Stelios Xanthos

Gamma-ray spectroscopy is an essential technique for identifying the composition of radioactive materials. This work presents a detailed algorithm for analysing gamma-ray spectra, focusing on peak detection, energy calibration, efficiency calibration and nuclide identification. The algorithm includes a systematic approach to match observed peaks with known nuclides, following a peak detection method, efficiency and energy calibration methods. The algorithm attempts to provide spectral analysis and nuclides identification on-site with portable spectroscopic devices. Therefore, it is tested with portable HPGe and CZT detectors demonstrating its ability to reliably identify isotopes in complex spectra, derived by multi radioactive sources identification scenarios. A conventional energy calibration is performed on the spectrum data to convert channels to energy values. Additional calibration is performed to adjust the height of the photopeaks, considering the efficiency curve of each detector. The algorithm identifies possible nuclides associated with each peak by searching for nuclides within a specified range of energy interval, minimum half-life, and minimum fraction yield of the photopeak. The matching process involves comparing photopeaks of the spectrum under investigation with a complete dataset of energies of γ-rays and their emitters. To exclude false positive identifications of nuclides, a criterion considers a certain number of the most intense γ-ray energies that each possible nuclide emits and investigates whether these energies are correlated with the photopeaks. In addition, another criterion checks the height of the photopeaks based on their fraction yield. The algorithm attempts to offer a solution for field applications, enhancing radiological crime-scene investigations and environmental monitoring.

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 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.

We present new data about the chemical and structural characteristics of bauxite residue (BR) from Greek Al industry, using a combination of microscopic, analytical, and spectroscopic techniques. SEM-EDS indicated a homogeneous dominant “Al-Fe-Ca-Ti-Si-Na-Cr matrix”, appearing at the microscale. The bulk chemical analyses showed considerable levels of Th (111 μg g⁻¹), along with minor U (15 μg g⁻¹), which are responsible for radioactivity (355 and 133 Bq kg⁻¹ for ²³²Th and ²³⁸U, respectively) with a total dose rate of 295 nGy h⁻¹. Leaching experiments, in conjunction with SF-ICP-MS, using Mediterranean seawater from Greece, indicated significant release of V, depending on S/L ratio, and negligible release of Th at least after 12 months leaching. STEM-EDS/EELS & HR-STEM-HAADF study of the leached BR at the nanoscale revealed that the significant immobility of Th⁴⁺ is due to its incorporation into an insoluble perovskite-type phase with major composition of Ca_0.8Na_0.2TiO₃ and crystallites observed in nanoscale. The Th L_III-edge EXAFS spectra demonstrated that Th⁴⁺ ions, which are hosted in this novel nano-perovskite of BR, occupy Ca²⁺ sites, rather than Ti⁴⁺ sites. That is most likely the reason of no Th release in Mediterranean seawater.

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 ²²⁶Ra 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. ²²⁶Ra 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.

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.

Following the Chernobyl accident, an area of ∼1000 m² 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 ¹³⁷Cs 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 ¹³⁷Cs distribution was reproducible by a sum of two exponentials. However, at least since 2005 the ¹³⁷Cs distribution can be successfully fitted by a single exponential function. The long-time (∼27 y) evolution study of the ¹³⁷Cs distribution in soil permit one to extract with the use of a simple compartment model, the mean vertical migration velocity of ¹³⁷Cs. Vertical migration of ¹³⁷Cs 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 ¹³⁷Cs distribution in soil and the model predictions indicate that the simple model used is realistic.

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.

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 ¹³¹I, ¹³⁴Cs and ¹³⁷Cs 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 (−3 for ¹³¹I). The range of concentration values in aerosol particles was 10–520 μBq m⁻³ for ¹³¹I, 10–200 μBq m⁻³ for ¹³⁴Cs and 10–200 μBq m⁻³ for ¹³⁷Cs and was 10–2200 μBq m⁻³ for ¹³¹I in gaseous phase. The ratios of ¹³¹I/¹³⁷Cs and ¹³⁴Cs/¹³⁷Cs concentrations are also presented. For ¹³¹I, the maximum concentration detected in grass was 2.2 Bq kg⁻¹. In the case of sheep milk, the maximum concentration detected for ¹³¹I 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.