Calculation of Attenuation Parameter for Ir-192 Gamma Source in Shielding Materials.

Purpose: Calculation of photon attenuation is necessary for the selection of shielding materials for an irradiation facility. Methods and Materials: In this work, a Monte Carlo simulation was utilized to assess the effectiveness of clay-polyethylene mixture and clay as the radiation shielding materials for high-energy gamma sources (Ir-192). Ordinary concrete was also studied as the benchmark.
Results: The calculated linear attenuation values for ordinary concrete are within 0.44% of the standard XCOM value for 380 keV photon. In the case of a multienergy Ir-192 gamma source, the calculated linear attenuation coefficient (μ) for ordinary concrete is 15.5% and 7.25% higher than clay and fabricated clay-polyethylene, respectively. Meanwhile, the μ value for fabricated clay-polyethylene is 8.3% higher than that of clay.
Conclusion: In conclusion, a 10 cm thickness of clay and clay-polyethylene mixture is sufficient to attenuate 87% and 89% of photons from Ir-192 source. The calculated linear attenuation coefficients for the three shielding materials are also consistently higher, about 7.5%, than that of the XCOM value for 380 keV photon. Copyright:

INTRODUCTION

Many studies conducted since 1970 have proved that lead (Pb-82) is effective in photon attenuation either to shield the emission of photons from radioactive sources or as a shielding material for the wall of an irradiation facility room[123] such as for diagnostic and radiotherapy room. However, the disadvantages of lead, along with its advantages, are the lead material itself is toxic and expensive.[456] Due to these reasons, many studies have been conducted to replace the lead element with other materials as a shielding material.[7891011121314] As an example, the authors had successfully revealed that the gadolinium compound doped with the polymer could be used as an alternative shielding material other than lead for 150 keV with a minimum thickness of 2 cm.[15] The authors calculated when gadolinium-doped polymeric compounds had been applied, more than 90% photon attenuation with 2 cm thickness for 150 keV incident photon will be adequate for shielding an X-ray laboratory. Recent studies led by Kaura et al.[16] and Agar et al.[17] proposed the utilization of metallic alloy as a potential gamma-shielding material by doping with a high atomic number (Z) element.

To compare with the XCOM values, the primary concern most of the time in many cases of photon shielding studies was about the transmitted photons that penetrate the shielding materials, both scattered and unscattered photons. Typically the transmitted photons count after where typically the amount of interest is measured or calculated from the attenuating material. For instance, Sayyed et al. had read attenuation properties for the chose germanate glasses-based shielding compound through hypothetical calculations.[18] Other investigations of radiation attenuation had been conducted by hypothetical calculation for different concretes[19] and by trial work for glass-based shielding compound.[2021] As well as attenuation coefficient (μ) and its related parameters, it is likewise important to assess the transmitted or reflected intensities because of the shielding material, especially for a recently evolved shielding material and photon energy (keV). As we need to guarantee the radiation dose rate is pretty much as low as possible according to background radiation, such assessment is significant when planning a radiation shielding facility or for source storage. It is upon this motivation that we conducted the study of our relatively newly developed clay[14] and clay-polyethylene,[22] benchmarked against ordinary concrete,[23] as radiation shielding materials for high-energy gamma sources (Ir-192).

In this study, the transmitted photons of ordinary concrete, clay, and fabricated clay-polyethylene materials were assessed by Monte Carlo simulation (EGS5 code).[2425] Transmitted photons using clay and fabricated clay-polyethylene materials were calculated for gamma-ray of multi-energy source of Ir-192. While concrete as a standard shielding material that usually utilized, we come out with another sort of shielding material, for example, clay and clay-polyethylene. Clay materials are used for construction and building purpose in many developed and developing nations. Clay products such as burnt bricks, ceramic wares, and tiles that commonly used for floor and material, are less expensive and tougher structure materials than concrete, particularly under tropical condition.[26]

CALCULATED TRANSMITTED PHOTON FOR MONOENERGY PHOTON

Monte Carlo code for electron and photon transport Electron Gamma Shower (EGS5 code)[2425] was used to calculate the transmitted primary photons. EGS5 code has been developed at the High Energy Accelerator Research Organization, Japan.[24] EGS code is a general-purpose package for the Monte Carlo simulation to transport electrons and photons in an arbitrary geometry with energies above a few keV up to several hundred GeV.[24] Simulation results obtained from this code were reported in many publications. As an example, simulations of the longitudinal and radial distributions of energy deposition of electrons of various energies made using EGS5 were compared with experimental results in the literature.[27] This example research confirms that the results from EGS5 simulation can be used for approximating the various energy depositions of electrons. Tajudin et al.[28] stated that they were successfully demonstrated two-energy calibrations (~200 keV and 662 keV) to be used for survey meter calibration from a monoenergetic radioactive source (Cs-137) and believed that their method would be very effective for accurate calibration of dosimeters which in turn essential for any instrument. Weber et al.[29] also agreed that simulations indicate an excellent polarimeter quality of such detector systems when used as Compton polarimeters. Moreover, good agreement is found between simulations and recently obtained experimental data. The photons interaction such as photoelectric absorption, Compton scattering, and pair productions were considered. The branching ratio of the gamma sources were sampled as the JRIA data book.[30] The photons were emitted in the parallel beam to the material and the Rayleigh scattering option was requested for the material. The materials data for concrete and clays and their density for the calculations correspond to the literature data.[31]

