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GAMMA RADIATION NATURALLY OCCURRING IN ENVIRONMENT

 

A Greek version of the reserch scientific activities of this topic can be found in odf.pdf  format.

 

 

OVERVIEW

Humans are exposed to a variety of natural sources of ionising gamma radiation. The exposure can be external and internal. The external exposure due to natural sources of gamma ionising radiation. Natural sources are divided to: a) from the cosmic space and b) encountered in natural human environment. Natural sources are the main source of human exposure to gamma radiation. Important natural sources are radioactive isotopes of elements present in the Earth's crust - and by extension the building materials -  40K and the suspended isotopes of radon and its progeny. 

Up to date (2017) several methods been applied to the gamma spectrometry analysis of the gamma-radiation environment of the island of Lesvos. Related dose rates have been calculated. Similar measurements have been conducted in Crete and Athens. Geographic Information System (GIS) and mapping have been employed as well. New technologies and methods have been introduced for assessing the effective doses due to 238U, 232Th and 40K. 

In the following the example case of Lesvos is presented.


INTRODUCTION

Many radionuclides exist naturally at trace levels in various soil and rock formations. Particular interest is being paid on the external exposure due to gamma radiation emitted by naturally occurring radionuclides. The significant sources of this external exposure are the 238U decay chain, the 232Th decay chain and 40K. This exposure depends primarily on geological and geographical conditions (1-3) and, in a significant part, is delivered outdoors (1). This part is frequently estimated by means of outdoor dose rate measurements.

 

Among the various methods developed for the measurement of outdoor dose rates due to the gamma radiation emitted from the natural occurring radionuclides, a well-established one is in-situ gamma spectrometry (4). This method presents the advantages of being quick, direct and convenient, especially when soil and rock samples are difficult to manipulate (4). Another quick method, which, however, provides rather raw estimations, is the surveying with detectors such as a Geiger Müller.

 

Outdoor dose rate surveys have been conducted during the past few decades in many countries (1). This study focused on Lesvos island which is very particular interest due to its geological background. Lesvos is the third largest Greek island (1630 km2) and is located at the northeastern part of the Aegean Sea. The study aimed to survey outdoor dose rates mainly due to the gamma radiation emitted from the natural occurring radionuclides 238U, 232Th and 40K.

 

MATERIALS AND METHODS

Instrumentation

The outdoor gamma dose rate measurements were conducted by employing two portable instruments: (i) a Geiger-Müller detector (Bicron, Micro Sievert), (ii) a portable 76.2 mm x 76.2 mm thallium-activated sodium iodide (NaI) scintillation detector (Model 802, Canberra Industries) equipped with a 1024-channel spectrometer unit (NaI Inspector, Canberra Industries)andappropriate software (Genie PC, Canberra Industries).

 

The Geiger-Müller detector was calibrated at purchase, for the measurement of the effective gamma dose rates. The thallium-activated sodium iodide (NaI) scintillation detector was enclosed in a single integral unit with a photomultiplier tube, a high-voltage supply and signal preamplifier. It was thermally insulated and housed in an aluminium cylinder containing beta absorbers for the elimination of any beta particles entering or escaping the detector housing. To automatically control of the system gain and the gain shifting caused by temperature effects and component ageing, a reference isotopic source of 137 Cs with an initial activity of 37 kBq was used.After exposure, the gamma ray spectrum was processed with the Genie PC2000 software. The system gain was thereafter adjusted according to the data stored in the existing database. Energy calibration was performed in the laboratory, using standard type D point sources of 22 Na, 54 Mg, 57 Co, 60 Co, 109 Cd, 133 Βa and 137 Cs, purchased from Isotopen Laboratory Products.

 

Survey methodology

At the beginning, total outdoor gamma effective dose rates were surveyed in the island of Lesvos. The dose rates were measured with the Geiger-Müller detector. For the purpose of surveying, the island was divided into 4 x 4 km2 grids using appropriate Geographic Information System (GIS) and mappring software (Arcview, ESRI). Within each divided grid three measuring locations were arbitrarily selected. Each of these locations was considered to be the centre of a 10 m equal edge triangle. In each of the three peaks of this triangle, one measurement of the total gamma effective dose rate was performed with the Geiger-Müller detector placed at 70 cm height andwith its windowfacingtheground. This measurement was considered to be the average value recorded by the detector within 5 minutes. This time was selected compensating for convenience and bias; the latter arbitrary considered enough to provide steady dose rate values to within ±15%. The triangle peaks were located on-site via GIS navigation (MAGELLAN GPS 320TM). The average of the measurements conducted in every triangle was considered to correspond to the total gamma effective dose rate value of each measuring location. The data of all of the measuring locations were processed with another GIS software (Arcmap, MAGELLAN) and the (OziExplorer, MAGELLAN) mapping programme so as to produce a map of outdoor total gamma dose rates in Lesvos Island. The Kriging method was employed for the map configuration (7-9).

