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IAEA-TECDOC-566

THE USE OF DATA TO DEFINE THE NATURAL ENVIRONMENT

A TECHNICAL DOCUMENT ISSUED BY THE INTERNATIONAL ATOMIC AGENCY, VIENNA, 1990 The SAEA does not normally maintain stocks of reports in this series. However, microfiche copie f thesso e reportobtainee b n sca d from

IN IS Clearinghouse Internationa! Atomic Energy Agency Wagramerstrasse5 0 10 x P.OBo . A-1400 Vienna, Austria

Orders shoul accompaniee db prepaymeny db f Austriao t n Schillings 100, in the form of a cheque or in the form of IAEA microfiche service coupons which may be ordered separately from the INI S Clearinghouse. THE USE OF GAMMA RAY DATA TO DEFINE THE NATURAL RADIATION ENVIRONMENT IAEA, VIENNA, 1990 IAEA-TECDOC-566 ISSN 1011-4289

Printed by the IAEA in Austria September 1990 FOREWORD

The accident at the at Chernobyl in the USSR in April 1986, and the confusion in the public mind over the and extent of the resulting fallout pointed up the neemorr fo de widespread awarenes naturae th f so l radiation background in which people live. An examination of the possibilities for acquiring these data led to the recognition of the of the exploration community over the yearimportann a s sa t yetsourcs a ., unusedeof , natural radiation data. These data have been collecte purpose th r efo d of locating uranium deposits in many countries, frequently publif o througe cus funde hth developmenr so t funds mose .Th t extensive effor thas U.S.Ate wa t th carrien .i whert dou e eth whol countrf eo s flowywa n with airborne gammy ara spectrometer surveys durin Nationae gth l Uranium Resources Evaluation (NURE) project. These data have been compiled and issued as maps that have been of great value in locating areas of potential hazard, in addition to their use in geological mapping and mineral exploration for a numbe metalsf ro . They fundamentaf wilo e lb l importancn ei the International Geological Correlation Programme (IGCP)/UNESCO projec Internationan to l Geochemical Mapping ultimate producinwhicth f o s m hha eai g Geochemical Mapf so the World. The IAEA is playing a coordinating role in the production of the Radioelement Geochemical Maps. This manual is designed to encourage and assist in the use of preexisting gamm y survera a y dat produco t a enaturae mapth f so l radiation environment. The manual is the work of three consultants; Dr. J. S. Duval of the United States Geological Survey, Dr. Anders Linden of the Swedish Geological Company, and Dr. Milan Matolin of Charles University in Prague, Czechoslovakia. These three combine a great many years of experierce in natural radiation measuremen developmene th n i methode th d tan f t o o st their high leve sophisticationf lo Agence .Th y wishe thano st k most sincerely these thre contributinr efo g their timd ean expertise to the task. The Agency staff member responsible for the project was Arthur Y. Smith of the Division of Nuclear Cycle. EDITORIAL NOTE

In preparing this material for the press, staff of the International Atomic Energy Agency have mounted and paginated the original manuscripts and given some attention to presentation. The views expressed necessarilynot do reflect those governments ofthe Member ofthe States organizationsor under whose auspices manuscriptsthe were produced. thisin The bookuse of particular designations of countries territoriesor does implynot any judgement publisher,the legalby the IAEA,to the status as of such countries territories,or of their authorities and institutions or of the delimitation of their boundaries. The mention of specific companies theirof or products brandor names does implynot any endorsement recommendationor IAEA. partthe the of on CONTENTS

1. INTRODUCTION ...... 7

2. THE NATURAL RADIATION ENVIRONMENT ...... 8 2.1. Source f naturao s l radiation ...... 8 . 2.2. Radiation units and conversion factors ...... 10 2.3. Occurrenc f potassiumeo , uranium, thoriu othed man r radiation sources environmene inth t ...... 1 1 . 2.3.1. K, U and Th in rock and soil ...... 11 2.3.2. K, U and Th and their daughter products in the , hydrospher biospherd an e e ...... 4 1 . 2.3.3. Cosmic radiation ...... 15 2.3.4. Natural nuclei concentrate ...... r relocate do n ma y db 5 1 . 2.3.5. Artificial nuclei in nature ...... 15 2.4. Measurin naturae gth l radiation environment ...... 6 1 . 2.5. to man from natural radiation ...... 17

3. GAMMA RAY SURVEYS ...... 18 3.1. Instrumentation ...... 8 1 . 3.2. Calibration and reporting units ...... 19 3.3. Geological gamm surveyy ra a s ...... 0 2 . 3.4. Suitable data ...... 0 2 .

. 4 STANDARDIZATION TECHNIQUES ...... 2 2 . 4.1. General concep f standardizatioo t n ...... 2 2 . 4.1.1. General considerations ...... 22 4.1.2. Conversion of radioélément concentrations to air dose rate ...... 23 4.2. Levellin f datgo a withi nsurvea y area ...... 4 2 . 4.2.1. Factors causing level differences ...... 4 2 . 4.2.2. A mathematical approach to levelling ...... 26 4.2.3. Levelling of data using image techniques ...... 28 4.3. Levellin f datgo a between survey areas ...... 2 3 . 4.4. Procedure for standardization of spectrometer surveys ...... 32 4.4.1. Gamma ray spectrometer surveys with calibrated equipment ...... 32 4.4.2. Gamma ray spectrometer surveys with uncalibrated equipment ...... 32 4.5. Procedures for standardizing total count surveys ...... 34 4.5.1. General problem f totao e l scounus relatee th t o surveydt s ...... 4 3 . 4.5.2. Total count surveys with calibrated equipment ...... 7 3 . 4.5.3. Total count surveys with uncalibrated equipment ...... 8 3 . 5. DATA PRESENTATION ...... 38 5.1. Graphical form ...... 38 compilatiop 5.2Ma . n ...... 9 3 . 5.2.1. Manual compilation ...... 39 5.2.2. Digital compilation ...... 41

6. INTERPRETATION AND USE OF THE MAPS ...... 42

REFERENCES ...... 45 l. INTRODUCTION The natural radiation environment is the major source of to man and consists of both internal and external sources mose Th t. significant internal sourcee sar radioactive th e element 222d Ran n K whics 40 takee har n inte oth body. The external sources are cosmic rays and naturally occurring radioactive isotope 40e K,th 238 232f d sUo T, an h decay series, some or all of which can be found in the ground, in construction materials, and in the air. Both internal and external radiation levels functioa var geologicae s ya th f no l materials, type of dwelling, and elevation above sea level. This report has been prepared to provide a guide for the use of existing gamm survey ara y dat defino at naturae eth l radiation environment wilt ,bu l onl concernee yb d wite hth gamma ray exposure resulting from radiation originating in geological materials. Chapte below, r2 , discusse naturae sth l radiation environment in greater detail. Because natural radiation levels represent the minimum exposure attainable, the usee comparisor ar yfo d n when judging the significance of exposure from man-made sources such as medical x-rays, nuclear weapons fall-out, and entire Th . e issu man-madf eo e radioactivity becomes more importan numbee th nucleaf s tro a r power plants, research reactors, medical and industrial isotope sources, etc. increases. The study of the natural radiation environment has been actively carried out since the discovery of radioactivity. In recent year sseriea symposif so a [1], [2], [3], [4], dedicate subjece th o dt t have produce significana d t volumf eo information. However, currently available data provide coverage for a small fraction of the 's land surface and maps covering much larger areas are needed. Examples of such maps for parts of Canada were presented by A.G. Darnley et al. e typ [5]datTh f eo . a neede produco dt e similar mape sar available as a result of airborne, carborne, and ground gamma y surveyra mann si y worlde partth f s.o Although mosf to these data were acquire uraniur fo d othed man r mineral exploration and geological mapping purposes, they can be used to provide background radioactivity information. The primary criteria for the use of these data is that they must be of good qualit seld yan f consistent. Chapte discusse3 r e sth characteristics of gamma ray surveys. Because the relationship between the quantities of natural gamma ray sources and air dose rate is known, existing data can be standardized in terms of the gamma ray exposure resulting from the radioactivity of geological materials. The process of standardization involves the use of conversion factors and, where necessary, back calibration. Chapter4 discusses standardization procedures. Standardized surve compileye b datn aca produco dt e regional maps of gamma ray air dose rates as well as single radioélément maps. Compilation procedures may use either manual or computer techniques. Chapter 5 discusses these compilation techniques. Regional maps of air dose rates provide a broad view of the terrestrial part of the natural radiation environment. These maps can be used to estimate the terrestrial radiation dose to the population, and to identify areas of potential natural radiation hazards. They also provid basea e against whic madn hma e contaminatio estimatede b n nca . Chapter6 discusses the interpretation and use of these maps. Finally must ,i stressee tb d that these maps retain their geological and geophysical information. More importantly, the diverse primary data will have been brought to a common datum enablin comparisoe gth geologicaf no l features over large regions. Although this work will primaril done yb s ea national compilations wilt ,i l ope possibilite nth e th f yo compilatio radioélémenf no t geochemical regionamapa n so d lan global scale.