Several studies have been conducted to compare the values of simulated linear attenuation coefficients from Monte Carlo calculation with the values from XCOM[3233] to get the value of discrepancies. The radiation attenuation is an inherent property of materials, which can be clarified by the exponential decay equation as:

where I is the initial attenuated intensity of photons while I is the attenuated intensity of photons. The quantity x represents the thickness of the material, and μ is the linear attenuation coefficient. Commonly, the unit for linear attenuation coefficient is cm−1. However, it is usually identified as the mass attenuation coefficient μ/ρ in a unit of cm2 g−1. The radiation attenuation in materials is determined by the interaction of photons with particles, electrons, or atoms in matter, including pair production, Compton scattering, coherent scattering, and the photoelectric effect.

The calculation of transmitted photons as a function of ordinary concrete thickness has been shown in Figure 1 for a pencil beam of 380 keV incident photon. It has to be noted that photons of Compton scattering that penetrate the clay-shielding material were disregarded in our calculation for comparison with the attenuation coefficients generated by XCOM.[34] The full square points were suited with an exponential function to get linear attenuation coefficient (μ) values of 0.228/cm. Good agreement was achieved between the calculated μ value and theory value, which is within 0.44% to legitimize the resulting calculations for multi-energy gamma sources. The error of 0.13% appeared in the Figure 1 for the calculated μ value comes from exponential fitting error.

Figure 1

Calculated μ of concrete for single-photon energy in comparison with the theoretical value. The calculated μ value is 0.228/cm for 380 keV incident photon to have a decent concurrence with the theory value within 0.44%

CALCULATED TRANSMITTED PHOTONS FOR MULTI-ENERGY GAMMA SOURCE

Clay had been effectively demonstrated to diminish transmitted photon compared with ordinary concrete for low-incident photon energy in our previous study due to predominant photoelectric absorption.[35] Photon's pencil beam was incident to the focal point of cylinder clay. In this study, we considered the photon's interaction, for example, photoelectric absorption, Compton scattering (including Rayleigh scattering), and pair productions. μ value was calculated for multi-energy gamma source of Ir-192, and according to the JRIA data book,[30] the branching ratio of the gamma source adopted in the calculation was sampled as in [Table 1].[30]

Table 1

List of gamma energies with its own branching ratio

Source	Energy (MeV)	Branching ratio (%)
Ir-192	0.296	28.7
	0.308	29.7
	0.317	82.7
	0.468	47.8
	0.589	4.5
	0.604	8.2
	0.612	5.3
	0.0664	7.3
	0.0764	1.9
	0.0104	4.1
	0.206	3.3
	0.485	3.2
	0.0626	3.4

Figure 2a-c shows the calculation of transmitted photons as a capacity of the material thickness (ordinary concrete, clay, and fabricated clay, respectively) for a pencil beam of Ir-192 gamma source. The full square points were fitted with an exponential function in Figure 2 to get linear attenuation coefficient (μ) values for each shielding material, as shown in Table 2 (0.243/cm). While the calculated values by EGS5 are for Ir-192, the XCOM values were obtained for a single-photon energy of 380 keV. According to Rijnders, 380 keV photon could be used as the average energy of Ir-192 gamma source.[36] As an example, from Figure 2a-c, a thickness of 10 cm is satisfactory to lessen up to 91% of the photons from Ir-192 source for ordinary concrete, 87% and 89% for our clay and fabricated clay, respectively. Among the three materials, ordinary concrete is the highest value of attenuation because of its higher density compared to clay and fabricated clay, which is 2.302 gcm-3. The attenuation value decreases as density decreases, where the density of fabricated clay is 2.03 gcm-3 with attenuation of 89%, while the density of the clay is 1.99 gcm-3 with attenuation of 87%.

Figure 2

Calculated μ of fabricated clay-polyethylene for single-photon energy compared to the theoretical value. The calculated μ value is 0.226/cm for 612 keV incident photon to have a decent concurrence with the theory value within 7.34%

Table 2

Calculated linear attenuation coefficient (µ) cm−1 for Ir-192 gamma source

Materials	XCOM (380 keV)	EGS5 (Ir-192)	Percentage increase (%) of calculated value
Ordinary concrete	0.227	0.243	7.04
Clay[14]	0.193	0.208	7.77
Fabricated clay-polyethylene[22]	0.210	0.226	7.62

From Table 2, the calculated values of linear attenuation coefficient for the three shielding materials are consistently higher, about 7.5% than the XCOM value for 380 keV photon. The higher calculated value by EGS5 was expected as the Ir-192 gamma source also consisted of a few percent low photon energies, as shown in Table 1, which are not considered in the theoretical value. The XCOM values are outputted from single-photon energy of 380 keV. Therefore, one who has no facility to do experiments or perform Monte Carlo simulation could simply use the XCOM database to estimate the shielding attenuation parameter with a few percent difference on account of multi-energy gamma sources. As the XCOM database is only for single-photon energy, the use for gamma sources needs to be verified either by experiment or simulation as the particular gamma source might have lower and higher energy photons that need to be considered.

