PERFORMANCE OF HANDHELD NAI(TL) SPECTROMETERS AS DOSIMETERS BY LABORATORY AND FIELD DOSE RATE MEASUREMENTS

In the framework of the IAEA Coordinated Research Project (CRP) J02012 on 'Advancing Radiation Detection Equipment for Detecting Nuclear and Other Radioactive Material Out of Regulatory Control', the properties of two commercial instruments (1) InSpector 1000 analyzer (Canberra), with a 2″ × 2″ NaI(Tl) scintillator and (2) RIIDEYE M-G3 analyzer (Thermo Scientific), with a 3″ × 3″ NaI(Tl) scintillator, were evaluated as dosimeters by laboratory and field measurements. In the Ionizing Radiation Calibration Laboratory (IRCL) of the Greek Atomic Energy Commission, the NaI(Tl) spectrometers were tested in order to measure Ambient gamma Dose Equivalent Rate (ADER). The NaI(Tl) scintillators were irradiated in a homogeneous field with 662 keV photons with different ADER values from 0.17 to 100 μSv h-1 at 0° incidence (irradiation field perpendicular to the detector's front window) and at 90° incidence. For each irradiation, the measured ADER by the spectrometers and the 'true' ADER values (provided by the IRCL) were compared. In addition, the angular dependence (0-359°) of the ADER response of the spectrometers was studied with a 152Eu source placed at 1, 2 and 3 m from the spectrometers. The ADER dependence as function of the distance from the 152Eu source (at 0° incidence) measured by the two detectors was compared with the theoretical one. In the field studies, ADER was measured by the spectrometers at seven locations belonging to the Greek Early Warning System Network (which is based on Reuter-Stokes ionization chambers). These locations have different ADER values ranging from 20 to 120 nSv h-1. In these locations, gamma ADER were also deduced (1) by in situ gamma spectrometry measurements with portable Germanium HPGe detectors and (2) by the Reuter-Stokes ionization chambers (by subtraction of the cosmic radiation). Gamma dose measurements were also performed with the InSpector 1000 and RIIDEYE M-G3 detectors in 25 locations (beaches) of Northern Greece and compared with the ADER values deduced by sand sample analysis with gamma spectroscopy. Beaches with sand are good candidates for such type of measurements since they are commonly flat and in principle the natural radionuclides are homogenously distributed.

RESPONSE OF THE GREEK EARLY WARNING SYSTEM REUTER-STOKES IONIZATION CHAMBERS TO TERRESTRIAL AND COSMIC RADIATION EVALUATED IN COMPARISON WITH SPECTROSCOPIC DATA AND TIME SERIES ANALYSIS

The Telemetric Early Warning System Network of the Greek Atomic Energy Commission consists mainly of a network of 24 Reuter-Stokes high-pressure ionization chambers (HPIC) for gamma dose rate measurements and covers all Greece. In the present work, the response of the Reuter-Stokes HPIC to terrestrial and cosmic radiation was evaluated in comparison with spectroscopic data obtained by in situ gamma spectrometry measurements with portable hyper pure Germanium detectors (HPGe), near the Reuter-Stokes detectors and time series analysis. For the HPIC detectors, a conversion factor for the measured absorbed dose rate in air (in nGy h-1) to the total ambient dose equivalent rate Ḣ*(10), due to terrestrial and cosmic component, was deduced by the field measurements. Time series analysis of the mean monthly dose rate (measured by the Reuter-Stokes detector in Thessaloniki, northern Greece, from 2001 to 2016) was performed with advanced statistical methods (Fast Fourier Analysis and Zhao Atlas Marks Transform). Fourier analysis reveals several periodicities (periodogram). The periodogram of the absorbed dose rate in air values was compared with the periodogram of the values measured for the same period (2001-16) and in the same location with a NaI (Tl) detector which in principle is not sensitive to cosmic radiation. The obtained results are presented and discussed.

RADON MIGRATION IN SOIL AND ITS RELATION TO TERRESTRIAL GAMMA RADIATION IN DIFFERENT LOCATIONS OF THE GREEK EARLY WARNING SYSTEM NETWORK

Radon concentration as function of the soil depth was measured during the years (2011-2015), in a location of the Aristotle University campus. Radium distribution in soil was found constant. On the contrary, as expected, radon concentration increases with soil depth. The experimental distribution was reproduced by solving the general transport equation (diffusion and advection). From the general radon migration (diffusion and advection) equation it was indirectly deduced, from the measured radon profile in the soil, the radon exhalation rate from the soil (26.7 ± 4.5 Bq m-2 h-1). In the same location, during 2010-2015, 113 direct radon exhalation measurements were performed and give an average value for the 6 years of 21.1 ± 3 Bq m-2 h-1. The comparison between the radon exhalation rates deduced by the indirect and direct method indicates the validity of the diffusion advection model predictions concerning the radon exhalation rate from soil. The relation between radon migration in soil and terrestrial gamma radiation was studied. In particular, in the present study was investigated the correlation between gamma radiation 1 m above soil and radon exhalation in six locations of the Greek early warning system network. A positive correlation between gamma dose rate in air and radon exhalation rate from soil was found.

