TY - JOUR
AU - , Di Lisa, Giuseppe Antonino
AB - Abstract The outdoor gamma background of the historic center of Rome was studied by in situ measurements and average values of the outcropping geological formations. The survey resulted in two maps of dose equivalent rate, related to pre-anthropic and present conditions. Presently, the average of the dose equivalent rate from outdoor gamma-ray field is equal to 0.31 μSv h−1, corresponding to an outdoor annual effective dose equivalent of 0.548 mSv a−1 and to an outdoor excess lifetime cancer risk [International Commission on Radiological Protection (ICRP). Recommendations of the ICRP, 21, 1/3, Publication 60, 1990] of 2.56 × 10−3. The originary radioactivity was enhanced by anthropic action up to a level of health risk comparable to that one deriving by fine particulate matter. The assessment of the evolution and dispersion of the outdoor gamma background offers a new perspective to study the urban architectural evolution. Such a mapping allows us to individuate mitigation actions and neighborhoods in which the monitoring of illicit trafficking of radioactive material can be efficiently tested. INTRODUCTION Rome, the capital of Italy, has a population of 3 974 318 inhabitants, including resident people and commuters. Every year, about 41 610 000 tourists visit the city(2). The city of Rome, which is also the capital of the Latium region, is administratively divided into 15 Districts (named locally ‘Municipi’) (Figure 1a). The study area coincides with the First District (Figure 1b) which, generally known as the historic center of Rome, is divided into 25 neighborhoods (named locally ‘Rioni’): Borgo, Campitelli, Campo Marzio, Castro Pretorio, Celio, Colonna, Della Vittoria, Esquilino, Ludovisi, Monti, Ostiense, Parione, Pigna, Ponte, Prati, Regola, Ripa, Sallustiano, S. Angelo, S. Eustachio, San Saba, Testaccio, Trastevere, Trevi and Trionfale. The First District is not only the heart of the Italian history (Roman Forum, Coliseum, Pantheon, Trevi Fountain, etc.) but also the core of the modern Italian State. Indeed, the Presidency of the Republic, the Chamber of Deputies, the Senate, the Italian Government, most of government ministries (90%), a multitude of public offices and finally even a foreign state, the Vatican state, are located inside this District. It is, therefore, surprising that up to now neither public environmental agencies nor other research institutions have published a map of the outdoor gamma-ray background (OGRB) of this ‘nerve center’ of Italian State. However, the only published paper that has addressed the problem of the environmental radioactivity in the First District concerns a survey of indoor radon inside the Chamber of Deputies(3). The importance of investigating also the OGRB is given by its general positive correlation both with indoor gamma-ray background and overall with the indoor radon concentration(4,5). The main responsible for this lacuna is a sense of awe toward the Italian public opinion, affected by a widespread form of radiophobia(6). As scientists, however, we are required to invert this trend by trying to involve the general public and by keeping them well informed about the OGRB of this crucial area of Italy. In this paper, four relevant outcomes, deriving from a better assessment of the local OGRB, but having a more general application, will be briefly discussed: (1) historical modification of the gamma background as a tool for urban development studies; (2) public health risk deriving from outdoor gamma-ray exposure in urban environment and comparison with other ‘urban’ risks; (3) risk mitigation actions in urban areas; and (4) finally, prevention of risk from illicit trafficking of radioactive material. Figure 1 Open in new tabDownload slide (a) Administrative division of Rome in districts. (b) Neighborhoods of the First District. Figure 1 Open in new tabDownload slide (a) Administrative division of Rome in districts. (b) Neighborhoods of the First District. Study area: geological background Rome, which extends for 128 500 ha, is characterized by a prevalently hilly topography and Mount Mario reaches the highest elevation (139 m a.s.l.). Rome is grown up on the banks of the Tiber River and the most of it extends to the east of the river. From the geological point of view, the deposits of the receding Pliocene sea are the earliest outcropping rocks (Monte Vaticano Formation, see Figure 2). The subsequent Monte Mario Formation comprises sandy clay of a circalittoral environment testifying to progressive uplift of the area culminating in a first pulse of uplift (around 800 ka), which was followed by the collapse of a NW-SE-oriented structural depression across the north–eastern area of Rome (Paleo-Tiber Graben) and by the interruption of Pleistocene alluvial deposits due to the eruption of the Sabatini volcanic district to the NW(7,8). A second uplift at 600 ka was coincident with start of the main eruptive phases of the Alban Hills volcanic apparatus (to the SE). Figure 2 Open in new tabDownload slide Geological formations and lithologies of the study area. Figure 2 Open in new tabDownload slide Geological formations and lithologies of the study area. This intense Quaternary explosive activity resulted in a large cover of ignimbrite units and volcanic ash. This volcanism has an alkaline potassic geochemical character with magmatic products enriched in incompatible elements and, in particular, in natural radioactive elements as 40K and radionuclides of the 238U and 232Th decay series(9–13). During the later stages of volcanism, modern topography was initiated by river erosion cutting down the easily erodible volcanic deposits. The most recent erosional phase is represented by the Quaternary Wurmian regression, which significantly lowered the base level of the Tiber River and its affluents. The urban sector of the Tiber River deepened to −60 meters above sea level and valleys were subsequently filled up by alluvial sediments (gravels, non-consolidated clays and sand-clay sediments) during Holocene(14). All these general geological features also occur in the First District of Rome. MATERIALS AND METHODS Classification of the land covers classes In order to identify land cover classes of the First District, extending over about 2054 ha (20.54 km2), we used one Sentinel-2A image recorded on 20 June 2017. The Sentinel missions are designed to provide routine observations for operational Copernicus services. Sentinel-2A is an EU Copernicus Programme orbital satellite which carries a multispectral instrument with a sun-synchronous 786-km orbit that allows covering all the land surfaces and coastal waters between −56 and +84° latitude with a 290-km swath width at a 10 day revisit time at equator. In particular, the level-1c data products are available to users. The mission is designed for land monitoring, emergency management, security and climate change(15). For object-oriented classification, we analyzed four bands (2, 3, 4 and 8a) corresponding to a Sentinel-2 level 1c product (spatial resolution 10 m and spectral resolution from 0.490 to 0.865 μm). Image processing was conducted using Definiens eCognition software(16) with scale parameter = 10, shape criterion = 0.5 and compactness = 0.5. Image objects were then assigned to three main classes: urban areas (i.e. impervious surface), green areas (i.e. vegetation) and water. After supervised classification, which assigns image objects to classes based on minimum distance measurements