Indoor Radon and Thoron Measurement using CR-39 and LR-115 detectors in dwellings of Beni Mellal-Khenifra area (Morocco)

: Radon and thoron are radioactive gases that are found naturally in the environment. They can seep into homes through cracks in the foundation, walls, and floors. Once inside, they can build up to levels that are harmful to human health. A study of radon and thoron concentrations in 65 homes in the Beni Mellal province of Morocco found that the levels varied widely, from 16 to 304 Bq. m -3 for radon and from 1 to 25 Bq. m -3 for thoron. The concentrations were found to be influenced by a number of factors, including the type of home, ventilation conditions, and the geological materials in the area. The average estimated dose received by the public due to radon and thoron in the homes studied was 45 ± 0.28 mSv.y -1 and 0.20 ± 0.01 mSv.y -1 , respectively. These doses are considered to be low-level radiation exposure, but they can increase the risk of cancer over time. These results suggest that it would be beneficial to prohibit the construction of houses near industrial areas and to use construction materials that are adapted to the criteria of dose limitation.


Introduction
Humans are constantly exposed radiation from radon, thoron, and their decay products.These radioactive elements are released into the air from construction materials, thermal water, and air pollution [1][2].Radon and its short-lived decay products are the main source of radiation exposure for people in their homes and workplaces.They account for almost half of the total radiation exposure that people receive from natural sources (UNSCEAR 2000) [3].Radon is a gas that occurs naturally in the ground and can cause lung cancer.The risk of lung cancer from radon exposure is due to the alpha-dose deposited by short-lived radon progeny.These particles are released when radon decays, and they can attach to dust and smoke particles in the air.When people breathe these particles in, they can damage lung cells.The concentration of radon progeny in the air is affected by a number of factors, including the type of soil, the temperature, the humidity, the wind speed, and the rainfall [4][5].
Global measurements of the quantities of radon and thoron in indoor air in houses [6][7][8][9].Numerous measurements of the quantities of radon and thoron activity in various nations, as well as epidemiological studies on the risk of lung cancer, have been reported in recent years [10,15].
The Beni Mellal-Khenifra region (Morocco) as one of the most air-polluted provinces in Morocco due to the industrial activities.In addition, the building materials used in this area are made from industrial waste or traditional dry stone, which contains significant amounts of uranium [16][17].Older houses in the Beni Mellal-Khenifra region are often built on unsealed floors and with traditional dry stone building materials that contain uranium.This can lead to high levels of radon and thoron in the indoor air.
Radon and thoron are radioactive gases that can cause cancer if people are exposed to them for a long time.It is important to measure the levels of radon and thoron in the indoor air of these houses in order to limit the risk of people getting cancer from breathing in the air.
The purpose of this study is to investigate the concentration of radon and thoron in the air of various rural areas of the Beni Mellal-Khenifra region of Morocco.This will enable us to calculate the annual effective radiation dose received by individuals exposed to 222 Rn and 220 Rn levels in different housing types.

Study area
The Beni Mellal-Khenifra region, with an elevation of 468 m, is located in central Morocco (Figure 1) between 32° 22° North and 6° 24° West longitudes.This area was chosen for the study.The five provinces that make up this nation are Khenifra, Khouribga, Fkih Ben Salah, Azilal, and Beni Mellal (Figure 1).Its population of 2,520,776 people, or 7.4% of all Moroccans, live in a 33208 km 2 area.The two primary economic pillars of the people are agriculture and cattle.The area experiences a semiarid to arid climate, characterized by a dry season extending from April to October and a rainy season spanning from November to March.Throughout the year, the region receives an average annual rainfall of 350 mm, but faces significant evaporation of about 1800 mm.The temperature fluctuates from 3.5°C during the winter to 38°C in the summer, with an average temperature of 17°C.