PERIODICITY ANALYSIS OF GAMMA RADIATION MEASUREMENTS IN THESSALONIKI, NORTHERN GREECE, IN THE PERIOD 1988-2015

Gamma radiation measurements were performed during the last 27 y, starting from 1988, with a NaI(Tl)-based Xetex 501A radiation monitor located outside the Nuclear Technology Laboratory of the Aristotle University of Thessaloniki in Northern Greece, and a time series was created. Measurements were also performed in the same place during 1995-98 and 2013-15 with portable high purity germanium (HPGe) detector. The total absorbed dose rate in air decreases exponentially with time. The total absorbed dose rate in air is the sum of the gamma dose rates due to (1) uranium series, (2) thorium series, (3) ⁴⁰K and (4) ¹³⁷Cs (due to the Chernobyl accident). In addition, a small contribution due to cosmic radiation is measured by the radiation monitor. From the time-dependence measurements with the HPGe detector, it was found that the time dependence of the absorbed gamma dose rate in air due to (1) uranium series, (2) thorium series and (3) ⁴⁰K is quite constant. On the contrary, gamma dose rate due to ¹³⁷Cs decreases exponentially with an effective half-life (t_½) of ∼13.5 y, stronger than expected due to the natural decay of ¹³⁷Cs. Time series analysis of the mean monthly total absorbed dose rate in air was performed. Fourier analysis reveals several periodicities, and applying Zhao-Atlas-Marks transform unravels the time distribution of those periodicities. There are three main discernible periodicities: 12 ± 0.2, 42.3 ± 2.9 and 53.2 ± 3.2 months. One of them is of a seasonal character (annual cycle) and can be linked to seasonal atmospheric variations and is strongly visible from 1988 to 2002 and 2008 to 2014. The other two (42.3 ± 2.9 and 53.2 ± 3.2 months) were found to be also related to meteorological parameters (air temperature), and they were very intense during the years 2002-4 when the annual periodicity was weak. Apart from the three main periodicities, there are also four others (14.7, 18.6, 21.3 and 27.3 months) with lower magnitudes; of which, three agree well with literature data periodicities in solar activity. Different possible mechanisms that can influence the gamma radiation measurements, due to solar activity, were discussed.

Environmental impact of phosphogypsum stockpile in remediated Schistos waste site (Piraeus, Greece) using a combination of γ-ray spectrometry with geographic information systems

From 1979 to 1989, ten million tons of phosphogypsum, a waste by-product of the Greek phosphate fertilizer industry, was disposed into an abandoned limestone quarry in Schistos former waste site, Piraeus (Greece). The quarry has been recently closed and remediated using geomembranes and thick soil cover with vegetation. A part of the deposited phosphogypsum has been exposed due to intense rainfall episodes leading to concerns about how could potentially released radioactivity affect the surrounding environment. This study seeks to assess the environmental impact of the phosphogypsum deposited in the Schistos quarry, using laboratory-based γ-ray spectrometry measurements and geographical information systems. Radioactivity concentrations were mapped onto spatial-data to yield a spatial-distribution of radioactivity in the area. The data indicate elevated (226)Ra concentrations in a specific area on the steep south-eastern cliff of the remediated waste site that comprises uncovered phosphogypsum and is known to be affected by local weather conditions. (226)Ra concentrations range from 162 to 629 Bq/kg, with an average activity being on the low side, compared to the global averages for phosphogypsum. Nevertheless, the low environmental risk may be minimized by remediating this area with geomembranes and thick soil cover with vegetation, a technique, which has worked successfully over the remainder of the remediated quarry.

Measurements and modelling of 137Cs distribution on ground due to the Chernobyl accident: a 27-y follow-up study in Northern Greece

Following the Chernobyl accident, an area of ∼1000 m(2) in the University farm of the Aristotle University of Thessaloniki was considered as a test ground for radioecological measurements. The radiocesium deposition in this area, due to the Chernobyl accident, was 20 kBq m(-2). The profile of (137)Cs in the soil of this area was measured systematically from 1987 to 2012. The form of the profile has changed over the years. During the 1987-2000 period the (137)Cs distribution was reproducible by a sum of two exponentials. However, at least since 2005 the (137)Cs distribution can be successfully fitted by a single exponential function. The long-time (∼27 y) evolution study of the (137)Cs distribution in soil permit one to extract with the use of a simple compartment model, the mean vertical migration velocity of (137)Cs. Vertical migration of (137)Cs in soil is a very slow process. The mean vertical migration velocity is estimated to be 0.14 cm y(-1).The relative good comparison between the time dependence of the (137)Cs distribution in soil and the model predictions indicate that the simple model used is realistic.

