction may result in gradual internal geochemical changes, pad decomposition, radon escape and instability in radiation. Instability of equipment function, causing gamma energy spectrum shift, contributes to instrument output data errors. Sensitivity of airborne gamma ray spectrometers play an important role in depression of radioactivity fluctuations and statistical data processing. Large volume Nal(Tl) scintillation detectors are preferable for regional airborne gamma ray spectrometry, though semiconductor detectors has been also tested [6]. Geometry of gamma ray detection and width range play an important role for compilation of regional terrestrial radiation maps. Flight height and line spacing of airborne measurement determine the percentage of area covered. Climatic changes, affecting the variation of air density and air radon concentration may result in inconsistency of data measured in various periods. Attenuation of gamma rays by biomass, soil moisture and water planes is essential namely for the airborne measurement, and may contribute to deviations of tens of percent relatively [7]. Unlike to simple processing of ground gamma ray measurent, the airborne data processing, implying steps of data filtering, background correction, air radon correction, stripping, altitude correction and conversion to element concentration, depends on series of introduced constants [8]. Compilation of radiometric maps may imply digitalization, gridding, filtering, interpolation of digital data and their graphical presentation, realized by contour lines or pixels [9]. Though the theory of radiometric measurement, field procedures and data processing has been intensively studied and developed in in the last 30 years, the nature of observed physical field and a series of input and correction constants may affect resultant values of dose rate reported in radiometric maps. 3. RADIOMETRIC MEASUREMENT OF THE CZECH REPUBLIC Airborne and ground radiometric measurement were applied for uranium exploration and radiometric mapping in the Czech Republic in past decades. Regional 1: (flight lines separation 2000 m, flight height 100 m, period ) and detailed 1: (flight lines separation 250 m, flight height 80 m, period ) airborne gamma total count measurement covered the whole area of the country. Airborne equipment was calibrated by means of a a6 Ra point source and the maps of contour lines were expressed in exposure rate (jxr/h). Detailed 1: airborne gamma ray spectrometry (from 1976 onward) with digital, spectrum stabilized airborne spectrometers DiGRS 3001 of cm 3 Nal(Tl) volume and GR 800D of cm 3 Nal (Tl) volume at flight height 80 m covered 50% of the country. The instruments were calibrated by calibration standards and at airborne natural calibration strips. Maps of total count and K, U, Th concentrations and their ratios were compiled. Car-borne, ground, logging and laboratory radiometric measurements were used too. 54