 

At a second phase, the outdoor gamma dose rates due to the natural occurring radionuclides 238U, 232Th and 40K were measured in-situ in various locations in Lesvos Island. The measurements were carried out using the NaI scintillation detector. In each location a flat surface of 20 cm x 20 cm was prepared. A gamma spectrum was derived with the NaI detector positioned right on the top of this surface and encased within a 2 cm thick cylindrical lead shield. The latter served for the reduction of the gamma ray contribution from cosmic rays and the surrounding environment. It also served so as to ensure detection efficiency during measurement. Based on the gamma attenuation properties of lead, the shield eliminates about 70% of the total environmental contribution (10). This contribution was quantified via background measurements taken at the same locations. The measured background was subtracted from the measured spectra using the Genie PC2000 software. A collecting time of 15 min was selected for the measurements compensating for accuracy and quickness. Particular care was taken so as to avoid outcrop surfaces showing evidence of weathering or presenting uneven geometry. Extra care was taken so as to avoid measurements in flat surfaces affected by vegetation or detrital cover.

 

The outdoor gamma dose rates from each radionuclide (238U, 232Th and 40K) were determined from the gamma ray spectra collected at the sites of measurement. Three energy windows (photopeaks) were investigated in each collected gamma ray spectrum. 238U was determined from the photopeak of 214Bi (609 keV), while 232Th, from the 208T1 (583 keV) one. The primary photopeak of 40K (1.46 MeV) was measured directly. The total energy window of the gamma ray spectrum was set between 0.12 and 3.00 MeV. The dose rates were calculated by application of a method quite similar as that reported by others(4) according to which, the outdoor gamma dose rate in air, D (nGy h-1) may be calculated by the equation

                                                            D= DRCF  ψ (1)

where DRCF (nGy h-1 per cm-2 s-1) represents the, detector independent, dose rate conversion factor and ψ (cm-2 s-1) the flux due to unscattered photons. According to this method (4), DRCF and ψ correspond to the photopeaks (214Bi, 208T1, 40K) used for the 238U, 232Th and 40K determination, while to the 238U, 232Th and 40K radionuclides. According to this reference the DCRF values are 165 nGy h-1 per cm-2 s-1 for 214Bi (238U series), 342 nGy h-1 per cm-2 s-1 for 208Tl (232Th series) and 43 nGy h-1 per cm-2 s-1 for 40K, with the note that the values for the 238U and 232Th series correspond to the average dose rate values due to the different gamma lines belonging to these series. The flux due to unscattered photons emitted by the radionuclides 214Bi, 208Tl and 40K, was calculated according to the equation

                                                       ψ= Rnet / [A e(E)] (2)

where Rnet (s-1) is the net area count rate under each photopeak (214Bi, 208T1, 40K), A is the surface area of the detector (7.62x7.62 cm2) and e(E) is an energy dependant unitless factor incorporating the detector detection efficiency, the peak-to-total ratio and the correction due to the beta absorber of the NaI scintillator (11). The factor e(E) was calculated from data provided by the manufacturer (11).

 

RESULTS AND DISCUSSION

The measured outdoor total gamma effective dose rates ranged between 0.0023 μSv h-1 and 0.28 μSv h-1. These measured rates are also within the international range (1). However, some limited number of measurements lie along the marginal area of this range. The highest outdoor total gamma effective dose rates (0.013 μSv h-1 to 0.28 μSv h-1) were detected in the Northeast part of the island, whilst intermediate rates (0.066 μSv h-1 to 0.13 μSv h-1) in the central region of it; north and northwest of Gulf of Kalloni. In these regions the main rock types are of volcanic origin, e.g. Sykaminea and Skoutaros Formations, and particularly of felsic type, e.g. Polychnitos Ignimbrites, Kapi Rhyolite and Sigri Pyroclastic Formation (12). A possible explanation for these high measured values may be that volcanic and felsic rock types are strongly enriched in 232Th and 238U(3, 13-15). It should be noted though, that the employed Geiger-Müller detector could not distinguish the gamma rays in respect, to their energy and therefore, no gamma spectrometric techniques were applicable. Hence, the reported data collected with the Geiger-Müller detector correspond to effective dose rates due to total gamma rays and not due to gammas of terrestrial only origin. However, since the facing ground orientation reinforces the detection of gamma rays of such origin, these data were interpreted as of some indication for the underlying ground geology.