to training data, the main land cover class is represented by urban areas (71.4% of the study area). In particular, it includes public, administrative and commercial buildings, museums, roads, rail networks and historical monuments (Coliseum, Imperial Fori, Pantheon, etc.). About 26.6% of the study area is covered by the green urban areas, including parks, botanical garden, tree-lined streets, and sport and leisure facilities. Finally, the water class (accounting only for 2% of the study area) consists of the segment of the Tiber River. The details of land cover classes of the study area are presented in Figure 3A. Figure 3 Open in new tabDownload slide (A) Classified image of the study area. (B) In situ measurements of gamma radiation in the First District of Rome. Figure 3 Open in new tabDownload slide (A) Classified image of the study area. (B) In situ measurements of gamma radiation in the First District of Rome. Gamma dose equivalent rate of geological formations of the First District of Rome Table 1 Gamma dose equivalent rate of geological formations of the First District of Rome Geological formation* Dose equiv. rate μSv h−1 ±1σ Number of measurements AEL Aurelia formation 0.20 0.02 6 CIL S. Cecilia formation 0.19 0.02 6 LTT Tuffs La Storta 0.28 0.02 9 MTM Monte Mario formation 0.18 0.01 6 MVA Monte Vaticano formation 0.041 0.005 6 PGLa Ponte Galeria formation 0.24 0.05 4 PTI Palatino unit 0.35 0.03 4 RED Red pozzolane 0.41 0.04 15 SFTba Alluvial deposits 0.08 0.01 10 SKP Saccopastore unit 0.35 0.04 6 SKF Tuffs sacrofano 0.33 0.04 23 TDC Tor De’ Cenci unit 0.40 0.04 4 VGU Valle Giulia formation 0.11 0.02 6 VSN1 Villa Senni formation 1 0.41 0.04 8 VSN2 Villa Senni formation 2 0.38 0.04 8 VTN Vitinia formation 0.14 0.02 6 Geological formation* Dose equiv. rate μSv h−1 ±1σ Number of measurements AEL Aurelia formation 0.20 0.02 6 CIL S. Cecilia formation 0.19 0.02 6 LTT Tuffs La Storta 0.28 0.02 9 MTM Monte Mario formation 0.18 0.01 6 MVA Monte Vaticano formation 0.041 0.005 6 PGLa Ponte Galeria formation 0.24 0.05 4 PTI Palatino unit 0.35 0.03 4 RED Red pozzolane 0.41 0.04 15 SFTba Alluvial deposits 0.08 0.01 10 SKP Saccopastore unit 0.35 0.04 6 SKF Tuffs sacrofano 0.33 0.04 23 TDC Tor De’ Cenci unit 0.40 0.04 4 VGU Valle Giulia formation 0.11 0.02 6 VSN1 Villa Senni formation 1 0.41 0.04 8 VSN2 Villa Senni formation 2 0.38 0.04 8 VTN Vitinia formation 0.14 0.02 6 *See Figure 2 for detailed geological description of geological formations. Open in new tab Table 1 Gamma dose equivalent rate of geological formations of the First District of Rome Geological formation* Dose equiv. rate μSv h−1 ±1σ Number of measurements AEL Aurelia formation 0.20 0.02 6 CIL S. Cecilia formation 0.19 0.02 6 LTT Tuffs La Storta 0.28 0.02 9 MTM Monte Mario formation 0.18 0.01 6 MVA Monte Vaticano formation 0.041 0.005 6 PGLa Ponte Galeria formation 0.24 0.05 4 PTI Palatino unit 0.35 0.03 4 RED Red pozzolane 0.41 0.04 15 SFTba Alluvial deposits 0.08 0.01 10 SKP Saccopastore unit 0.35 0.04 6 SKF Tuffs sacrofano 0.33 0.04 23 TDC Tor De’ Cenci unit 0.40 0.04 4 VGU Valle Giulia formation 0.11 0.02 6 VSN1 Villa Senni formation 1 0.41 0.04 8 VSN2 Villa Senni formation 2 0.38 0.04 8 VTN Vitinia formation 0.14 0.02 6 Geological formation* Dose equiv. rate μSv h−1 ±1σ Number of measurements AEL Aurelia formation 0.20 0.02 6 CIL S. Cecilia formation 0.19 0.02 6 LTT Tuffs La Storta 0.28 0.02 9 MTM Monte Mario formation 0.18 0.01 6 MVA Monte Vaticano formation 0.041 0.005 6 PGLa Ponte Galeria formation 0.24 0.05 4 PTI Palatino unit 0.35 0.03 4 RED Red pozzolane 0.41 0.04 15 SFTba Alluvial deposits 0.08 0.01 10 SKP Saccopastore unit 0.35 0.04 6 SKF Tuffs sacrofano 0.33 0.04 23 TDC Tor De’ Cenci unit 0.40 0.04 4 VGU Valle Giulia formation 0.11 0.02 6 VSN1 Villa Senni formation 1 0.41 0.04 8 VSN2 Villa Senni formation 2 0.38 0.04 8 VTN Vitinia formation 0.14 0.02 6 *See Figure 2 for detailed geological description of geological formations. Open in new tab Gamma radiation measurements The gamma survey was carried out by in situ measurements performed by means of a portable gamma spectrometer, the ‘Exploranium GR-135 Plus-The identifier’, equipped with a NaI crystal. The crystal has a diameter of 3.8 cm and a length of 5.5 cm. The detector displays dose rates, but single radionuclide concentrations can also be acquired from spectral analysis with the 1024 channel multichannel analyzer (MCA). In dose rate mode, the instrument is calibrated to give directly the ambient dose equivalent rate (ADER) at a 10-mm depth of human tissue, H*(10), in nSv h−1 at a pre-selected time(17). The ADER is recommended by the International Commission on Radiological Protection (ICRP) as the operational quantity for assessing effective dose in area monitoring(18–20). Measurement time at each point was set to 55 s and measurements were repeated until an uncertainty (type A uncertainty) less than 3% was reached for the local gamma dose equivalent rate. Level of uncertainty was relative to a gamma range between 100 keV and 3 MeV. The energy calibration has been done for every 24 h (or less) with a 137Cs calibration source. Intercalibration with a secondary standard calibration laboratory (Department of Science, Roma TRE University), which uses an independently calibrated digital rate meter connected to a scintillator (model 2241-3, Ludlum Measurements, Inc.), operating in the same aforementioned gamma range was carried out in December 2018 within the European Project ‘LIFE-RESPIRE’ (Radon Real Time Monitoring System and Proactive Indoor Remediation/LIFE16ENV/IT/000553). ADERs in 27 selected sites were measured at the same point and time (with dose rates in the range between 0.2 and 0.8 μSv h−1) and data were corrected according to calibration coefficient. Uncertainty from this intercalibration (type B uncertainty) was of about 5% so that the overall uncertainty can be estimated around 8% (1−σ). To acquire data as generic as possible and to be able to investigate different situations, the field survey was carried out in sites with different land cover classes: urban areas, green areas and near water class (see 2.1 section) and inside the land cover classes, in points with different ground coverage (i.e. bare soil, leucititic basalt pavement, cement and asphalt cover). To minimize the influence of climatic factors, and in particular of hydrometeors that might give transient conditions of high background(21,22), all data have been collected in similar conditions of fine weather during two field surveys performed in the summers of 2016 and 2017. The result of the first field survey is represented by 219 points covering the whole First District of Rome, with a density of 10.6 measurements per km2 (Figure 3B). In the second field survey, average values of ADER related to 15 local geological formations (Table 1) were obtained by measuring their ADER in 127 points of outcropping. In this case, the points were located: (1) inside of the First District for those formations here outcropping in wide spaces far from buildings and (2) outside the boundaries of the First District for those geological formations that do not outcrop in it according the former ideal conditions. Gamma radioactivity maps The pre-anthropic gamma dose equivalent rate map (Figure 4a) is here defined as the gamma radioactivity map linked to original geological features and independent on human activity transformations. In order to determine this map, the average value of the ADER corresponding to each mapped geological formation (Table 1) was attributed to each of the 219 measurement sites according their location in Figure 2. Differently, the present gamma dose equivalent rate map (Figure 4b) was created using directly the 219 in situ measurements. Surfer software was used to show the distribution of the ambient gamma dose equivalent rate due to geological formations only (pre-anthropic map), as well as to anthropogenic land use-cover, (present map), as contour maps. The data imported into Surfer software included: geographical coordinates (Longitude and Latitude), measurements of gamma dose equivalent rate converted from nSv h−1 to μSv h−1 and map image data. The Kriging method of interpolation was used to produce an accurate grid and efficient mapping of gamma radioactivity within the First District of Rome(23,24). A gamma radiation change map (Figure 5) was also created by reciprocal subtraction of the values of the previous two maps. Figure 4 Open in new tabDownload slide Maps of gamma radiation (dose equivalent rate in μSv h−1) of the First District of Rome: (a) pre-anthropic map; (b) present map. Figure 4 Open in new tabDownload slide Maps of gamma radiation (dose equivalent rate in μSv h−1) of the First District of Rome: (a) pre-anthropic map; (b) present map. Figure 5 Open in new tabDownload slide The gamma radiation change map (variation of dose equivalent rate in μSv h−1) between pre-anthropic and anthropic condition in the First District of Rome. Figure 5 Open in new tabDownload slide The gamma radiation change map (variation of dose equivalent rate in μSv h−1) between pre-anthropic and anthropic condition in the First District of Rome. Figure 6 Open in new tabDownload slide (A) Schematic drawing with top and section view of the gamma backscatter density gauge used in this work: S: source, B: PVC base, L = lead shield, D = detector, P: piston air. (B) Calibration curve. (C) Inverse relationship between bulk density and total gamma emission rate. (D) Maps of total gamma emission rate and bulk density before and after the repair work of a road over the Pozzolane Rosse Formation. Figure 6 Open in new tabDownload slide (A) Schematic drawing with top and section view of the gamma backscatter density gauge used in this work: S: source, B: PVC base, L = lead shield, D = detector, P: piston air. (B) Calibration curve. (C) Inverse relationship between bulk density and total gamma emission rate. (D) Maps of total gamma emission rate and bulk density before and after the repair work of a road over the Pozzolane Rosse Formation. Densitometry and total gamma emission monitoring for mitigation actions The Compton Mobile Range Densitometer with 40K source was used for investigating the variation of bulk density and total gamma emission in a stretch (18 m) of a road above the Red Pozzolane Formation, before and after the repair and improvement of the upper level (asphalt binder + aggregate) of road surface. This instrument (Figure 6a) is a gamma backscatter density gauge(25,26) designed by IGAG (CNR)(27) to reveal the values of bulk density and gamma emission of soil, road surfaces(28) and basements. The densities values are derived from the detection of gamma rays which, emitted by a natural source of 40K, are backscattered by Compton Effect from the investigated material according to its bulk density. According to the known Compton effect law, after the photon–electron interaction, the gamma photon is deviated from the incident direction according to a random angle and with a lower energy than the initial one, energy that depends on the cosine value of the diffusion angle. Since the photonic density in a natural medium is proportional to its bulk density, then there is a relationship between bulk density of the interacting medium and percentage of the backscattered gamma photons. The relationship has a bell shape with a maximum which depends on the energy of the gamma rays of the source. By identifying the range of density values for which the function is monotonic (1–2.5 kg dm−3), a polynomial relationship (Figure 6b) can be obtained between bulk density and the number of backscattered photons. The system includes the following elements: source of monoenergetic gamma rays from 40K (1461 keV): 40 kg of potassium hydroxide with a total activity equal to 0.99 MBq. It has some advantages compared with traditional sources consisting of 137Cs or 60Co: (a) higher gamma ray energy (1461 keV in comparison of 661 keV of 137Cs) and consequently a lower attenuation before backscattering. It results in a major investigated thickness of the material; (b) lower radioactivity (less than 1 MBq) compared to traditional sources (generally 100 MBq). It can be used without any notification based on the current national and European provisions concerning ionization sources; (c) square frame shaped form which allows referring the bulk density value to an area of about 1 square meter; (d) the source is made up of 20 elongated bags containing 2 kg of KOH each. They can be easily removed from their allocation, allowing the instrument to measure the total gamma emission range from the ground (background). the NaI scintillator annularly shielded by a lead shield to minimize the detection of gamma rays emitted directly by the source and for detecting only gamma rays backscattered or emitted by the investigated material; PVC base: with an area of 1 m2, it is used to allocate the source and, at the center, the scintillator. Its thickness is about 4 cm to support the overall weight of the source and of the lead shielding. The low density of the material is necessary for minimizing the attenuation of gamma rays emitted by the source and backscattered by the investigated material; MCA for the analysis of the gamma spectrum: analogic digital converter that provides a gamma spectrum. It is powered by electric batteries and also supplies the scintillator voltage. The analyzer can be pre-programed to establish the counting time and the number of spectra to be recorded; software for converting the acquired spectrum into density values; mobile unit: for the transport and installation of the instrument at the analysis site through a system of compressed air pistons; GPS system: for associating the measured spectrum to the measurement site and to trace consequently isodensity maps. The system is calibrated for a density range between 1 and 2.5 T m−3 with an error dependent on the counting time (about 5% for a counting time of 120 s). The probability that a photon will undergo Compton scattering is proportional to n, the electron number density in the material. This in turns can be expressed(25) as: $$\begin{equation} n={N}_{\mathrm{A}}\ Z \rho /M \end{equation}$$ (1) where NA is the Avogadro’s constant, Z is the mean atomic number, M is the mean atomic mass of the material and ρ is its bulk density. It is well known that the ratio Z/M is equal to 0.50 for most common isotopes. Hence, the electron number density is almost constant with composition(29). Hydrogen (Z/M = 1) is the only significant exception though its abundance in most materials is low enough for its effect to be small. In soils, which can display a relevant amount of moisture content (Z/M in pure water = 0.55), the minimization of hydrogen effect is reached during dry season after at least one week of absence of rain. In these conditions, which are recommended for carrying out calibration and measurements, in the first 40 cm of investigated depth, moisture content is lower than 10%(30) (Z/M of soils with 10% of moisture