Follow-up study of indoor radon in Greek buildings

The Nuclear Technology Laboratory of the Aristotle University of Thessaloniki (NTL-AUTh) and the Greek Atomic Energy Commission (GAEC) have a continuous collaboration on indoor radon measurements in Greek buildings since 1999. In the present work, the existing database was enriched with 590 indoor radon measurements in 295 houses and 76 indoor radon measurements in 38 workplaces. In total in the present work, 1948 indoor radon measurements in 974 buildings performed by the NTL-AUTh and GAEC from 1999 to 2012 in 8 of the 13 administrative regions of Greece are presented and discussed.

Environmental measurements and inspections on imported foods and feedstuffs in Greece after the Fukushima accident

The radionuclides released during the accident at the Fukushima Daichii nuclear power plant following the Tōhoku earthquake and tsunami on 11 March 2011 were dispersed in the whole north hemisphere. Traces of (131)I, (134)Cs and (137)Cs reached Greece and were detected in air, grass, sheep milk, ground deposition, rainwater and drainage water. Members of Six Greek laboratories of the national network for environmental radioactivity monitoring have collaborated with the Greek Atomic Energy Commission (GAEC) and carried out measurements during the time period between 11 March 2011 and 10 May 2011 and reported their results to GAEC. These laboratories are sited in three Greek cities, Athens, Thessaloniki and Ioannina, covering a large part of the Greek territory. The concentrations of the radionuclides were studied as a function of time. The first indication for the arrival of the radionuclides in Greece originating from Fukushima accident took place on 24 March 2011. After 28 April 2011', concentrations of all the radionuclides were below the minimum detectable activities (<10 μBq m(-3) for (131)I). The range of concentration values in aerosol particles was 10-520 μBq m(-3) for (131)I, 10-200 μBq m(-3) for (134)Cs and 10-200 μBq m(-3) for (137)Cs and was 10-2200 μBq m(-3) for (131)I in gaseous phase. The ratios of (131)I/(137)Cs and (134)Cs/(137)Cs concentrations are also presented. For (131)I, the maximum concentration detected in grass was 2.2 Bq kg(-1). In the case of sheep milk, the maximum concentration detected for (131)I was 2 Bq l(-1). Furthermore, more than 200 samples of imported foodstuff have been measured in Greece, following the EC directives on the inspection of food and feeding stuffs.

Implementation of CFD modeling in the performance assessment and optimization of secondary clarifiers: the PVSC case study

The water industry and especially the wastewater treatment sector has come under steadily increasing pressure to optimize their existing and new facilities to meet their discharge limits and reduce overall cost. Gravity separation of solids, producing clarified overflow and thickened solids underflow has long been one of the principal separation processes used in treating secondary effluent. Final settling tanks (FSTs) are a central link in the treatment process and often times act as the limiting step to the maximum solids handling capacity when high throughput requirements need to be met. The Passaic Valley Sewerage Commission (PVSC) is interested in using a computational fluid dynamics (CFD) modeling approach to explore any further FST retrofit alternatives to sustain significantly higher plant influent flows, especially under wet weather conditions. In detail there is an interest in modifying and/or upgrading/optimizing the existing FSTs to handle flows in the range of 280-720 million gallons per day (MGD) (12.25-31.55 m(3)/s) in compliance with the plant's effluent discharge limits for total suspended solids (TSS). The CFD model development for this specific plant will be discussed, 2D and 3D simulation results will be presented and initial results of a sensitivity study between two FST effluent weir structure designs will be reviewed at a flow of 550 MGD (∼24 m(3)/s) and 1,800 mg/L MLSS (mixed liquor suspended solids). The latter will provide useful information in determining whether the existing retrofit of one of the FSTs would enable compliance under wet weather conditions and warrants further consideration for implementing it in the remaining FSTs.

