A mobile γ–ray spectrometer (AMESOS) has been developed using a 3”×3” NaI(Tl) detector, a custom–made mounting holder, and portable electronics to perform in situ measurements of radioactivity. The spectrometer was calibrated using standard point sources and its absolute efficiency was determined. As a field test operation, AMESOS was deployed on the premises of the University of Athens Zografou campus focusing on estimating the NORM levels. Data were analyzed and used to create radiological maps for the metropolitan UoA campus for the first time. Besides natural radioactivity levels, trace concentrations of ¹³⁷Cs were also detected, attributed to the Chernobyl fallout in Greece. An overall steady performance of the spectrometer was observed throughout the field operation. AMESOS is ready to be deployed for in situ studies of environmental radioactivity and radwaste management.

AMESOS, environmental radioactivity, mobile spectrometer, radiological map

Mobile γ–ray spectrometry is a well–established technique in environmental radioactivity measurements, which can provide results in harsh or remote environments (ICRU 1994), especially when sampling is difficult or when information is needed in a very short time scale. Nowadays mobile spectrometers are rather simple to deploy in the field and can be carried by humans (Kock and Samuelsson 2011, Cresswell et al. 2013, 2018), mounted on robotic instruments (Jilek et al. 2016), air vehicles (Simon and Graham 1998, Kock and Samuelsson 2011), drones (Kurvinen et al. 2005, Pöllänen et al. 2009), or dive into deep oceans aboard marine submersibles (Hattori 2000, Hattori et al. 2001, Tsabaris et al. 2011, Androulakaki et al. 2016). Depending on the application, different types of detectors can be used, such as scintillators (e.g. NaI(Tl)) or solid–state detectors (e.g. HPGe), coupled to integrated electronics modules for data acquisition or custom–made designs to accommodate special needs. The major drawback of mobile systems has been the limitations in supplying stable, uninterrupted power, which generally shortens the duration of measurements in the field. However, as technology leaps forward, power autonomy becomes less and less of an issue, thus extending the range of applications where a mobile spectrometer can be exploited.

Scintillators are typically favored over solid–state detectors (Ge(Li) or HPGe) for mobile spectrometers, since the latter traditionally require cooling with liquid nitrogen to operate, are generally more expensive, and are overall more sensitive in field deployment, despite the higher resolution they offer (Clouvas et al. 2004). On the other hand, scintillators are sensitive to environmental conditions, especially temperature changes that may cause gain shifts during operation (Gilmore 2008).

In any case, a careful characterization of the spectrometer is required, especially in terms of its absolute efficiency. The response function of the spectrometer depends on the type of the detector, the geometrical characteristics of the spectrometer and the energies of the photons emitted by the isotopes of interest. Standard point sources are typically used for γ–ray spectrometers, but for mobile systems aiming at measurements in wide–open areas, a characterization under realistic conditions is often additionally necessary.

In this paper, a mobile spectrometer, AMESOS: A MobilE Spectrometer for environmental radioactivity Studies¹, has been developed in the National and Kapodistrian University of Athens (UoA) to support a basic program of in situ environmental radioactivity measurements (see photo of the actual setup in Fig. 1. So far, there has been no such spectrometer in the pool of instruments at the Nuclear Physics Laboratory at UoA and AMESOS is expected to significantly expand the in–house research program in environmental radiation studies. The technical aspects of the spectrometer and its full characterization prior to field deployment are described later in the text. As a proof of good operation in the field, AMESOS was used to measure the natural radioactivity levels in the UoA Zografou Campus, the largest university campus in Greece, hosting ≈70,000 students and staff. To the best of our knowledge this study has been carried out systematically for the first time. The results are presented and discussed in the respective section.

Figure 1. 

A photo of AMESOS during an in-situ measurement in the UoA campus.

AMESOS utilizes a 3”×3” NaI(Tl) crystal coupled to an ORTEC digiBASE 14–pin PMT base with integrated bias supply, a preamplifier, and a MCA with digital signal processing (see ref. ORTEC 2020). The digiBASE ensures low power consumption and gain stability, as well as it allows for easy communication with a dedicated portable PC equipped with the MAESTRO32 software via USB. The software enables online analysis, while it also allows for nuclide identification and photopeak integration during the measurement. Typical settings of operation of AMESOS during the field measurements and the lab tests are +700 V for the HV bias, 1024 channels and spectrum energy range ≈ 0–2000 keV.