content = 0.505). The algorithm used in the calibration and determination of the bulk density is based on the calculation of the backscattered gamma rays. It is obtained by subtracting the background (measured by precedently removing the 40K source) from the number of gamma rays with energy lower than 1000 keV that constitute the majority of the backscattered rays. The calibration was carried out over substrates of known density included in the aforementioned density range. The depth investigated by the system refers to about 40 cm of thickness when the PVC support surface is directly leaning against the material to be analyzed. The recorded spectra, in addition to provide through a simple algorithm the values of average bulk density of the investigated thickness, can also be used to obtain information on the composition in uranium, thorium and potassium or more in general on the total gamma emission of the same layer (i.e. background from the underground). This additional information is represented by the number of total counts in the spectrum from 0 to 2700 keV in a fixed unit of time (cpm). RESULTS The range of the present outdoor gamma dose equivalent rates measured in the First district of Rome is between 0.09 and 0.49 μSv h−1 with an average of 0.31 ± 0.08(1−σ) μSv h−1. It is interesting to compare the pre-anthropic map (Figure 4a) with the present radioactivity map (Figure 4b) in order to highlight the historical change of the gamma dose equivalent rate in the First District of Rome. The gamma radiation change map (Figure 5) shows that the gamma dose equivalent rate has changed differently in the study area due to increased anthropic pressure/activity. The pre-anthropic average dose equivalent rate from outdoor gamma radiation is equal to 0.22 μSv h−1, while the average of corresponding present value is around 0.31 μSv h−1. Considering that the dose from cosmic contribution is 0.03 μSv h−1, the anthropogenic enhancement of the original outdoor terrestrial gamma radiation is about 47%. A statistical method, based on Shannon diversity index(31) (H), was applied for comparing the results. From the pre-anthropic and the present radioactivity maps, for each neighborhood, the obtained dose equivalent rates were regrouped in eight frequency classes, on the basis of their relative areal frequency in each neighborhood, varying from 0.1 to 0.50 μSv h−1, with a bin size of 0.05 μSv h−1. The Shannon diversity index (H) is an index of dispersion, commonly used to characterize species diversity in a community or in a landscape. In this case, we used it to characterize each neighborhood of the historic center of Rome. The Shannon diversity index is given by: $$\begin{equation} H=-\sum\limits_{i=1}^k\ {p}_i{\log}_{\mathrm{e}}\left({p}_i\right) \end{equation}$$ (2) where pi, in this particular case, is the areal frequency of the ith class of the ADER in the specific neighborhood, obtained from the radioactivity map, and k is the number of frequency classes (k = 8). The Shannon equitability index, EH, varying from 0 to 1, is simply the Shannon diversity index divided by the maximum possible diversity, expressed as loge (k): $$\begin{equation} {E}_H=H/{\log}_{\mathrm{e}}\ (k) \end{equation}$$ (3) The values of EH and the gamma dose equivalent rates for each neighborhood of the First District in pre-anthropic and present conditions are displayed in Table 2. First District of Rome: gamma dose equivalent rates and Shannon equitability index in pre-anthropic and in present conditions Table 2 First District of Rome: gamma dose equivalent rates and Shannon equitability index in pre-anthropic and in present conditions Neighborhood Pre-anthr. Dose (μSv h−1) Present dose (μSv·h−1) Pre-anthropic EH Present EH 1. Della Vittoria 0.17 0.29 0.60 0.76 2. Trionfale 0.13 0.26 0.07 0.65 3. Prati 0.13 0.26 0.00 0.53 4. Borgo 0.14 0.26 0.27 0.68 5. Ponte 0.13 0.35 0.00 0.58 6. Parione 0.13 0.31 0.01 0.61 7. S. Eustachio 0.13 0.28 0.01 0.62 8. Regola 0.13 0.31 0.00 0.70 9. Pigna 0.13 0.28 0.03 0.24 10. S.Angelo 0.16 0.35 0.50 0.57 11. C. Marzio 0.16 0.32 0.56 0.54 12. Colonna 0.16 0.33 0.49 0.51 13. Trevi 0.21 0.36 0.63 0.68 14. Ludovisi 0.26 0.33 0.36 0.63 15. Sallustiano 0.27 0.29 0.15 0.53 16. C. Pretorio 0.35 0.31 0.38 0.55 17. Esquilino 0.36 0.38 0.81 0.46 18. Monti 0.32 0.31 0.76 0.60 19. Celio 0.31 0.32 0.75 0.65 20. San Saba 0.33 0.29 0.83 0.63 21. Ostiense 0.34 0.34 0.66 0.48 22. Testaccio 0.14 0.29 0.26 0.66 23. Ripa 0.26 0.35 0.86 0.68 24. Trastevere 0.17 0.33 0.61 0.74 25. Campitelli 0.24 0.38 0.73 0.58 Neighborhood Pre-anthr. Dose (μSv h−1) Present dose (μSv·h−1) Pre-anthropic EH Present EH 1. Della Vittoria 0.17 0.29 0.60 0.76 2. Trionfale 0.13 0.26 0.07 0.65 3. Prati 0.13 0.26 0.00 0.53 4. Borgo 0.14 0.26 0.27 0.68 5. Ponte 0.13 0.35 0.00 0.58 6. Parione 0.13 0.31 0.01 0.61 7. S. Eustachio 0.13 0.28 0.01 0.62 8. Regola 0.13 0.31 0.00 0.70 9. Pigna 0.13 0.28 0.03 0.24 10. S.Angelo 0.16 0.35 0.50 0.57 11. C. Marzio 0.16 0.32 0.56 0.54 12. Colonna 0.16 0.33 0.49 0.51 13. Trevi 0.21 0.36 0.63 0.68 14. Ludovisi 0.26 0.33 0.36 0.63 15. Sallustiano 0.27 0.29 0.15 0.53 16. C. Pretorio 0.35 0.31 0.38 0.55 17. Esquilino 0.36 0.38 0.81 0.46 18. Monti 0.32 0.31 0.76 0.60 19. Celio 0.31 0.32 0.75 0.65 20. San Saba 0.33 0.29 0.83 0.63 21. Ostiense 0.34 0.34 0.66 0.48 22. Testaccio 0.14 0.29 0.26 0.66 23. Ripa 0.26 0.35 0.86 0.68 24. Trastevere 0.17 0.33 0.61 0.74 25. Campitelli 0.24 0.38 0.73 0.58 Open in new tab Table 2 First District of Rome: gamma dose equivalent rates and Shannon equitability index in pre-anthropic and in present conditions Neighborhood Pre-anthr. Dose (μSv h−1) Present dose (μSv·h−1) Pre-anthropic EH Present EH 1. Della Vittoria 0.17 0.29 0.60 0.76 2. Trionfale 0.13 0.26 0.07 0.65 3. Prati 0.13 0.26 0.00 0.53 4. Borgo 0.14 0.26 0.27 0.68 5. Ponte 0.13 0.35 0.00 0.58 6. Parione 0.13 0.31 0.01 0.61 7. S. Eustachio 0.13 0.28 0.01 0.62 8. Regola 0.13 0.31 0.00 0.70 9. Pigna 0.13 0.28 0.03 0.24 10. S.Angelo 0.16 0.35 0.50 0.57 11. C. Marzio 0.16 0.32 0.56 0.54 12. Colonna 0.16 0.33 0.49 0.51 13. Trevi 0.21 0.36 0.63 0.68 14. Ludovisi 0.26 0.33 0.36 0.63 15. Sallustiano 0.27 0.29 0.15 0.53 16. C. Pretorio 0.35 0.31 0.38 0.55 17. Esquilino 0.36 0.38 0.81 0.46 18. Monti 0.32 0.31 0.76 0.60 19. Celio 0.31 0.32 0.75 0.65 20. San Saba 0.33 0.29 0.83 0.63 21. Ostiense 0.34 0.34 0.66 0.48 22. Testaccio 0.14 0.29 0.26 0.66 23. Ripa 0.26 0.35 0.86 0.68 24. Trastevere 0.17 0.33 0.61 0.74 25. Campitelli 0.24 0.38 0.73 0.58 Neighborhood Pre-anthr. Dose (μSv h−1) Present dose (μSv·h−1) Pre-anthropic EH Present EH 1. Della Vittoria 0.17 0.29 0.60 0.76 2. Trionfale 0.13 0.26 0.07 0.65 3. Prati 0.13 0.26 0.00 0.53 4. Borgo 0.14 0.26 0.27 0.68 5. Ponte 0.13 0.35 0.00 0.58 6. Parione 0.13 0.31 0.01 0.61 7. S. Eustachio 0.13 0.28 0.01 0.62 8. Regola 0.13 0.31 0.00 0.70 9. Pigna 0.13 0.28 0.03 0.24 10. S.Angelo 0.16 0.35 0.50 0.57 11. C. Marzio 0.16 0.32 0.56 0.54 12. Colonna 0.16 0.33 0.49 0.51 13. Trevi 0.21 0.36 0.63 0.68 14. Ludovisi 0.26 0.33 0.36 0.63 15. Sallustiano 0.27 0.29 0.15 0.53 16. C. Pretorio 0.35 0.31 0.38 0.55 17. Esquilino 0.36 0.38 0.81 0.46 18. Monti 0.32 0.31 0.76 0.60 19. Celio 0.31 0.32 0.75 0.65 20. San Saba 0.33 0.29 0.83 0.63 21. Ostiense 0.34 0.34 0.66 0.48 22. Testaccio 0.14 0.29 0.26 0.66 23. Ripa 0.26 0.35 0.86 0.68 24. Trastevere 0.17 0.33 0.61 0.74 25. Campitelli 0.24 0.38 0.73 0.58 Open in new tab Results of measurements bulk density and total gamma emission in a stretch (18 m) of a road above the Pozzolane Rosse