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

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INTRODUCTION
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.

GEOLOGICAL ASPECTS
The studied area is the South zone from Natal city, Rio Grande do Norte, Brazil (05 o 47' 42" S, 35 o 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.

MATERIALS AND METHODS
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: AU, ATh and AK 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.
 Radium Equivalent Activity (RaEq) [13,14,15]:  External Fatality Cancer Hazard (FCHExt) [10]: Where: AEDExt 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: 1 ppm of Th = 4.06 Bq/kg of Th-232 (8) 1 % of K = 313 Bq/kg of K-40 (9) 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 nothreshold), 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). The statistical summary of the gamma radiation variation is given in Table 1  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.4). 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].

RESULTS AND DISCUSSION
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][16][17][18][19][20].

Figure 4: Gamma radiation spectrometry of the street corner intersections from the South zone of Natal city, Rio Grande do Norte, Brazil: A) The Absorbed Dose interpolative distribution (Natural Neighbors Method) (ƞGy/h); B) Absorbed Dose Histogram (ƞGy/h).
Geological legend as in Figure 2.

CONCLUSION
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: 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.