1 Introduction

In nuclear science, radiation protection is considered one of the most essential topics. Shielding from gamma rays can be considered as the most difficult one due to the enormous amount of energy held by gamma photons, and since they have no mass and charge, they can readily penetrate into the matter. Radiation shielding is commonly used to protect medical patients and workers from exposure to direct and secondary radiation during diagnostic imaging in hospitals and radiological facilities. The effectiveness of radiation shielding varies significantly with the attenuation properties of the component materials, material thickness, and radiation energy ^[1], thus there is always a need to develop materials that can be used as shielding material. In a case of nuclear radiation shielding, huge amount of shielding materials are required, therefore, it is necessary to investigate the efficiency of the available materials, experimentally, before using them ^[2]. Generally, it is not fare to consider the concrete as a simple mixture of cement, water and aggregates. It often contains varies mineral components, chemical admixtures, fibers, etc. These components are usually affecting the characteristics of concrete shielding structures ^[1]. Also, the cement industry is one of the most common structural materials used in constructions such as home, hospital, etc. ^[3]. For this reason, the type and quantity of aggregates and admixtures are important components for radiation protection properties of concrete and for its physical and mechanical properties. Different researches in the field of radiation shielding have been published using different construction materials, different geometries, and different nuclear radiation sources. Moreover, a lot of studies are related with linear and mass attenuation coefficients of different materials such as building materials and concrete ^[4-9]. Mahdy et al., ^[10] investigated the influence of magnetite on the compressive strength and shielding properties of concrete. Magnetite was implemented as an aggregate material to produce a concrete mixed with three different percentages of silica fume. It was concluded that magnetite and silica fume had improved both the compressive strength and shielding properties. Akkurt et al., ^[11] used concrete containing different fine and coarse normal aggregates mixed with barite as shielding materials for gamma ray. It was concluded that the type of used aggregate was more important than its proportion in concrete mixed. Wasan et al. ^[12] investigated the suitability of using reactive powder concrete without steel fiber in shielding structures by measuring linear and mass attenuation coefficient using beta particles and gamma ray with different energies. In the present work, cube samples of reactive powder concrete without and with different volume ratio of micro steel fiber (1% and 1.5%) have been prepared to investigate the adequacy of using this element as gamma ray shield.

The attenuation of radiation is expressed as

where

I₀ is number of particles of radiation counted during a certain time

duration without any absorber.

I is number counted during the same time with a thickness x of

absorber between the source of radiation and the detector

μ is linear absorption coefficient.

When discussing the mass attenuation coefficient, equation (1) is rewritten as

where

ρ is the density.

(μ/ρ) is mass attenuation coefficient (μ_m)

ρx is area density (mass thickness d_m).

The half value thickness (X_1/2) for the samples is calculated according the following formula

where

X_1/2 is the average amount of material needed to absorb 50% of all radiation.

2 Material and Methods

2.1 Casting and Curing of the Samples

The schedule of the experimental test in this research includes casting three groups of standard cubes (50 mm × 50 mm × 50 mm side length) using the adopted mix of reactive powder concrete as demonstrated in Table 9. The process of mixing was similar to the one recommended by the ACI Committee Report 544. Water curing was used for 28 days after which three cubes were tested (dry surface) to evaluate the compressive strength. Each group of the castellated samples contains eight cubes, as presented in Table 10. Three of them were tested to evaluate the compressive strength of reactive powder concrete at an age of 28 days and the other five cubes were tested after being attenuated with a gamma ray.

2.2 Experimental setup

The planning of the experimental program with the electronic configuration is schematically shown in Fig.1.

Fig. 1Full-screen View

(Color online) Schematic of experimental setup

The assembly was placed in a lead castle. Two collimators with diameter of 5 mm were used as shown in Fig.1. The distance between source and detector was approximately 35 cm. Energy calibration was performed using a set of standard gamma sources. Measurements have been carried out using a collimated beam of ¹³⁷Cs, ⁶⁰Co and ²⁰⁷Bi gamma sources with energies (0.662, 1.17, 1.33, 0.569, and 1.063) MeV. The leakage gamma ray intensities behind the samples have been captured by using a sodium iodide crystal NaI(TI) scintillation detector with a dimension of 2”× 2”. The incident and transmitted intensities were determined for a fixed preset time at 1000 sec in each measurement.

