Journal of Medical Physics and Applied Sciences

Keywords

Granitic rocks; Gamma rays; Radioactive material; Monzogranite; Radiometric measurements; Uranium

Introduction

All minerals and raw materials contain radionuclides of natural origin. The most important for the purposes of radiation protection are the radionuclides in the 238U and 232Th decay series and the non-series radioactive element 40K. For most human activities involving minerals and raw materials, the levels of exposure to these radionuclides are not significantly greater than normal background levels and are not of concern for radiation protection. However, certain work activities can give rise to significantly enhanced exposures that may need to be controlled by regulations [1]. Material giving rise to these enhanced exposures has become known as naturally occurring radioactive material (NORM). Uranium exploration and mining include several activities that give rise to the enhanced exposures to ionizing radiation. Seila area, southeastern Desert of Egypt is located between latitudes 22°13′48′′-22°18′36′′N and longitudes 36°10′12′-36°1836′′E. The younger granite at Seila area is represented by Gabal Qash Amir, Gabal El Seila and isolated granite stocks. These rocks are affected by ENE-WSW shear zones and sub-parallel fault system dipping 50°-70° to the south and extending about 9 km, with thicknesses between 2 to 40 m. The ENE-WSW trend was intersected by the N-S sinestral strike slip and dip slip fault systems. Shear zones and fault systems are filled with quartz veins, fine grained granite and basic dykes. The ENE-WSW shear zones display radioactive anomaly along the sheared fine grained granite and basic dykes. Generally, these rocks are pale pink slightly leucocratic, medium to coarse grained, cavernous and exfoliated monzogranite and they include U-bearing minerals such as biotite, zircon and muscovite [2]. These minerals have enhanced concentrations of the terrestrial radioactive elements; ²³⁸U, ²³²Th and ⁴⁰K [3]. The Egyptian nuclear materials authority (NMA) established some of projects to explore the radioactive elements in Egypt. One of them was established at Seila area. So, the radioactive exposure of workers in this project from terrestrial radioactive elements; uranium, thorium and potassium should be evaluated. In this study, the author suggests some modes and configurations to evaluate the external effective doses from the gamma rays received by the workers at Seila area during exploration of uranium and handling the rock samples collected from this area.

Field Works and Measurements

Among several locations chosen by the Nuclear Materials Authority NMA for uranium exploration development at Seila area, the one at Lat. 22°17'55.95", Long. 36°14'8.76" is the subject of this study (Figure 1). This location represents a vertical face cutting the fractured granite. The area of the face is almost 3.5 × 3.5 m².

Figure 1: Location of the studied U-exploration site at Seila area.

Radiometric measurements

A grid pattern containing 12 points was demonstrated on the granitic face for radiometric measurements (Figure 2). The spacing between the vertical lines is 1.5 m while the spacing between the horizontal lines is 1m. Radiometric measurements were carried out at the chosen spots on the grid pattern using RS-230 BGO (Bismuth Germanate Oxide) Super-Spec portable radiation detector. This spectrometer has high accuracy (Probable measurement errors about 5%) in full assay capability with data in K%, eU (ppm) and eTh (ppm) while no radioactive sources required for proper operation. The detector is a product of an independent private company (Radiation Solutions Inc. Mississauga, Ontario, Canada). The values of eU and eTh in ppm as well as K in percent were converted to activity concentrations, (Bq/kg), using the conversion factors given by the International Atomic Energy Agency IAEA [4]. The activity concentration of a sample containing 1 ppm by weight of eU yields is 12.35 (Bq/kg) of ²³⁸U, 1 ppm of eTh yields 4.06 (Bq/kg) of ²³²Th and 1% of K yields 313 (Bq/kg) of ⁴⁰K.

Figure 2: A grid pattern on the vertical granitic face to investigate the concentrations of U, Th and K. The picture was taken looking east.