Materials and Methods
The houses in the Beni Mellal-Khenifra area are predominantly constructed using cement, hollow bricks, and plastered bricks, with some older houses made of clay.To assess the concentrations of radon and thoron in these homes, CR-39 and LR-115 type II solid-state nuclear track detectors (SSNTDs) were used.These detectors film, measuring 2 cm x 2 cm, were affixed to glass slides and placed at a height of approximately 1.5 meters from the ground level on the walls of various dwellings.The sensitive surfaces of the detectors were positioned to face the surrounding air to accurately capture radiation levels.After a three-month exposure period, the detector films were removed and subjected to an etching procedure.For LR-115 type II detectors, a 2.5 N NaOH solution at 60 °C was used for 120 minutes, while CR-39 detectors were etched at 70 °C for 7 hours using a 6.25 N NaOH solution.
The SSNTDs were meticulously cleaned when the etching was finished, dried, and examined under a binocular microscope to determine the track density [16].Under our experimental conditions, and assuming that radon, thoron, and their corresponding daughters are in secular equilibrium, the total track density rates (tracks cm -2 d -1 ) due to the αparticles of radon and thoron daughters, registered on the CR-39 and LR-115 are, respectively, given by [16]: The track density rates on the LR-115 type II and CR-39 detectors are denoted as DG LR and DG CR , respectively.The alpha activities of radon and thoron inside the dwelling air are represented by Ac( 222 Rn) and Ac( 220 Rn) in Bq/m 3 .The branching ratio in % is denoted by ki.The range of alpha particles of energy Eαi inside the soil samples with index i is Rαi, which is calculated using the TRIM program.The critical angles of etching for CR-39 and LR-115 type II are represented by θc and θc′, respectively.The difference between the maximum and minimum ranges of alpha particles in the soil samples is represented by ΔRs.For LR-115 type II, Rmax and Rmin denote the maximum and minimum ranges of alpha particles in the soil samples, which correspond to the lower and upper ends of the energy window, respectively.This energy window depends on the residual thickness of the LR-115 SSNTD [16].
Combining Eqs. ( 1) and ( 2), we obtain the following relationship between track density rates to radon and thoron ratios: Measuring D G CR and D G LR track density rates and knowing θ c ,θ c ′ , it is possible to evaluate the Ac ( 220 Rn)/Ac ( 222 Rn) and consequently the indoor radon and thoron activity inside the studied dwelling.
The following formula was used to measure the radiation doses caused by inhaling radon and thoron, respectively (3): =   (

222
) × (0.17 and their corresponding progeny.For the calculation of inhalation dose due to 222 Rn and its progeny, an equilibrium factor of 0.4 was utilized, while an average equilibrium factor of 0.07 was employed for 220 Rn (3).As a general guideline, an occupancy factor (OF) of 0.8 is commonly used, as recommended by UNSCEAR [3].

Results and discussion
Between April and June 2023, the concentrations of 222 Rn and 220 Rn, along with the effective dose rate, were assessed in different dwellings across the Beni Mellal-Khenifra area.
The findings of this study, presented in Tables 1, 2, 3, 4, and 5, revealed that the levels of radon and radon effective dose rates were notably higher in the cities of Khouribga and Fkih Ben Salah.In the city of Khouribga, the concentrations of 222 Rn in various dwellings displayed a range of 180 to 304 Bq.m -3 , with a calculated geometric mean of 254.60 Bq.m -3 .Additionally, the concentrations of 220 Rn in the same homes varied from 13.14 to 25.84 Bq.m -3 , with a geometric mean of 20.68 Bq.m -3 .Conversely, in Azilal city, the radon concentrations in homes ranged from 16 to 102 Bq.m -3 , with a calculated geometric mean of 50.31Bq.m -3 .For thoron concentrations in the same dwellings, values fluctuated between 1.23 and 8.57 Bq.m -3 , with a geometric mean of 4.21 Bq.m -3 .
Inadequate ventilation in dwellings can be a contributing factor to elevated levels of radon gas.When indoor spaces lack proper airflow, radon gas may accumulate, resulting in increased exposure levels.In the study area, the dwellings were frequently closed during the daytime due to the residents' occupation as farmers or agricultural laborers, as they spent significant portions of their day working in the fields.This practice further limited ventilation in the homes, potentially leading to higher concentrations of radon gas indoors.On the other hand, inadequate ventilation prevents the buildup of thoron gas ( 222 Rn) inside the homes.
In contrast, poor ventilation inhibits the accumulation of thoron gas ( 220 Rn) inside the dwellings.Thoron gas also comes from the ground, but it has a shorter half-life than radon gas.This means that it decays more quickly and has less time to build up in the air [17].All the houses in the study had radon levels that were in the range of the intervention level, which is 200-600 Bq.m -3 .This is the level at which the International Commission on Radiological Protection (ICRP) recommends that action be taken to reduce radon exposure [18].In most of the dwellings, the annual effective dose received by the residents was in the range of the action level, which is 3 -10 mSv/y.This is the level at which the ICRP recommends that people take steps to reduce their radon exposure [18].
The average relative uncertainty of radon activities and radon effective dose rates determination is about 8.0%.A map of the concentrations of radon and thoron in indoor air was created using the data gathered (Figure 3).High radon levels can be found in parts of the phosphate-rich Khouribga region.The Azilal and Khenifra regions have low levels of radon and thoron in indoor air as a result of a clay layer on the surface that, when wet, functions as an effective radon transport barrier.These lowest levels in the Azilal region aren't just attributed to the local geology, but also to weather patterns there, particularly the numerous temperature inversions that take place in winter.
Figure 4 shows the seasonal fluctuations in 222 Rn and 220 Rn concentrations in the investigated homes.The concentrations of 222 Rn and 220 Rn are reported to be higher in the winter.In order to keep heat in their homes throughout that season, residents used to cover doors and windows, which led to the buildup of 222 Rn and 220 Rn gases.The lower 222 Rn concentrations seen during the rainy season, on the other hand, can be explained to the fact that the water-filled soil pores prevent radon from escaping into the air. in mud-walled homes.These naturally occurring radioisotopes have the potential to produce radioactive particles, which can lead to greater indoor radon and thoron concentrations in homes made of this particular material.This emphasizes the need of taking the qualities of the building material into account when designing buildings to reduce exposure to elevated quantities of these radioactive gases [19].Radon and thoron concentrations in indoor air vary widely across different countries (Table 6).
The highest levels of indoor radon concentration have been reported in Eastern Sicily, South-east Italy, and the province of Naples in Italy.