2. THE NATURAL RADIATION ENVIRONMENT Natural radiation arises from sources which always have been a part of man's environment. Natural radiation is of three types, alpha, groune betgammad th mai e aan n dth I n . sources are potassium, the uranium decay series and the thorium decay series. Cosmic radiation coming from outer internad spacean r ai ,l e radiatioradoth humae n ni th n ni body are also contributers to the natural radiation environment. In addition to natural radiation, different source madn ma e f sradiatioo n exis wells ta . Descriptionf so the natural radiation environment are given in Adams and Gasparin . [7]Kogad al an , t ne ] [6 i

1 Source2. Naturaf so l Radiation Natural radiation externa includen ma o t l s terrestrial gamma rays, gamma rays from building material and cosmic radiation. Internal exposure results from radionuclei entering the body through ingestion and inhalation of naturally occurring radioactive substances in food, and air. The natural in the environment are of two general classes; cosmogenic and primordial. Cosmogenic producee sar d throug interactioe hth cosmif no c rays with atoms in the atmosphere, and do not contribute significantly to external gamma radiation doses. The terrestrial componen naturaf to l radiation arises from the primordial radionuclides potassium, uranium and thorium and their decay products. Naturally occurring potassium contain decae sisotope Th f th 0.011y o . f K o e9% uraniu thoriud man stablo mt e isotopes gives decay series comprising several tens of radionuclides. Natural uranium is a mixture of the isotopes 235U and 238U with 235U comprising naturaf 0.7o % 2 l uranium. Natural thorium parene ,th t elemen thoriue th f to m decay e isotopseriesth f mado s e ,i p eu 232decae Th Th 40.f Kyo , 232d 238 ThUan , daughter productd san half- are shown in figures 2.1 - 2.3 [11. There are two gaseous isotopes, 222 Rn and ??0Rn, in the uranium and thorium decay series. These radionuclides and their decay products contribute significantl internao yt l alpha exposure.

Figure 2.1: The decay of potassium-40.

""IT

Figure 2.2: The uranium-238 series. Radionuclides produced in quantities of less than 1% are not shown.

Figure 2.3 thorium-23e :Th 2 series. Radionuclides producen i d quantities of less than 1% are not shown. The estimate of annual effective dose equivalents from natural radiation sources base globan do l average shows si n ni Table 2.1 [46]. This indicates that radon in indoor air is largese th t contributo annuae th o lrt effective dose equivalent. Variations in the geological environment will change the relative proportions of these factors. In some countrie inhalatioe sth n componen bees tha n s reportea e b o dt annuae th f lo effectiv% 0 8 hig s ha e dose equivalent [9].

Table 2.1: The global average annual effective disequivalents from natural radiation sources. Relative contributrions in % are indicated in brackets. (After [46]).

Sourcf eo External Internal Total mSv mSv mSv

Cosmic rays 0.410 (17) 0.410 (17) Cosmogenic radionuclides 0.015 (1) ) 0.01(1 5 Primordial radionuclides

*°K 0.150 (6) 0.180 (7) 0.330 (13) 0.006 0.006 238U - series 0.100 (4) 1.239 (51) 1.339 (55) 232Th - series 0.160 (7) 0.176 (7) 0.336 (14)

Total 0.820 (34) 1.616 (66) 2.436 (100)

Indoor gamm radiatioy ara n levels type depenth f e o n do building materials used and on their composition. The ratio of indoo outdooo rt r gamma radiatio bees nha n reported mostly [462 - a]. s1

2 Radiatio2. nersion UnitCo d nsan Factor s Th uniI activitr eS becquere e tfo th s i y l (Bq ). Activit y correspondq B e ofon decaa o st y atoe r raton mpe f eo . The unit formerly used was the (Ci) , which corresponds to 3.7x1010 Bq. 1 Ci has approximately the activity of 1 gram of 226Ra. The definition of exposure to is based amoune onth ionizatiof to airn ni originae .Th e l th uni s twa relationshie roentgeTh . basi o t ) unitI R (R ncS : f po sis

2.58xlO~= R l kg'C exposure .Th exposure th rat s ei r epe

1 unit4 . In geological work the unit is still common. The in any substance is expressed in terms of energy transfer uni I absorber S tfo e Th . d dos ths ei « (Gy) . 1 Gy is equal to 1 (J) kg". Previously absorbed

10 dos s expresseewa unitn i d radsf absorbese o Th . d dose rate (Gy s"1) is the absorbed dose per unit time. The dose rate of gamma radiatio relatee b nn exposuro ca absorbedt r ai n ei d rate and is used to specify the terrestrial source of gamma radiation. quantitA y that takes into accoun characteristice tth f so particulae th r typ radiatiof e o potentia s it d causinr nan fo l g damage to body cells and tissues is the dose equivalent. The SI uni dosf to e equivalen sievere th s ti t e (SvTh ). conversion from Gy to Sv is possible by multiplying the absorbed dose by the relative biological effectiveness (RBE) radiatioe ofth n concerned naturar Fo . l gamma rayRBEe sth , also known as the quality factor, has the value of 1. tere Th m "effective dose equivalent" expressee sth effects of man whole body irradiation. The estimated conversion facto convero rt t dose equivalen isotropio t e tdu c gamma radiation to effective dose equivalent is 0.7 [46]. Exposure rate can be converted to absorbed dose rate in 2.413= 1 y s"h" R pG 91 ) . /i Dos l calculates ( i e r e ai th s a d product of absorbed dose rate in air and time of exposure. Due to the quality factor for gamma radiation being equal to 1, the dose and dose equivalent are numerically identical. Effective dose equivalen calculates ti multiplyiny db e gth dose equivalent by the conversion factor 0.7.

Table 2.2 unitI :S olded san r unit radiationr sfo . Quantity SI unit Older unit Conversion factor Activity curie (Ci 10'x )7 12 = q B 1 Ci (Bq) q B 9 10 x 7 3 = i C 1 q kB 7 3 /iC1 = i Exposure /kg roentge) (R n 1 C kg'1 = 3876 R (C kg'1) l R = 2.58 x 10"4 C kg"1 8.6= lR 910"x y G 3 (standard air) Absorbed gray (Gyd )ra 1 Gy = 100 dose 0.0= d y 1G ra 1 Absorbed gray/sec. 1 = 1 h" 2.413s" R /x y 1 9pG dose rate (Gy s"1) Dose rem 1 Sv = 100 rem equivalent (Sv) 1 rem = 0.01 Sv

3 Occurrenc2. Potassiumf eo , Uranium, Thoriu othed man r Radiation Source Environmene th n si t

rocn soid i kan h l T d an 2.3.1 U , :K contene Th potassiuf to rockn mi havn sca ranga e 0 f eo witK averagn % ha usualls 5 i o t t e bu 1 y valu K % f eo 0 1 o t e contenTh . uraniuf aboutK o % thoriu d t2 man rockn i m s

11 varies from near zer severao ot l hundred partf so millior spe n (ppm) dependh T d [10] an rocn so contene U k»Th , K typ f to e geologicae andth l environment. Tabl list3 e2. contente sth s that may be expected in some ordinary rock types. For a specific rock or a specific locality the content of K, U and diffey Thma r significantly from these average values [7], [10], [11].

Table 2.3! Typical concentration of potassium, uranium and thoriu somn i m e common rock correspondind san g absorbed dose rate in air and exposure rate (calculated at 1 in above the ground) .

Rock K U Th Absorbed dose Exposure % ppm ppm rate in air rate 1 s" y pG HR h"1

Granite 3-4 3-5 10-30 25-50 10-20

Gabbro < 1 < 1 2-3 4-7 2-3

2 1- 2 1- Limeston 1 < e 4-7 2-3

Sandstone 1-2 3-5 10-15 15-25 5-10

Average 2.5 2.7 9.6 20.0 8.3

In most worle areabedroce th th df s o coveres i k d with soil. Therefore the radiation from the soil is the most important contributor to the terrestrial radiation. The content of radioactive minerals in the soil depends upon the content in the original parent rocks. Sometimes the radioactive minerals are concentrated and sometimes spread more homogeneously in the soils. Because many uranium and thorium mineral smale sar fragild commos an li t ei o finnt d them concentrated in the fine grained fraction, such as clay. In soils the content of K, U and Th is of the same order as in rock usuallt sbu y somewhat lower [12] mosn .I t casee sth uranium decay series is not in radioactive equilibrium in soils whereas thorium decay series ofte . Uraniuis n m disequilibrium occurs when some of the radionuclides in the decay series are moved by chemical or physical processes. Table 2.4 lists the average concentrations of K, 238U and 232Th in soils with corresponding absorbed d dosan er ratai n ei exposure rate [7], [8].

12 Table 2.4: Average concentration of K, U and Th in soil with corresponding absorbed dose ratexposurd ean e rate (calculate abovm l Ground)e et th da . Soil K eU Th Absorbed dose Exposure % ppm ppm rate in air rate 1 s" y pG pR h"1 *D 1.2 2.0 6.3 12 5 *2) 1.4 1.6 6.0 12 5 *1) adapted from UNSCEAR [8] adapte) *2 d from Koga] [7 n

transpore th modeA r fo luraniuf to roc n soid i m kan y lb mechanical and chemical processes in glaciated terrain is show Tabln i n e 2.5 [13] uraniue Th . m contenexample th n ti e referuraniun a o st m rich granite.

