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.

PROCEDURES TO MEASURE MEAN AMBIENT DOSE EQUIVALENT RATES USING ELECTRET ION CHAMBERS.

The capabilities of electret ion chambers (EICs) to measure mean ambient dose equivalent rates were investigated by performing both laboratory and field studies of their properties. First, EICs were 'calibrated' to measure ambient gamma dose equivalent in the Ionizing Calibration Laboratory of the Greek Atomic Energy Commission. The EICs were irradiated with different gamma photon energies and from different angles. Calibration factors were deduced (electret's voltage drop due to irradiation in terms of ambient dose equivalent). In the field studies, EICs were installed at eight locations belonging to the Greek Early Warning System Network (which is based on Reuter-Stokes ionization chambers) for three periods, averaging 5 months each. In the same locations, in situ gamma spectrometry measurements were performed with portable germanium detectors. Gamma ambient dose equivalent rates were deduced by the in situ gamma spectrometry measurements and by soil sample analysis. The mean daily electret potential drop (in Volts) was compared with the mean daily ambient dose equivalent, measured with a portable HPGe detector and Reuter-Stokes high-pressure ionization chambers. From these measurements, 'field' calibration factors (electret's voltage drop due to gamma radiation in terms of ambient dose equivalent) were deduced and found in very good agreement with the values deduced in Laboratory. The influence of cosmic radiation and the intrinsic voltage loss when performing long-term environmental gamma measurements with EICs, was estimated.

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 H*(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 (t½) 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.