3 Results and Discussion

In order to test the radiation shielding of reactive powder concrete without and with steel fiber, linear attenuation coefficient, mass attenuation coefficient, and half value thickness are studied. Figure 2 show the intensity of radiation emitted from ¹³⁷Cs, ⁶⁰Co and ²⁰⁷Bi gamma sources with energies (0.662, 1.17, 1.33, 0.569, and 1.063) MeV, respectively, as a function of d_m (g/cm²) without micro steel fiber (sample A). The different component of concrete and the percentage of these components, have played an important role in attenuation properties.

Fig. 2Full-screen View

The logarithmic intensity of a gamma ray as a function of d_m for different energy (sample A)

The addition of micro steel fiber to the reactive powder concrete showed an obvious increase in density.

Figure 3 shows the intensity of radiation with a micro steel fiber volume ratio of 1% (sample B) at different gamma ray energies in MeV (0.569, 0.662, 1.063, 1.17, and 1.33) as a function of d_m (g/cm²). Also, the results of the sample with volume ratio of 1.5% (sample C) are shown in Fig. 4. It can be detected that the intensity is inversely proportioned to the thickness of the samples. The slope of the absorption graph gives the experimental gamma ray mass attenuation coefficient (μ_m) of the absorber reactive powder concrete in terms of (cm²/g). This is shown in Fig. 5a. This figure shows the variation of mass attenuation with the gamma ray energies in MeV for different samples of reactive powder concrete without and with steel fiber. It is shown that the mass attenuation coefficient decrease with increasing gamma ray energies, and the higher values of μ_m were found at sample C with 1.5% steel fiber, while the lowest values were found for sample A without steel fiber. These results give the conclusion that the mass attenuation coefficient increases with increasing the steel fiber percentages by volume (1% and 1.5%).

Fig. 3Full-screen View

The logarithmic intensity of a gamma ray as a function of d_m for different energy (sample B)

Fig. 4Full-screen View

The logarithmic intensity of a gamma ray as a function of d_m for different energy (sample C)

Fig. 5Full-screen View

(Color online) Attenuation coefficient of reactive powder concrete without and with a volume ratio of (1% and 1.5%) as a function of energy

Figure 5b shows the effect of gamma ray energies in MeV on linear attenuation coefficient (μ) for different samples of reactive powder concrete without and with micro steel fiber. It is shown that the linear attenuation coefficient decrease with increasing the gamma ray energies. At the same time, the linear attenuation coefficient increase with increasing the volume ratio of micro steel fiber. The behavior of the linear attenuation coefficient curve is opposite to that of the X_1/2 curve, as shown in Fig. 6. This demonstrates that as the percentage of micro steel fiber increased, the properties of the reactive powder concrete attenuations improved and give better shielding for the reactive powder concrete. It is clear from Fig. 6. that larger thickness of materials is needed to stop higher energy photons.

Fig. 6Full-screen View

(Color online) Half value thickness of reactive powder concrete without and with a volume ratio of (1% and 1.5%) as a function of energy

The compressive strength for all the tested samples were compared with those of no gamma effect, and it was found that there is no significant reduction in the compressive strength of the reactive powder concrete samples shown in Table 11.

4 Conclusion

In this work, linear attenuation coefficient, mass attenuation coefficient, and half value thickness of reactive powder concrete without and with different volume ratios of steel fiber (1% and 1.5%) were investigated. It was detected that the density of the sample can be modified by increasing the steel fiber volume ratio. This inevitably increases the attenuation of gamma radiation hence the half- value thickness of the sample reduces. Depending on these results, reactive powder concrete can be used in radiation shielding since it exhibits a positive photon attenuator at different energies (0.569, 0.662, 1.063, 1.17, and 1.33) MeV. The gamma radiation shielding capabilities can be enhanced by using steel fiber volume ratio of 1.5%. In addition, the compressive strength of the tested samples has no significant changes in its compressive strength after being affected by gamma radiation.