Table 2.5 transpore th :modeA r fo luraniuf to chemicay mb d lan mechanical processes [13]. GRANITIC ROCK 15 ppm eU Transfer of uranium precipitation of U and Ra crushing and surface onth fracturef eo s leachiny gb and fissures. water High uranium and radon conten fracturn ti e groundwater

MORAINE 8 ppm eU Transfer of uranium (5-15 ppm eU) Precipitatioa R d an U f no sorting and surface onth fissuresf eo , leaching pebbles and grains. by water

BOULDERS 15 ppm eU Transfe uraniuf ro m GRAVEL U e m pp 0 1 SAND < 3 ppm eU

sorting Loss of the main part of by water the fine grained uranium I minerals I I CLAY 4-8 ppm eU Deposition, transformation of primary uranium minerals to clay minerals, precipitation and absorption clan o ya R d an o fU s

13 2.3.2: K, U and Th and their daughter products in the atmosphere, hydrospher biospherd ean e The concentrations of K, U, Th and their daughter product waten si muce rar h lower tha rockn i n soil d san s (see Table 2.6). For obvious reasons the series in water are not in equilibrium.

waten i h TablrT [7]d ean , 2.6U [14], :K , [15] Element Sea water Fresh water % K0.03 5 <0.001 Bq/kg 10 <1 238U ppm 0.003 0.001 Bq/kg 0.04 0.01

Bq/kg

e maiTh n contributor radioactivite th d o san t r ai n yi water are the radon isotopes and their decay products. Radon emanates frogroune th msoia dvi l capillaries inte oth atmospher intr groundwatere o oth . Becaus ehalfa 222s R-nha lif 3.82f eo 5 days wherea shalf-lifa 220s Rnha onlf eo y 54.5 , 220Rn is generally less important to the equivalent play ma minoya dos t ebu r role indoors. Thereforee th n ,i following discussion the term radon will refer to Rn. In open lake riverd s an rado e sth n conten lows ti , usually less than 1 Bq/kg. In small creeks the radon content may rise to higher values due to the influx of radon bearing groundwaters concentratioe Th . radof no groundwaten i n r depend radiue th n smo concentratio bedroce th f no soir k o l through whic watere hth s have passedreacy ma hd ,an level f so several thousand Bq/k higherr go . The concentration of radon and its daughter products in outdoo varier rai s with place, time, height above ground an d meteorological conditions. Over land, a reasonable estimate concentratioe ofth radof no grount na d leveBq/m5 s i l3 [46]. In special meterological conditions examplr ,fo e unden ra inversio layerr n ai concentratio e ,th radof n o risy nma e considerably concentratioe Th . othef no r radioactive elements atmosphere inth negligibls ei y low. mose Th t common radioisotop th n ei 40e s Kei Th . average potassium conten plantf to abous sn i i tt 0.0bu K 5% time0 1 woodn i higs e sa b s grasse yha y plantsma a t r si Fo . mature forest, potassium gives about 1000 Bq/m2 [7]e .Th

concentratio radioisotopef no animaln si s averages % abou 2 t0. [7]. The average activity Ko f40 i n the human body is about 60 Bq/kg.

14 2.3.3: Cosmic radiation High energy cosmic radiation from the and other source outen si r irradiate earth'e earthe th sth t A s. surfac eradiatioe roosth f to n dose arises from secondary cosmie Th intensit.y cra y shows small fluctuations due to the 11 year solar cycle and the latitude. The intensity increases with altitude, doubling about every 200 0m [12]absorbee Th . levea dse s i doslt a er ratai n ei " [12]s y exposure pG . giveTh 9 s na elevea se rats i lt ea given as 3.6 p,R h"1 by Lemmela [16]. 2.3.4: Natural nuclei concentrate relocater do n ma y db numbeThera e othef ear ro r source exposurf so o et radiation which are a direct consequence of man's technological activities. Examples of these are coal and peat fired power plants, geothermal energy production, mininr fo g uranium and phosphate and the use of phosphate fertilizers on stonf o brickd e ean us fieldsbuildins sa e Th . g materials may lead to elevated outdoor and indoor radiation levels. These activitie increasy sma gamme activity eth ra a y measured fertilizerf o groune e onth us fror aire e o dy mth Th .s ma increase the gamma ray activity over cultivated areas. The accumulated concentration of 22TRa from fertilizers in West Germany gives a mean absorbed dose rate in air of about 0.2 pGy s'1 [17].

2.3.5: Artificial nucle naturn i e In most regions aerial deposition of artificial radionuclide occurreds sha . Nuclear tests conducten i d atmosphere th e since 1945 have resulte introductioe th n di f no artificial radionuclides into the environment. Many radionuclides produce nuclean i d r weapons testing emit gamma rays and so contribute to the external radiation. The most important one from this point of view is 137Cs but other short- lived isotopes were importan limitea r fo t d timee Th . relative distribution of the fallout is a function of latitude shows i Figurn d ni an e 2.4e integrateTh . d mean deposition norte th hn i temperats C toda degree0 f 5 yo e- zon0 s(4 e latitude kBq/m9 estimates i )2. e 2b [8]o dt .

0 2.3. 0 5 2. kßq/sm q. Figure 2.4: The integrated mean deposition of 137Cs in the northern and southern hemispheres.

15 Other causes of tire aerial distribution of man made gamma ray emitters are accidents in the nuclear power industry, loss of medical or industrial applications sources, and the fall of satellites powered by nuclear reactors. Considerable contributions to the terrestrial gamma ray component were caused by the reactor accidents at Windscale, UK (1958) and Chernobyl, USSR (1986), the loss of isotope sources Mexico (1985), Brazi nucleaa fale l f th o l (1987) y b r d powere,an d satelite in Canada, (1978). Shortlived isotopes will, of course, change the absorbed dose rate in time. For example e absorbeth d dos measureer ai rat n ei Sweden i d n shortly after e Chernobyth l acciden 198n i t 6 was som,n i e places higs ,a s ha y 198b 1 declined [18]t 8s" ha bu 1 y .s" pG abouo 250 t dy 0 0pG 15 t Fall-out nuclide radioactive sar emid ean t gamma rays that interfere with the detection of the gamma radiation of natural radioactive elements. The most prominent fall-out isotopes, detectabl theiy eb r gamma radiatio earth'e th n no s surface in different stages of contamination are given in Table 2.7. Important fall-out radionuclides with long half- live 137e Cssar , 134106d CRusan .

Table 2.7: Significant fall-out radionuclide sources- gammf so a rays Isotope Half- Energ gammf o y a rays keV

95Nb 35. 15 d 766 95Zr 65. 5 d 724, 757 ^°3Ru 39. 5 d 497, 610 106RU 368. 2 d 622 131j 8. 05 d 284 , 364,637 132I 2. 38 h 523, 630, 668 , 773,955 134Cs 750 d 569 , 605,796 137Cs 30. 12 Y 662 u°La 40. 27 h 487, 816, 925, 1596

4 Measurin2. Naturae gth l Radiation Environment e disintegratioTh naturaa f no l radioactive elemens i t accompanied by the emission of a and ß particles. The daughter nuclei are often in a state of excitation. The excess energ emittes i y gamms a d a rays that hav maso en r so charge. Because gamma rays are more penetrating than a or ß particles they are most often used to characterize the terrestrial componen naturae th f to l radiation environment. However certain ,i n cases ,detectioe th suc s ha radonf no , measurede sar . The measurement of gamma rays for geological purposes starte larga n do e e 1950scalth sn ei whe n active uranium prospecting began. Later, measurements of natural radiation were made to identify geological boundaries, for the classification of rocks and, indirectly, for prospecting for

16 minerals other than uranium. Natural radiatioe b y nma measure mann i d y differen geologicar t fo way t sbu l purposes only a few techniques have been used. Recently ground and airborne gamma ray surveys have been used to identify areas with possible radon hazard as well as to calculate the equivalent dose from the and fall-out [12], [18], [19]. r normaFo g^mme lth rockaf o soild ray% san 0 ss9 measure spectrometea y db r survey originate froe uppermosmth t 0.15 m. Thus the gamraa ray survey is a surface measurement technique mosn I .t case bedroce sth coveres i k soiy db o ls that the gamma ray survey shows the activity of the soil and not the bedrock. Although radioactive elements are not usually present in snor so levelt wa s detectabl geologicay eL l instruments, planes and do have a shielding effect on the measurements. For airborne surveys in Canada the shielding effect treef s o calculates swa % cauro 5 t d1 ea underestimation of the exposure rate [12]. Variations in the vegetotio-t cover can also affect ground surveys. For example, the radioactivity of a soil covered by moss and humus will be less tha;i the radioactivity of the bare soil. A equivalent o 0.0 :t watef o 8 m r will reduc exposure eth e raty eb abou % [20] O b t . Because soil moistur similaa s eha r effect and becaus variet ei As s a functio timf no rainfalld ean , surveys carried out at different of the year may not giv e sameth e estimat exposure th f eo e rate.

5 Equivalen2. tfron DosMa m o eNaturat l Radiation Cosmic radiation give annuan sa l effective dose equivalent of 0.41 mSv[46]. The terrestrial component of radiation varies considerably. World estimate averagf so e outdoor absorbed dose rate in air of 55 nGy h"1 (15.28 pGy s"1) giv annuan ea l effective dose equivalen 0.3f to 4 mSve .Th data of table 2.4 corresponds to 0.27 mSv for a person who stays out-of-door yeal sal r roundabsorbee Th . d dose ratn ei air of 45.3 pGy s"1 gives 1 mSv a"1. Indoor radiation varies with building material. Estimates of average indoor absorbed dose rate in air is 70 1 (19.4h" s"y y 4pG 1)nG [46] effectivl , aa givinm a n ga e dose equivalen persoa o stayo t 0.4f to nwh v s3mS indoor yeal sal r round. Estimate occupancf so outdoorr yfo factor2 0. s e sar indoorr fo 8 ans0. d [46], correspondin annuan a o gt l effective dose equivalent from terrestrial radiation of 0.41 mSv. Inhalation of radon in the air is probably the most important contributo radiatioe th o rt n dose receive many db . A radon daughter content of 1 Bq/m3 gives an annual equivalent dos 0.08f eo v [22]1 mS totae .Th l world population global mean annual dose equivalen bees tha n v estimatemS 2 1. t a d [46]. The regional variation is large. For example, in Sweden, the estimated average dose equivalent from radon is 3.7 mSv a"1.

