Determination of Radon and Thoron Concentrations in Different Parts of Some Plants Used in Traditional Medicine Using Nuclear Track Detectors

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Figure 2. Thoron decay products

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2. Materials and Methods

2.1. Geological Setting and Samples

Soil and medicinal plant samples were collected from different fields throughout Beni-Mellal area (Middle of Morocco, Figure 3) during the spring and summer period of 2011. The Beni-Mellal area has a Mediterranean climate with continental influences and situated between 32°20’20.61” North latitude and 6°21’47.41” Western longitude with elevation of 514m. The mean annual precipitation is 520mm, and the mean annual temperature 12°C. The soil samples collected from each place were dried at 70°C for 48h until they reach a constant mass, the ground to pass through 2mm sieve and then were thoroughly mixed and pulverized with an agate ball mill. The soil of the experimental areas had 34.2% clay, 20% loam and 38% sand. Above-ground plant material was weighed, dried at 100°C until constant weight and milled before the radioactivity quantification.

Figure 3. Map showing the localization of the studied samples in the Beni-Mellal region (Morocco)

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2.2. Method of Study

According to a method developed by ^[15] when the CR-39 and LR-115 type II SSNTD are separately placed in close contact with a homogeneous studied sample (soils and the studied plants) in a hermetically sealed cylindrical plastic container for 1 month (Figure 4), one can evaluate radon (Ac (²²²Rn)) and thoron (Ac (²²⁰Rn)) alpha-activities inside the sample by using the following equations:

(1)

and

(2)

where D_G(LR) and D_G(CR) are the track density rates (tracks cm⁻² s⁻¹) registered on the LR-115 type II and CR-39, A_c(²²²Rn) and A_c(²²⁰Rn) are the radon (²²²Rn) and thoron (²²⁰Rn) α-activities (Bq.g^-1) inside the studied material sample, k_i is the branching ratio corresponding to the disintegration of the nuclei of uranium and thorium groups, R_iα is the range of α-particle of energy Eα_i in the material sample and it is calculated by means of the TRIM programme ^[16], d_s is the sample density, and are the critical angles of etching for the LR-115 type-II and CR-39 are respectively given by:

(3)

and

(4)

where , R_D are the ranges of α-particles in LR-115-II and CR-39 SSNTD in µm. ΔR_S=R_max(E_max)−R_min(E_min), R_max and R_min are the ranges of α-particles in the sample which corresponds to the energy windows E_max and E_min at the upper and lower ends, both of which are functions of the detector and critical angle.

Combining Equations. (1) and (2), we obtain the following relationship between radon-to-thoron ratios and track densities:

(5)

where: and

From where one obtains the Radon activity given by:

(6)

And therefore, we determine the thoron activity.

Figure 4. Arrangement of the solid state nuclear track detectors on a studied sample in a cylindrical plastic container

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2.3. Calculation of the Transfer Factor

Plants are the primary recipients of radioactive contamination to the food chain following atmospheric releases of radionuclide. Vegetation may be the subject to direct or indirect contamination ^[17]. Absorption of radionuclide from soils into plants is usually quantified in terms of the transfer (or concentration) factor, which is defined as the ratio of the radioactivity per unit dry weight of plant to the radioactivity per dry weight of soil in the rooting zone ^[18].Transfer factor (TF) is defined as the ratio of radionuclide concentrations in vegetation and soil.

The soil to plant transfer factors was determined according to the relation.

(7)

The dry weight was preferred because the amount of radioactivity per kilogram dry weight is much less variable than the amount per unit fresh weight. It reduces uncertainties ^[19].

2.4. Dose Calculation Method

Effective dose is a useful concept that enables the radiation doses from different radionuclides and from different types and sources of radioactivity to be added. It is based on the risks of radiation induced health effects and the use of the International Commission on Radiological Protection (ICRP) metabolic model that provides relevant conversion factors to calculate effective doses from the total activity concentrations of radionuclides measured in foods ^{[20, 21]}. Estimates of the radiation induced health effects associated with intake of radionuclides in the body are proportional to the total dose delivered by the radionuclides while resident in the various organs.

Based on reference ^[9], we determine the effective dose rate (noted Dα, in mSv.y^-1) due to radon and thoron inside the studied medicinal plants. Otherwise, the Dα gave the committed effective radiation (of radon and thoron) dose due to ingestion of terrestrial studied plant. It’s given by:

(8)

where DCF is the dose conversion factor (the factor to convert Bq.g^-1.MeV to mRad.y^-1).

3. Results and Discussion

3.1. Distribution of Radon and Thoron in the Studied Samples

Concentrations of radon and thoron have been determined inside various natural material samples (soils and the corresponding medicinal plants). Data obtained are shown in Table 1, Table 2 and Table 3. From the statistical error on track counting one can determine the error on track density rate and then evaluate the relative uncertainty of the radon and thoron determination that is of 8%.