Conclusion
The LR-115 type II and CR-39 SSNTD detectors were carefully deployed to monitor radon and thoron levels in households across the Beni Mellal-Khenifra region of Morocco.This crucial initiative was undertaken to assess the potential radiological impacts on the population and to implement robust measures to protect public health from the harmful effects of prolonged exposure to natural radiation.Research has shown that people living in phosphatic regions are exposed to high levels of radiation, especially inside their homes.Intriguing comparative analysis has also shown that mud-brick structures have markedly higher levels of radon and thoron gases than their hollow-brick counterparts, which increases the amount of radiation exposure in these homes overall.Notably, the cumulative annual effective doses recorded within the confines of this study area's dwellings have markedly surpassed the global average of 2.4 mSv/y, emphasizing the severity of the situation.
These values were discovered to be within the reference range of 3 -10 mSv/y, necessitating an immediate and coordinated effort to implement stringent regulatory measures and proactive interventions to mitigate the potential health risks posed by elevated radiation levels in the local population.

Figure 1 :Figure 2 :
Figure 1: Study area's local mapThe region of Beni Mellal-Khenifra and the vicinity is mainly covered by phosphates, limestones alluvium, and Brown soil (Figure2).The phosphate layer is sedimentary, with alternating layers of phosphate and marl.

Figure 5 illustrates how the kind of wall construction affects the fluctuations in activity concentrations of Rn 222 and Rn 220 .
The presence of radioisotopes in the mud used to make tile may be the cause of the increased levels of Rn 222 and Rn 220

Figure4:
Figure4: Radon and thoron concentrations exhibit changes according to the different seasons.

Table 1 :
the findings present the indoor levels of radon and rate of radon's effective dose recorded in various residences within Khouribga city during the exposure period from April to June 2023.
Table2: the findings present the indoor levels of radon and rate of radon's effective dose recorded in various residences within Beni-Mellal city during the exposure period from April to June 2023).

cm -2 d -1 ) Activity concentrations (Bq.m -3 ) Inhalation dose (mSv.y -1 )
Table3: the findings present the indoor levels of radon and rate of radon's effective dose recorded in various residences within Fkih Ben Salah city during the exposure period from April to June 2023.

Table 4 :
the findings present the indoor levels of radon and rate of radon's effective dose recorded in various residences within Azilal city during the exposure period from April to June 2023.

Table 5 :
the findings present the indoor levels of radon and rate of radon's effective dose recorded in various residences within Khenifra city during the exposure period from April to June 2023.

Table 6 :
The lowest levels have been reported in Alexandria city in Egypt and Shiraz city in Iran.The indoor radon concentration in dwellings in Bathinda district of Punjab state in India and Malwa region is similar to the levels reported in the present study.Radon concentration in the indoor air of different countries.

Table 6 :
Radon concentration in the indoor air of different countries.