17 3. GAMMA RAY SURVEYS Gamma ray surveys for geological purposes have been designed in many different ways. To be of use in defining the natural radiation environmen necessars i t ti y thae tth measurements be self consistent and have a direct correspondenc terrestriae th o et l radiation. There ear several conditions that must be satisfied. The following sections discuss the instrumentation, measurement techniques, calibration and reporting units.

3.1 Instrumentation Instrument measurinr sfo g gamma rays convert energy int electrican oa l current mose Th .t commonly used radiometric field instrument geologicar sfo l wor Geigere kar - Müller (GM) counter scintillatiod san n counters. Most recent gamma ray surveys use Nal(Tl) scintillation detectors. For a the most significant factors are the detecto e energe sizth th f eo d y ran discriminatio n threshold. A Nal(Tl) detector interacts with radiation mainly by Compton scattering and the . The output electrical signa proportionas i l absorbee th o lt d gammy ra a energy and thus the scintillation detector can be used in a gamma ray spectrometer. Because the number of gamma rays absorbed increases as the volume of the detector increases, larger detectors provide greater sensitivity. The energy discrimination threshold control outpue sth t count rate. Greater detail gammn so a survey instrumentatio gives ni n i n IAEA [23] and IAEA [24]. In a total count gamma ray scintillometer instrument all gamma ray signals above an energy discrimination threshold are recorded discriminatiochangy e An th . n i e n threshold will affec response tth instrumente th f e o smal r Fo l. scintillation counters the discrimination threshold is usually keV 0 givo 15 t ,betweet ea d higan t 0 se nh3 sensitivity. Such instruments usually hav systeo en stabilizo mt gaie d eth nan thus the discrimination threshold which means that the instrument response may change with time. Instruments with stabilization systems eliminate this problem. In a gamma ray spectrometer a gamma ray energy spectrum is measured which can be used to define the amounts of the different radioéléments. This is done by measuring the count rate in distinct energy windows specific for the gamma ray energy of particular element. Some systems use a multichannel analyzer channels6 wit 25 mans ha s ya , while simpler systems use 3 or 4 distinct energy windows. Typical energy windows used for determining K, U and Th are shown in Table 3.1 [25]. e functioTh gamma spectrometey f no ra a r requires stabilization to avoid the effects of small energy drifts. These cause more severe errors in a spectrometer than in a total count system. Stabilization can be done by controlling temperature th detectore th f eo energy ,b y stabilization using a reference or an LED, or by the use of

18 Table 3.1: Spectral energy windows commonly used for gamma ray spectrometres. Element Isotope Gamm y ara Energy windows analysed used energy (MeV) (MeV)

K 40 1,4K 6 1.3 1.57- 7

U 2KBi 1.76 1.66 1.8- 6

Th 208T1 2.62 2.40-2.80

measured natural radiation lattee th n rI .case , witha multichannel analyzer possibls i t ,i correco et energr tfo y surveye th f o . d en drifte th t sa

3.2 Calibration and Reporting Units Many of the early radiometric surveys carried out around the world were reported in counts per unit time which apply e instrumenth onl o yt t actually used provido T . e consistent quantitative radiometric measurements it is necessary to calibrat instrumentse eth . Gamma ray spectrometers can be calibrated using a source containin knowga n radioélémentamoune th f to . These sources must be large enough to be considered of infinite size. Calibration sources are constructed as large concrete pads, usually spikethree th edf o naturawite hon l radioélémentsA . fourth, unspike alss i od constructedpa d [26]e .Th measurements on the pads show the response of the instrument to the three natural radioéléments in each of the spectrometer energy windows. Portable gamm spectrometrey ra a e sar calibrated on pads of 3 metres horizontal dimensions. The calibratio airbornn a f no e radiometric syste dons i my eb making measurement largen so r calibratio tesa n to pad d san strip. A test strip is a natural area with accurately known ground concentrations of the radioéléments. Sensitivities of e spectrometeth altitudd ran e dependenc responsf eo e ear determined on the test strip. Calibration facilities and procedures are described by Lovborg [27], Grasty [28], [29] an d26]& IAE5 .[2 A Total count instruments can be calibrated using calibration pads. Point sources have t beeno no d uset dbu give an adequate calibration for field measurements. Spectrometric measurements have been reported either in counts per unit time for the three energy windows or as concentrations of potassium, uranium and thorium. In some cases other reporting units have been used such as multiples

of 'background' radioactivity. Total count measurements have been reported as counts per unit time, as /xR h",1 ppm equivalent uranium (ppm eU), unit of radioélément concentration (Ur) or as multiples of "background".

19 3 Geologica3. l Gamm Surveyy aRa s Regional geological gamm surveyy ara s have most frequently been airborne surveys and sometimes carborne surveys exceptionan I . l cases ground survey methods have been used desige particulaa Th . f no r regional survey dependn so many factors; the area to be covered, available techniques and equipment, funds and personnel resources. Corrections for the influence of different effects must be applied to gamma ray data. These effects may include cosmic radiation, instrument background, altitude variations, Compton scatteringd ,an source geometry. The effect of radon in the air has been estimated using upward looking detectors in airborne surveys. Airborne surveys are usually flown on a regular parallel grid using flight line spacings from 0.1 to 10 km. In mountainous terrain contour flying is often used and sometimes irregularly spaced flight lines. Nominal terrain clearances vary from 30 to 150 metres. Navigation and flight path recovery commonl combinatioa e yus visuaf o n l techniques, electronic positionin flighd gan t path cameras, either video or photographic. varioue Methodth r sfo s corrections noted above are discussed by Darnley [30], IAEA [24] and Grasty et al. [12]. Regional ground surveys include surveys that have covered large areas with carborn hanr eo d held gamm measurementsy ara . In general these will have been carborne total coun gammr to a ray spectrometer surveys. Carborne survey restrictee sar o dt existing roadaffecte e trackd ar san d roay san db d materials and geometry. For typical rocks or soils with an air dose rate of 10 - 2 jit1 R - h" 4 1)( tota1 s~ ly counpG 0 3 t measurements with hand held instruments willy havpG accuracn 5 ea ordee ± th f rn o i y s"1 (± 2 pR h"1) . For similar materials a spectrometer with a Nal(Tl" 3 3"X ) detector will give standard deviatione th n si usinh eT ga m pp 6 0. ± d an U e m pp 3 0. ± , K % 1 0. orde± f ro measuring minutes4 tim f eo . e resultTh gammf so surveyy ara presentee sar maps da r so profiles. Maps produced include contour maps, maps showing profiles along flight lines, grey scale maps and so on. Maps prepared for early surveys were usually hand drawn which implies interpretation by the cartographer.

3.4 Suitable Data Many of the regional gamma ray surveys made throughout the world contain much valuable informatio naturae th n no l radiation environment. Because the techniques and instruments usethesr fo d e surveys were different compilationy an , f so these data will require careful analysi originae th f so l specifications of the surveys. If a gamma ray survey is to be use defino dt naturae eth l radiation environmen date tth a collection and reduction techniques should result in values proportional to the air dose rate. No amount of subsequent analysis or interpretation can compensate for improper data

20 collectio reductionr no . Minimum requirement fule sar l documentatio proped nan r field procedures [30], [24], [12]. It is also important to check the accuracy of the geographical positionin datae th .f g o Airborne survey particularle sar y suitabl regionar efo l compilations because, typically, they cover large areas. Carborne and other regional ground surveys that cove least ra t several hundred square kilometre alse sar o suitable. With the exception of detailed surveys of small areas gamm y surveyra a s cove r percenw aree onlfe th a ya f to surveyed generan I . coult i l saie b d d tha objective tth a f eo gamma ray survey should be to provide data for all of the lithological units within the survey area. In airborne or carborne survey date recordee sth aar d continuousl thud yan s densite th samplinf yo g alon measuree gth d lin highs i e . Regional ground surveys consist of point measurements at densities such as 1 per 1 or 2 km2. airbornr Fo e gamm surveyy ra a flighe sth t line spacing should be proportional to the final map scale. In the United States and Sweden a factor of 10"5 has been used successfully. require0 00 0 00 exampler scalf slina o Fo 1 p ee1: ma ,a spacing of 10 km or less. Surveys with irregularly spaced flight lines should hav similaea r average sampling density. Because the area measured by a carborne survey is much smaller than that of an airborne survey the sampling of the carborne survey should be more dense. The sampling density of a ground survey should similarl more yb e dense. In addition to careful analysis of the technical quality ane parameterdth survea f decisioe so yth o includnt e eth survey in a regional compilation should consider other factors that might affect efforts to back calibrate the survey. Sometimes the shielding effects of an area may have changed since the survey was carried out. Examples are logging activitie drainine th d swampsf an go s whic changy hma e the soil moisture. Another important factor is radioactive fall-out because the air dose rate due to fall-out will change with time. If the contribution of the fall-out to the total count measuremen 1 (approx s" s lesi t r y o s pG ßR" thal . h 5 t2. kBq/m1 137f o Cs2 ) this neglecteeffece b n tca d (see Figure 2.4) .

21 4. STANDARDIZATION TECHNIQUES Regional measurements of ground radioactivity for geological purposes were carried out by different methods, instruments different ,a t time unded san r different environmental conditions. Early measurements used mostly uncalibrated equipmen reported an t date unitn th di a s thae tar relativa e measur radioactivitye th f eo resulte .Th morf so e recent gamm spectrometry ara y survey reportee sar s da radioélément concentrations. Standardization proceduren sca unify disparate data and express them in air dose rate units generae Th . ) (Gl ys procedur usee b do et include s checking the original radiometric data and their levelling within the surve conversioe ydosr th areai datd e e o aan th ratat f no e directl meany b recalibratior f syo o n (back calibration).