CALCULATED AMBIENT DOSE EQUIVALENT AND PHOTON SPECTRA FOR MULTI-ENERGY GAMMA SOURCE

Table 3 shows the measured, theory, and calculated dose rate for Ir-192 source (10.3 Curie) at a distance of 83 cm. The table has proved a good agreement between the calculated (simulation) and the experiment, where the validation was met. Therefore, the experimental value can be used to measure dose at any location. The source activity when the experiment was carried out was 10.3 Curie. The dose rate was measured by an ionization chamber survey meter of 1000 cm3 effective volume (Victoreen 451P-RYR). The agreement of calculated with theory and measured value with the average difference of 1.37% ambient dose rate in air are adequate to justify or validated our next simulations for photon dosimetry with concrete and our fabricated clay materials for Ir-192.

Table 3

Measured, theory, and calculated dose rate for Ir-192 source (10.3 Curie) at a distance of 83 cm

Measured	75.504 mSv/h (±0.98%)
Theory[37]	76.894 mSv/h
Calculated (EGS5)	75.327 mSv/h (±1.11%)

From Figure 3, the calculated dose rate for fabricated clay is always higher than an ordinary concrete with a percentage of difference <4% for the thickness <8 cm. However, the dose rate for clay is higher than concrete up to 15% at a thickness of 20 cm, as the effect of concrete has a higher density than developed clay. In previous, we have successfully described the fabricated clay for neutron shielding purposes.[31] Nevertheless, as an example, at a thickness of 20 cm, both samples successfully decreased the dose rate to 92%. All the calculated dose rates have an error of <1%.

Figure 3

Calculated ambient dose rate (mSv/h) of fabricated clay-polyethylene for single-photon energy compared to ordinary concrete and theoretical value

The average energy () of a photon spectrum was calculated by the equation:

where f (E) is the number of photons with energy E, each photon was multiplied by its corresponding energy and all the product was integrated over the whole spectrum. The integral of all the products were divided by the total number of photons to yield the mean energy.

From Figure 4, the average photon energy of Ir-192 at 0.83 cm distance in the air without a sample is 349.5 keV. With 8 cm of concrete and clay, the transmitted average photon energies are 272.2 keV and 282.9 keV, respectively. Furthermore, the photon peaks of 468 keV and above from the source were attenuated within a factor of 5, while the photon peaks of less 400 keV, including the 317 keV with 82.7% branching ratio, were significantly reduced by almost a factor of 7.

Figure 4

Average photon energy (keV) of fabricated clay-polyethylene (blue dotted line), concrete (black dotted line), and without a sample (full red line). With 8 cm of concrete and clay, the transmitted average photon energies are 272.2 and 282.9 keV, respectively

As the gamma source energy used is considerably high, there is a possibility for backscatter radiation which is the radiation scattered with a large angle after undergoing Compton scattering interaction inside the material. Figure 5 shows an example of reflected photon scored from an 8 cm thickness of concrete and fabricated clay due to Ir-192 source incident photon emitted in the 4 π direction. The average reflected photon energy is 130.5 keV. The impact of reflected photon energy and its dose rate will be further investigated.

Figure 5

Reflected photon spectra for Ir-192. The average reflected photon energy is 130.5 keV

In another example, Tajudin et al.[14] had shown how to diminish reflected photon spectra from the clay material for Am-241 gamma source by utilizing an iron (Fe-26) element. At whatever point vital, if the added reflected dose is sufficiently high to legitimize attempting to lessen it, at that point, it turns into a matter of cost and comfort in choosing what approach may be ideal for diminishing reflection.

CONCLUSION

In this study, the linear attenuation coefficient of ordinary concrete and both newly-developed clay and the clay-polyethylene mixture were calculated using the Monte Carlo method for multi-energy gamma source Ir-192. It was found that a 10 cm thickness of ordinary concrete could allow 9% transmitted photon, while it was 13% and 11% for the same thickness of clay and clay-polyethylene, respectively. In terms of attenuation coefficients μ, the value for ordinary concrete was higher by 15.5% and 7.25% compared to clay and clay-polyethylene, respectively, due to its higher density. However, clay offers a much lighter mass than ordinary concrete for a similar photon attenuation value. For calculated ambient dose equivalent, the agreement of calculated with theory and measured value with the average difference of 1.37% ambient dose rate in the air is adequate to satisfactory to legitimize or approved our next simulations for photon dosimetry with concrete and our fabricated clay material for Ir-192. From the simulation, all the calculated ambient dose rates have an error of <1%, and the average reflected photon energy is 130.5 keV. The impact of reflected photon energy and its dose rate from both types of clays will be further investigated.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.