 

 

The results of the in-situ measurements of the outdoor gamma dose rates due to the natural occurring radionuclides 238U, 232Th and 40K are presented in Table 1. The locations were dispersed mainly in the northeast and the central part of the island, where the highest total gamma effective dose rates were detected. As can be observed from Table 1, the outdoor gamma dose rates due to all the examined natural occurring radionuclides i.e. 238U, 232Th and 40K vary among the various locations. The measured values due to all radionuclides (238U, 232Th and 40K) ranged between (1.7±0.8) nGy h-1 and (154±7) nGy h-1 with an average of (86±6) nGy h-1. The outdoor gamma dose rates due to the 238U radionuclide ranged from (0.30±0.25) nGy h-1 up to (26±3) nGy h-1 with an average of (10±2) nGy h-1. The corresponding rates due to the 232Th radionuclide were between (1.1±0.7) nGy h-1 and (91±3) nGy h-1 with an average of (50±5) nGy h-1 whereas the ones due to 40K ranged from (0.30±0.16) nGy h-1 up to (42±2) nGy h-1 with an average of (25±8) nGy h-1. All the above values are within the international range (1). These values are also within the range of values reported both for Greece (4, 5, 16)and the near area (17) nevertheless, the great majority of these values lie in the upper part of the reported ranges. The average contribution of each of the examined radionuclides (238U, 232Th and 40K) to the total gamma dose rate was found equal to (12±4) %, for 238U, (58±6) % for 232Th and (29±7) % for 40K respectively. The 40K contribution is comparable to the average contribution reported for Greece (4). However, the contribution of 232Th is about 4 to 5 times higher than the corresponding one of 238U; both values deviating from the average values reported for Greece (4). Nevertheless, the elevated 232Th contribution is in accordance to the 4-5 times elevated 232Th concentrations compared to the corresponding ones for the 238U radionuclide, reported for igneous rocks. (2,3).

 

The reported dose rate estimations are subjected to confinements related to the materials and methods employed. On the one hand, the spectrometry measurements in contact with the ground are limited to a small volume of soil or rock, and, hence, the reported dose rate values may have been over- or underestimated. Future measurements higher from the ground level will reveal any possibly existing positive or negative bias. On the other hand, the outdoor gamma dose rates are also biased by the soil moisture and, consequently, by the climate. However, the climate in Lesvos during the reported measurement time intervals does not present intense variations. Therefore, any possible influence is, more or less, similar to both the reported outdoor total effective, and the in-situ outdoor gamma dose rate values.

 

REFERENCES

1.United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. Report to the General Assembly. (United Nations Sales Publication). (2000).

2.Tzortzis, M., Tsertos, H., Christodes, S., Christodoulides, G.. Gamma-ray measurements of naturally occurring radioactive samples from Cyprus characteristic geological rocks. Radiat. Measur.37, 221–229 (2003).

3.Tzortzis, M., Tsertos, H.. Determination of thorium, uranium and potassium elemental concentrations in surface soils in Cyprus J. Environ. Radioact. (in press).

4. Clouvas, A., Xanthos, S. and Antonopoulos-Domis, M. Extended survey of indoor and outdoor terrestrial gamma radiation in Greek urban areas by in-situ gamma spectrometry by a portable Ge detector. Rad. Prot. Dosim. 94(3), 233-246 (2001).

5. Probonas, M. and Kritidis, P. The Exposure of the Greek Population to Gamma Radiation of Terrestrial Origin. Radiat. Prot. Dosim. 46(2),123–126 (1993).

6.Anagnostakis, M.J., Hinis, E.P., Simopoulos, S.E. and Angelopoulos, M.G. Natural Radioactivity Mapping of Greek Surface Soils. Environ. Int. 22(Suppl. 1), S3–S8 (1996).

7. Durrani. S.A., Khayrat, A.H., Oliver, M.A. and Badr, I. Estimating soil radon concentration by Kriging in the Biggin area of Derbyshire (UK). Radiat.Meas. 28(1-6), 633-639 (1997).

8. Van Groenigen, J.W.. The influence of variogram parameters on optimal sampling schemes for mapping by kriging. Geoderma; 97, 223-236 2000.

9.Boulgaraki, B. and Tsetoura, H. Background radiation measurements in Lesvos Island, Greece. Batchelor Thesis, Dept of Environmental studies, University of the Aegean, Mytilene (2003). (in Greek).

10.Chen, F. Q. M. and Chan, L. S.In-situ gamma-ray spectrometric study of weathered volcanic rocks in Hong Kong. J. Earth Surf. Process. Landforms 27, 613-625 (2002).

11. Health, R.L. Scintillation spectrometry. Gamma x-ray spectrum catalogue. IDO-16880-1 (1997).

12. Pe-Piper, G. and Piper, D.J.W. Geochemical variation with time in the Cenozoic high-K volcanic rocks of the island of Lesvos, Greece: significance for shoshonite petrogenesis, Jour.Volc. Geoth. Res. 53, 371-387 (1992).

