Environmental Implications of the Laboratory Gamma Ray Spectrometric

Home , Black sand

XA9745962 ENVIRONMENTAL IMPLICATIONS OF THE LABORATORY GAMMA RAY SPECTROMETRIC MEASUREMENTS MADE ON THE BLACK SAND BEACH DEPOSIT OF THE ABU KHASHABA RADIOACTIVE PROSPECT, ROSETTA, EGYPT

A.M. ELSIRAFE, H.A. HUSSEIN, R.A. ELSAYED Nuclear Materials Authority of Egypt, El Maadi, Cairo A.M. SABRI Geophysics Department, Faculty of Science, Ain Shams University, Cairo Egypt

Abstract

Laboratory gamma-ray spectrometric measurements carried out on the surficial black sand beach deposits of Abu Khashaba radioactive prospect located 5km east of Rosetta, north Nile Delta, as a part of the comprehensive exploration and assessment program of these radioactive placer deposits has generated a vast amount of radiometric data. These data, initially used for radioelement exploration, provided valuable baseline information necessary for environmental studies and monitoring the impact of any potential environmental radioactive pollution that might occur as a result of nuclear activities in the surrounding regions. Analysis of the acquired spectrometric measurements indicated that both thorium and uranium are the two radioelements that significantly contributed to the measured total natural gamma-ray flux, rather than potassium. This is mainly attributed to the nature of the mineralogical composition of these beach deposits. On the other hand, qualitative interpretation of the produced maps of the three radioelements: potassium, uranium and thorium, supplemented by the examination of their frequency distributions and application of the homogeneity statistical chi-squared test has shown pronouned uranium and thorium comparable zonal distribution patterns that are consistent with the spatial distribution of the heavy mineral concentrations in the studied black-sand deposits. In contrary, these deposits showed uniformally distributed potassium concentrations all over the studied area of Abu Khashaba beach. The two identified radiometric zones displayed a normally distributed uranium and thorium concentration distributions and a contrasting mean background concentration values for these two elements. Concentrations of natural radioelements: potassium, uranium and thorium in the investigated radioactive black-sand deposits were used in producing the environmental total gamma radiation exposure rate map of the studied Abu Khashaba beach and estimation of the mean background levels of the total gamma radiation dose rates.Two zones of contrasting average gamma exosure rates were outlined over the studied Abu Khashaba beach. The first zone displayed an average exposure rate of 13.9 /*R/h (average dose rate 115.6 millirem/year) whereas, the second zone displayed a relatively lower average exposure rate of 3.9 fiR/h (average dose rate 32.1 millirem/year). The estimated average total gamma dose rates remain in the safe side and within the maximum permissible safe radiation dose without harm to the individual. However in order to have better information about the actual levels of radiations, the contributions from other radionuclides that have not been taken into account should be considered.

1. INTRODUCTION

1.1. Genesis, Mineralogy and Distribution of the Egyptian Black-Sand Deposits

The Egyptian black-sands are beach placer deposits formed by the mechanical concentration of resistant minerals released by weathering of the Precambrian igneous and metamorphic source rocks of the upper reaches of the River Nile. These erosional products have been carried down through the course of the Nile, and were subjected to natural physical and chemical separation and sorting during transportation. By normal surface processes, mainly the combined effect of waves and wind, these transported weathering products were accumulated to form huge deposits, intermittently and in variable proportions, occupying the beach of the Mediterranean Sea of Egypt along a stretch of about 350 km long, extend from El Arish eastwards to Abu Qir westwards. The geology of these

389 deposits along the Mediterranean coast is directly related to the development of the Nile Delta and the past configuration of the Nile branches [1]. The Egyptian black-sands has drawn an increasing interest since they contain different concentrations of potentially economic heavy minerals that are considered as nuclear raw materials as well as their importance in many of the metallurgical industries.

The Egyptian black-sand deposits are loose fine sands consisting a complex mineral associations of light and heavy fractions. The light mineral fraction is mainly consists of quartz which is the principal constituent (average 95%) and felspars which constitute about 5% of this fraction. The percentage of the heavy mineral fraction varies from 8% up to 97% of the total mineral constituents of the black sands. This fraction is mainly formed of a complex association of minerals characterized by their dark color and high specific gravity. The most abundant of these minerals are: ilmenite, magnetite, zircon, garnet, rutile and monazite. Other minerals are also present, but in trace amount such as chromite, cassiterite, wolframite, corundum, beryle, uranothorite and gold [1 and 2].