Gamma radiation measurements and Monte Carlo computations following myocardial perfusion imaging with 201Tl

In the current study, the time-dependent retention of (201)Tl-thallous chloride (111 MBq) was measured in a 56-y-old man undergoing myocardial perfusion imaging. For 23 d following the (201)Tl injection, total-body retained activity was measured by (i) in situ gamma spectrometry using a portable high-purity germanium (HPGe) detector and (ii) ex vivo urine radioassay using a shielded HPGe detector. The time-dependent decrease in total-body activity followed a monoexponential function, exp(-0.011 t), with an excellent correlation (R(2)=0.9988) between the experimental data and the fitted values. The effective half-life, Teff, of (201)Tl (physical half-life, Tph: 72.9 h) was therefore 63 h and the biological half-life, Tb, 463 h=19.3 d, identical to those measured in the same patient in 1997 (i.e. 14 y ago). The time-dependent decrease in the urine activity concentration, which followed a monoexponential function, exp(-0.0115 t), corroborated the foregoing results. The correlation (R(2)=0.9939) between the experimental data and the fitted values was again excellent. The effective half-life, Teff, was 60.26 h and the biological half-life, Tb, 348 h=14.5 d. Monte Carlo simulation using a simple model of the patient as a unit-density cylinder filled with water and containing a uniform distribution of (201)Tl yielded photon flux results in reasonable agreement with the measured data.

Environmental radioactivity measurements in Greece following the Fukushima Daichi nuclear accident

Since the double disaster of the 9.0 magnitude earthquake and tsunami that affected hundreds of thousands of people and seriously damaged the Fukushima Daichi power plant in Japan on 11 March 2011, traces of radioactive emissions from Fukushima have spread across the entire northern hemisphere. The radioactive isotope of iodine (131)I that was generated by the nuclear accident in Fukushima arrived in Greece on 24 March 2011. Radioactive iodine is present in the air either as gas or bound to particles (aerosols). The maximum (131)I concentrations were measured between 3 and 5 April 2011. In aerosols the maximum (131)I values measured in Southern Greece (Athens) and Northern Greece (Thessaloniki) were 585±70 and 408±61 μΒq m(-3), respectively. (131)I concentrations in gas were about 3.5 times higher than in aerosols. Since 29 April 2011, the (131)I concentration has been below detection limits. Traces of (137)Cs and (134)Cs were also measured in the air filters with an activity ratio of (137)Cs/(134)Cs equal to 1 and (131)I/(137)Cs activity ratio of about 3. Since 16 May 2011, the (137)Cs concentration in air has been determined to be about the same as before the Fukushima accident. Traces of (131)I were also measured in grass and milk. The maximum measured activity of (131)I in sheep milk was about 2 Bq l(-1) which is 5000 times less than that measured in Greece immediately after the Chernobyl accident. The measured activity concentrations of artificial radionuclides in Greece due to the Fukushima release, have been very low, with no impact on human health.

A comparison study between radon concentration in schools and other workplaces

The Nuclear Technology Laboratory of the Aristotle University of Thessaloniki has since 1999 an open research project of indoor radon measurements in Greek workplaces. Since now 1380 measurements in 690 workplaces have been performed. Most (75 %) of the workplaces were offices in schools. The remaining 25 % were offices, mainly in public buildings. In the present study, a possible correlation between radon concentration in schools and other workplaces is investigated and discussed.

Critical modeling parameters identified for 3D CFD modeling of rectangular final settling tanks for New York City wastewater treatment plants

New York City Environmental Protection is in the process of incorporating biological nitrogen removal (BNR) in its wastewater treatment plants (WWTPs) which entails operating the aeration tanks with higher levels of mixed liquor suspended solids (MLSS) than a conventional activated sludge process. The objective of this paper is to discuss two of the important parameters introduced in the 3D CFD model that has been developed by the City College of New York (CCNY) group: (a) the development of the 'discrete particle' measurement technique to carry out the fractionation of the solids in the final settling tank (FST) which has critical implications in the prediction of the effluent quality; and (b) the modification of the floc aggregation (K(A)) and floc break-up (K(B)) coefficients that are found in Parker's flocculation equation (Parker et al. 1970, 1971) used in the CFD model. The dependence of these parameters on the predictions of the CFD model will be illustrated with simulation results on one of the FSTs at the 26th Ward WWTP in Brooklyn, NY.

Indoor radon levels in Greek schools

Radon and gamma dose rate measurements were performed in 512 schools in 8 of the 13 regions of Greece. The distribution of radon concentration was well described by a lognormal distribution. Most (86%) of the radon concentrations were between 60 and 250 Bq m(-3) with a most probable value of 135 Bq m(-3). The arithmetic and geometric means of the radon concentration are 149 Bq m(-3) and 126 Bq m(-3) respectively. The maximum measured radon gas concentration was 958 Bq m(-3). As expected, no correlation between radon gas concentration and indoor gamma dose rate was observed. However, if only mean values for each region are considered, a linear correlation between radon gas concentration and gamma dose rate is apparent. Despite the fact that the results of radon concentration in schools cannot be applied directly for the estimation of radon concentration in homes, the results of the present survey indicate that it is desirable to perform an extended survey of indoor radon in homes for at least one region in Northern Greece.

