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= R_net / [A e(E)] (2)

where R_net (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.

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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.