1.2. Location and General Topography of Abu Khashaba Beach

Abu Khashaba beach occurs in the extreme northwestern corner of the Nile Delta, on the Mediterranean Sea coast (Fig. 1). It is situated 5 km to the north of Rosetta (Rashid) City and lies on the eastern side of Rosetta branch of the River Nile, about 7.5 km from the estuary. This area represents the most wider coastal plain, extends for about 2.5 km in the back beach. The present

zDITERRANEAN S

31 00 -31 00' X 00' 31 00

Scale 1 : lpOO.OOO >...,... :

FIG. 1. Map of north Nile Delta, Egypt. The arrow points to location of the studied part of Abu Khashaba beach, east of Rosetta (Rashid) city.

390 study is only restricted to the western part of Abu Khashaba beach (Fig. 1) and covers a total surface area of 2.4 km2. The altitude of Abu Khashaba area is slightly higher than the sea level, and the topography is ahnost flat where the coastal plain occupies most of the surface except the southern part where accumulations from eolian sands occur as small dune belts and sand bars. The highest concentration of the heavy minerals along the Mediterranean coast of Egypt is found along Abu Khashaba beach where these extend eastward for a distance of about 40 km along the coast (Fig. 2). These conditions favour the area of Abu Khashaba beach as a good prospect for radioactive raw materials.

1.3. Scope and Objectives of the Study

Environmental monitoring and meaningful interpretation of man-mad radioactive pollution is impossible without knowledg about the natural abundance of the radioelements and the natural background radioactivity leveles in the environment [3]. In that regard comperhensive assessment program of the mineral potentials of the radioactive black sand deposits of Abu Khashaba prospect, carried out by the Egyptian Nuclear Materials Authority, provided a vast amount of potassium, uranium and thorium concentration measurements acquired from the systematic aerial, ground and laboratory gamma-ray spectrometric measurements conducted over these beach placer deposits. The present research work aims essentially to make use of the laboratory spectral gamma-ray measurements, in establishing the environmental natural gamma radiation levels along the suveyed Abu Khashaba beach. This will provide baseline information necessary for the detection and determination the amount and extent of potential environmental changes in the terrestrial background radiation that might result from nuclear testing or accidents involving release of nuclear radiations and accompanied radioactive contamination of the ground.

FIG. 2. Photomap of north Nile Delta, Egypt produced by the processing of the digital data acquired with the Landsat Satellite Multispectral Scanner (MSS) of resolution 82 x 57 m, restored to 50 x 50 m. This map has been produced by the Egyptian Remote Sensing Center, Cairo. Black-sand deposits (white to light blue colored zones) extend along andparrallel to the Mediterranean shoreline, east of Rosetta (Rashid).

391 2. FIELD WORK (SAMPLING PROCEDURE)

A total of 272 samples, representing the upper 50 cm of the black-sand depoists at the eastern part of Abu Khashaba beach (2.4 km2), were collected on a quadrate grid pattern using 100 m sampling interval. The regular grid pattern of sampling was designed to overcome the variability in the heavy mineral content of these black-sand deposits. These samples were taken by forcing a cylindrical rigid plastic tubes, 50 cm length and 2.5 cm diameter, vertically into the ground at the sampled location. In this way continous core samples from the sampled locations were obtained. Tubes were then sealed and labeled.

3. LABORATORY GAMMA-RAY SPECTROMETRIC MEASUREMENTS

3.1. Sample Preparation

Preparatory processes were carried out on the collected black-sand samples as a necessary step prior to the execution of the laboratory spectral radiometric measurements. In that regard each sample was thoroughly rubbed and mixed well in order to avoid the non-homogeneity in the distribution of its mineral constituents. Then a proper weight (300-350 gm) of each sample was placed in standard- size cylindrical plastic container 9.5 cm in diameter and 3 cm height. These containers were carefully sealed to prevent contamination. Samples in the sealed containers were stored for three weeks where the radon buildup is back to the normal state and attains radioactive equilibrium.