Formation, carried out before and after the repair and improvement of the deteriorated upper level of the road surface, are presented in Figure 6C and D. After repair work, bulk density increased from 1.88 to 2.05 T m−3 and total gamma emission rate decreased from 1710 to 1480 cpm. DISCUSSION Historical modification of the gamma background in the framework of the urban development A preliminary objection to the method of study proposed here could arise from the real value of what we call ‘pre-anthropic gamma dose equivalent rate’. With this term, we refer to the value that had to be present in a point in pre-Roman times, about 3000 years ago. It is obvious that in this strict sense, the pre-anthropic radioactivity map is an artefact and it should be, therefore, considerate with some precaution. However, we are approaching reality as the internal variation of gamma dose equivalent rate of the individual geological formations is minimal. Looking at Table 1, we can see that 11 out of 15 formations have a low internal variability, less than 12%. They are almost all formations of volcanic origin and generally have a considerable natural radioactivity. Only four formations (PGLa, VGU, VTN and SFTBa) linked to river deposition cycles have an internal variability of between 13 and 21%. Fortunately, these are the formations with low natural radioactivity. Consider for example the Prati neighborhood that extends mainly on the alluvial deposits of the Tiber (SfTBa, internal variation: 13%). It is easy to observe from pre-anthropic and present radioactivity maps (Figure 4a and b) and from Table 2 that this neighborhood today has a gamma dose equivalent rate which is 100% higher than pre-anthropic conditions and seven times greater than internal variation of the SftBa Formation. Figure 7 Open in new tabDownload slide Variation (Δ) of Shannon equitability index vs. variation of gamma equivalent dose rate between pre-anthropic and anthropic conditions in the neighborhoods of the First District of Rome. Light gray: group (A); deep gray: group (B); gray: group (C). Figure 7 Open in new tabDownload slide Variation (Δ) of Shannon equitability index vs. variation of gamma equivalent dose rate between pre-anthropic and anthropic conditions in the neighborhoods of the First District of Rome. Light gray: group (A); deep gray: group (B); gray: group (C). The gamma radiation change map (Figure 5), created by reciprocal subtraction of the values of the previous two maps, shows that the gamma dose equivalent rate has changed differently in the study area due to increased anthropic pressure/activity. In particular, the comparison between gamma dose equivalent rate and Shannon equitability index in natural, pre-anthropic condition and in the present state shows two main effects of the anthropic impact on the outdoor gamma background: a general increase (1) in terms of absolute values of gamma dose equivalent rate and (2) in terms of diversity. The comparison also shows that the neighborhoods can be grouped, according to the bivariate diagram of Figure 7, in three groups (A–C) on the basis of the difference (Δ) of the Shannon equitability index (i.e. diversity) and of the gamma dose equivalent rate between pre-anthropic and anthropic conditions. The first group, (A), corresponds to the S-SE area of the historic center, where the present gamma dose is weakly increased or decreased respect to the original one with significant variations in diversity. A view to Figures 5 and 7 shows that in these zones, 16 and 20 neighborhoods have recorded a lowering of gamma dose. A careful check of the satellite image demonstrates that this decrease is mainly due to wide zones covered by asphalt, a material that, in this specific context, attenuates the gamma radiation of the Rome soil, which is originally very high in this sector of the historic center due to the local geological background represented by volcanic products enriched in 238U, 232Th and 40K. In particular, the decrease of gamma dose in San Saba neighborhood is evident, being likely the consequence of the enlargement of roads and their asphalt cover along the Almone Valley, occurred in a very recent time(32). A second group, (B), displays a general increase of gamma dose equivalent rate and diversity. It comprises some neighborhoods of the First District which are along the Tiber alluvial area characterized by low pre-anthropic gamma doses. In these zones, the material of oldest buildings (in which the mortar and/or the same bricks consist of prevailing volcanic material) and also of the rock pavement, the latter consisting of Alban Hills leucititic basalt blocks(33) locally named ‘sampietrini’(Figure 9A–C), have caused strong positive variations due to their high content of gamma natural emitters (40K, 232Th—238U and their progeny). A third group, (C), comprises both neighborhoods near to the Tiber alluvial area and others located at East respect to the previous group (B). In group (C), the occurrence of leucititic basalt as road pavement is extremely spread and also the building density(34) is very high; however, the original middle-high value of the local soil has minimized the increase in diversity with respect to the group (B). These findings confirm that urban spaces are complex environments in which the outdoor gamma radiation does not depend only on the level of radionuclides in soil but also on radionuclide content of building materials used for the construction of roads, sidewalks and buildings(35) and on effects of topographic enclosure in built environments(36). It is intriguing to consider that anthropic modifications could result in an enhancement of the diversity, as it happens for the most part of historic center of Rome, where different buildings made of different materials got all mixed up over time, or in a lowering of it, as in the case of the Esquilino neighborhood. Soon after the capture of Rome in 1870, in fact, the Kingdom of Italy tried to direct the development of Rome in the eastern direction and so the Esquilino was born as a neighborhood for the new bureaucracy, mostly coming from Turin, reproducing uniform building types in a regular plot typical of this city(37). These observations open an original perspective for analyzing and classifying the architectural evolution of different neighborhoods of an urban area over time in terms of variation of gamma dose and Shannon equitability index. Outdoor gamma radioactive risk, its mitigation and comparison with other ‘urban’ risks The average of 0.31 ± 0.08(1−σ) μSv h−1 for the present outdoor gamma dose equivalent rates measured in the First district of Rome is not in line with the UNSCEAR 2000 report for the Latium(38,39) outdoor radiation dose rate (0.18 μSv h−1), on the contrary the pre-anthropic average dose equivalent rate from outdoor gamma radiation, equal to 0.22 μSv h−1, it is close to the officially reported value. This difference depends at least from three factors: the official values are the result of (i) an old survey carried out in 1972, (ii) in different sites of Latium (with sites also located in a limestone context characterized by low dose rates) and not exclusively in Rome where the lithology is predominantly volcanic and (iii) in areas distant from buildings which as, yet discussed, increase the dose rate. Since the detector used in this survey measures directly the H*(10) as operational quantity for assessing the effective dose (μSv h−1), the outdoor annual effective dose equivalent (AEDEout) for each neighborhood (expressed as mSv a−1) was simply calculated by the following equation: $$\begin{equation} {\mathrm{AEDE}}_{\mathrm{out}}=D\cdotp T\cdotp F/{10}^3 \end{equation}$$ (4) where D is the mean ADER for each neighborhood (in μSv h−1), T is the number of hours in one year (8760 h a−1) and F is the outdoor occupancy factor (assumed conventionally equal to 0.2). The resulting ‘neighborhood area-weighted’ average of the AEDEout of the First District is equal to 0.548 mSv a−1 which is five times higher than the population-weighted world’s average(38) of 0.106 mSv a−1. The calculated AEDEout value is close to the range of values (0.58–0.67 mSv a−1) found for the Caffarella Valley(40), located within the Eigth District, immediately south of the historic center. The excess lifetime cancer risk from outdoor gamma radiation (ELCRout), defined as the probability that an individual will develop cancer over his lifetime of exposure to outdoor gamma radiation(41), was calculated by using the following equation: $$\begin{equation} {\mathrm{ELCR}}_{\mathrm{out}}={\mathrm{AEDE}}_{\mathrm{out}}\times \mathrm{DL}\times \mathrm{RF} \end{equation}$$ (5) where DL is the expected duration of life for Italy (82 years) and RF is the risk factor (Sv−1), i.e. fatal cancer risk per Sievert. For stochastic effects from low dose background radiation, ICRP 103 suggested an RF value of 0.057 Sv−1 for the public exposure(18). The values of AEDEout and ELCRout calculated for each neighborhood of the First District are tabulated in Table 3. The mean of ELCRout for the historic center of Rome, weighted on the areas of its neighborhoods, is equal to 2.56 × 10−3. A comparison (Figure 8) of AEDEout and ELCRout recorded in different areas of the world(42–55) shows that the value of the historic center of Rome is in the middle-high level of the reported range and more than five times higher than the world average. First District of Rome: radiological units Table 3 First District of Rome: radiological units Neighborhood Present doseout equivalent rate (μSv h−1) AEDEout (mSv a−1) ELCRout 10−3 Neighborhood area (ha) (total = 2054 ha) 1. Della Vittoria 0.29 0.508 2.373 318.66 2. Trionfale 0.26 0.455 2.127 88.86 3. Prati 0.26 0.455 2.127 133.11 4. Borgo 0.26 0.455 2.127 45.27 5. Ponte 0.35 0.613 2.864 35.81 6. Parione 0.31 0.543 2.536 21.98 7. S.Eustachio 0.28 0.490 2.291 20.17 8. Regola 0.31 0.543 2.537 33.76 9. Pigna 0.28 0.490 2.291 24.48 10. S.Angelo 0.35 0.613 2.864 16.43 11. C. Marzio 0.32 0.560 2.619 92.98 12. Colonna 0.33 0.578 2.699 33.32 13. Trevi 0.36 0.630 2.946 50.13 14. Ludovisi 0.33 0.578 2.699 35.3 15. Sallustiano 0.29 0.508 2.373 30.25 16. C. Pretorio 0.31 0.543 2.536 96.03 17. Esquilino 0.38 0.665 3.109 159.47 18. Monti 0.31 0.543 2.536 169.75 19. Celio 0.32 0.560 2.619 89.55 20. San Saba 0.29 0.508 2.373 112.76 21. Ostiense 0.34 0.595 2.782 45.39 22. Testaccio 0.29 0.508 2.373 68.27 23. Ripa 0.35 0.613 2.864 88.85 24. Trastevere 0.33 0.578 2.699 183.08 25. Campitelli 0.38 0.665 3.109 60.57 Weighted average* 0.31 0.548 2.563 Neighborhood Present doseout equivalent rate (μSv h−1) AEDEout (mSv a−1) ELCRout 10−3 Neighborhood area (ha) (total = 2054 ha) 1. Della Vittoria 0.29 0.508 2.373 318.66 2. Trionfale 0.26 0.455 2.127 88.86 3. Prati 0.26 0.455 2.127 133.11 4. Borgo 0.26 0.455 2.127 45.27 5. Ponte 0.35 0.613 2.864 35.81 6. Parione 0.31 0.543 2.536 21.98 7. S.Eustachio 0.28 0.490 2.291 20.17 8. Regola 0.31 0.543 2.537 33.76 9. Pigna 0.28 0.490 2.291 24.48 10. S.Angelo 0.35 0.613 2.864 16.43 11. C. Marzio 0.32 0.560 2.619 92.98 12. Colonna 0.33 0.578 2.699 33.32 13. Trevi 0.36 0.630 2.946 50.13 14. Ludovisi 0.33 0.578 2.699 35.3 15. Sallustiano 0.29 0.508 2.373 30.25 16. C. Pretorio 0.31 0.543 2.536 96.03 17. Esquilino 0.38 0.665 3.109 159.47 18. Monti 0.31 0.543 2.536 169.75 19. Celio 0.32 0.560 2.619 89.55 20. San Saba 0.29 0.508 2.373 112.76 21. Ostiense 0.34 0.595 2.782 45.39 22. Testaccio 0.29 0.508 2.373 68.27 23. Ripa 0.35 0.613 2.864 88.85 24. Trastevere 0.33 0.578 2.699 183.08 25. Campitelli 0.38 0.665 3.109 60.57 Weighted average* 0.31 0.548 2.563 *The average was weighted according to each neighborhood area value. Open in new tab Table 3 First District of Rome: radiological units Neighborhood Present doseout equivalent rate (μSv h−1) AEDEout (mSv a−1) ELCRout 10−3 Neighborhood area (ha) (total = 2054 ha) 1. Della Vittoria 0.29 0.508 2.373 318.66 2. Trionfale 0.26 0.455 2.127 88.86 3. Prati 0.26 0.455 2.127 133.11 4. Borgo 0.26 0.455 2.127 45.27 5. Ponte 0.35 0.613 2.864 35.81 6. Parione 0.31 0.543 2.536 21.98 7. S.Eustachio 0.28 0.490 2.291 20.17 8. Regola 0.31 0.543 2.537 33.76 9. Pigna 0.28 0.490 2.291 24.48 10. S.Angelo 0.35 0.613 2.864 16.43 11. C. Marzio 0.32 0.560 2.619 92.98 12. Colonna 0.33 0.578 2.699 33.32 13. Trevi 0.36 0.630 2.946 50.13 14. Ludovisi 0.33 0.578 2.699 35.3 15. Sallustiano 0.29 0.508 2.373 30.25 16. C. Pretorio 0.31 0.543 2.536 96.03 17. Esquilino 0.38 0.665 3.109 159.47 18. Monti 0.31 0.543 2.536 169.75 19. Celio 0.32 0.560 2.619 89.55 20. San Saba 0.29 0.508 2.373 112.76 21. Ostiense 0.34 0.595 2.782 45.39 22. Testaccio 0.29 0.508 2.373 68.27 23. Ripa 0.35 0.613 2.864 88.85 24. Trastevere 0.33 0.578 2.699 183.08 25. Campitelli 0.38 0.665 3.109 60.57 Weighted average* 0.31 0.548 2.563 Neighborhood Present doseout equivalent rate (μSv h−1) AEDEout (mSv a−1) ELCRout 10−3 Neighborhood area (ha) (total = 2054 ha) 1. Della Vittoria 0.29 0.508 2.373 318.66 2. Trionfale 0.26 0.455 2.127 88.86 3. Prati 0.26 0.455 2.127 133.11 4. Borgo 0.26 0.455 2.127 45.27 5. Ponte 0.35 0.613 2.864 35.81 6. Parione 0.31 0.543 2.536 21.98 7. S.Eustachio 0.28 0.490 2.291 20.17 8. Regola 0.31 0.543 2.537 33.76 9. Pigna 0.28 0.490 2.291 24.48 10. S.Angelo 0.35 0.613 2.864 16.43 11. C. Marzio 0.32 0.560 2.619 92.98 12. Colonna 0.33 0.578 2.699 33.32 13. Trevi 0.36 0.630 2.946 50.13 14. Ludovisi 0.33 0.578 2.699 35.3 15. Sallustiano 0.29 0.508 2.373 30.25 16. C. Pretorio 0.31 0.543 2.536 96.03 17. Esquilino 0.38 0.665 3.109 159.47 18. Monti 0.31 0.543 2.536 169.75 19. Celio 0.32 0.560 2.619 89.55 20. San Saba 0.29 0.508 2.373 112.76 21. Ostiense 0.34 0.595 2.782 45.39 22. Testaccio 0.29 0.508 2.373 68.27 23. Ripa 0.35 0.613 2.864 88.85 24. Trastevere 0.33 0.578 2.699 183.08 25. Campitelli 0.38 0.665 3.109 60.57 Weighted average* 0.31 0.548 2.563 *The average was weighted according to each neighborhood area value. Open in new tab Figure 8 Open in new tabDownload slide Comparison of AEDEout and ELCRout relative to the gamma radiation of Rome (First District) with values of other areas of the world. WA = world average. (a) Balad, Iraq(49); (b) Tulkarem, Palestine(43); (c) Tanke-Ilorin, Nigeria(52); (d) Gange River, India(47); (e) Northern Pakistan(45); (f) Jelhum Valley, Pakistan(46); (g) Alkananada River, India(47); (h) Kirklareli, Turkey(42); (i) Nile Delta, Egypt(53); (l) Chihuahua City, Mexico(48); (m) Birjand, Iran(55); (n) Beach sands, Kerala, India(44); (o) Rome, Italy (this work); (p) Kuala Lumpur, Malaysia(54); (q) Bajelsa state, Nigeria(51); (r) Kerala coastal regions, India(40). Figure 8 Open in new tabDownload slide Comparison of AEDEout and ELCRout relative to the gamma radiation of Rome (First District) with values of other areas of the world. WA = world average. (a) Balad, Iraq(49); (b) Tulkarem, Palestine(43); (c) Tanke-Ilorin, Nigeria(52); (d) Gange River, India(47); (e) Northern Pakistan(45); (f) Jelhum Valley, Pakistan(46); (g) Alkananada River, India(47); (h) Kirklareli, Turkey(42); (i) Nile Delta, Egypt(53); (l) Chihuahua