The detector is placed vertically (Fig. 1), with its end cap facing the ground, inside a custom–made hollow aluminum holder of cylindrical shape (wall thickness is 3 mm), standing on a 3–mm thick plexiglass window mounted firmly at the bottom end of the holder. An additional 3–mm thick lead jacket fills the void space between the detector and the holder and is used as a thin radiation shield from the side. Three removable, extensible legs are mounted on the outer surface of the holder, forming a tripod, which allows for positioning the detector at various heights up to 160 cm from the ground. Horizontal alignment is checked with a bubble meter before starting a measurement. A handheld GPS device is used to track the exact coordinates of the setup for each measurement. The whole apparatus is lightweight and can be easily carried around in a backpack or a carriage.

Recorded spectra can be analyzed offline with the specialized software codes SPECTRW (Kalfas et al. 2016) and XSA (Kumbartzki 2018). In such a configuration, the laptop’s power autonomy is the only potential limiting factor of AMESOS operation, which can be overcome easily if long–life battery packs or solar cells are used. During operation, each measurement in the field was performed for a time interval of ≈3 h, which was proven sufficient to run without any power outage interruptions.

Prior to field deployment, the spectrometer was fully studied in terms of its calibration, linearity, energy resolution and absolute efficiency. Separate sets of measurements were carried out to study these properties, as described below.

Lacking an extended–area radionuclide standard, such as those offered by IAEA (IAEA 1987), the calibration was carried out using point sources available at the UoA Nuclear Physics Laboratory. Three standard point sources of known activities, i.e. ²²Na (511 and 1274.54 keV), ⁶⁰Co (1173.23 and 1332.49 keV), and ¹³⁷Cs (661.66 keV) (Dulieu et al. 2017), were individually placed directly below the detector at point “O” (see Fig. 2). The detector was place at a height h = 1.60 m from the ground.

Figure 2. 

A rough sketch of the experimental setup with AMESOS detecting radiation emitted from a potential source on the ground at a random distance r away from the detector. For the present study, the point sources were placed on the ground, exactly at the point “O”.

Spectra for each source were collected for approximately one hour in this series of measurements. The first step in characterizing the spectrometer was to perform an energy calibration and check on its linearity. For the calibration, besides the five photopeaks from the sources, two additional summing peaks (1785.54 keV / ²²Na and 2505.72 keV / ⁶⁰Co, respectively) were used, reaching the typical energy range needed for environmental radioactivity studies (up to ≈3000 keV). The energy values of the photopeak centroids are plotted in Fig. 3a as a function of the respective channel numbers recorded in the raw spectra. A 2^nd–order polynomial function was used to fit the data and test AMESOS linearity. The fitting curve is also shown in Fig. 3a. The parameters a₀, a₁ and a₂ of the fitting function:

Figure 3. 

(top) The energy calibration data for AMESOS and the corresponding fit using eq. (1). A good linear response is apparent; (bottom) The FWHM as a function of energy. The solid line is a 2^nd–order polynomial fit to the data using eq. (2).

E = a₀ + a₁ * (chn) + a₂ * (chn)² (1)

with (chn) being the channel number, are shown in Table 1. The value of a₂ is very small, reflecting on the good linearity of the spectrometer in the selected energy range. From the same spectra, the energy resolution of the detector was determined in terms of the full width at half maximum (FWHM) as a function of photopeak energy, E. The experimental data are shown in Fig. 3b together with a fitted 2^nd–order polynomial function of the form:

h (E) = c₀ + c₁E + c₂E² (2)

An overall good fit was produced resulting in the parameter values shown in Table 1.