4.1 General Concept of Standardization Contemporary field gamm spectrometry ra a y techniques enable us to determine the contents of the natural radioactive element surfacn si e rock materials. These radioactive elements are sources of ionizing radiation in the environment. Theory and experiments verify that surface radioactivity measurements expressee b dosr n ai e ca e e termraten th th i d f f s.I o origina directle b t lno datyn convertedaca , procedures using standard geophysical field radiometric equipmene b n tca applied to determine the necessary conversion factors.

4.1.1: General considerations materiale Th informatiod san n require thesr fo d e standardization procedures includ radioactivite eth f o p yma the area, and the conversion constants for air dose rate. If the data cannot be directly converted to the air dose rate, back calibration procedures will be necessary. It is clear that any data to be included in a regional compilation must be of good quality radioactivite .Th shoulp yma d show eithee rth data as concentrations of K, U, Th, corrected count rates in energh T d yan windowsU totae , th K lr e ,o gamm th y ra a activity expresse specifin i d c (e.g. p.R h' 1)r relativo e units. date Th a shouldigitan i e db l thet formno y f .I mus e tb digitized at some point. The map should be accompanied by a descriptio techniquee th f no s used methode ,th s use mako dt e the corrections that have been applied to the data, and the methods use mak o mapde t eth . Regional radiometric survey carriee sar undet dou r varying climatic conditions and with a variety of equipment conditions. These varying condition causn sca e levelling problems within a single survey as well as between different surveys .factore Somth f eo s that cause these effecte sar airborne radon, soil moisture, and errors in background correction. Suitable procedures can often be used to eliminate these level differences.

22 A suitable instrument for the back calibration is a

portable gamm spectrometey ara r with gain stabilizatioa d nan Nal(Tl) scintillation detector that is at least 76 x 76 mm (31

volumen i cm inches' 3 0 . 35 x Calibratio r )o n methodr sfo 3 portable gamm spectrometery ara s have been described [24], [25]. Lovborg [27] give detailesa d discussioe th f no technical problems associated with instrument calibration based on theory and numerous experimental data gathered in an IAEA sponsored intercalibration project. Darnley [30] describes airborne gamma ray spectrometry, while Grasty [29] illustrate practicasa l approac establishmene th o ht a f to calibration facility. Recommendations for the construction and use of calibration facilities for field gamma ray spectrometers have been prepared by the IAEA [26]. Back calibratio bases ni d upon comparisonp ma e th f so dat grouno at d measurements obtained witcarefulle hth y calibrated portable gamm y spectrometerra a .

4.1.2: Conversion of radioélément concentrations to air dose rate Potassium, uranium and thorium in rocks are sources of gamma radiation expressee b .n Theica n r i drai effece th n ti exposure termth f so eabsorbee th rat r eo d dose rat airn ei . Conversion factors from radioélément concentration rockn si s and soils to ground level exposure rate have been estimated from photon transport calculations applied to infinite soil medir anai d a [31], [32] numericae .Th l solutio thif no s complex mathematical problem evaluates the effects of primary and scattere whicr dai hgamme th n agroune i th ray d n an di s depen transporn o d energh tT eguation, U y, K emissio d san n spectra. Lovborg [27] published conversion factors calculated from up-to-date data of the 1981 version of the Evaluated Nuclear Structure Data File [33] factore .Th convertinr fo s g natural radioélément concentration into radiation exposure rate are nearly identical for rocks and soils and are given in Table 4.1.

Table 4.1s Calculated factors for converting radioélément concentrations of an infinite homogeneous plane into absorbed exposurdosr o er ai rat en ei ratt ea an altitude of 1m above the soil plane [27].

Conversion to K % 1 1 ppm eU 1 ppm eTh

Air dose rate (pGy s'1) 3.633 1.576 0.693 Exposure rate (/xR h"1) 1.505 0.653 0.287

The values given in Table 4.1 were verified by experimental measurement exposure th f so e rate witha calibrated chamber over various geological materials with known concentration naturae th f so l radioéléments. Generally good agreement was achieved between

23 the experimental results and the calculated values [12]. The calculated air dose rate will be incorrect if the radiation source is not infinite because the downward backscattered radiation of an infinite plane contributes up to 13% to the air dose rate. If artificial radionuclides are present they will also contribute to the total gamma ray dose rate. Such contributions may have occurred in existing gamma ray survey data.

2 Levellin4. Datf go a Withi Survena y Area Raw radiometric data from airborne and ground surveys often reflec measuremene th t t condition individuan o s l days sa wel variations a l s cause sucy b d h effect gais a s n drifd tan variable background radiation correctet no f .I d durine gth data processing, these effects can produce level differences e radioactivitith n y maps. 4.2.1: Factors causing level differences Background radiation includes cosmic rays, internal radioactivit equipmene th f yo airborn d tan e radone .Th background, determined over water, may differ from values measured at altitudes greater than 800 m because of difference amounte th n airbornf si o s e rado cosmid an n c rays. Also amounte ,th airbornf so e largr radoo a ense ove e rth lake generalls si y less thaamounte nth s over inland terrestrial areas. Contamination of the airplane, car or of the instrument itself can also change the background.

Table 4.2: Background values detected by an airborne gamma ray spectrometer with installed Nal(Tl) detector

29x10 (416, 2mm 8 cm )Atomi. c Energy Establishment, Egypt [34]. 3 Energy windows(MeV) h T U K TC

0.36-2.84 1.34-1.59 1.68-2.32 2.42-2.84 Area Altitude Count rate

m c/s c/s c/s C/S

Lake Karon 60 91.7 7.0 6.6 2.2 The Mediterranean 60 66.3 6.3 5.8 1.8 Nile Delta 850 68.3 5.7 6.0 2.4

The accumulation of airborne radon under a temperature inversion layer causes a detectable variation of gamma rays emitted by radon disintegration products as a function of time. Maximum gamma radiation has been observed between four and nine A.M. [35]. Rainfall will caus temporarea y increase measuree inth d radioactivity (Figure 4.1). However^ daily and seasonal precipitatio resultine th d nan g higher soil

24 "re (mtn*)

/ \ TOO ^0 ————— O-——"" ~li~~—-~-fL- I fSO

^jyf^ "~~-^ s* C

1 1 1 11 1 1 I 1 I 1 1 to *3 hourt

Imin') J /OO _ // NV X r a >><3i * ao — °-"~"^/ , ~~~°— --^ ?n o—— —o—-^" 1^ ^^ ^° —^,^^^ —

1 ! 1 I 1 1 1 1 I ! 1 1 1 }Q OG S3 hours

SO SO _.._. A

30 - ——_° — g, ^^-^^ ^-x>-^-— ^

1 1 1 1 1 1 1 1 ] ! I ! 1

1'5 A> « /* O! O¥ OS OS 1O 1! «• IS ta /I

"Th

235 Jl

Q^ ^^J^~~-~^rfS'^ ^^X_ — — -°-«- jy*^~ ~~ ' — " - -No^ -o-__^- 18,5

1 1 1 1 ! 1 1 1 1 ! 1 1 ia it

* ngntfantfarrj conditions

Figure 4.1: Diurnal change of gamma radiation and the effect of radon decay products accumulated by , after 16 hours. Measured count rates in the TC, h energT d yan K, window,U portably sb e gammy ara spectrometer overactivityw lo sand. (After Matolin [35]).

moisture caus increasen ea d attenuatio gammf no a rays. Snow similacovea s rha r attenuation decreaseeffecte th t ,bu d mobilit radof yo frozesoit n i nwe lr no generate increasn sa e e surfacinth e gamm activityy ra a . Instabilit radiometrif yo c equipment, especially gain changes cause spectral energy shifts, which give rise to variation recordee th n si d count rates. Illustrative figures were presente Lovbory db g [27] complee .Th x proble shorf mo t

25 and long term gamma emission instabilit padf yo t sa calibration facilities affect equipmene sth t calibration constants used for survey data processing. The results of airborne measurements are significantly affected by attenuation of gamma rays in wood biomass (see Chapter 2.4). However, seasonal changes in vegetation influenc surface eth e gamma radiation less tha % [36]n3 , [37], [38], [39]. Summary description analysed san errof so r factors in gamma ray spectrometry were presented by Filimonov [40] and IAEA [24].

4.2.2: A mathematical approach to levelling Radiometrie maps can be corrected for the effects of variable conditions using a mathematical approach to levelling. The procedures used to correct for inconsistencies levele inth groupf so profilef so s from different daye sar different from those used for single profiles. These procedure digitaf o s e assumus l e dataeth . The determination of a profile group correction is based comparisoe onth datf no a frosurvee mth y line dato st a from tie-lines. Because the radioactivity field has significant horizontal gradients locatioe point,e th th comparisof f sno o n must be accurate. Airborne surveys using electronic navigation methods may have positioning error in the order of 5 - 20 m whereas other methods of navigation have typically lower accuracies. Profile group corrections of airborne survey are determined by applying the general condition

h hr 2 f [(nw d, - S ki) e*< - (n'- > d,- k '-') e^'^] (4.1n mi = ) 1 i=

where: n,-, n's are the values of the count rates in one of the spectrometer energy windows for the survey lines tie-lined ,an s respectivel theit ya h rit intersection. These data are assumed to have been correcte backgrounr fo d strippingd dan , i is the index of the intersection point of the e th n survei N . y .. lin tie-lined 2 ean , 1 = ,i grou profilesf po ,

k, k' are the indices of the group of survey lines and tie lines, respectively, altitude th e referencd ar ean r h e, h altitude, respectively. w- is a weighting factor for the data at the ith intersection,

e correctioth k ,- d ,e dar k, j n factore b o st calculated.