Development of a flocculation sub-model for a 3-D CFD model based on rectangular settling tanks

To assess performance and evaluate alternatives to improve the efficiency of rectangular Gould II type final settling tanks (FSTs), New York City Department of Environmental Protection and City College of NY developed a 3D computer model depicting the actual structural configuration of the tanks and the current and proposed hydraulic and solids loading rates. Fluent 6.3.26™ was the base platform for the computational fluid dynamics (CFD) model, for which sub-models of the SS settling characteristics, turbulence, flocculation and rheology were incorporated. This was supplemented by field and bench scale experiments to quantify the coefficients integral to the sub-models. The 3D model developed can be used to consider different baffle arrangements, sludge withdrawal mechanisms and loading alternatives to the FSTs. Flocculation in the front half of the rectangular tank especially in the region before and after the inlet baffle is one of the vital parameters that influences the capture efficiency of SS. Flocculation could be further improved by capturing medium and small size particles by creating an additional zone with an in-tank baffle. This was one of the methods that was adopted in optimizing the performance of the tank where the CCNY 3D CFD model was used to locate the in-tank baffle position. This paper describes the development of the flocculation sub-model and the relationship of the flocculation coefficients in the known Parker equation to the initial mixed liquor suspended solids (MLSS) concentration X0. A new modified equation is proposed removing the dependency of the breakup coefficient to the initial value of X0 based on preliminary data using normal and low concentration mixed liquor suspended solids values in flocculation experiments performed.

Experimental and theoretical study of radon distribution in soil

Radon concentration as a function of the soil depth (0-2.6 m) was measured during the years 2002-2003 and 2003-2004 on the Aristotle University campus. Radium distribution in soil was found constant. On the contrary, as expected, radon concentration increased with soil depth. However, the radon concentration did not follow the well known monotonous increase, which levels off to a saturation value. In both radon distributions, radon concentration increased up to a soil depth of about 80 cm, seemed to remain constant at depths of 80-130 cm, and then increased again. The experimental distribution was reproduced by solving the general transport equation (diffusion and advection). The main finding of the numerical investigation is that the aforementioned, experimentally observed, profile of radon concentration can be explained theoretically by the existence of two soil layers with different diffusion-advection characteristics. Soil sample analysis verified the existence of two different soil layers. Different boundary conditions of the radon concentration at the soil surface were used for the solution of the diffusion-advection equation. It was found that the calculated radon concentration in the soil is, away from the soil surface, the same for the two boundary conditions used. However, from the (frequently used) boundary condition of zero radon concentration at the soil surface, the experimental profile of the radon concentration at the soil surface cannot be deduced. On the contrary, with more appropriate boundary conditions the radon concentration at the soil surface could be deduced from the experimental profile. The equivalent diffusion coefficient could be uncovered from the experimental profile, which can then be used to estimate the radon current, which is important, for example, for the estimation of radon entrance to dwellings.

Indoor radon measurements in areas of northern Greece with relatively high indoor radon concentrations

Indoor radon concentrations were measured in 77 schools of the prefecture of Xanthi in northern Greece. The arithmetic mean radon concentration is 231 Bq m(-3) with a range between 45 and 958 Bq m(-3). Thirty five schools have mean radon concentration above 200 Bq m(-3) and nine schools have mean radon concentration above 400 Bq m(-3). From continuous radon gas measurements in the school with a relative higher radon concentration (958 Bq m(-3)) was deduced the 'true' radon concentration, defined as the radon concentration in the school during the hours of the presence of teachers and scholars. The mean 'true' radon concentration for a time period of about 2 weeks was 104 Bq m(-3). The mean radon concentration for the same 2 weeks was seven times higher (700 Bq m(-3)). Greek and International regulations for radon in workplaces refer to only the mean annual radon concentration. It would be preferable for schools to replace the mean annual radon concentration with the 'true' radon concentration.

Measurements and Monte Carlo calculations of photon energy distributions in MAYAK PA workplaces

Photon energy distributions were measured in different workplaces of the Mayak Production Association (MPA), which was the first plutonium production plant in the former Soviet Union. In situ gamma spectrometry measurements were performed with a portable germanium detector. The spectral stripping method is used for the conversion of the in situ gamma-ray spectra to photon fluence rate energy distribution. This method requires the simulation of the portable germanium detector, which has been performed based on the MCNP code of Los Alamos. Measured photon fluence rate energy distributions were compared with calculated photon energy distributions (with the MCNP code) in two different workplaces: in the first workplace the geometry exposure was known. On the contrary, in the second workplace, as in most workplaces of MPA, the exposure geometry was unknown. The results obtained from the comparison between the experimental and calculated photon fluence rate energy distributions are presented and discussed.