13. Faure, G. Principles of Isotope Geology. (London: second ed. John Wiley & Sons) (1986) ISBN: 0471864129.

14. Menager, M.T., Heath, M.J., Ivanovich, M., Montjotin, C., Barillon, C.R., Camp, J. and Hasler, S.E. Migration of uranium from uranium-mineralised fractures into the rock matrix ingranite: implications for radionuclide transport around a radioactive waste repository. In: Fourth International Conference of Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere (Migration1993), Charleston, USA,12–17 December1993.Radiochemica Acta 66/67, 47–83 (1993).

15.Chiozzi, P., Pasquale, V. and Verdoya, M. Naturally occurring radioactivity at the Alps-Apennines transition. Radiat. Meas. 35, 147-154 (2002).

16 Sakellariou, K., Angelopoulos, A., Sakellariou, E., Sandilos, P., Sotiriou, D., Proukakis C. Indoor Gamma Radiation Measurements in Greece. Radiat. Prot. Dosim. 60, 177–180 (1995).

17.Karahan, G. and Bayulken A. Assesment of gamma dose rates around Istanbul (Turkey). J.Environ.Radioac., 47, 213-221 (2000).

 

Table 1. Results of the in-situ measurements of the outdoor gamma dose rates due to the natural occurring radionuclides 238U, 232Th and 40K together with the geology of the measurement sites. The last column presents the ranges of the outdoor total gamma effective dose rates measured with the Geiger-Müller detector at the sites of column 2.

Table 1

           
   

Outdoor dose rate (μGy h-1)

   
             

Dose from Background

   

Total

238 U

232 Th

40 K

 

Radiation (μGy h-1)

a/a

Measurment location

(x 10-3)

(x 10-3)

(x 10-3)

(x 10-3)

Rock type

(x 10-3)

1

Kakopetria

1.7±0.8

0.30±0.25

1.1±0.7

0.30±0.16

Post Miocene sediments

0 - 33

2

Kidwnies

99±6

11±2

57±5

32±2

Polychnitos & Skopelos Ignimbrites

66 - 96

3

Pedi

124±6

13±3

74±6

37±2

Polychnitos & Skopelos Ignimbrites

130 - 280

4

Sikaminea

96±6

8±2

62±5

27±2

Sykaminea formation

130 - 280

5

Kapi

70±5

10±2

41±4

19±1

Sykaminea formation

96 - 130

6

Pelopi A

75±5

3±1

42±4

30±2

Sykaminea formation

130 - 280

7

Pelopi B

114±6

14±2

65±5

34±2

Sykaminea formation

130 - 280

8

Stipsi A

33±3

4±1

20±3

9.0±0.9

Sykaminea formation

130 - 280

9

Stipsi B

154±7

26±3

91±6

37±2

Sykaminea formation

130 - 280

10

Ipsilometopo

90±6

10±2

56±5

24±2

Sykaminea formation

130 - 280

11

Petsofas A

58±4

9±2

32±4

17±1

Post Miocene sediments

96 - 130

12

Petsofas B

113±6

19±3

64±5

30±2

Kapi rhyolite formation

130 - 280

13

Petra

48±4

8±2

21±3

19±1

Skoutaros formation

96 - 130

14

Anaxos

50±4

9±2

26±3

15±1

Skoutaros formation

96 - 130

15

Skoutaros

106±6

19±3

63±5

24±1

Skalohorion formation

130 - 280

16

Dafia-Filia

77±5

11±2

45±4

22±1

Skalohorion formation

96 - 130

17

Skalochorion

93±6

12±2

59±5

23±1

Skalohorion formation

96 - 130

18

Filia

65±5

7±2

39±4

20±1

Skalohorion formation

66 - 96

19

Klio

100±6

10±2

64±5

26±2

Sykaminea formation

130 - 280

20

Lepetimnos

138±7

14±2

82±6

42±2

Sykaminea formation

130 - 280

21

Molibos

58±5

9±2

41±4

7.9±0.8

Skoutaros formation

66 - 96

22

Bafeios

55±4

8±2

30±4

18±1

Undivided lower lavas

66 - 96

23

Eftalou

75±4

8±2

34±4

35±2

Undivided lower lavas

66 - 96

24

Orato Rigma

105±5

13±2

52±5

40±2

Undivided lower lavas

130 - 280

25

Skala Sikaminias A

111±6

8±2

70±6

33±2

Undivided lower lavas

130 - 280

26

Skala Sikaminias B

126±6

10±2

82±6

34±2

Sykaminea formation

130 - 280

 

Average:

86±6

10±2

50±5

25±8

   

tb

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