Calculation and determination of radioactivity in the old district of Najaf by using the track detector CR-39 and geographical information systems (GIS) methods

1.   Introduction

Humans are exposed to radiation permanently from two main sources: natural and made-man (artificial) sources, and nuclear radiation, which are released by charged particles, such as alpha particles and natural uranium ^[1,2,3], This type of radiations is what this study will focus on, as well as uncharged particles and electromagnetic radiation that work to ionize and excite atoms and molecules. Living organisms, cells, tissues, and organs are affected by radiation and differ in their sensitivity and how they are affected by different types of nuclear radiation ^[1,3,4].

Since this information is gathered through aerial photography or satellite imaging, remote sensing and geographic information systems are among the most crucial techniques utilized to gather data on this topic ^[5,6]. Due to its highly analytical data, remote sensing technology is linked with geographic information systems. Applications utilizing these systems include earth observation, monitoring changes to land and water cover, managing land resources, mapping, and other applications are just a few of the many domains where geographic information systems (GIS) and remote sensing (RS) technologies ^[7].

Below, some of the previous studies that that studied radioactivity using nuclear methods (CR-39) and image processing are addressed.

N.K. Libeesh, et al. ^[8], used geochemical (XRF) and remote sensing (RS) in the Salem District (Pakkanadu, India) by collecting samples from different locations and utilized the visible near Infrared (VNIR) and Short Wave Infrared (SWIR) regions of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), to retrieve the different band combinations and determine the shielding efficiency by gamma rays in the energy range (0.015–15 MeV). Additionally, it was found that the dunite rocks are characterized by the maximum shielding properties.

Lubna A. Alasadi and Ali A. Abojassim ^[9], measure the natural radioactivity of the background gamma radiation produced by (238 U, 235 U, 232 Th and 40 K), and the alpha emitters produced by (222 Rn, 226 Ra, and 238 U) in the collected soil samples from the cities of Najaf and Kufa in the Najaf governorate. The radiological hazard index was determined due to gamma and alpha emitters from soil samples and maps were drawn by the geographic information system (GIS).

Nikezic and Yu ^[10], used a computer program developed to calculate the light scattered from an assembly of alpha particle tracks in a PADC film. The ray-tracing method was applied to simulate light propagation through the tracks. The distribution of the scattered light was found to be independent of the equilibrium factor between radon and its progeny.

Fuminobu Sato, et al. ^[11], developed a monitoring system for the sites of time-lapse recording from tracks formed on the surface of the solid-state nuclear track detector, by irradiating the CR-39 nuclear track detector with alpha particles, this irradiation of CR-39 clarified the process of track formation and created an image model for developing the track through image processing programs for the time-lapse image data. Following the track's development through image analysis and computational simulation of the incident of the energetic particles on CR-39, the incidence angle was used to reveal the formation of the track resulting from that fall.

Mostofizadeh A, et al. ^[12], used MATLAB software as a method to study edge detection and used nuclear track density CR-39. Experimental and statistical results showed that not only did each particular edge detection method affects the accuracy of measurements but considering the overlapping phenomenon -including double and triple tracks but also demonstrated a significant difference in accuracies across of the mentioned techniques before and after image enhancement.

Hadad, et al. in ^[13], used an automated counting system adopted on a high-resolution scanner and related image processing software to study the CR-39 detector, once the electrochemical etching was performed the CR-39 detectors were scanned and computed by a developed software (called NTC) with high accuracy.

Sa'ad R Yousif et al. ^[14], used the gamma-ray spectrum using Nal (TI) detector to calculate the radioactivity for samples of soil taken from the Badra oil field (Iraq). The rustles are subjected on the map using the GIS software with the datum WGS1984 by means of the IDW interpolation system.

This research aims to detect and determine radioactivity using nuclear methods (CR-39) track detectors and assist GIS, in the old Najaf discrete, which is the center of Najaf Province. We have chosen this topic of radioactivity due to its relevance in tourism and cancer risk. Additionally, these sites were focused on in this study to investigate their level of ionizing radiation. It included the use of two interpolation methods, namely, the IDW and TIN ^[15,16], in studying the distribution of radioactivity concentration from the point and behavior bulk and its effect on determining the most radioactive areas.