Follow up study of radiocesium contamination in a Greek forest ecosystem

Radiocesiun distribution in the different parts of a Quercus conferta Kit ecosystem in Northern Greece was measured in 2005-2006, twenty years after the Chernobyl accident. The comparison between the results of this study and those previously measured (1993-1995) in the same ecosystem gives information about the long-term behavior of 137Cs in forest ecosystems. The major part of the 137Cs inventory is still in the upper layers of the soil. The radiocesium distribution in soil is fixed and has been in equilibrium at least since 1993, when the first measurements were performed. The major contamination mechanism of leaves and wood is root uptake.

Pilot study of indoor radon in Greek workplaces

Radon and gamma dose rate measurements have been performed in 561 workplaces in 19 prefectures of Greece. The distribution of radon concentration can be well described by a log-normal distribution. Most of the radon concentrations are between 50 and 200 Bq m(-3) with an arithmetic mean of 123 Bq m(-3). The maximum measured value of radon gas concentration is 695 Bq m(-3). About 10% of the workplaces exceed 200 Bq m(-3). Only a small fraction ( approximately 1%) of workplaces exceed the European Commission action level (400 Bq m(-3)). Despite the relative small fraction of workplaces which exceed the value of 400 Bq m(-3), it is clear from the results of the present work that for certain prefectures, further and more extensive research is needed.

Twenty-year follow-up study of radiocesium migration in soil

The profile of (137)Cs present in undisturbed soil due to the Chernobyl accident was measured repeatedly for approximately 20 y. The vertical migration of (137)Cs in soil is a very slow process. The mean vertical migration velocity is estimated at approximately 0.1-0.2 cm y(-1). A method based on in situ gamma spectrometry measurements and Monte Carlo computations, aimed at estimating the profile of (137)Cs without performing any soil sampling, is investigated.

Simultaneous measurements of indoor radon, radon-thoron progeny and high-resolution gamma spectrometry in Greek dwellings

Simultaneous indoor radon, radon-thoron progeny and high-resolution in situ gamma spectrometry measurements, with portable high-purity Ge detector were performed in 26 dwellings of Thessaloniki, the second largest town of Greece, during March 2003-January 2005. The radon gas was measured with an AlphaGUARD ionisation chamber (in each of the 26 dwellings) every 10 min, for a time period between 7 and 10 d. Most of the values of radon gas concentration are between 20 and 30 Bq m(-3), with an arithmetic mean of 34 Bq m(-3). The maximum measured value of radon gas concentration is 516 Bq m(-3). The comparison between the radon gas measurements, performed with AlphaGUARD and short-term electret ionisation chamber, shows very good agreement, taking into account the relative short time period of the measurement and the relative low radon gas concentration. Radon and thoron progeny were measured with a SILENA (model 4s) instrument. From the radon and radon progeny measurements, the equilibrium factor F could be deduced. Most of the measurements of the equilibrium factor are within the range 0.4-0.5. The mean value of the equilibrium factor F is 0.49 +/- 0.10, i.e. close to the typical value of 0.4 adopted by UNSCEAR. The mean equilibrium equivalent thoron concentration measured in the 26 dwellings is EEC(thoron) = 1.38 +/- 0.79 Bq m(-3). The mean equilibrium equivalent thoron to radon ratio concentration, measured in the 26 dwellings, is 0.1 +/- 0.06. The mean total absorbed dose rate in air, owing to gamma radiation, is 58 +/- 12 nGy h(-1). The contribution of the different radionuclides to the total indoor gamma dose rate in air is 38% due to 40K, 36% due to thorium series and 26% due to uranium series. The annual effective dose, due to the different source terms (radon, thoron and external gamma radiation), is 1.05, 0.39 and 0.28 mSv, respectively.

In situ gamma spectrometry measurements and Monte Carlo computations for the detection of radioactive sources in scrap metal

A very limited number of field experiments have been performed to assess the relative radiation detection sensitivities of commercially available equipment used to detect radioactive sources in recycled metal scrap. Such experiments require the cooperation and commitment of considerable resources on the part of vendors of the radiation detection systems and the cooperation of a steel mill or scrap processing facility. The results will unavoidably be specific to the equipment tested at the time, the characteristics of the scrap metal involved in the tests, and to the specific configurations of the scrap containers. Given these limitations, the use of computer simulation for this purpose would be a desirable alternative. With this in mind, this study sought to determine whether Monte Carlo simulation of photon flux energy distributions resulting from a radiation source in metal scrap would be realistic. In the present work, experimental and simulated photon flux energy distributions in the outer part of a truck due to the presence of embedded radioactive sources in the scrap metal load are compared. The experimental photon fluxes are deduced by in situ gamma spectrometry measurements with portable Ge detector and the calculated ones by Monte Carlo simulations with the MCNP code. The good agreement between simulated and measured photon flux energy distributions indicate that the results obtained by the Monte Carlo simulations are realistic.