City, Mexico(48); (m) Birjand, Iran(55); (n) Beach sands, Kerala, India(44); (o) Rome, Italy (this work); (p) Kuala Lumpur, Malaysia(54); (q) Bajelsa state, Nigeria(51); (r) Kerala coastal regions, India(40). The comparison of the above calculated ELCRout with the ELCR deriving from the contribution of both ultrafine and coarse particles from light duty and heavy duty vehicles in urban areas (ELCRp) is particularly interesting for the risk perception(56). By using the model(57) given by Scungio et al. to calculate ELCRp, when specific different factors such as the compactness of the urban fabric(34,58) i.e. the percentage of ground occupied by buildings (about 70%), aspect ratio of historic center’s streets (H/W ̴ 3), wind velocity ( ̴ 3 m s−1), outdoor occupancy factor (0.2) and duration of life (82 years) are considered, an upper level of 2.07 ×·10−3 for ELCRp can be estimated for at least six neighborhoods (Campo Marzio, Colonna, Trevi, Pigna, S. Eustachio and Regola) of the historic center of Rome. This value is fully comparable with the one previously estimated for the ELCRout. A further evidence of this result can be obtained by the comparison with the total ELCR, deriving from outdoor and indoor exposure to ultrafine particles (UFPs) and PM10 for the whole Italian population, which is estimated(59) between 1.0 and 2.2 × 10−3. Like in many other cities, the local administration is very focused on the health risk posed by fine and UFPs from vehicular traffic and domestic heating. The similarity of this health risk with the one deriving from outdoor gamma radiation should push for a better balance of mitigation actions devoted to these two different risks. Mitigation actions In our opinion, two feasible and realistic mitigation actions against the risk, previously discussed, can be adopted. The first is the substitution of the leucititic basaltic rock pavement (Figure 9A–C) by an asphalt cover mixed with limestone grains or, in order to preserve the original aesthetic appearance, by a similar basaltic rock pavement made of blocks extracted from caves of low radioactive Italian volcanic districts as, for example Mount Aetna. The volcanics of Mount Aetna indeed have a gamma emission from U and Th series equal to 50% of Alban Hills leucititic lavas(60). Since the pavement road contributes at least up to 50% of the outdoor gamma emission, this action should reduce significantly the AEDEout and consequently the ELCRout in the neighborhoods of the group (C). Figure 9 Open in new tabDownload slide (A) Via Nazionale, paved with ‘sampietrini’ blocks. (B) A strict view of the same street. In the lower right box, the shape of a block. (C) An old quarry of the Alban Hills from where the ‘sampietrini’ were extracted. (D–E) The typical spider web cracking of deteriorated asphalt. (F) A large hole that highlights the lack of intermediate layers that should be present in an ideal road section; 1: asphalt + binder; 2: base course; 3: subbase; 4: subgrade. Figure 9 Open in new tabDownload slide (A) Via Nazionale, paved with ‘sampietrini’ blocks. (B) A strict view of the same street. In the lower right box, the shape of a block. (C) An old quarry of the Alban Hills from where the ‘sampietrini’ were extracted. (D–E) The typical spider web cracking of deteriorated asphalt. (F) A large hole that highlights the lack of intermediate layers that should be present in an ideal road section; 1: asphalt + binder; 2: base course; 3: subbase; 4: subgrade. The second concerns the correct restoration of the deteriorated upper level (asphalt + binder) of road surfaces which, as Roman citizens sadly know, is plagued by disconnections, holes and crumbling (Figure 9C and D). Besides of it, many roads lack of at least 45 cm of subbase and base course that should be interposed between the asphalt mantle and the subgrade. All these factors have the effect of lowering the density of the road surface diminishing its role in attenuating the gamma radioactivity from the subgrade. In fact, as also the experimental study carried out on a stretch of road that crosses the formation of Pozzolane Rosse highlights, bulk density of the upper level of road surface is inversely related to the total gamma emission (Figure 6C). It is encouraging to note (Figure 6D) that a correct restoration of even the most superficial level has the effect of increase the density of road surface and of lowering the total gamma emission by 11%. Considering that subbase and base course are lacking in many roads of historical center and even when they occur are often made of enriched U–Th–K local volcanic material, it can be reasonably thought that there is still a lot of margin of lowering of gamma emission from the subgrade. Figure 10 Open in new tabDownload slide Procedure for the selection of the neighborhoods where testing active gamma monitoring. At right: the diagram equitability vs. gamma dose identifies the two neighborhoods with the lowest (9) and the highest (13) product of the two variables. A further criterion is the proximity of the two neighborhoods. At left: a uniform increase of the dose of 0.2 μSv h−1 for the two neighborhoods, due to a sudden radioactive contamination, makes the cut-off value, previously set to have a specificity equal to 1 (FAR = 0), less effective (lowering of sensitivity) for the neighborhood with the highest value of equitability index. Figure 10 Open in new tabDownload slide Procedure for the selection of the neighborhoods where testing active gamma monitoring. At right: the diagram equitability vs. gamma dose identifies the two neighborhoods with the lowest (9) and the highest (13) product of the two variables. A further criterion is the proximity of the two neighborhoods. At left: a uniform increase of the dose of 0.2 μSv h−1 for the two neighborhoods, due to a sudden radioactive contamination, makes the cut-off value, previously set to have a specificity equal to 1 (FAR = 0), less effective (lowering of sensitivity) for the neighborhood with the highest value of equitability index. The two proposed actions could reduce the outdoor gamma dose (0.55 mSv a−1) by around 35%, a reduction that, in absolute terms, corresponds to 0.2 mSv a−1. On the basis of indoor gamma measurements at Rome, the average annual effective dose equivalent from indoor gamma radiation(61) should approach 2.0 mSv a−1. Therefore, in the First District, the average total annual effective dose equivalent from gamma radiation should be equal to 2.5 mSv a−1. The average indoor radon dose in Rome calculable from the average indoor radon concentration in Latium(62) (100 Bq m−3), although not considered in this work, adds further 2.5 mSv a−1 to the total natural dose. Considering finally the cosmic contribute, the total average dose from natural radiation could reach 5.2 mSv a−1. A reduction of only 0.2 mSv a−1 (less than 4%) could, therefore, appear irrelevant, and the benefit of the proposed mitigation should be considered negligible. Also by considering the synergistic effect of particulate matter(12) and 212Pb, the effect of mitigation does not change substantially. In fact, the particulate matter at Rome, due to the high local thoron flux, is particularly enriched in 212Pb (about 42 000 Bq g−1 from data on fine particulate(12)) and its contribution to the outdoor dose of population can be estimated around 0.2 mSv a−1 (0.75 total outdoor dose). Since the proposed mitigation actions should have a similar effect on 212Pb concentration in particulate matter, then reduction actually does