S. Xanthos

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.

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 CERNs GEANT library. Using the detectors 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.

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.

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.

Radon and thoron progeny measurements in dwellings of northern Greece

Radon progeny concentration is measured with a total alpha counting system in 185 dwellings in the town of Thessaloniki. The Equilibrium Equivalent radon Concentration (EEC Rn) follows a lognormal distribution with an arithmetic and geometric mean value of 28 Bq/m and 16 Bq/m respectively. Simultaneous in situ alpha and gamma spectroscopy measurements were performed in 60 schools of Thessaloniki. The mean Equilibrium Equivalent Concentration of radon and thoron is 22 Bq/m and 0.9 Bq/m respectively. The minimum, maximum and mean total absorbed dose rate in air due to gamma radiation is 13 nGy/h, 88 nGy/h and 47 nGy/h respectively. The contribution of the different radionuclides to the total gamma dose rate in air is 39% due to K, 33.5% due to thorium series, 27 % due to uranium series and about 0.5% due to Cs from the Chernobyl accident. For schools that were under construction or constructed the first years after the Chernobyl accident, Cs act also as an indoor source term, however with small contribution to the total dose rate. The mean annual effective dose equivalent due to gamma radiation is 0.29 mSv, comparable to the 0.2 mSv due to thoron progeny and about five times smaller than the value of 1.36 mSv due to radon progeny. The mean annual effective dose equivalent due to thoron progeny is about 13% of the total effective dose equivalent due to radon progeny and thoron progeny. No correlation was found between radon and thoron progeny and the absorbed gamma dose rates in air due to uranium and thorium series respectively. Radon and thoron progeny was measured every 4 hours in the Nuclear Technology Laboratory of the Aristotle University of Thessaloniki for more than a year. The mean EEC Rn on a weekly basis varies from 16 Bq/m up to 155 Bq/m. The time dependence variation of EEC thoron progeny is smaller than the radon progeny. The mean annual effective dose equivalent due to radon and thoron progeny is 3.8 mSv and 1 mSv respectively. The mean annual effective equivalent dose due to thoron progeny is about 21% of the total effective dose equivalent due to radon progeny and thoron progeny. The effective dose equivalent due to thoron progeny is sometimes comparable to the effective dose equivalent due to radon progeny.

Measurements with a ge detector and monte carlo computations of dose rate yields due to cosmic muons

Abstract— 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.

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.

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.

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.

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.