Radiological maps of outdoor and indoor gamma dose rates in Greek urban areas obtained by in situ gamma spectrometry

The results obtained from 259 indoor and outdoor in situ gamma spectrometry measurements with a portable Ge detector and 707 total gamma dose rate measurements with an NaI detector in urban areas of 16 Greek islands are presented. From the in situ gamma spectra, the absorbed dose rate in air due to Uranium series, Thorium series, (40)K and (137)Cs are derived and discussed. The results obtained from the present work in conjunction with those reported previously were used for the realization of a complete indoor and outdoor gamma radiation map of Greek urban areas using in situ gamma spectrometry with portable Ge detector.

Measurements with a Ge detector and Monte Carlo computations of dose rate yields due to cosmic muons

The present work shows how portable Ge detectors can be useful for measurements of the dose rate due to ionizing cosmic radiation. The methodology proposed converts the cosmic radiation induced background in a Ge crystal (energy range above 3 MeV) to the absorbed dose rate due to muons, which are responsible for 75% of the cosmic radiation dose rate at sea level. The key point is to observe in the high energy range (above 20 MeV) the broad muon peak resulting from the most probable energy loss of muons in the Ge detector. An energy shift of the muon peak was observed, as expected, for increasing dimensions of three Ge crystals (10%, 20%, and 70% efficiency). Taking into account the dimensions of the three detectors the location of the three muon peaks was reproduced by Monte Carlo computations using the GEANT code. The absorbed dose rate due to muons has been measured in 50 indoor and outdoor locations at Thessaloniki, the second largest town of Greece, with a portable Ge detector and converted to the absorbed dose rate due to muons in an ICRU sphere representing the human body by using a factor derived from Monte Carlo computations. The outdoor and indoor mean muon dose rate was 25 nGy h(-1) and 17.8 nGy h(-1), respectively. The shielding factor for the 40 indoor measurements ranges from 0.5 to 0.9 with a most probable value between 0.7-0.8.

Long-term measurements of radon equilibrium factor in Greek dwellings

The time variation of the radon equilibrium factor F was measured every four hours in the Nuclear Technology Laboratory of the Aristotle University of Thessaloniki (in northern Greece) during October 1998-April 1999. The time dependence of the mean weekly value of radon equilibrium factor F is relatively small. During October 1999-May 2000 the radon equilibrium factor was measured in 25 apartments randomly distributed in Thessaloniki. The mean value of the equilibrium factor F is 0.47 +/- 0.09, close to the typical value of 0.4 adopted by UNSCEAR.

A combination study of indoor radon and in situ gamma spectrometry measurements in Greek dwellings

The aim of the present study was to investigate of a possible correlation between indoor radon and indoor gamma dose rates deduced by in situ gamma spectrometry measurements by using a portable HPGe detector. Indoor radon and high resolution in situ gamma spectrometry measurements were performed in 60 apartments in Thessaloniki, the second largest city of Greece. Geometric mean radon concentration is 52 Bq m(-3). The mean total absorbed dose rate in air due to gamma radiation is 56 +/- 9 nGy h(-1). The contribution of the different radionuclides to the total indoor gamma dose rate in air is 41% due to 40K, 36% due to the thorium series and 23% due to the uranium series. No correlation was found between indoor gamma dose rate due to the uranium series and indoor radon for ground and first floor apartments. For upper floor apartments (above the second floor) a weak correlation is observed. The mean annual effective dose due to radon is 1.15 mSv, i.e., more than four times higher compared to the effective dose due to gamma radiation (0.27 mSv).

Extended survey of indoor and outdoor terrestrial gamma radiation in Greek urban areas by in situ gamma spectrometry with a portable Ge detector

The results obtained from more than 1000 indoor and outdoor in situ gamma spectrometry measurements in 41 towns (from all geographic subdivisions) of the Greek mainland (not islands) are presented. From the in situ gamma spectra the absorbed dose rate in air due to uranium series, thorium series, 40K and 137Cs are derived and discussed.

Derivation of indoor gamma dose rate from high resolution in situ gamma ray spectra

The dose build up factor B related to the ratio of primary to scattered gamma radiation in indoor environment was calculated with Monte Carlo simulations using the MCNP code for different indoor geometries and gamma source distributions. The main conclusion is that the B factor does not depend strongly on parameters such as dimensions of the rooms, the thickness of walls, the density of the building materials, and the gamma source geometry. The calculated dose build up factors were used within the framework of the full absorption peak analysis method in order to deduce the dose rates in air from about 100 indoor in situ gamma spectrometry measurements performed at the town of Thessaloniki, Greece. A spectral "stripping method," which has been recently developed, was applied to the same 100 spectrometry measurements. The results of the dose rates obtained by the two methods are compared and discussed.