26 The weighting factors are proportional to the count rates. An example of group profile correction is shown in Figure 4.2. Figure 4.2a shows a contour map of an airborne survey prior to correction and Figure 4.2b shows the corrected data.

4.2a

Figure 4.2: Contou energC T re ymapth f windoso w fron ma airborne survey. Bound dailf so y operatioe nar marke dashey db d lines. 4.2 acompile- d from original data. 4.2 bidentica- levellep lma y db profile group correction. (After Zabadal [47]).

27 Level shift individuaf so l survey correctee lineb n sca d by applying the concept of general continuity of the radioactivity field. This concept is applicable for closely spaced flight lines. The distance between flight lines depends upon the . In terrain with highly variable geology the spacing might have to be as small as 200 m but could be as muc s 100morhr a o arean 0i em homogenouf so s geologye .Th average differences of observed values at neighbouring points of adjacent survey lines should be close to zero. M . 5 wi.j i * ni,j ~ fj> 2 n mi = <4'2> . J — i poine th observatiof ts o i profilewheree th i n no : , profile th s i e number j , fulle th ys i correcte n d count rate,

a weightin s i WTj - g factor, e correctioth s i e calculatedb f - o nt .

An example of single profile correction is shown in Figure 4.3. Both of the above procedures are presented only to introduce the concepts. A detailed description is given in [47].

4.2.3: Levellin datf go a using image techniques Image levelling techniques that utilize image processing equipment and procedures can make the task of back calibration easier. These techniques presen imagn a date n tth ei a format odisplana y screen image .Th e format enables rapid recognitio levellinf no g error estimatiod san correctiof no n factors. In some cases the use of this approach can eliminate the nee mako dt e ground spectrometer measurementsn .I particular, spectrometric data can frequently be standardized conveniently by this method because level differences are ofte resulna scalinf to g differences cause calibratioy db n errors, soil moisture, and/or snow cover. However, if the levelling differences requir additioe eth constana f no t followed by a scaling factor, the imaging method is not appropriate and the mathematical methods described in section 4.2.1 should be used. In the case of total count surveys, the latter type of correction is likely to be required. Figure 4.4 shows grey-tone images of the spectral data fro aerian ma l gamm y surveyara indicates .A e figurth n eo d e imagee lefth th t n so sho date deliverews th a e th y db survey contractor obvious i t .I st no tha date e tth aar internally consistent. The images on the right side of Figure 4.4 show the data after levelling corrections have been applied. Each of the images shown in the figure was produced by first griddin date gth a wit appropriatn ha e grid interval

28 4.3a

4.3b

Figure 4.3: airbornn a Maf o p e gamm y surveyra a . p 4.3ma a- compiled from original data showing inconsistent valuelinee on .n so same 4.3aftep th e ma b - r individual profile correction. (After Zabadal [47]).

29 110' 109* 110' 109'

48"3Q' 48'3Q'

48' 0 ° 48e 0*

POTflSSIUM DflTfl POTflSSlUM DflTfl 1 110 109°

48°50-

48° 0*

URflNIUM DflTfl 110' 109- URflNIUM DftTfl 110* 109°

48°30' 48830'

» 0 ° 48 48° 0'

THORIUM DflTfl THORIUM DflTfl

Figure 4.4: Spectral gamma ray data corrections (left) and corrections (right)

30 and then convertin date gth a frooriginae mth l data unito st integer values in the range from 1 to 254 as shown in Figure 4.5. The images have also been contrast-enhanced to emphasize e flighth t lines with levelling problems. Becaus date eth a for this data set have been corrected by the contractor for background radiation, altitude variations airbornd ,an , Bi e the assumption was made that the levelling problems are the result of calibration errors or soil moisture content. The corrections were, therefore, mad multiplyiny eb date r gth fo a individual flight lines by a constant. Each of the radioéléments (K, eU, eTh) were corrected separately and independently. The criterion used to judge that the levelling problems were corrected was that the individual flight lines can no longer be seen in the images. Experience with this correction techniqu shows eha n that difference smals 5 sa s la imagese percen th seee b n ni n . tca Becaus precisioe eth f no e originath l measurement certain i , sis n cases, probablo yn better than about 20-30 percent, these corrections mighe tb viewed as cosmetic, although the image format actually increase statisticae sth l validit datae correctioth e .Th f yo n procedures frequentl removt no lineal o yed al r trends parallel to the flight lines and, in addition, the gridding procedures sometimes produce linear trends perpendicular to the flight lines. Strik trenr eo d filtering technique appropriate b y sma e to reduce those trends paralle perpendiculad an l e th o rt flight lines. Such filters were applied to the corrected image potassiur sfo uraniumd man .

2 5 _ o

u. o O i: cc "-ö S l

PARAMETER VALUE"

100 200 DENSITY NUMBER

Figure 4.5: Conversio originaf no l data unit o integest r valuerange o 254t th 1 en . si

31 3 Levellin4. f Dato g a Between Survey Areas The results of regional gamma ray surveys in adjoining or widely separated area diffen sca thein i r r levels. Compilation of large scale radiometric maps from the result f individuaso l surveys stage nationaf so l programmes will require levellin date th a f go frodifferene mth t areas. Levelling is based on regional airborne gamma ray tie-lines comparisonn o d ,an statisticae th f so l radiometric characteristics of ground and laboratory group assays with field data, or on systems similar to back calibration.

4.4 Procedure for Standardization of Spectrometer Surveys The radioactivity of the ground has been measured by various type equipmentf so , both calibrated an d uncalibrated. Procedures for standardization will differ for the individual cases. 4.4.1: Gamm spectrometey ra a r surveys with calibrated equipment Airborne, carborn portabld ean e gamm y spectrometerra a s should be calibrated using calibration pads at a calibration facility resulte .Th gammf so y spectromete ra a r surveys with calibrated equipment are reported in concentration e radiatioeThd th an .f U I e , nK sourcf so e correspond infinitn a o st e plane source constante ,th s given in Table 4.1 can be used to convert the data to air dose rate. The individual effects of contributions are then compilatioe summeth used r radioactivite an dfo d th f no y map. Assuming average errors in determining the radioélément concentrations, the air dose rate map can be

contoured wit contouha r s"y interva.pG 5 4- l 1 4.4.2: Gamma ray spectrometer surveys with uncalibrated equipment e resultTh gammf so spectrometey ra a r surveys with uncalibrated equipmen reportee b n tca fulls a d y corrected count rates (c/s eacr )fo h energy window correctione .Th s applied for airborne gamma spectrometer data include background corrections, stripping corrections and altitude corrections. The corrections applied for portable gamma ray spectrometer include background corrections and stripping corrections. Conversio thesf no e corrected count rateo st dosr ai e rate requires back calibration. Dependine th n go geological settinaree f interestth o a f go , back calibratio wayso tw carriee n .b i n t nca ou d Large, radioactively homogeneous geological bodies of differing radioactivit aree realizef th o a n i y d regional radiometric survey can serve for back calibration. It is recommended tha minimuta fivf o m e extensive radioactively different level areas be measured. Twenty or more stations measuree ar eacn i d h wit calibrateha d portable gammy ra a spectrometer, with the detector positioned l m above the ground surface to accumulate a statistical group of data.

32 Using a Nal(Tl) detector volume of 350 cm,3 a minimum measurement time of 4 minutes at each station is recommended. The concentrations of K, U, Th are determined meae andth n valu eacr s i fo eh , elementTh , U , ,K calculated for the individual areas. This will result in separate conversion equatione th eacr f sfo ho radioéléments conversioe .Th n factor then se ca nb calculated usin lineaga r meae regressioth no t t nfi radioélément values for the individual areas versus average valuefulle th yf so correcte d count rate respectivn si e energy windows. The resulting regression equation can be considered acceptable if the errors between the calculated and measured concentration lesl valueal s e thasar n 20%. The regression equation should ideally intersect the origin of the axes of the variables. Any shift of the regression verticaa lin n i e horizontar o l l direction indicate incorrecn sa t background subtractioe th n i n original surve bacr yo k calibration data regressioe .Th n is, none-the-less, valid. Once acceptable regression equations have been calculated enabling conversio fullf no y corrected count rateo st radioélément concentrations, the survey data can be converte dosr ai e o ratedt . The second method employs similar procedures using data along selected line segments. Fiv morr eo e segments with minimum lengths of 1000 m should be selected. These segments should have relatively constant radioactivity and the average radioactivity of the different segments should be as different as possible. The ground data measured by a calibrated portable gamm y spectrometera a r shoule db obtaine t intervala d alonm 5 e lin2 gth e f so segment with detectoe th abovm groundrl e eth . Usin Nal(Tla g ) detector minimua , m c m0 volum measuremen35 f o e t minute4 tim f o e s at each station is recommended. The average values of K, U, Th are then determined for each line segment. The calculatio conversioe th f no n factors use regressiosa n analysi describes sa d above e location.Th e linth ef so segments mus accuratee tb orden .I consideo rt e linrth e segments or areas as radioactively distinct, they should fulfil the following criterion:

- n2 > 1.3 (s1 + s2) (4.3)

where: n.|, n2 are the mean values of the fully corrected segmente th counf o areasr tso o tw rate r , fo s

s1, s2 are the standard devic.tions of the count segmente th f o areasr so o ratetw .r sfo

e resultIfth gammf so spectrometey ra a r surveys with uncalibrated equipment are reported as raw window count rates, the back calibration requires more complex data th processing. The raw window count rate rM in the i window

33 is the sum of the background count rate b,- and three products of sensitivities ss-- of the instrument and concentrations C- [24, 26].