From the Table 1 we found that the radon and thoron concentrations in soil samples varies from a minimum of 65.4 ± 5.7 and 26.8 ± 2.4Bq kg^-1 to a maximum of 96.1 ± 8.6 and 41.8 ± 3.8Bq kg^-1, respectively. We notice that alpha-activities due to radon and thoron are clearly different in the studied soil samples collected from various places. This difference can be explained by the transport conditions and of the uranium and thorium dissolution at the time of the sedimentation of these soils (because ²²²Rn and ²²⁰Rn are members of the ²³⁸U and ²³²Th decay series, respectively).

Radon and thoron activities have been determined inside parts of various medicinal plants (Table 2 and Table 3) widely used in Moroccan traditional medicine. These activities are different from one plant to another. It is to be noticed that roots of each studied plant show higher radon and thoron concentration than the other parts (stems, leaves fruits and seeds) of the plant materials: i.e. that the distribution of radon and thoron in different parts of the studied plants showed the decreasing trend as root>leaf>stem>fruit (or seed). In this case, one can write:

Table 1. Data obtained for the radon and thoron alpha-activities in soil samples corresponding to the different studied plants.

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From the Table 2, we found that the radon activity in the parts of studied medicinal plants varies from a minimum of 0.87 ± 0.0.06Bq.kg^-1 (in the seed of Coriandrum sativum L.) to a maximum of 6.20 ± 0.47Bq.kg^-1 (in the root of Marrubium vulgare L.). While we note that the thoron concentration in most of the studied plants is very low compared to that of radon. From the Table 3, we found that the thoron concentration in the parts of studied medicinal plants varies from a minimum of 30 ± 2.30mBq.kg^-1 (in the seed of Coriandrum sativum L.) to a maximum of 195 ± 16mBq.kg^-1 (in the root of Marrubium vulgare L.). These results show that radon and thoron can be transferred from soil into plants via the root absorption. The concept of the ²²²Rn (and ²²⁰Rn) soil-pathway is shown in Figure 5 with its three parts: the soil solids (minerals, particulate organic matter), the soil solution and the plant. The exchange between the soil solids and the soil solution is the chemical reaction. The uptake of these radionuclides from the soil solution is controlled by plant physiology. Thus, it is known that radionuclides occur naturally in soil incorporated metabolically into plants. In addition to root uptake, direct deposition may occur on foliar surfaces, and when this happens the radionuclides may be absorbed metabolically by the plants. Uptake of the radon and thoron by plants depends to a considerable degree on whether it remains within reach of the roots of plants and the extent to which it is chemically available. The rates at which the various radionuclides will migrate through different soils under varying conditions of pH and moisture are not well known.

Figure 5. A conceptual presentation of the soil-plant transfer of radionuclide

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Table 2. Activity of ²²²Rn (Bq kg^-1, dry weight) inside different parts of the studied medicinal plants

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Table 3. Activity of ²²⁰Rn (mBq kg^-1, dry weight) inside different parts of the studied medicinal plants

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3.2. Transfer Factor

The radon and thoron activities in the studied soils and inside parts of medicinal plants can be used for the determination of soil-to-plant transfer factor (transfer coefficients between soil and plants). In the literature this transfer factor (TF) is also known as the relative concentration factor ^[22], or plant-soil concentration ratio ^[23].

The TF values for ²²²Rn and ²²⁰Rn to different parts (roots, stems, leaves, fruits and seeds) of various plants used in traditional folk medicine are shown in Table 4 Table5. The calculated ²²²Rn TF values for all plant species were in the order of 10^-2 varying from 1.12x10^-2 to 6.63x10^-2. While, the calculated ²²⁰Rn TF values for all studied plants were in the order of 10^-3 varying from 0.87x10^-3 to 4.85x10^-3. Differences between ²²²Rn and ²²⁰Rn TF values for various plant species are due to different characteristics of the plants. ²²²Rn and ²²⁰Rn TF values from soil to roots of vegetables were found to be higher than stems, leaves and fruits (or seeds). The reason for the high variability of soil-to-plant transfer factors is obvious. This macroscopic parameter integrates a number of soil chemical, soil biological, hydrological, physical and plant physiological processes. The comparison with investigations reviewed given by IAEA demonstrated the wide variation of obtained transfer factors, depending on environmental conditions and selected plant species ^[19].

Table 4. TF values of radon (²²²Rn) to different parts of the studied medicinal plants

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Table 5. TF values of thoron (²²⁰Rn) to different parts of the studied medicinal plants

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Table 6. Effective dose rates (Din mSv.y-1) due to radon and thoron inside various parts of the studied plants

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