Monte Carlo calculation of dose rate conversion factors for external exposure to photon emitters in soil

The dose rate conversion factors D(CF) (absorbed dose rate in air per unit activity per unit of soil mass, nGy h(-1) per Bq kg(-1)) are calculated 1 m above ground for photon emitters of natural radionuclides uniformly distributed in the soil. Three Monte Carlo codes are used: 1) The MCNP code of Los Alamos; 2) The GEANT code of CERN; and 3) a Monte Carlo code developed in the Nuclear Technology Laboratory of the Aristotle University of Thessaloniki. The accuracy of the Monte Carlo results is tested by the comparison of the unscattered flux obtained by the three Monte Carlo codes with an independent straightforward calculation. All codes and particularly the MCNP calculate accurately the absorbed dose rate in air due to the unscattered radiation. For the total radiation (unscattered plus scattered) the D(CF) values calculated from the three codes are in very good agreement between them. The comparison between these results and the results deduced previously by other authors indicates a good agreement (less than 15% of difference) for photon energies above 1,500 keV. Antithetically, the agreement is not as good (difference of 20-30%) for the low energy photons.

Contribution of 137Cs to the total absorbed gamma dose rate in air in a Greek forest ecosystem: measurements and Monte Carlo computations

The absorbed gamma dose rate in air 1 m above soil due to natural gamma emitters and 137Cs from the Chernobyl accident was determined inside a Quercus conferta Kit ecosystem in Northern Greece by combination of Monte Carlo simulations with the MCNP code and in-situ gamma spectrometry measurements. The total absorbed gamma dose rate in air is about 64 nGy h(-1), where 40% of this value is due to 137Cs and 60% to natural gamma emitters. The Monte Carlo simulations indicated that the gamma absorbed dose rate in air due to 137Cs is mainly due (70%) to unscattered radiation and to a lesser extent (30%) to the scattered radiation. The results obtained with the Monte Carlo simulations for the unscattered radiation were in very good agreement with the experimental values deduced by in-situ gamma spectrometry measurements. From the combination of the Monte Carlo simulations and in-situ gamma spectrometry measurements a conversion factor C = 1 nGy h(-1)/kBq m(-2) was deduced for 137Cs. This factor must be used with caution and only for forest sites similar to the one used for this work.

Monte Carlo based method for conversion of in-situ gamma ray spectra obtained with a portable Ge detector to an incident photon flux energy distribution

A Monte Carlo based method for the conversion of an in-situ gamma-ray spectrum obtained with a portable Ge detector to photon flux energy distribution is proposed. The spectrum is first stripped of the partial absorption and cosmic-ray events leaving only the events corresponding to the full absorption of a gamma ray. Applying to the resulting spectrum the full absorption efficiency curve of the detector determined by calibrated point sources and Monte Carlo simulations, the photon flux energy distribution is deduced. The events corresponding to partial absorption in the detector are determined by Monte Carlo simulations for different incident photon energies and angles using the CERN's GEANT library. Using the detector's characteristics given by the manufacturer as input it is impossible to reproduce experimental spectra obtained with point sources. A transition zone of increasing charge collection efficiency has to be introduced in the simulation geometry, after the inactive Ge layer, in order to obtain good agreement between the simulated and experimental spectra. The functional form of the charge collection efficiency is deduced from a diffusion model.

Radiocesium contamination in a submediterranean semi-natural ecosystem following the Chernobyl accident: measurements and models

Radiocesium dynamics in a Quercus conferta Kit ecosystem in Northern Greece have been extensively studied over the years 1993-1995. Radiocesium distribution in the different parts of the ecosystem was measured. A total 137Cs inventory of 243+/-66 MBq ha(-1) due to the Chernobyl accident was measured in all parts of the ecosystem. Almost 90% of this inventory is still in the upper layers of the soil and the forest floor. In particular 13.4% is in the forest floor, 52.6% in the Ah horizon, and 23.4% in the upper 5 cm of the soil. Only 2.2% of this inventory is in the above ground biomass. The mean total 137Cs deposited on the forest floor from the above ground biomass is 0.18 MBq ha(-1) y(-1). Cesium leaching from the forest floor is negligible. The radiocesium distribution in soil is fixed and in equilibrium, at least since 1993. Most of radiocesium is not available for migration. Cesium migration in soil was modeled by a) an "equivalent diffusion" model with different initial conditions and b) a "compartment" model derived from a diffusion-advection model. A compartment model for the contamination of living biomass is proposed. The total absorbed dose rate in air as well as the contribution due to 137Cs from the Chernobyl accident was determined inside the forest, by in-situ gamma spectrometry.