3.2. Instrument and Methodology

Laboratory determination of the total gamma-ray activity (TC) and absolute concentration of the naturally occurring three radioelements thorium (Th), uranium (U) and potassium (K) in the investigated black-sand samples have been performed with a Bicron multichannel analyzer of a calibrated gamma-ray spectrometer with 285 cm3 Nal (Tl) crysta. Thorium and uranium were measured from the gamma-ray photons emitted by Pb-212 (at 238.6 keV) and Pb-214 (at 52 keV) respectively in their decay chain, whereas, potassium was measured directly from the gamma-ray photons emitted by K-40 at 1460.8 keV. The utilized energy regions (windows) for eTh, eU and K were determined on indicated energy lines of the spectra, generated by means of laboratory thorium, uranium and potassium reference standared samples provided by the IAEA [4]. Assaying of the black- sand samples was carried out using long period counts, 1000 second for each, in a shielded environment to determine the gross counts for the total gamma-ray activity and the gross counts of the three radioelements Th, U, and K.

3.2. Data Processing and Presentation

The measured spectrometric data for the analysed 272 black sand samples were processed by computer program "ANALYSIS" [5]. Background corrected and stripped count rates were converted to ground concentrations of potassium (K in percent), uranium and thorium (U and Th in ppm), in addition to the integral gamma-ray activity expressed in units of counts/second/gram, using the sensitivity constants of the used spectrometer determined from the calibration procedure. Estimated absolute concentrations of the three radioelements K, U and Th, as well as the integral gamma-ray activity were finally displayed in the form of colored contour maps (Figs 3-6) using the the Geosoft Graphics System.

4. DATA ANALYSIS

Laboratory spectral gamma-ray measurements have been subjected to conventional statistical methods of analysis, in order to facilitate the manipulation of the large volume of the data and to

392 reach valid conclusions and reasonable predictions concerning the nature of concentration distributions of the naturally occuring radioelements potassium, uranium and thorium. Hereafter is the procedure steps followed throughout this statistical treatment:

1. Data sets for the individual laboratory spectrometric variables that were monitored over the investigated black-sand deposits were displayed in the form of frequency histograms in order to recognize potential natural grouping within each data set, that consequently might assist the delineation of the spatial distribution of the heavy mineral concentrations in the investigated black sand deposits.

FIG. 3. Map showing spatial distribution of Potassium concentration (K in %) in the black-sand deposits of Abu Khashaba beach, east ofRosetta.

393 FIG. 4. Map showing spatial distribution of uranium concentration (U in ppm) in the black-sand deposits of Abu Khashaba beach, east ofRosetta.

2. State of homogeneity of the data distribution of the individual four measured spectral radiometric variables has been mathematically tested through the application of the statistical chi-squared Test (Tables I and II).

3. Correlation analysis has been applied, as a bivariate statistical analysis, in order to explore the mutual relations and strength of association between each pair of the four measured radiometric variables, through calculation of the linear Person product moment correlation coeeficient (Table III).

4. Calculation of simple statistics that describe the characterstics of the distributions of the data aquired for the four spectral radiometric variables. These included the range (maximum- minimum), the mean (x), standard deviation (Sd) and coefficient of variability (CV). These computed statistics are listed in Tables I and II.

394 FIG. 5. Map showing spatial distribution of thorium concentration (Th in ppm) in the black-sand deposits of Abu Khashaba beach, east ofRosetta.

5. DISCUSSION AND INTERPRETATION

5.1. Qualitative interpretation

Visual examination of the three colored contour maps namely: TC, Th and U (Figs. 3-6) revealed a remarkable consistency in the spatial distribution features of the concentration of these variables over the studied black sand deposits at Abu Khashaba beach. The most striking gross feature of these spatial distributions is the pattern of repetitional zones of high and low concentration levels, striking approximately in the east-west direction more or less parallel to the present shore line. The well-defined sharp boundaries between these zones reflect an abrupt change in the magnitude of the spectrometric responses and imply that there is an abrupt lateral mineralogical variations between these zones in the area under consideration. This distinct zonation pattern is in good agreement with the know sedimentary history of these deposits [1,6 and 7] and can be observed in the field as alternating dark and light coloured bands that reflect heavy and light mineral concentrations. On the

395 other hand, the radioactive potassium contour map (Fig.3) revealed independent features with respect to its distribution pattern. It does not display much variations with respect to the whole area. Zonation is less obvious and trends are difficult to define. However the central area of the map, show a remarkable local elevation in tthe potassium content. Field obseravtion revealed that this is spatially restricted to a zone of high clay and evaporite contents.

