Gamma radiation of the street corners from South zone of Natal city, Rio Grande do Norte, Brazil

All living organisms on the planet are exposed to the Earth's natural radiation. The exposure to natural ionizing radiation is a constant and inexorable life feature. The background radiation comes from soils, air, food, outer space and even our own bodies. The ionizing radiation present in the environment is mainly due to the primordial natural radionuclides activity, namely U-238, Th-232, K-40, associated to natural occurrence radioactive materials (NORM) in the Earth's crust [1, 2, 3]. The external exposures to NORM arise from terrestrial radioisotopes present at trace levels in soil, sediments and building materials. Consequently, geological formation, the soil type and bulk materials of buildings (bricks, concrete, cement, aggregates) in a given area control the natural radioactivity and the external exposure associated with gamma radiation. These factors strongly influence the dose distribution of natural terrestrial radiation [2, 3].

The natural radiation is the biggest contributor to the dose absorbed by the world population, therefore, it is vitally important to evaluate the dose of gamma radiation from natural sources. The U-238, Th-232 and K-40 concentrations varies widely and depends on their location [2, 3]. Most of the external gamma dose rate (95%) above typical soils comes from primordial radionuclides embedded in the soil [4, 5]. The objective of this research was to investigate the natural radioactivity of street corner intersections from the South zone of Natal city, Rio Grande do Norte, Brazil, through the analysis of the absorbed dose rate in the air, obtained by in situ gamma ray spectrometry, in order to determine the activity equivalent concentration of U-238, Th-232 and K-40 and the fatality cancer hazard index.

The studied area is the South zone from Natal city, Rio Grande do Norte, Brazil (05o 47' 42" S, 35o 12' 34" W) and has 896,708 inhabitants (2021 census; Fig. 1). Geologically, Natal city grew up over dune terrain, which is composed of unconsolidated sedimentary rocks and more rarely consolidated, they are composed of very friable material and easily transported by wind action. These unities are: Vegetated coastal wind deposits (N4elv), Un-vegetated coastal wind deposits (N4eln) and Beach littoral deposits (N4lp) (Fig. 2).

The measurement stations were randomly chosen, located at the intersection of street corners from the South zone of Natal city.

For in situ surface gamma radiation measurements were used three portable gamma radiation spectrometers model RS-230® with BGO crystal (Radiation Solution Inc., Canada). These instruments are factory calibrated, with a 5-year stability guarantee and does not require a radioactive source for field calibration. The measurements were performed by a researcher-collector who suspended the RS-230 spectrometer 1 meter above the ground and far from any masonry construction [9].

At each station, in situ gamma spectrometry was performed with the protocol of 5 measurements with 3 minutes duration and with an interval of 10 minute between them [9]. The data average of each station was calculated and taken as the absorbed dose rate of the sampled station [9]. For civil security reasons, measurements were only carried out in the southern area of Natal city. This research took place during the year 2019.

For the analysis of natural and artificial radioactivity, several health risk indexes have been used by research organizations [3, 9, 10, 11, 12, 13, 14, 15], in order to arrive at a better and safer conclusion about the health status of an irradiated environment or irradiated person, whose maximum limits recommend that the gamma radiation of natural radionuclides in a given area generates an Annual Equivalent Effective Dose below of 1 mSv/y, namely:

Where: A_U, A_Th and A_K are the activity concentrations of U-238, Th-232 and K-40, and 0.462, 0.604 and 0.0417 are the conversion factors dose for radionuclides, respectively.

Where: AD_Ext the external absorbed dose (ƞGy/h); OFext the external occupancy factor (20%); T is the annual duration time in hours (24 h x 365 day) and f is the conversion coefficient (0.7 for adults adopted by [3]. Maximum value allowed: 1 mSv/y.

Where: C_U, C_Th, C_K are the Activity concentrations (Bq/kg) of U-238, Th-232 and K-40. Maximum value allowed: 1 Bq/kg.

Where: C_U, C_Th, C_K are the Activity concentrations (Bq/kg) of U-238, Th-232 and K-40. This formula is based on the estimation that 1 Bq/kg of U, 0.7 Bq/kg of Th and 13 Bq/kg of K produce the same dose of Gamma radiation. Maximum value allowed: 370 Bq/kg.

Where: C_U, C_Th, C_K are the activity concentrations (Bq/kg) of U-238, Th-232 and K-40. Maximum value allowed: 1 Bq/kg.

Where: AED_Ext is the External Annual Effective Dose, LT is the estimated life time of 70 years, FH is the Fatality Hazard Factor for Cancer by Sievert = 0.06 for the public recommended by [3].

According to [9] for the purpose of some calculations foreseen in gamma radiation dosimetry, the following equivalences between the activity concentrations of U (ppm), Th (ppm) and K (%) and the respective value in Bq/kg should be used:

Attention is drawn to the fact that the international community is adopting as a policy of protection against radiation the motto that "there is no minimum threshold of dose of radioactive exposure that is safe". This motto, designated by the English acronym of LNT (Linear no-threshold), carries with it the assumption that an unsafe dose of radiation is always received. Consequently, we should always consider radiation exposure values as low as possible as a policy to protect against ionizing radiation. This policy is called ALARA (As Low As Reasonably Achievable).