Measurements of dose equivalent rate

The RDS-100 survey-meter, ALNOR, Turku, Finland, was used for measuring the gamma equivalent dose rate. This detector can detect gamma rays in the energy range of 50 kev to 3 Mev and equivalent dose rate measurements range 0.05 (μSv/h)-99.99 (mSv/h). It contains Geiger Muller tube calibrated against a ⁶⁰Co γ-source of activity 7.4 × 10⁸ Bq at the national institute of standards and technology (NIST). Measurements were achieved at the chosen locations on six lines parallel to the studied granitic face. These lines were at distances 0.5, 2.5, 5, 7.5,10 and 12.5 m from the face.

Results and Discussion

Concentration of the radionuclides

Darnley defined the "uraniferous granites" as those containing at least twice the Clarke value (4 ppm U), hence, they would contain 8 ppm or more, regardless of the presence of associated U-mineralization or not [5]. Accordingly, the data in Table 1 clarified that the studied granite face at Seila area can be considered uraniferous granites. U-content varies between 13 and 224.9 with an average value of 94.16 ppm. It seems that U-content varies from the higher values at the north of the grid pattern (N line) to the lower values at the south (S line) and from upwards (first line) to downwards (fourth line). However, the rock samples were collected in cloth bags and loaded on a truck to be transported to NMA branch at Inchas. Table 2 represents the concentration of U, Th and K in the rock samples on the truck at six points. Comparing the average values of the studied measurements in Tables 1 and 2, It is clear that the data of U and Th were affected by the physical condition of the rocks. U-content decreased from 94.16 ppm in the local bedrock at the studied granitic face to only 32.16 ppm in the crushed rock samples on the truck, while Th-content decreased from 8.93 to 6.08 ppm. This conclusion suggests an intercomparison between all the radiometric survey meters and devices to check the reproducibility of the measurements.

Table 1: The eU, eTh and K contents in the studied granite face located at Seila area.

Table 2: The eU, eTh and K contents in the rock samples at six points on the truck.

Absorbed dose rate D and equivalent dose rate H

The absorbed dose rate D (nGy/h) at 1 m due to the exposure to gamma rays emitted from the studied granitic face at Seila area is calculated according to the formula [6].

D=0.462 A_U+0.604 A_Th+0.0417 A_K (1)

Where A_U, A_Th and A_K are the mean activities of ²³⁸U, ²³²Th and ⁴⁰K in (Bq/kg), respectively

The workers of the NMA at Seila area are all adults. Consequently, the conversion factor from the absorbed dose rate D (nGy) to the equivalent dose rate H (nSv/h) equals unity. Table 3 represents the activity concentrations (Bq/kg) of ²³⁸U, ²³²Th and ⁴⁰K. The values of the equivalent dose rate H (μSv/h) using Equation 1 and the relevant conversion factor are shown in Table 3. The equivalent dose rate H varies between 0.14 and 1.38 with an average of 0.61 (μSv/h). Indeed, Table 3 represents the value of H at 1 m from each point on the studied granitic face. Alternatively, the survey meter located at 1 m from the granitic face receives gamma rays emitted from each point on the whole area of the face and replies a value representing the average value of H. Figure 3 represents the gradient of the measured equivalent dose rate H (μSv/h) with the distance from the granitic face. Fitting the measured data suggests an exponential gradient of H with the distance x from the studied granitic face according to the relation:

H=0.0907+1.3e^-x/6.98 (2)

Figure 3: Variation of the average values of the measured equivalent dose rate H (µSv/h) with the distance from the granitic face at Seila area.