The measured spectrometric responses of the studied black-sands are considered as operative physical properties that reflect valuable information about the main causative to such properties. Accordingly, these responses can be interpreted on the basis of general geological considerations, particularly the mineralogical composition of these placer beach deposits.

As inferred from the spectral gamma-radiation response of the investigated black-sand deposits, both thorium and uranium are the two elements that significantly contributed to the observed total gamma-radiation response, as indicated from their highly comparable spatial distribution pattern, rather than potassium. The three heavy minerals zircon, monazite and uranothorite are the major

FIG. 6. Map showing spatial distribution of the natural total gamma radiation (TC in c/s/gm) of the black sand deposits at Abu Khashaba beach, east ofRosetta.

396 TABLE I. COMPUTED SIMPLE STATISTICS OF THE DISTRIBUTIONS OF THE LABORATORY SPECTRAL MEASUREMENTS OF THE TOTAL GAMMA RADIATION (TC IN c/s/gm), THORIUM (Th in ppm), URANIUM (U in ppm) AND POTASSIUM (K) CONCENTRATIONS IN THE 272 BLACK SAND SAMPLES COLLECTED FROM THE STUDIED ABU KHASHABA BEACH AND RESULTS OF APPLICATION OF THE HOMOGENEITY CHI-SQUARED TEST

Variable N Range X Sd CV(%) X^-test values Type of distribution Min. Max. obs. crit.

TC 272 5.10 29.9 14 7.5 54 63 18 Not normal

Th 272 1.10 72.5 24 13.9 58 55 22 Not normal

U 272 0.61 15.0 5.2 3.1 60 52 24 Not normal

K 272 0.01 1.43 0.49 0.25 51 22 27 Normal

TABLE II. COMPUTED SIMPLE STATISTICS OF THE DISTRIBUTIONS OF THE LABORATORY SPECTRAL MEASUREMENTS OF THE TOTAL GAMMA RADIATION ( TC IN c/s/gm), THORIUM (Th in ppm), URANIUM (U in ppm) AND POTASSIUM (K) CONCENTRATIONS IN THE IN THE TWO INTERPRETED RADIOMETRIC UNITS OUTLINED OVER THE BLACK SAND DEPOSIT OF ABU KHASHABA BEACH (FIG. 8), AND RESULTS OF APPLICATION OF THE HOMOGENEITY CHI-SQUARED TEST

Unit Variable N Range X Sd CV (%) X2-test values Type of distribution Min. Max. obs. crit.

TC 204 9.5 29.9 16.4 3.7 22.2 19.5 27.6 Nrmal

I eTh 190 10.1 72.5 29 10.1 33.7 12.5 21 Normal eU 208 2.5 15.0 6.6 2.3 33.3 8.6 22.4 Normal

TC 75 5.1 10.8 7.8 1.4 17.9 7.3 12.6 Normal n eTh 70 1.1 12.6 6.6 2.9 44 3 11.1 Normal eU 61 0.61 3.1 1.7 0.68 40 2.9 7.8 Normal

N.B. Units I and II are the high and low radiation units shown in black and grey on Fig. 9.

TABLE m. COMPUTED CORRELATION MATRIX, IN DIAGONAL FORM, OF THE FOUR SPECTRAL RADIOMETRIC VARIABLES MEASURED IN THE 272 BLACK SAND SAMPLES COLLECTED FROM ABU KHASHABA BEACH, EAST ROSETTA, EGYPT Rad. Variable Total gamma Thorium Uranium Potassium

Total gamma 1.000 Thorium 0.975 1.000 Uranium 0.962 0.915 1.000 Potassium -0.401 -0.399 -0.451 1.000

397 contributors to thorium and uranium in the Egyptian black-sands [8]. Zircon and monazite minerals showed strongly direct correlation with each other at Rosetta area [9]. Accordingly, both uranium, thorium and the total gamma radiation colored maps (Figs. 4-6) discriminate and outline alternating zones that contrast in their content of the radioactive heavy minerals especially zircon and monazite in the investigated Abu Khashaba area. On the other hand, potassium did not display any response that could be related to the heavy mineral concentrations. [10] attributed the main occurrence of radioactive potassium in the Egyptian beach deposits to the light fraction, mainly represented by the clay minerals and feldspars.