not overpass 0.27 mSv a−1 (about 5% of the total dose from natural radiation). However, considering the problem from the point of view of individuals, these mitigation actions could be relevant. There are in fact many categories of workers most exposed to outdoor range radiation, such as ecological operators, city police, policemen, drivers of public and private vehicles, tour operators, street hawkers, food service operators, public service maintenance workers and even several thousand homeless. For these categories, the outdoor occupancy factor could reach 0.4, and therefore, the outdoor gamma dose amounts to 1.5 mSv a−1, the indoor gamma dose to 1.5 mSv a−1, the radon indoor dose to 1.87 mSv a−1, the outdoor 212Pb dose to 0.4 mSv a−1 and finally, also adding the cosmic radiation, a total of 5.45 mSv a−1 would be reached. In this case, the reduction of outdoor gamma radiation and of the outdoor 212Pb would amount to 0.66 mSv a−1 that is 12% of the total dose, a percentage that is starting to become significant. Prevention of risk from illegal trafficking of radioactive material In Italy, unfortunately, at the best of our knowledge, provisions are in force regarding the active radiation protection of the population only in the case of transport of fissile materials(63) and there are no active preventive protection plans regarding the illicit trafficking of radioactive material(64). Therefore, the following considerations remain at a very theoretical level. In prevention of illicit trafficking of radioactive material, a weakly gamma rays-emitting source must be detected in active or passive mode in the presence of local background. Obviously, when using a detector with spectrometric capability, the alarm is triggered by a modification of the spectrum normally expected from natural radionuclides and this technique avoids the problem of high background. However, in the simplest case, the measurement may be represented by gamma rays gross counts. The ambient background limits the detection of gamma-rays from the radioactive source, and some threshold(65) must be set as a decision criterion for the presence or absence of it. When quantifying the performance of a detection system, set at a particular threshold, observations are classified based on whether or not the measurements lie above or below the threshold. The use of receiver operating characteristics (ROC) graphs is commonly adopted to select a threshold or cut-off for alarms. ROC graphs show the detection probability, (DP), as a function of the false-alarm rate, (FAR). DP, also named sensitivity, is equal to $$\begin{equation} \mathrm{DP}=\mathrm{true}\ \mathrm{positive}\ \mathrm{signals}/\mathrm{total}\ \mathrm{positive}\ \mathrm{signals} \end{equation}$$ (6) whereas FAR is equal to $$\begin{eqnarray} \mathrm{FAR}&=&1-(\mathrm{true}\ \mathrm{negative}\ \mathrm{signals}/\mathrm{total}\nonumber\\ && \mathrm{negative}\ \mathrm{signals}). \end{eqnarray}$$ (7) The ratio in Equation 7 is also called specificity. An ideal monitoring system displays both sensitivity and specificity equal to 1. The searching of the optimum cut-off for gamma detectors cannot be addressed without the knowing of the value of local gamma background and the extent of its dispersion. Consequently, the present map, from which equivalent dose rate and equitability (a measure of dispersion) are determined for any neighborhood, facilitates the preliminary setting up and testing of these monitoring systems(66). In passive monitoring at fixed points, the FAR is easily minimized by setting the cut-off at a value higher than the superior limit of the class of equivalent dose rate. Then, the DP can be maximized by selecting as detection points especially the ones with the lowest local equivalent dose rate and, possibly, close to natural or anthropogenic barriers which can backscatter gamma rays from the radioactive source toward the detector. By far more efficient than passive monitoring, the active monitoring (pedestrian or by mobile cars) notably reduces the average distance from detector and source and consequently increases the DP, since detection is proportional to the inverse of the square of the distance. In active monitoring, the threshold is variable and monitor’s designers equalize the FAR at each measured background intensity by automatically adjusting the cut-off according to the local threshold. This causes the monitor’s DP to decrease at higher background intensity(67). Alternatively, the designer can make constant DP at any background intensity, but in this case, FAR increases at higher background intensity. In both cases, best (worst) performance is obtained in the lowest (highest) possible radiation intensity environment with the lowest (highest) possible amount of radiation intensity variation (i.e. dispersion or Equitability index). Among the neighborhoods of the historic center of Rome, Pigna and Trevi (Figure 10), for their characteristics of gamma radiation intensity and dispersion, appear those, respectively, where the best and the worst performance of active monitoring are expected and where, therefore, active monitoring systems of radioactive risk prevention can be tested with more proficiency also considering the logistic advantage of their proximity. CONCLUSIONS A detailed map of the present outdoor gamma radioactivity of the historic center of Rome in terms of ADERs and its comparison with the original (pre-anthropic) outdoor gamma radioactivity have highlighted the enhancement of the original radioactivity in the historic center of Rome due to anthropic action occurred during the historic urban development of the city. The mapping of the background gamma radiation when coupled with the analysis of dispersion expressed as Shannon equitability index is a promising tool for studying architectural evolution of different neighborhoods of Rome. The average of the present dose equivalent rate from outdoor gamma radiation is equal to 0.31 μSv h−1 corresponding to an AEDEout of 0.548 mSv a−1 and to an ELCRout of 2.56 × 10−3, which is a level of health risk presently comparable to the upper limit of the estimable risk deriving by the inhalation of fine and ultrafine particulate matter from the vehicle traffic. Mapping of outdoor gamma equivalent dose rates is also useful for planning appropriate and feasible mitigation actions of health risks from outdoor gamma radiation by highlighting where the doses could be eventually reducible either by the replacement of the road pavement than by a partial (as experimentally proved during the gamma backscattering survey) or, better yet, by a radical reconstruction of the road section. Finally, such a mapping can aid for assessing a reference base level and a selective criterion for the passive and active monitoring of illegal trafficking of radioactive material in this important area of the Italian state. Declaration of interests None. Funding This research was institutionally funded by the Institute of Environmental Geology and Geoengineering, IGAG- CNR Rome, within the project: ‘Geochemical prospecting and Geochemical mapping’. 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TI - PRE-ANTHROPIC AND PRESENT OUTDOOR GAMMA EQUIVALENT DOSE RATE OF THE HISTORIC CENTER OF ROME (ITALY)
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncz247
DA - 2019-12-31
UR - https://www.deepdyve.com/lp/oxford-university-press/pre-anthropic-and-present-outdoor-gamma-equivalent-dose-rate-of-the-QGrWIrCnzI
SP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -