1. Introduction

Ionizing radiation is energy emitted from radioactive atoms in the form of particles (such as alpha, beta, and neutron) or electromagnetic waves (such as X-rays and gamma rays). Gamma rays and X-rays are chargeless and massless photons; thus, they are the most penetrating forms of ionizing radiation. Humans are exposed to ionizing radiation through different processes, such as diagnostic treatment of various cancers, nuclear power generation, and medical radiation exposure from X-rays. It is well-known that exposure to ionizing radiation leads to the ionization of several biologically important macromolecules, including membranes, proteins, lipids, and nucleic acids [1, 2].Ionizing radiation passing through living tissues results in the generation of reactive O species, which interact with DNA, causing cell damage, dysfunction, or death [3].In recent decades, various studies have been performed to protect humans from the hurtful effects of ionizing radiation and to develop a novel and active radio protector [4].In this regard, numerous compounds have been investigated for their capability as a radio-protective material, such as vitamins, amino acids, plants, glasses and herbs [4–9].Radio-protective materials should be highly effective, cheap, orally administered, and without toxicity implications. Natural products such as plants have all of these properties. It is well-known that medicinal plants are utilized in different traditional systems of medicine because they are considered non-toxic and are exceedingly accepted by people. The use of medicinal plants as radio-protective materials requires scientific evaluation and assessment. The precise values of photon interaction parameters, such as the mass attenuation coefficient and effective atomic number, are needed to clarify the interaction of photons with medicinal plants. The mass attenuation coefficient (µ/ρ) for a medicinal plant indicates how easily it can be penetrated by a photon. This coefficient is considered to be one of the most important parameters characterizing the interaction between ionizing radiation and a specific medium and can be used to calculate other parameters, such as the effective atomic number and electron density [10, 11]. There are different methods for calculating the mass attenuation coefficients for a specific photon energy and material, including a theoretical approach employing the WinXCOM program [12], an experimental method using transmission geometry [13–15], and simulation codes such as Monte Carlo N-Particle Transport Code System-extended (MCNPX), Geant4, and FLUKA[10, 16, 17].

In the present study, we used the transmission geometry to measure the mass attenuation coefficients of different parts (root, flower, stem, and leaf) of three medicinal aromatic plants (Teucrium chamaedrys L. subsp.sinuatum, Rheum ribes and Chrysophthalmum montanum). Additionally, we used the MCNPX code and the WinXCOM program to validate the obtained values. Using the obtained mass attenuation coefficients, we calculated related parameters, such as the effective atomic number and electron density.

2. Materials and methods

2.2. Experimental work

The spectrometer system comprised a Si(Li) semiconductor detector (SLP-04160P-OPT-0.3) having an energy resolution of 2.7% at 5.9 keV, a 12.5-mm² active area, and a 5-mm sensitive depth.A4K multichannel analyzer was used to measure the mass attenuation coefficients of the medicinal aromatic plants. An ²⁴¹Am photon source having activity of 370 kBq was used to irradiate the plant samples. This source provided the following useful photon energies: 13.92, 17.75, 20.78, 26.34, and 59.54 keV.

The radioactive source enclosed in a lead cylinder with a 3-mm orifice was collimated to produce a narrow beam. The experimental narrow-beam geometry setup is shown in Fig. 1.The distance between the source and detector was 15 cm. The detector was housed in a lead shield to minimize the detection of background radiation from the surroundings. Each plant sample was placed between the source and detector. Then, the photon intensities I₀ (incident) and I (after attenuation) were measured by recording corresponding counts for a fixed preset time. For each sample, gamma-ray intensities without (I_o) and after attenuation (I) were used to calculate the mass attenuation coefficients using the Lambert–Beer law[17].More extensive information regarding the experimental process is presented in our previous works [19–21]. A typical spectrum of 13.92 keV with and without attenuation by the S2-S, S2-L, and S2-R samples is shown in Fig. 2.

Fig. 1.Full-screen View

(Color online) Narrow beam transmission geometry

Fig. 2.Full-screen View

(Color online) A typical spectrum of 13.92 keV with and without attenuation by S2-S, S2-L and S2-R samples

2.3. MCNPX code

The Monte Carlo method is one of the best ways to resolve physical problems, medical difficulties, and unknown parameters when the experimental or clinical conditions are limited or difficult to attain. MCNPX version 2.6.0 (Los Alamos National Lab, USA) was utilized for the calculations of the mass attenuation coefficients of different parts of three medicinal aromatic plants. The MCNPX code is a general-purpose Monte Carlo radiation transport code for simulating the interaction of radiation with matter in a wide energy range [22]. MCNPX can ensure fully three-dimensional simulation and utilizes extended nuclear cross-section libraries and physics models for particle types. The validation of the MCNPX Monte Carlo code for the investigation of radiation mass attenuation coefficients and other shielding parameters for different types of materials—such as glasses, concrete, and living biological tissues—has been reported [23–33]. The computational modeling of radiation interaction problems, including the radiation attenuation and shielding properties,depends on the material definitions. The input structure of the MCNPX code has three major partsdefining the significant details of the simulation. A completed MCNPX input file describes the problem geometry and defines the materials with their chemical compositions, as well as the structure of the radiation source,including details such as the source geometry andradiation distribution. The simulation geometry is constructed by defining cells. Each cell in the simulation is bounded by one or more geometric surfaces. In the present investigation, the square prism geometry was employed for modeling the parts of medicinal aromatic plants. The edge lengths of this square prism geometry were defined as 5 cm, while the axial z-length was set differentlyfor each simulation because it represents the thickness of the parts of the medicinal aromatic plants. The mass attenuation coefficients of each part of the medicinal aromatic plant sample were measured in a narrow beam transmission geometry using a point isotropic source with a collimated and monoenergetic beam. The radiation energy value of the point isotropic source was defined in the photon energy range of 13.92–59.54 keV for each calculation. The geometric center of the detection area on the central axis was considered as the location of the point isotropic source that emits gamma rays perpendicular to the front face of the medicinal aromatic plant sample in the direction of the z axis. Another important definition is the material definition, considering the atomic number, mass number, elemental mass fraction, and density of compounds or mixtures. The elemental mass fractions and densities of each medicinal aromatic plant sample are presented in Table 2. In the present MCNPX simulation, to obtain the absorbed dose amount in the detection field, the average flux tally (F4) was utilized. This type of tally mash gives the sum of the average flux in the cell. The initial quantity of gamma rays is set as 10⁸ particles. The mass attenuation coefficient calculations were performed using Intel® Core™ i7 CPU 2.80 GHz computer hardware. The error rate in the output file was observed to be <0.1%. Analysis of a recent investigation was performed using the D00205ALLCP03 MCNPXDATA package, whichcomprisesDLC-200/MCNPDATA cross-section libraries. This library typically includes ENDF/B-VI data in the range of 20 to 150 MeV. Fig. 3 shows the total simulation geometry for the present investigation.A screenshot showing the design of the MCNPX simulation setup is displayed in Fig.4.

Fig. 3.Full-screen View

(Color online) Total simulation geometry of the present investigated samples

Fig. 4.Full-screen View

(Color online)The screenshot of MCNPX simulation setup

3. Results and discussion

Different photon attenuation coefficients—such as the mass attenuation coefficients (µ/ρ), effective atomic number (Z_eff), and electron density (N_e)—of different parts (root, flower, stem, and leaf) of three medicinal aromatic plants (Teucrium chamaedrys L.subsp. sinuatum, Rheum ribes, and Chrysophthalmum montanum) were investigated. The µ/ρ values of the present samples were obtained in three ways: (a) experimentally, using a transmission arrangement; (b) theoretically, using the WinXCOM program; and (c) using a Monte Carlo simulation (MCNPX code). The experimental and WinXCOM results for the µ/ρ of the present samples at 13.92, 17.75, 20.78, 26.34, and 59.54 keV are illustrated in Fig.5, while the experimental and MCNPX results for the µ/ρ of all the samples are shown in Fig.6. The experimental, WinXCOM, and MCNPX results for the µ/ρ are summarized in Table 3.

Fig. 5.Full-screen View

(Color online) Comparison of mass attenuation coefficients calculated by WinXCOM with respect to experiment for the present medicinal aromatic plants

Fig. 6.Full-screen View

(Color online) Comparison of mass attenuation coefficients calculated by MCNPX code with respect to experiment for the present medicinal aromatic plants

The tendency of µ/ρ is almost the same for all samples owing to the similarity of the elemental compositions among the samples (all the samples contain H, C, N, and O in different proportions).

Figs.5 and 6 clearly show that the experimentally measured µ/ρ values of the medicinal aromatic plants agree well with the WinXCOM and MCNPX results. The present experimental WinXCOM and MCNPX results are similar to the observations of Singh et al. [25],who studied the µ/ρ of polymers in the energy range of 59.5–1,332.5 MeV using Monte Carlo simulation (MCNP-4C) and reported good agreement among theirMCNP-4C,XCOM,and experimental results. The µ/ρ values for all samples decrease with the increase of the photon energy. This means that with the increase of the energy of the incident photons, we obtain smaller attenuation; thus, more photons can penetrate the present samples.

The differences among the µ/ρ values due to the medicinal aromatic plant compositions are obvious, particularly in the low-energy region. As shown in Table 3, S1-S (stem of Teucrium chamaedrys L. subsp. sinuatum) has higher values of µ/ρ than the other samples because of the larger amount of O in this sample (73.925%).Additionally, Table 3 shows that the S1-R sample has the lowest µ/ρ values, as this sample contains mainly C (92.989%), while the percent of O in this sample is low (1.182%).It is important to investigate the experimental uncertainty in the measurement of µ/ρ.The following equation was used for this:

where x and ρ represent the thickness and density of the sample, respectively, and ΔI₀ and ΔI are the uncertainties of I₀ and I, respectively.

Eq. (2) was used to calculate the experimental uncertainty in the measurement of µ/ρ, and the results are summarized in Table 3. Clearly, the uncertainties in the experimental µ/ρ measurements are within the range of 1.09% to 10.73% (except for S1-R and S3-F at 26.34 keV). The uncertainties in the measurements are mainly due to statistical uncertainties in the determination of the intensity without (I₀) and with (I) the absorber, as well as to the scattered photons reaching the detector.

By usingthemeasured and calculateddatafor µ/ρ, the effective atomic numbers (Z_eff) were calculatedas follows [34]:

where f_i represents the fractional abundance of the element i relative to the number of atoms, and Z_i and A_i are the atomic number and atomic weight, respectively. The results for the Z_eff of the selected samples are plotted with respect to the photon energy in Fig. 7.In addition, the experimental Z_eff values are presented in Table 4, together with theoretical values. The Z_eff results indicate good agreement between the experiment and theory. As shown in Table 4, the values of Z_eff for all the medicinal aromatic plants change with respect to the photon energy. In the case of Teucrium chamaedrys L. subsp. sinuatum, Z_eff ranges from 5.692 to 4.047, 6.725 to 4.613, and 7.744 to 5.348 for the root, flower, and stem, respectively. In the case of Rheum ribes, Z_eff ranges from 6.419 to 3.954, 6.226 to 4.499, and 5.795 to 3.107 for the stem, root, and leaf, respectively. In the case of Chrysophthalmum montanum, Z_eff ranges from 6.790 to 4.547, 6.720 to 4.338, and 6.247 to 4.512 for the stem, leaf, and flower, respectively. Notably, the atomic numbers (Z) of the elements of the present medicinal aromatic plants change from 1 (H) to 8 (O), and a total of four elements (H, C, N, and O) are considered. The values of Z_eff for the present plants lie within the range of Z for the constituent elements (1<Z_eff<8). As indicated by the µ/ρ results, S1-S (stem of Teucrium chamaedrys L. subsp. sinuatum) has higher values of Z_eff.

Fig. 7.Full-screen View

(Color online) The experimentally measured effective atomic numbers for the present medicinal aromatic plants in comparison with those obtained by WinXCOM

Z_eff is related to another parameter called the effective electron density (N_E), which represents the number of electrons per unit mass of the interacting materials and can be calculated using the following relation [35]:

where ˂A˃ is the mean atomic mass, and N_A is the Avogadro constant. The determined N_E for the present samples are plotted in Fig.8 and listed in Table 5. As shown in Fig.8, the experimental and theoretical (WinXCOM) values agree well. It is also clear from Fig.8that the N_E for all samples decreases with increasing energy owing to the dominance of the photoelectric effect in this energy region.

Fig. 8.Full-screen View

(Color online) The experimentally measured effective electron densities for the present medicinal aromatic plants in comparison with those obtained by WinXCOM

4. Conclusion

This work aimed to measure the mass attenuation coefficient (µ/ρ) for different parts (root, flower, stem, and leaf) of three medicinal aromatic plants using an²⁴¹Am photon source in a stable geometry. The µ/ρ for all samples under investigation wasalso calculated using the MCNPX code and the WinXCOM program. The µ/ρ values obtained using the three aforementioned methods were similar. Furthermore, the effective atomic numbers (Z_eff) and the electron densities (N_E) for the medicinal aromatic plants were calculated. The calculations indicate thatS1-S (stem of Teucrium chamaedrys L. subsp. sinuatum) has the highestvalues of µ/ρ and Z_eff. The numerical methods and simulation tools employed in this study can be very useful for similar future studies, particularly for cases where it is not possible to perform experimental studies. Monte Carlo simulations are a viable alternative method when experimental investigations are not possible. The recent Monte Carlo method can encourage the scientific community to undertake similar studies and to better evaluate the biological structures and their radiation interaction properties.