3 4. BACK CALIBRATION Original total count airborne maps, checked and completed by airborne gamma ray spectrometry, were back calibrated in order to level the data and to convert them into gamma dose rate. A ground portable gamma ray spectrometer GS-256 was calibrated at calibration facilities Bratkovice (Czech Republic) and Langenlebam (Austria) and verified by intercomparison measurement (in Berlin 1992) with a Geological Survey of Canada gamma ray spectrometer GR-256, calibrated in Canada. 122 regional traverses 1-5 km long, evenly distributed in the area of the Czech Republic, situated in low, medium and highly radioactive rocks, were measured and results were expressed in gamma dose rate (ngy.h" 1 ). Regression analysis between these data and data of airborne measurement, converted into dose rate (1 /tr/h = 8.69 ngy.h" 1 ), determined multiplication correction constant 0.85 for data of the original airborne map. Correction constant reflects the inconsistency in geometrical conditions and gamma energy spectra of m Ra airborne total count equipment calibration, and the field measurement. Coefficient of correlation showed acceptable agreement of airborne and ground data sets used in regression. 5. COMPILATION OF THE RADIOMETRIC MAP OF THE CZECH REPUBLIC Original airborne exposure rate maps of contours, on the scale 1: , has been converted to vector form by digitizing and expressed by data in a regular grid 300 x 300 m over the territory. Digital data were convened into dose rate and the multiplication correction constant 0.85 was applied. The new terrestrial gamma dose rate map, on the scale 1: , was compiled by computer processing with the step of contours 10 ngy.h 1. The map has been published in 1995 [10]. Regional radioactivity of rocks of the Czech Republic, formed by magmatic, sedimentary and metamorphic rocks, is in the range ngy.h"', with the mean 65.6 ±19.0 ngy.h" 1 (Fig. 1). 6. RADIOMETRIC MAP DATA VERIFICATION A series of factors, which have been described in the paragraph 2, affect resultant data of radiometric maps. In 1995, reported data in published radiometric map of the Czech Republic were preliminary checked by comparison with ground gamma ray specrometry measurement of 81 regional traverses 1-5 km long, situated in the whole area of the Czech Republic. Ground gamma ray measurement was carried out with calibrated spectrometer GS-256 at 761 sectors of 200 m length in dynamic mode. Results were expressed in dose rate, averaged for each traverse, and compared to dose rate values of the map 1: , read with the step of 5 ngy.h" 1. The average difference of compared data sets 2.1 ngy.h" 1 shows a good procedure of regional dose rate data levelling, while the mean deviation ±13.8 ngy.h" 1 illustrates expected differences at individual sites and geological setting. Coefficient of correlation of compared data sets (N = 81) is Analysis of dose rate data verification must take into consideration the limited width range of ground measurement, which is compared with integrated data of airborne measurement and map data interpolation, resulting in filtered regional average values. Magnitude of determined mean deviation, and its statistical significance, is a measure of accuracy, with which the radiometric map can be used for the assessment of the radiation environment. 55

4 i J r ^}j$» f»!«ol rock* 0 IC JO M <? SO J »C IS* (M JW ii0 no».h F/G. 7. Map of terrestrial gamma dose rate of the Czech Republic.

5 REFERENCES [1] INTERNATIONAL ATOMIC ENERGY AGENCY, Application of uranium exploration data and techniques in environmental studies, IAEA-TECDOC-827, Vienna (1995). [2] INTERNATIONAL ATOMIC ENERGY AGENCY, The Use of Gamma Ray Data to Define the Natural Radiation Environment, IAEA-TECDOC-566, Vienna (1990). [3] LOVBORG, L., The Calibration of Portable and Airborne Gamma Ray Spectrometers Theory, Problems and Facilities, Riso-M-2456, Riso (1984). [4] MATOLIN, M., DEDACEK, K., The Causes of Differences in the Maps of Total Gamma Ray Activity of Rocks, Acta Universitatis Carolinae-Geologica, 4, Prague (1974) [5] LOVBORG, L., Monitoring of Pads in Various Countries for the Calibration of Portable and Airborne Gamma Ray Spectrometers, Riso-N-02-82, Riso (1982) 35 p. [6] SANDERSON, D.C.W., ALLYSON, J.D., TYLER, A.N., "Environmental Applications of Airborne Gamma Ray Spectrometry", Application of uranium exploration data and techniques in environmental studies", IAEA-TECDOC-827, IAEA, Vienna (1995) [7] RUBIN, R.M., LEGGET, D., WELLS, M., Effects of Overburden, Biomass and Atmospheric Inversions on Energy and Angular Distribution of Gamma Rays from U, K, Th and Airborne Radon Sources, Radiation Res. Associates, Report-GJBX , Fort Worth (1980) 259 p. [8] INTERNATIONAL ATOMIC ENERGY AGENCY, Airborne Gamma Ray Surveying, Technical Reports Series No. 323, IAEA, Vienna (1991). [9] SCHWARZ, G.F., RYBACH, L., KLINGELE, E.E., "Data processing and mapping in airborne radioactivity surveys", Application of uranium exploration data and techniques in environmental studies, IAEA-TECDOC-827, IAEA, Vienna (1995) [10] MANOVA, M., MATOLIN, M., Radiometric map of the Czech Republic 1: , Czech Geological Survey, Prague (1995) 19 p. SfgXY t BLANK 57