5.2. Quantitative interpretation

The area of Abu Khashaba beach consists mainly of black-sand sediments. The whole area under study, therefor, can be considered to comprise a single lithological unit. With this approximation, data sets for the individual laboratory spectrometric variables that were monitored over the investigated black-sand deposits were displayed in the form of frequency histograms. This step aimed to recognize the potential natural grouping within each data set, that consequently might assist the delineation of the spatial distribution of the heavy mineral concentrations within the studied area.

Examination of the frequency distribution histograms of the variables namely; TC, Th and U revealed a bimodal distributions and consequently two levels of responses with respect to these variables. This means that these radiometric variables possess heterogeneous distributions allover the investigated area. This has been confirmed quantitatively through application of the numerical x2-test, [11] as shown on Table I. On the bases of these information, supplemented by the aforementioned qualitative interpretation of the maps, the whole investigated area has been statistically treated and discussed in terms of two geographical areas or zones (Fig. 8) of different radiometric responses. These zones are approximately outlined on the interpreted zonation map (Fig.8). Table II summarizes the computed statistics of the distributions of the four spectral radiometric in these interpreted zones.

Analysis of the frequency distributions and application of the x-test, (Table II) revealed that the TC, Th, and U are normally distributed within the two outlined low radiation and high radiation zones of the black sand deposits of Abu Khashaba beach (Fig. 8). This consequently reflects the homogeneity of the distribution of the radioactive heavy minerals, monazite zircon and uranothorite within these zones.

The frequency histogram of potassium measurements, revealed a unimodal distribution. This implies that there is no much variations in the radioactive potassium content within the whole area. This result has been quantified through application of the x2-test (Table I) which revealed that the radioactive potassium concentration is normally distributed all over the studied area, with mean background (X) of 0.49%. Consequently, the entire black-sand deposits in the area can be considered to represent a uniform radiometric unit with respect to its radioactive potassium content.

6. ENVIRONMENTAL IMPLICATIONS OF THE SPECTRAL GAMMA RADIATION MEASUREMENTS

6.1. Conversion of Radioelement Concentration Measurements to Total Gamma Exposure and Dose Rates

Concentrations of natural radioelements: potassium, uranium and thorium in the investigated radioactive black-sand deposits were used in producing the environmental total gamma radiation exposure rate map of the studied Abu Khashaba beach (Fig. 8) and estimation of the mean background levels of the total gamma radiation dose rates (Table IV). Conversion factors recommended by Grasty [12] were used. Two zones of contrasting average gamma exosure rates were outlined over the studied

398 FIG. 7. Photograph, facing south, showing zonal distribution of the heavy mineral concentrations in the black sand deposits of Abu Khashaba beach. Dark colour in this photo indicates zones of high concentration while light colour indicates low concentration zones.

FIG. 8. Map showing spatial distribution of the terrestrial total gamma radiation exposure rate (in over the black sand deposits of Abu Khashaba beach.

399 TABLE IV. LIST OF THE COMPUTED MEAN BACKGROUND VALUES OF THE THREE RADIOELEMENTS POTASSIUM, URANIUM AND THORIUM CONCENTRATIONS AS WELL AS AVERAGE VALUES OF THE NATURAL GAMMA RADIATION EXPOSURE RATES OF THE TWO ENVIRONMENTAL RADIOMETRIC UNITS IDENTIFIED OVER THE BLACK-SAND DEPOSITS AT ABU KHASHABA BEACH, EAST OF ROSETTA, NORTH NILE DELTA

Rad. Unit Potassium Uranium Thorium Exposure rate Dose rate (%) (ppm) (ppm) foR/h) (millirem/y) High 0.49 6.6 29 13.9 115.6 Low 0.49 1.7 6.6 3.9 32.1

Exposure rate (in fiR/h) = %K x 1.505 + ppmU x 0.625 + ppmTh x 0.310 Dose rate (in millirem/y) = Exposure rate x 8.33

Abu Khashaba beach. The first zone (shown in black) displayed an average exposure rate of 13.9 /xR/h (average dose rate 115.6 millirem/year) whereas, the second zone (shown in grey) displayed a relatively lower average exposure rate of 3.9 fiR/h (average dose rate 32.1 millirem/year).

The radiation doses received by people are frequently measured in "rems". For gamma- radiation, the conversion from exposure rate to dose rate is: 1 fiR/h ~ 8.33 millirems/year. The International Commission of Radiological Protection (ICRP) has recommended that no individual should receive more than 5000 millirems/year from all natural and artificial radiation sources in his or her environment [13].