2.   The study area description

The old District of Najaf was chosen as a study area to measure the radioactivity concentrations of radioactive radon gas, which is located in Najaf Province, southwest of the capital Baghdad (Iraq). The study area is in (31º57´8.08 N to 32º7´34.86 N) latitudes and (44º5´0.45 E to 44º22´53.57 E) longitudes, as it is characterized by the flatness of the land. It contains distinctive sites such as the Cemetery (Wadi Al-Salam), and the areas adjacent to the Najaf water depression. It is a shrine for many citizens because it contains religious sites, including the shrine of Imam Ali. The urban areas are located on the eastern side of the district, with little urbanization in the western site ^[4,17], as shown in figure 1.

3.   Material and methodology

In this research, the CR-39 nuclear track detector was used for the purpose of calculating the concentration of radon gas. The CR-39 detector was discovered in 1978 by (Cartwright & Shirk) at the University of California, USA, and it is one of the organic nuclear track detectors, and its molecular formula (C₁₂H₁₈O₇) is symbolized by (CR) ^[3,4,17], which is an abbreviation of (Columbia Resin), its density is (1.32 gm.cm^-3) and it has an ionization potential of (70.2 eV) and it is insoluble in chemical solvents ^[1,18,19]. The CR-39 detector is one of the best-recorded detectors for nuclear tracks, due to its characteristics ^[1,19,20].

Its high sensitivity to radiation (alpha particles, protons, and neutrons) and different energies, as well as its ability to record the effects of returning protons, resulting from their collision with neutrons with high energies ranging between (0.1–20 Mev) ^[18]. This makes this detector sensitive to fast moving neutrons. In addition to optimal detector sensitivity, the detector exhibits the following benefits:

● High analytical ability.

● Sheer optical.

● Not dissolved in chemical solutions abrasive. If the abrasive solution works to break the polymeric chains and reduce their thickness.

● Highly homogeneous.

● Insensitive to big doses of gamma-rays and x-rays.

● Low cost and easy to use.

● Thermally hard material, the refractive index is lower than that of glass.

It is not affected by atmospheric factors such as temperature and humidity when stored for long periods under natural conditions, and it does not dissolve in chemical solutions, but decomposes through a decrease in its thickness during the chemical etching process. So, the following steps were taken.

● 15 samples of soil are collected from the Najaf province (Old a Najaf District) (Iraq). At depth of (15 cm).

● The samples are cleaned of impurities and dried in an electric oven for 2 hours, then grind to a fine powder in a (250 μm) sieve.

● Using the nuclear track detector CR-39, with a thickness of (500 μm) and dimensions of (1 × 1 cm²), as shown in figure 2.

● (10, 20, 30) grams of the soil sample is adopted to calculate nuclear radioactive concentration using a cylindrical cup with a length of (7 cm and 5 cm in diameter).

● Fixing the CR-39 detectors of the 1 cm² area at the top of the cup to prevent the alpha particles emitted from the Thoron gas Rn²²⁰ from reaching the detector.

● The soil samples are lefts for 1 month as exposure time.

● The CR-39 detector is Etched chemically using a NaOH solution by (6.25 N) at (60 ℃) for (4 hours).

The radiation background of the detector was measured. They were placed in an empty test tube of samples and sealed in the same manner and for the same time periods, in order to obtain the actual density of the effects of the alpha particles emitted from the samples under study without the impact of the surrounding environment.

Optical microscope with a magnification of (400 X) was used to count the average tracks of ten locations on the detector surface, the density of the track can be calculated by the following equations (1–3).

The detector (CR-39) was calibrated with a radium standard source Ra²²⁶. Which emits radon Rn²²² at a concentration (A = μCi = 18.5 × 10⁴ Bq).

Calculated the radon concentration in samples by using an equations (1) and (2).

C_x: Is the concentration in the samples, C_s: Is the concentration in the standard sample.

ρ_x: Is the density of tracks in the unknown sample with (track/mm²) and ρ_s: density of tracks in the standard sample with (track/mm²).