+i b Snj= is ijcj (4-4)

A minimum of four areas with different levels of radioactivity mus measuree tb d wit calibrateha d gammy ara spectrometer in the region of the regional survey and the mean concentration / CK h determineT ysC ,C eacr fo dh area. Using the original raw count rates ni and the calculated concentrations C- from the four investigated areas, the background rat ed sensitivitie ban ^ e originasth s-r -fo l instrumen determinee b n tca d from equation (4.4). Knowing d Sjje originaan th , window ra l w count e rateb n sca converte radioélémeno dt t concentrations whic alsn hca e ob used to evaluate the air dose rate (Table 4.1).

5 Procedure4. Standardizinr sfo g Total Count surveys e singlTh e measured value totaf so l count surveys, which are dependent on instrument construction, make the task of definin environmentae gth l radioactivity more difficult. 4.5.1: General problem totaf o e ls us coun relatee th t o t d surveys The response of total count instruments to the radiation groune th generalls n i di h T t proportionad yno an U , K f o l to the contributions of these elements to the air dose rate. The response of a total count instrument depends on typ detectorf eo dimensions energe ,it th n yo d san discrimination threshold used thir .Fo s reasos i t i n importan understano tt e limitationdth usinf so g total count survey estimato sdost r ai e e rateeth . detectioe Th n efficienc detectorf o y dependens si n to their propertie detectee energe th th f d o ys an d gamma rays. detectioe Th n efficienc scintillatioa f yo n counter decreases significantly with increasing gamma ray energy [25]. Total count instruments measure gamma rays of greater thaenerge nth y discrimination threshold, e responseanth d totaf so l count h instrumentT , U , K o st radiation differ [24] relationshie Th . p betweee nth corrected count rate and concentrations C of K, U, Th in the ground is:

nTC = aKC< + aUCU + aThCTh (4-5)

where: nTC is the total count rate,

a

e radioélémentth Ce K,ar h CyT C , s concentrations.

34 The sensitivities are expressed in terms of the count rat unir epe t concentratio infinitn a n ni e plane source, and their values depend on the properties of the detector energe anth d y discrimination threshold. Total count instruments typically have energy discrimination thresholds keV0 totae 15 e rang,Th i- nth l 0 e3 coun t energy threshold of gamma ray spectrometers is usually set in the range 400 - 800 keV. Equation writte e (4.5formb e th n )n :ca ni

nTC = aU C ————— CK + CU + ———— CTh 1 <4'6) au au

wher euraniue ath K /as i ym equivalen potassiuf to ad Than m/a u is the uranium equivalent of thorium. The term uranium equivalent refers to the concentration of uranium that gives an identical count rate in a total count measurement as 1% K or "• ppm Th. The ratio a^/a^ changes significantly with detvolumr co penergd ean y discrimination threshold, but aTh/" is nearly constant.

Table 4.3: Typical uranium equivalents of potassium and thorium for field total count instruments. Data based on Nal(Tl) detector; infinite plane source; concentrations m [41]pp h .ppmU expresseT , % K s a d

Nal(Tl) Energy discr. aK/au aTh/au volume threshold cm keV

29 30 1.57 0.37 82 50 1.56 0.38 348 400 2.37 0.41 348 500 2.67 0.41

Table 4.1 shows the air dose rate caused by unit evidens i t I r t. ai concentratiotha Th e d tth an U , K f no dose rate from 2.30 ppm U and 0.44 ppm U are equivalent to . Tabl Th show3 m 4. equivalene pp sth 1 d an t1K % concentration with respect to instrument response. It follows from Tables 4.1 and 4.3 that the response in the total count energy window is nearly proportional to air dos einstrumenn a ratr fo e t wit Nal(Tln ha ) detectox 6 r7 76 mm (348 cm3) and an energy discrimination threshold in V [42] Ke r instrument 0 Fo .e rang40 th f eo s with different technical parameter measuree sth d count t rateno o sd correspond linearly to air dose rate.

35 If total count instruments are calibrated over extensive containinU g plane source over so r infinite rock plane source in which radioéléments are expressed in uranium equivalent sensitivite ,th thee b nn expresseca ya s c/ n i d

per ppm eU. Equivalenu t uranium concentrations can be determine dividiny db correctee gth d count e rateth y sb constant a(J. Equivalent uranium concentrations multiplied by the conversion constant 1.576 (Table 4.1) giv dosr eai e rate values, tha onle tar y correcinstrumenn a r tfo f to technical parameters specified above. Respons othef eo r instruments to radiation of K, U and Th is not linearly proportional to air dose rates of K, U and Th and the results are determined with error increasing with K/U and Th/U ratios. 7 illustrat4. Figured an magnitud e 6 eth s 4. e th f eo relativ dosr ai e e erroratth en ri determine n a y db arbitrary total count instrument calibrated ove uraniura m pade relativTh . e erro calculates ri e ratie th th f o s da difference between the experimentally determined air dose rate and the true air dose rate to true air dose rate (De - Dt)/Dt. n examinatioA e figure th date n th i a s f no show e sth importance of size of detector and discrimination threshold. Instruments having value af s

-ItO

GK/CU

Figure 4.6: Dependenc relative th f eo e errore E(%th f o ) determined air dose rate on the K/U ratio in totaa rock r lfo s count instrument calibrated ove padU havind ra an , g uranium equivalenf to potassium aK/au.

36 0*3

-1Q -

-

CTn/C(J (ppm eTh/pp) meU

Figure 4.7: Dependenc relative th f eo e erroe rth E(%f o ) determined air dose rate on the Th/U ratio in rocks for a total count instrument calibrated over a U pad and having uranium equivalent of thorium aTh/au.

Total count instrument sdiscriminatiow witlo ha n threshold (30 - 50 keV, aK/au = 1.6; a^/a^ = 0.37) underestimat e tru dosr eth eai e y abou ratb r C^C^2 efo t = abouy b relatively % Cr 0 Th13 1 tfo /C = yd an % n .A 25 o t instrument with a discrimination threshold of 400 keV estimate dosr ai e e ratsth e wit erron ha r less tha eve% 3 n n for extreme K:U:Th ratios instrumenn .A t witha discrimination threshold greater than 400 keV (aK/au > 2.3; aTh/a(J > 0.44) will overestimate the air dose rate by up to 25%. The curves of Figures 4.6 and 4.7 can be used to estimat e likeleth dosr yai e e error e ratth th f n i esi energy discrimination is known. Calibration procedures by means of a 226Ra will resuloverestimation a n ti dosr ai e f no rates n .A instrument with an energy discrimination threshold of 30 keV gives nearly twice the correct air dose rate [43]. For this reason, such calibration procedures are not recommended. If proper calibration facilities are not available a natural site that has been characterized by laborator r fielyo d gamm y spectrometera a r measuremenn tca be usecalibrationr fo d . 4.5.2: Total count surveys with calibrated equipment e sensitivitTh instrumenn a f yo t calibrated over extensive uranium enriched plane source or over infinite rock plan U ande e,m sourcpp r expresses pe ei s c/ n i d consequently resulte ,th survef so reportee yar s a d equivalent concentrations of U (ppm eU). The conversion of these data to air dose rate can be performed by means of the constant given in Table 4.1. If the air dose rate or exposure rate over the U calibration source is known, the results of the survey can be converted directly to air dose rate or exposure rate.

37 4.5.3: Total count surveys with uncalibrated equipment Instruments that have not been calibrated over an infinite plane sourc consideree ear uncalibratee b o dt r fo d the purposes of this manual. The results of surveys with uncalibrated total count instruments, reported in count rate (c/s), provide onl relativa y e measure th f eo environmental gamma radiation. Conversion to air dose rate units requires back calibration. The back calibration procedure usee e similab ar do s t thoso rt e describen i d section 4.4.2. In the case of the total count back calibration procedure the portable spectrometer data are first converted to the air dose rate contributions from the three radioéléments. These contribution thee sar n summeo dt obtain the total estimated air dose rate. The total air dose rate versu totae sth l count survey datusee o ar at d obtai regressiona n equation, whic uses i hconvero t d e tth surve dosyr ai date o rateat befors .A e (see section 4.4.2) the resulting regression equation can be considered acceptabl difference th f ei e betwee calculatee nth d an d measure dosr dai e rate lesl valueal s e thasar n e 20%th f .I differences do exceed 20% then the back calibration data e inadequatear . This means that eithe surve~ rth y date ar a unacceptable for inclusion in a regional compilation or additional back calibration datrequirede aar . Comparison of available relative results witdosr hai e rates acquired by measurement with portable calibrated gammy ra a spectromete survee th n yri area determines conversion constants.

. 5 DATA PRESENTATION The presentation of the data from gamma ray surveys is a subject of some importance. The process of preparing the datpresentatior fo a n requires considerable effor wild tan l depend on the use to which the presentation material will be put. commoa Map e sar n for datf o m a presentatior nfo gamma ray surveys but the type and complexity of the map presentation will depend on the audience. Generally, technical workers using the maps will require a less polished but more complete depiction of the data, whereas maps prepare publir fo d c distribution will requir lesea s detaile mort dbu e graphically forceful presentation.