Table 3: Activity concentration of the radionuclides ²³⁸U, ²³²Th and ⁴⁰K (Bq/kg) and the equivalent dose rate H (μSv/h) at 1m from the surface of the studied granites at Seila area.

where H is the equivalent dose rate (μSv/h) and x is distance (m). From Equation 2, the equivalent dose rate at 1 m from the granitic face is 1.22 (μSv/h) which is almost twice the average value obtained in Table 3. This is because the equivalent dose rate measured by the survey meter collects gamma rays from not only the studied granitic rock face but also from the surrounding rock walls and the remnants of the rock samples on the ground (Figure 3). Equation 2 can be interpreted at two extreme values of the distance X. The first value of X is assumed at infinity. Mathematically, this value eliminates the second term in Equation 2 and the value of H equals 0.0907 ± 0.012 (μSv/h). Physically, this value is set as the average background of the equivalent dose rate due to the gamma rays received down into the northern neighboring Wadi at a distance far enough from the studied granitic face, X=50 m. However, the average background of the equivalent dose rate measured at the western neighboring Wadi was found to be 0.08 ± 0.012 (μSv/h) [7]. The other value of x represents the contact between the survey meter and the granitic face (X=0). The average value of the equivalent dose rate H is calculated at (X=0) from Equation 2 to have the value of 1.39 (μSv/h).

Effective dose rate and its different modes

The effective dose rate E (μSv/h) due to the terrestrial gamma rays is calculated according to the equation [8]

E= ΣH w_T (3)

Where H is the equivalent dose rate (μSv/h) and w_T is the organ or tissue weighting factor (Table 4).

Table 4: Recommended tissue weighting factors.

The equivalent dose rate H is assumed constant at a specific distance from the studied granitic face. Accordingly, Equation 3 becomes:

E= H Σ w_T (4)

Standing mode: The "Standing" mode is described as a worker stands before the rocks to study its mineral composition or takes its picture at any distance, etc. This mode represents a whole body radiation. So, the summation in Equation 4 is unity as shown in Table 4. Accordingly, the effective dose rate E equals numerically the equivalent dose rate H and Equation 2 tends to the form:

Es= 0.0907+1.3 e ^–x/6.98 (5)

Where Es is the effective dose rate for the standing mode. Equation 5 applies to the standing mode all over the studied area from the contact with the granitic face (X=0, Es=1.39 μSv/h) to the distance at (X=12.5, Es=0.31 μSv/h). These values are much below the recommended occupational effective dose rate of 10 (μSv/h) [8].

Carrying mode: The "carrying" mode represents the carrying of the rock samples in the bags to load on the truck. Three configurations are classified as carrying mode (Figure 4). The Right hand-Left hand (R-L) configuration represents the exposure to the nearest organs; bone and gonads (Σw_T=0.09). The "Hug" configuration represents the exposure to colon, lung, stomach, breast, bladder, oesophagus, small intestine, liver, heart, kidney and gonads (Σw_T=0.71). The "shoulder" configuration represents the exposure of bone-marrow, thyroid, brain, eye and salivary glands (Σw_T=0.19). It is clear that the Hug configuration is the most risky one as it includes the exposure of most organs in the human body. The suggested configurations were exercised at the vicinity of the studied granitic face which means that the worker who carries a bag receives a total effective dose rate E which is the summation of the Standing mode Es as a component and the other component Ec resulting from carrying the bag i. e:

Figure 4: Three configurations suggested to the carrying mode.

E = Es + Ec (6)

Where E is the total effective dose rate (μSv/h), Es is the effective dose rate resulting from the Standing mode (μSv/h) and Ec is the effective dose rate from carrying the sample bags (μSv/h).

The effective dose rate from the carrying mode is calculated as follows:

Ec = H_g ΣW_T (7)

Where H_g is net equivalent dose rate (μSv/h) measured in contact with each of random twelve bags during moving to the truck and ΣW_T has the values 0.09, 0.71 and 0.19 for the different carrying configurations as clarified above. The cloth bags containing the rock samples and the truck were located at 12.5 m from the granitic face i.e. H=0.31 (μSv/h). This value should be subtracted from the direct contact reading HD of the survey meter to obtain H_g. Accordingly, Equation 6 becomes:

E = 0.31 + H_g ΣW_T (8)

In this study the author equates the probability of the three carrying configurations to calculate the total effective dose rate E. Table 5 represents the values of H_g, Ec for the three suggested carrying configurations and E during the loading of rock samples bags at Seila area. From Table 5, all values of the effective doses; Ec or E are much below the recommended dose rate [8]. The importance of the definition of E as proposed by (Equation 6) is that it relates the contact reading of any radioactive sample H to the resulting biological effect (w_T) through a precisely described configuration to prevent any undesired overestimation of the occupational exposures. For example, grab #9 yields a contact equivalent dose of 1.12 (μSv/h). This value gives an effective dose of 1.12 (μSv/h) if assuming whole body radiations. In contrast, using Equation 6 the effective dose reduced to half its value 0.58 (μSv/h) (Table 5).