Where C_Rn is radon concentration (Bq･m^-3) (ρ) is the track density (track･mm^-2), T is the exposure time (day), and (η) is the calibration coefficient of CR-39 nuclear track detectors obtained from the experimental calibration $\left({{{0.2568}^{\mathit{tracks}{\rm{. }}{m^{ - 2}} \cdot da{y^{ - 1}}/}}/{\rm{ }}\mathit{Bq}{\rm{. }}{m^{ - 3}}} \right)$ of radon ^[1,2,17].

Calculating the radioactivity for each study area based on the calculated values for the sample areas using the two interpolation methods, the IDW method, and the TIN method.

4.   Results and discussion

The study applied to the old Najaf District by selecting 15 sample sites within it, the chosen sites tend to be primarily religious, archaeological, and heritage landmarks. They were distributed over the district at an almost equal inter-distance to ensure the covering of the district. The locations of chosen sites were located using the UTM WGS84 38N projection system. Note that the study area was divided into four main areas, represented by the urban area, which represents the largest part, the old cemetery area, the plant areas, and areas with a small population density. The small population density areas can be considered uninhabited compared to the urban area, as illustrated in Table 1 and figure 2 a, b.

The radioactivity concentrations were calculated for each site according to the steps shown in the methodology, where the drying and grinding process was carried out to a granular size of 250 μm, and the exposure process was performed for 30 days from the soil to three different weights, as shown in figure 4.

The counting of the accumulated tracks on the track detector CR-39 was carried out using a light microscope with a magnification capacity of 400 X, where a counting rate of five places was taken in the detector, as illustrated in figure 5.

Which covers most of the area of the detector, after which the impact density and radioactivity concentration was calculated for each site and according to the equations (1, 3) for each site, as shown in Table 2. The radioactivity concentration weights for three weights are distributed over the study area, as illustrated in figure 6.

In order to find the effect of the amount of radioactivity concentration calculated for the location of the samples on the rest of the study areas, the interpolation method was performed using inverse distance weight (IDW), and the method of interpolation using the triangle

Irregular network (TIN), where we note in general the division of the study area into two parts in terms of the intensity of radioactivity in the region on the western side of the study area, characterized by a relatively high concentration of radioactivity compared to the eastern region and the various weights that were relied upon.

The use of IDW allows us to deal with the sites that were selected in the study area as point sources where the radioactivity is concentrated and whose effect is transmitted to the neighboring areas in a radioactive manner. The increase in weight of the soil sample confines the area of high radioactivity in areas The Tomb of the Martyr Al-Sadr I and Gate 4 the Ancient Cemetery and two valleys located in the ancient cemetery of Najaf Wadi Al-Salam), then the use of 10 g of soil showed that all the areas of the western samples in the study area have radioactivity. High compared to nearby areas, as shown in figure 7.

The use of the IDW interpolation system does not describe the distribution of the concentration of the radioactivity area, as the distribution of the soil samples that were taken are part of the geological and environmental reality of the study area, and therefore the correlation between the sites of the samples to study the concentration of radioactivity must be taken into account. Thus, the use of (TIN) enabled us to calculate the amount of radioactivity concentration for each region confined between the nearest three points, to show the distribution of radioactivity in the form of regions, not as point sources, which is closer to the normal behavior of the distribution of radioactivity, as shown in Figure 4. Where we notice an increase in the concentration of radioactivity in the western regions of the study area, specifically in the area of the old Najaf cemetery (Wadi Al-Salam) and the areas adjacent to the increasing amount of soil used in calculating the concentration of radioactivity reduced the difference between high and low areas in terms of radioactivity concentration, but it did not change the radioactivity concentration sites in the old cemetery area and the areas adjacent to the Najaf water depression, as shown in figure 8.

5.   Conclusions

Depending on the results we can drive the following conclusions: (1) The relative increase in radioactivity concentration in the study area's western region, which is the old cemetery of (Wadi Al-Salam) and the adjacent areas of the Najaf water Depression and lower in the eastern residential areas. (2) The use of low weights leads to a difference in the concentration of radioactivity between the different sampling areas, which decreases with increasing the amount of soil. (3) The use of the interpolation method TIN is more realistic and closer to the real distribution of the concentration of radioactivity because it deals with it on a bulk basis, unlike the method IDW, which deals with it on a point source basis.

Conflict of interest

The authors declare no conflict of interest.