Further, the absolute efficiency, ε (E), of the spectrometer was further deduced, using equation (Gilmore 2008, El Khatib et al. 2016, 2017):

$\epsilon =\frac{N_{\text{meas }}}{N_o{P}_{\gamma }}$ (3)

where Ν_meas is the measured counting rate of the detector for a particular γ–ray photopeak of energy E, corrected for dead time (less that 0.1% throughout the measurements); N_o is the γ–ray emission rate from the source, and P_γ is the relative emission probability of a particular γ–ray with energy E. Uncertainties include the statistical error from the counting rate, the uncertainty of P_γ found in evaluated nuclear databases (Dulieu et al. 2017), and a 3% relative error in the initial activity of the calibration sources, as provided by the manufacturer. Experimental data points are plotted as a function of energy to extract the absolute efficiency of the detector (Fig. 4).

Figure 4. 

Absolute efficiency of the AMESOS spectrometer as a function of photopeak energy. Uncertainties result in error bars smaller than the symbols (solid triangles). The continuous line represents the best fit to the experimental data, using eq. (4).

The efficiency data were fitted with Eq. (4), a simplified version of Eq. (1) in McNelles and Campbell (1973). The results are shown as a function of energy E (in keV) in Fig. 4.

f (x) = b₀ * e-^b1E + b₂ (4)

The resulted values of the fit parameters are also shown in Table 1.

A final step before field deployment is finding the correspondence between the recorded photopeak counting rate (counts per second, cps) to spatial activity [Bq m^-2]. For this task, a circular area of uniform surface distribution with a radius of approximately 2.4 m can be assumed as the source for all counts registered in the detector, for the particular height used in the present study (h = 1.6 m). This radius value has been found by extrapolating the results by Billings et al. (2003). Using a source area of S = πR² = 18.25 m², all activities measured in AMESOS can now be converted to spatial activities.

After characterizing the spectrometer, a series of measurements were performed in the field. As a proof of good operation, AMESOS was deployed on the premises of the metropolitan Zografou campus of UoA. The metropolitan campus covers the largest area among university campuses in Greece and hosts about 65,000 students and 3,000 personnel on a daily basis (UoA 2020). The campus is located at the western foot of Mt. Hymettus in Athens and has a large percentage of undisturbed land covered by grass, bushes and trees. (Fig. 5). The area mainly consists of a) limestones aged Upper Cretaceous (Cenomanian), which are intensely fissured and traversed by a dense and chaotic network of secondary calcite veins infused with Fe-oxides, b) Pleistocene terrestrial and fluvial-terrestrial deposits of various lithological composition (IGME 1996). To the best of our knowledge, a systematic study of the natural radioactivity levels has never been performed on its premises, providing additional motivation to deploy AMESOS at several locations and create radiological maps of naturally occurring radioactive materials (NORM), and of persisting ¹³⁷Cs concentrations –if any– due to the Chernobyl fallout in 1986.

Figure 5. 

An aerial map of the metropolitan Zografou Campus with the locations of in situ measurements with AMESOS. The dashed line grossly marks the campus borders.

In situ measurements were performed at twenty (20) different locations of the campus, marked on the campus map shown in Fig. 5. Undisturbed forest lands, as well as parking lots, building backyards etc, were selected as measurement locations, so as to obtain a representative radiological map of the area in the end. A typical spectrum is shown in Fig. 6. The duration of measurement was kept at Δt≈3 h per location. The corresponding GPS coordinates are shown in Table 2 together with the corresponding spatial activities for ²²⁶Ra, ⁴⁰K and ¹³⁷Cs. All data were combined further to construct radiological maps.

Figure 6. 

A typical spectrum collected from location ca14. The 609-keV line of ²¹⁴Bi, the 662-keV line of ¹³⁷Cs, and the 1461-keV line of ⁴⁰K, are indicated by the arrows.

The distribution maps for ²²⁶Ra, ⁴⁰K and ¹³⁷Cs are shown in the Fig. 7 top, middle and bottom panels, respectively. ²²⁶Ra content is estimated from its daughter nucleus ²¹⁴Bi, which has a half-life of t_1/2 = 19.8(1) min and is one of the most important γ emitters in the ²³⁸U decay chain. In this study, it has been assumed in secular equilibrium with the longer lived ²²⁶Ra (t_1/2 = 1600(7) a). The ²²⁶Ra spatial activities are in the range of 0.240–0.749 kBqm^-2 with a weighted average value of 0.478(2) kBqm^-2.

Figure 7. 