Table 5: Equivalent dose rates H_D and H_g (μSv/h), the effective dose rate for the three carrying configurations Ec (µSv/h) and the total effective dose E (µSv/h) resulting from the loading of granitic rocks from Seila area.

Transport mode: The "transport" mode includes both occupational exposure represented by the driver in the cabin of the truck and public exposure due to the passage of the truck in the surrounding environment carrying the granitic rocks in its box. To estimate the resulting effective doses due this carriage, the equivalent doses were measured inside the cabin of the truck and in contact with sides of its box before loading the sample bags (Figure 5a) and after loading (Figure 5b).These measurements were carried out at the NMA Field Center at Abu Ramad city located about 22 km to the east of Seila area. From the data projected on (Figures 5a and 5b) the additional equivalent dose rate at the driver in the cabin is 0.05 (μSv/h). Assuming whole body radiation for the driver results in an additional effective dose rate of only 0.05 (μSv/h). Assuming 2000 working hours per year, the annual effective dose is 0.1 (mSv). This is much below the recommended occupational dose rate which ranges between 1 and 6 (mSv/y) during the transportation of natural radioactive materials [9]. On the other hand, the additional equivalent dose rate in contact with the box of the truck is 0.15 (μSv/h). This is lower than the recommended limit of 6 (μSv/h) [9].

Figure 5:Comparison between the equivalent dose rates (μSv/h) in the cabin of the truck and in contact with sides of its box before and after loading the granitic rocks from Seila area. Big digits represent the six points measured for eU, eTh and K.

Conclusions

All modes of exposures to the terrestrial gamma rays at Seila area result in occupational effective doses which are below the recommended limits. Precisely described exposure configurations can prevent any undesired overestimation of the effective doses received by the workers. It is unlikely that the transport of the granitic rocks from Seila area causes any considerable exposures to the occupants or to the environment.

References

1. IAEA (2011) Radiation protection and safety of radiation sources: International basic safety standards. International Atomic Energy Agency. IAEA Safety Standards No. GSR Part 3 Intrim Edition Vienna.
2. Ali KG (2011) Structural control of El Sela granites and associated uranium deposits, south eastern desert, Egypt. Arab J Geosci 6: 1753-1767.
3. El Afifi EM, Hilal MA, Khalifa SM, Aly HF (2006) Evaluation of U, Th, K and emanated radon in some NORM and TENORM samples. Radiat Meas41: 627-633.
4. IAEA (1989) Construction and use of calibration facilities for radiometric field equipment. International Atomic Energy Agency IAEA Technical Reports Series No. 309 IAEA Vienna.
5. Darnley AG (1982) Hot Granites: Some general remarks. In "Uranium in granites" Maurice YT ed. Proceedings of a workshop held in Ottawa OntarioGeolSurv Canada paper 81-23.
6. UNSCEAR (2008) Exposures of the public and workers from various sources of radiation. UNSCEAR Report.
7. Abdel-Razek YA,Masoud MS, Hanafi MY, El-Nagdy MS (2015) Effective radiation doses from natural sources at Seila area, South Eastern Desert, Egypt. J TaibahUniv Sci.
8. ICRP (2007) Recommendations of the International Commission on Radiological Protection. ICRP Publication 103 Ann. ICRP 37: 2-4.
9. IAEA (2007) Radiation protection programmes for the transport of radioactive material. Safety Standards Series No TS-G-1.3.