Concerning the studied Abu Khashaba area, the mean natural dose rates from the terrestrial gamma-radiation, to which individuals are exposed in the two outlined natural radiation zones (Fig. 8), was computed as 115.6 and 32.1 millirems/year. These values remain in the safe side and within the maximum permissible safe radiation dose without harm to the individual, with continuous external irradiation of the whole body. But, on the other hand, people in the area of Abu Khashaba beach are exposed to other irradiation sources. These include other potenial terrestruial man-made radionuclides that have not been taken into account (e.g Cs-137), radiation in the atmosphere and cosmic radiation. Hence, in order to reach more precise estimation for the total gamma dose rate levels in the study area, contributions of the aforementioned radiation sources should be considered. In that regard in-door radon survey in the populated areas in the surroundings of Abu Khashaba beach is highly recommended, since it was noticed from the field observations that the people in this area frequently use the beach black sands as a building material.

400 (metres)

FIG. 9. Environmental natural total gamma radiation unit map of the studied Abu Khashaba beach, as inferred from the qualitative interpretation of the exposure rate map (Fig. 7) supplemented by quantitative statistical analysis of the measured concentrations of the three radioelements: potassium, uranium and thorium.

25 • potassium (%) 20 • uranium (ppm) rathorium (ppm) 15 ^exposure rate (uR/ h) 10

0 radiation unit low radiation unit

FIG. 10. Bar chart showing mean background concentration of the three radioelements potassium, uranium and thorium as well as average natural gamma radiation exposure rates of the two radiometric units identified over the black-sand deposits at Abu Khashaba beach.

401 REFERENCES

[I] HAMMOUD, N. S., Concentration of monazite from Egyptian black sands, employing industrial techniques, M.Sc.Thesis, Faculty of Science, Cairo University (1966). [2] EL GIMMIZI, M. A., Note on the occurence of gold and cassiterite in the Egyptian beach placer deposits, Internal Report, Nuclear Materials Authority, Cairo, Egypt (1984). [3] INTERNATIONAL ATOMIC ENERGY AGENCY, Application of uranium exploration data and techniques in environmental studies, IAEA-TECDOC-827, Vienna (1995). [4] MATOLIN, M., Construction and use of spectrometric calibration pads for laboratory gamma-ray spectrometry, Nuclear Materials Authority (NMA) of Egypt, Report to the Government of the Arab Republic of Egypt, Project EGY/4/030-03, IAEA (1990). [5] SRP, P., MATOLIN, M., Computer program "ANALYSIS" for calibration of the laboratory gamma-ray spectrometer and analysis of the radioactive elements in rock samples, Charles University, Prague, Czechoslovakia (1990). [6] HAMMOUD, N. S., Physical and chemical properties of some Egyptian beach economic minerals in relation to their concentration problems, Ph.D Thesis, Faculty of Science, Cairo University (1973). [7] DABBOUR, G. A., Geological and mineralogical studies on rutile in the black sand deposits from the Egypyian Mediterranean coast, Ph.D Thesis, Faculty of Science, Cairo University (1980). [8] ZAGHLOUL, Z.M., Note on the occurrence of uranothorite and radioactive zircon in the black sands of Rosetta, Egyptian J. Geology, V.4 (1960) 89-91. [9] AMMAR, A.A., WASSEF, S.N., MELEIK, M. L., FOUAD, K. M., DABBOUR, G. A., Remote, surface and laboratory detection of radioactive minerals of Rosetta black sand beach deposits, Egypt, INT. J. Remote Sensing, V. 4, No. 4, P. 739-754 (1983). [10] ALY, M. M., WASSEF, S. N., YANNI, N. N., HATHOUT, M. H., Natural field radioactivity correlated with thorium, uraium and radioactive potassium in beach sands of east Rosetta, Egypt, Egyptian Desert Institute Bull., No. 1-2, P. 1-12 (1982). [II] CLARK, G. M., COOKE, D., A basic course in statistics, English Language Socity/Edward Arnold (Pulishers) Ltd., London (1983). [12] GRASTY, R. L., GARSON, J.M., CHARBONNEAU, B. W., HOLMAN, P. B., Natural background radiation in Canada, Bulletin 360, Geological Survey of Canada, Ottawa (1984). [13] INTERNATIONA ATOMIC ENERGY AGENCY, Gamma-ray surveys in uranium exploration, Technical Reports Series No. 186, IAEA, Vienna (1979).

402