5.1 Graphical Form The data should be presented as contour maps using the unit picf so o gray seconr spe d (pGy s"1) wit contoua h r s (sey intervaepG 5 sectio f lo n mape 4.5.1)e th sar f I . intende usefue b geologicar o dt fo l l applicationse ,th contour interval might smaller e havb o et generaln I . e ,th data shouldigitan i e db l forgreater fo m r eas datf eo a handling.

38 Standard gridding and contouring practices should be followed. In particular, care should be taken to eliminate area invalif so d contours (e.g. over large lakes) rulA . e of thumb to determine when to delete the contours would be to do so only for areas of 1 square centimetre or larger at the final map scale.

5.2 Map Compilation The process of compiling a variety of surveys that have been standardized as described in Chapter 4, above, can be either manual (analog automatir )o c (digital) generaln I . , digital compilatio preferree b o t s i nd over manual methods because the digital process will result in a data base that can be manipulated and augmented as new data become available however, If . larg,a e fractio e availablth f no e data aie not already in digital form, the effort required digitizo t consideree b date y eth a ma d excessivd ean preference give manuao nt l compilation e availabilitTh . y of suitable hardware and software for data processing and digitizing will als factore ob determininn si e gth preferred compilation method.

5.2.1: Manual compilation The first step in a manual compilation procedure is to redraft an existing contour map using the conversion equation defined by the recalibration procedures described equivalene ar in s Chapter example5 c/ Fo o 0 t 5 . f r4 i , pGy s" , the 50 c/s contour level would be redrafted and y s"labelle pG 1mosn .I 5 ts a dcases , however e existin,th g contours on the contour maps will not coincide exactly with contou1 contoure s" th y rpG s5 interval needea r n fo dI . this case correce ,th t positio contoue th f no r line will determinee havb o et d using linear interpolation between existing contours. Figures 5.la and 5.1b show an example omapa f exposurf so e rate determine lineay db r interpolation. bees ha n p completelAftema e rth y redrafted must i ,e tb changed to the final map scale. The scale change can be done photographically or using any of a variety of projectors commonly use chango dt scalep ema s (e.g.a Salzman projector) correce th t a ts i scale. p Aftema ,e rth it is physically placed in the correct location on the regional map. Ideally the recalibration process will eliminat levellinl eal g differences between different surveys. In practice, level differences will still be present. However, if such differences exceed 20 percent, bace th k calibration shoul consideree db d suspecd tan shoul examinee db possiblr fo d e errors. e leveth Eve f li n differences are saall, the contour lines are unlikely to match eithe arean ri overlaf so joit a nr po lines n I . areas of overlap, a decision must be made regarding which surve consideres i y more th de reliablcontoure th d ean s from the other survey should be eliminated. At the join lines the contour lines will need to be redrafted to produce smooth contours. Figure 5.2 illustrates this procedure.

39 - CO * 7 7 76 ' 45-

3T 50' -

• - 45 T 3

77 ' DO- 76 ' 45'

77 ' CO- 76' '45

- ' 45 7 3

77 ' Off 50- ' 45 * 6 7

Figure 5.1: Maps showing the conversion by interpolation of e contourth countn si r seconspe d (5.la aboveo )t contours in pGy s"-i (5.1b below).

40 " 00 • 7 7

37* 50- -

• ' 45 " 37

77 ' Off 55' 76 ' 45'

Figure 5.2: Adjustmen contourf t o e are overlath f o an si p between two dilferent surveys.

5.2.2: Digital compilation e datalreade th aar f I digitan i y l form e regriddin,th g and compilation procedures are straight forward. Figure 5.3 illustrates the general procedure where the data would be gridde overlappinn i d g e resultinblockth d san g grids inserted into the final grid. If the data are not in digital form, they will have to be digitized. If the original flight line location knowne sar date ,th a should be digitized only along the flight lines. Otherwise, a regular grid coulactuae th usee db r lo d contour lines coul digitizede necessars db i t i f digitizo I yt . e maps, care mus takee tb ensuro nt e tha digitizee tth d data accurately represent the original map. One way to verify e th t a e accurac th p digitizine ma th w f mako yo t ne es a i g same scale and with the same contour intervals as the origina compard an p lema them. Prio digitizingo rt e ,th maps should first be back calibrated and redrafted. In the case where spectral datavailablee ar a elemene ,th t data should also be compiled into separate maps because they will be useful for geological applications. In all cases

41 any digital data should be carefully preserved in a digital data base.

6. INTERPRETATION AND USE OF THE MAPS Geochemists have long recognize significance dth f eo geochemical surveys to environmental studies. In many countries this awarenes spreadinw no s si scientifio gt c policy makers. More importantly, scientific concern about the rate of global-change is beginning to attract media attention and is creating some popular pressure for government action. Recognising that most existing geochemical surveys were conducte geological/minerar fo d l exploration purposes neea , maximiso w dt ther no s e ei eth environmental usefulness of existing data in order to provide baseline geochemical information of use in global change environmental, agricultural and health studies as well as geological research. The radioélément exploration pare thif on to dat e s aar materia greaf o e ltb than tca value, put to proper use. A statistical relationship between strong irradiation and health effects has been proved. The relation between low level radiatio effect s humae it th d n s o nan organis a s i m subject of controversy. Organizations such as the ICRP, WHO, UNSCEAR, BEIR accept the assumption that the number of health effects, cause radiatioy db proportionas i n e th o lt absorbed radiation dose [44]. Figure 6.1 shows (A) proved, assume) an(B d d proportion probabilite th f so healtf yo h effects to the absorbed dose equivalent in the human body. Natural environmental radiation also contributes to the overall irradiatio humae th nf no body . dosr ai basia euse e b e r s rat a dMapth n sfo eca f s o calculation of the annual effective dose equivalent. Since conversio dosr ai e f nrato e dat wholo at e body absorbed dose rate is calculated by means of the conversion factor qualite th 7 [46 0. d y]an facto gammf ro a radiatio equas i n l to 1, the annual effective dose equivalent caused by terrestrial gamma radiatio e relationshipgives th i n y nb : = 2.2 1 s" 1 y a" 10" pG v 1 mS 2 (6.1) The IRCP has recommended 1 mSv a"1 as the limit on the annual effective dose equivalent from sources other than natural radiation recenA . t ICRP report recommendee dth limitation of that component of the dose absorbed from the natural radiation environmen reducee b tn wheca d t i n[22] .

Standardized radioactivity maps denoted in units of air assessmene dosth eusee r b ratfo dn environmentaf eca to l radioactivit delineatioe th r fo d areaf ynan o greatef so r radiation risk in a country. For most rocks and soils the

contributio naturaf no l terrestrial radioactivit annuao yt l effective dose equivalent is less than 0.5 mSv a" bu1 t

42 2 Sv

>0 !00 200 Avtiägt radiation doit equivalen perjor t (rempe )n

Figure 6.1(A): Simplified graph showin healte th g h effects resulting from radiation. (After IAEA [44]).

Figure 6.1(B): Portio Figurf no e 6.1(A), enlarged.

levels up to 2 mSv a" are frequent. In exceptional cases terrestrial radioactivit1 y causes effective dose equivalents of 5 to 10 mSv a"1. Examples of the evaluation of environmental radioactivity have been published in Canada and Sweden [12], [45], Distinct regional radioactivity was determine Variscae th n Alpini d nan e Systemn si Czechoslovakia [48] with valuedosr ai e f srato 19.f eo 4 and 13.9 pGy s"1 respectively with an average value of 17.0 y s"pG 1. Simila dosr ai date n ao rat e have been published focountrie4 r2 s [46]. Examinatio dosr ai e f nrato e maps along with regiona locad lan l gamm spectrometry ra a y maps showing uranium concentration, enables the recognition of areas of increased radiation and radon risks.

43 Maps of the natural radiation environment also serve as a basis for comparison in the event of a nuclear accident. Repeated measurements give information on the decrease of radiatio originao nt l levels with time following sucn ha accident. If the data are of a quality sufficient to permit the production of concentration maps of the individual radioéléments, either directly after ,o r back calibration followin proceduree gth s outline thin i d s manual, theie rus e earte fieldith nth hf s o science muce sar h increased. With this in view the International Geological Correlation Programme (IGCP)/UNESCO Project 259, "International Geochemical Mapping" has recently been launched. The purpose of the project is to encourage and facilitate the compilation of an international series of systematic surficial geochemical map national/internationay sb l organization smose baseth tn o dappropriat e type samplf so e material expectes i t .I d thaprojece tth t will provide evidenc extene th whicf o eo t h different portione th f so earth's crust are characterised by distinctive geochemical assemblages and patterns. In conjunction with other geological/geophysical evidence the data will assist in correlating areas separated through past plate tectonic displacements. The maps will show abundance levels, delineate regional trends and geochemical provinces, and permit the recognitio f largno e scale anomalous features. They will compliment existing international geological and geophysical map series, and will be published at scales suitabl direcr efo t comparison Internationae .Th l Atomic Energy Agenc assumes yha d special responsibilite th r fo y Radioelement Geochemical Mapping subprogramme. Radioelement maps contai wealtna f informatioo h e us f o n in mineral exploration for a number of commodities, including molybdenum, tungste goldd potassiue nan .Th m alteration that frequently accompanies certain types of copper mineralizatio oftes i n n clearly demarcatn eo potassium distribution maps obtained through airborne gamma ray spectrometer surveys. Geologists carryint ou g geological mapping have learne value dth combininf eo e gth information containe radioélémenn o d t maps wite hth magnetic maps they have long used. As with other geochemical element distribution maps the radioélément maps when prepare regionala n o d , continenta globar o l l scale, frequently depict major earth scale structural and tectonic features not seen in any other way.

44 REFERENCES

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