Radiological maps of ²²⁶Ra (top), ⁴⁰K (middle) and ¹³⁷Cs (bottom).

The spatial activity of ⁴⁰K falls between the values of 4.64 kBqm^-2 and 13.87 kBqm^-2 with a weighted average value of 7.10(1) kBqm^-2, which is in fair agreement with a reported value of 9.5(3) kBqm^-2 (Vosniakos et al. 1997) and others (Anagnostakis et al. 1996, Clouvas et al. 2004). Early studies of potassium in Earth’s crust report spatial concentrations of ⁴⁰K ranging 0.51–15 kBqm^-2 (Eisenbud 1973), in good agreement with the present results.

On the other hand, measurable ¹³⁷Cs activities have been recorded in only half of the locations studied, all being locations with undisturbed soils. It is known that soon after fallouts, radiocesium is trapped in the top layer of soils and sediments at depths not exceeding 5 cm (Fawaris and Johanson 1995, Isaksson and Erlandsson 1995, Ioannides et al. 1996, Giannakopoulou et al. 2012, Kato et al. 2012).

Besides the natural decay of radiocesium, any source of soil disturbance may result in additional reduction of ¹³⁷Cs activity. An additional motivation behind the initial plan to deploy AMESOS in UoA was to target such undisturbed soils, in an attempt to study the persistence of Chernobyl fallout in soils of Athens and elsewhere. Athens with its neighboring suburbs form a megacity of 5 million people, where undisturbed land is nowadays rare to find and study in terms of the long–term impact of man–made radioactivity. In that aspect, the metropolitan UoA campus is a great natural lab to assess the situation, almost 35 years after the 1986 Chernobyl fallout in Greece.

The ¹³⁷Cs values are significantly lower than those recorded in Athens immediately after Chernobyl (Simopoulos 1989, Kritidis et al. 1990) or later (Kritidis and Florou 1995, Anagnostakis et al. 1996, Petropoulos et al. 1996), showing a significant reduction of its retention in soils. The present ¹³⁷Cs measurements fall in the range 0.211–0.511 kBqm^-2 and are ≈20 times lower than the average value of 9.0 kBqm^-2 in Greek soils reported in 1990 by (Kritidis et al. 1990) and ≈10 times lower than the average 4.0(2) kBqm^-2 reported for Attica prefecture 10 years after Chernobyl fallout (Vosniakos et al. 1997). The reduction may be attributed to environmental processes, other than the natural decay of radiocesium, such as soil erosion, surface runoff, migration to deeper soil layers and plant root uptake (Kritidis and Florou 1995, Arapis and Karandinos 2004). This reduction is also in agreement with recent studies of undisturbed soils in locations of Northern Greece (Mertzimekis et al. 2014) where the Chernobyl fallout peaked in Greece (Simopoulos 1989, Kritidis et al. 1990, ICRU 1994, Kritidis and Florou 1995, Vosniakos et al. 1997, Arapis and Karandinos 2004, Clouvas et al. 2004).

A mobile spectrometer, AMESOS, was developed and characterized at the University of Athens to be used for environmental radioactivity studies. The spectrometer was found to behave well during field operation performing γ–ray spectroscopic studies. The first application of the spectrometer focused on assessing the NORM distributions (²²⁶Ra and ⁴⁰K) in the National and Kapodistrian University of Athens Zografou campus for the first time. In addition to the NORM levels, the man–made ¹³⁷Cs levels were examined in the studied locations and were found above detection limits in half of them. Despite the respective areal concentrations are small, the effects of the Chernobyl fallout for more than 30 years in the undisturbed soils of Athens seem to persist. Based on the measurements, a set of radiological maps were constructed for the UoA campus to serve as reference for future studies in the area. Overall, the deployment of AMESOS in the field can be considered successful. The mobile spectrometer will be further used for both research and educational purposes in the near future.

Raw gamma-ray spectra collected during this research are openly available at: https://doi.org/10.6084/m9.figshare.14794449. All other derived data are available from the corresponding author on reasonable request.

The authors would like to thank Ms. Martha Costos for improving the language of the manuscript. This work is partially supported by RAMONES, funded by the European Union’s Horizon 2020 research and innovation programme, under grant agreement No 101017808.