Data collection totaled 630 points, were measured in the street corner intersections from the South zone of Natal city, Rio Grande do Norte, Brazil (Figure 3). The natural gamma radiation level showed great variation, due to the street paving type, where: the busy streets have asphalt capping on cobblestone, with little traffic have granite cobblestone capping and quiet streets have no capping and outcrop sands of old dunes.

The statistical summary of the gamma radiation variation is given in Table 1 and its distribution interpolated by the "Natural Neighbors" method and respective histogram are given in Figures 4, 5, 6, 7, 8, 9, 10, 11 and 12.

The data analysis allows suggest that about 90% of the Absorbed Dose in the Air from the external gamma radiation of the studied street corners denote an average radiometric potential lower than the world average value 59 ƞGy/h [3], but with a peak of 150 ƞGy/h (Fig. 4Fig.). The areas with the highest levels of U and K correspond to the areas with asphalt/cobblestone capping, while the areas with the highest levels of Th correspond to the sand streets and dune's pathway of the protected areas (Fig. 5, 6 and 7). However, in the Natal Navy Base area we have positive peaks for K and very lower values for Th (Fig. 3, 6 and 7). While in the area of the Dunes stadium we have positive peaks for U and very lower values for K (Fig. 3, 5 and 7). We associate these two facts with the different NORM compositions of the bulk materials (bricks, concrete, cement, aggregates) used in buildings [14]

The mean Annual Effective Dose (Fig. 8) is lower than the global mean value of 0.6 mSv/year [3]. While 91.43% of the calculated fatality cancer risk (Fig. 12) is below the global average of 0.29 [3].

The fatality cancer risk over a 70-year lifetime was calculated. However, it was not possible to assess the health problems in the population, because of the lack of reliable and standardized statistics on morbidity and mortality in the Natal population. The present study was limited to inferring the levels of natural background gamma radiation.

In turn, the Radium Equivalent Activity and External Hazard index turned out to be below the accepted global average limits, respectively of 370 Bq/kg and 1 Bq/kg (Figs. 10 and 11). While 98.68% of the Gamma Activity Index is below the global average allowed limit of 1 Bq/kg. The measured external hazard index values were lower than the external hazard index of the other municipalities in Rio Grande do Norte, Brazil [15-20].

The in situ spectrometric measurements of gamma radiation were performed on the street corner intersections from South zone of Natal city (Brazil) and our data show that:

• The External Absorbed Dose ranged from 11 to 150 ƞGy/h (MG: 40; Median: 39; SD: 18);

• Uranium ranged from 0.1 to 6.2 Eq.A. ppm (MG: 1.6; Median: 1.4; SD: 1);

• Thorium ranged from 0.7 to 32 Eq.A. ppm (MG: 6; Median: 5; SD: 3.3);

• Potassium ranged from 0.1 to 3.9 Eq.A. % (MG: 1.2; Median: 1.2; SD: 0.7);

• Annual Effective Dose ranged from 0.01 to 0.18 mSv/y (GM: 0.05; Median: 0.05; SD: 0.02);

• Gamma Activity Index ranged from 20.53 to 310.70 Bq/kg (GM: 0.31: Median: 0.30; SD: 0.14);

• External Hazard Index ranged from 0.01 to 0.13 Bq/kg (GM: 0.03; Median: 0.03; SD: 0.01;

• Fatality Cancer Hazard Index ranged from 0.06 to 0.77  (GM: 0.21; Median: 0.20: SD: 0.09)

The 99.84 % of values of the Absorbed Dose and the 100% of the Annual Effective Dose of external radiation measured on the street corner intersections from South zone of Natal city show values lower than the world average of 143 ƞGy/h and 0.48 mSv/y, respectively, indicated by the UNSCEAR [3]. While the Radio Equivalent Activity and External Risk indexes showed values below the global average limits, respectively of 370 Bq/kg and 1 Bq/kg and Gamma Activity Index with 98.68% of values below the global average limit allowed of 1 Bq/kg [3].

Areas with higher U and K contents correspond to streets with asphalt/cobblestone capping, while areas with higher Th contents correspond to sandy streets and dune's pathways in protected areas.

Ours data are comparable to the recommended values and they fall within the safety limits. This fact denote a lower radiometric risk to the population and the harmful effects of radiation are not caused to the public and tourists who roam the streets of the Natal city.

The health risk indexes from street corner intersections were lower than the other municipalities in Rio Grande do Norte, Brazil [17-22].

This research was supported by the National Council for Technological Research and Development of Brazil (CNPq) and received laboratory support from the Nuclear Engineering Institute (IEN), the Poços de Caldas Laboratory of the National Nuclear Energy Committee (LAPOC/CNEN) and the Laboratory of Natural Radioactivity of the Geosciences Institute of the Coimbra University, Portugal.