Evaluation of Radiation Shielding Properties for Concrete with Different Aggregate Granule Sizes

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1 Evaluation of Radiation Shielding Properties for Concrete with Different Aggregate Granule Sizes ALI BASHEER AZEEZ 1 *, KAHTAN S. MOHAMMED 1, ANDREI VICTOR SANDU 2,3 *, ABDULLAH MOHD MUSTAFA AL BAKRI 1, HUSSIN KAMARUDIN 1, IOAN GABRIEL SANDU 2 1 University Malaysia Perlis (UniMAP), School of Materials Engineering, Center of Excellence Geopolymer &Green Technology (CEGeoGTech), 01000, Kanger, Perlis, Malaysia 2 Gheorghe Asachi Technical University of Iasi, Faculty of Materials Science and Engineering, 71 D. Mangeron Blv., , Iasi, Romania 3 Romanian Inventors Forum, 3 Sf. P. Movila Str., L11, III/3, , Iasi, Romania The radiation shielding characteristics of concrete samples incorporate different aggregate sizes ranging from 5 to 15mm were examined. To achieve this goal, gamma-ray absorption coefficient of these samples were calculated. The aggregate weight % in the concrete for all samples was similar. The radiation sources utilized in this study were Cs137 and Co60. Reference concrete mixes ratios of 1:2:4 consisted of Portland cement, sand, gravel and water of 5 kg/m 3, 10 kg/m 3, 20 kg/m 3, and 2 kg/m 3 respectively. The weight ratio of water to cement (w/c) was of 0.4. The measurements were performed using gamma ray spectrometer of 3 3 NaI (Tl) detector with a Multi Channel Analyzer (MCA). The spectrometer communicates with the PC by Genie200 software. The results showed that gamma-ray attenuation coefficient is inversely proportional to the aggregate size. This is attributed to the close packing levels of the aggregates within the sample. Usually the packing level of large aggregates is inferior to that for aggregates of smaller granule size. The attenuation coefficients ranges were of 0.247, 0.328, 0.33 and for samples of aggregate granule sizes of 5mm, 10mm, 15mm and for combined aggregates sizes of 3mm, 5mm and 10 mm respectively for the CS 137 source of photon energy of MeV. The best result was exhibited by the samples containing three aggregate sizes of 3, 5 and 10 mm. Reduction of about 10% in the half value level (HVL) was acquired for the samples of the combined aggregate sizes. The mean free path (mfp) values for the examined samples were calculated and related to the attenuation coefficients. Keywords: attenuation coefficient, radioactive, aggregate sizes, concrete, NaI (Tl) Concrete that is concrete composed of Portland cement, sand, aggregate (stones, gravel, etc.), and water [1], is one of the most common materials used in the construction of commercial buildings. Currently ordinary concrete (density about 2350 kg/m 3 ) is widely used for superficial and orthovoltage radiotherapy rooms [2]. Currently the usage of rays such as x-rays and gammarays is increasing in various fields from medical applications to food industry. Hence, protection from their detrimental effects became crucial issue. There are three general rules for protection: exposure time, distance, and shielding. In most cases, shielding is the main rule to be performed [3] although materials such as lead and iron are effective anti-ray shields, mechanical and economical considerations limit their usage to some special areas [4]. On the other hand, concrete is paramount material utilized for radiation shielding in the facilities having radiation generating equipment and radioactive sources [5]. The problem of shielding against ionizing radiation has always attracted a great deal of attention. Radiation shielding of a nuclear reactor is a costly and very complex process [6]. A nuclear reactor usually requires two shields; a shield to protect the walls of the reactor from radiation damage and at the same time reflect neutrons back into the core and a biological shield to protect people and the environment. The biological shield reduces the level of gamma radiation and neutrons to current dose limits. The biological shield may contain some heavy materials such as iron and steel punching. The biological shield consists of many centimeters of very high-density concrete [7-13].The concrete shielding features may fluctuate relying on the built-up of the concrete. Aggregates are the predominant component (about 70-80% of the whole weight of normal concrete). Various types of artificial and natural aggregates are utilized to promote the features of the concrete. The utilization of aggregates of the highest possible density in the concrete mix is an effective measure in shielding enhancement. So far adequate experiences is acquired in concretes for shielding goals, as each country has to benefit from its own experience relying on the abundance, cheapness and potency of the local materials. The aim of this work is to develop dense concrete by manipulating the size of the aggregate constituents to attain the optimum close packing levels of the aggregates component to improve the overall concrete density and eventually to improve the shielding properties. Experimental part Materials and methods Four concrete sample sets were fabricated utilizing different aggregate granule sizes of 5, 10, 15mm and a mix of combined sizes of 3, 5 and 10 mm the weight percentages of all concrete constituents were kept constant. Ordinary black Portland cement was used to prepare the tested samples. The shielding properties of these types of concretes have been investigated. These samples of * alibasheer2013@yahoo.com; sav@tuiasi.ro REV. CHIM. (Bucharest) 64 No

2 concrete mixes were produced according to ASTM no C637 [14]. Reference concrete mixes ratio 1:2:4 consisted of cement 5kg/m 3, sand 10kg/m 3, gravel 20kg/m 3, and water 2kg/m 3 were utilized. The water to cement (w/c) ratio was 0.4.The measurement was performed using gamma ray spectrometer of 3 3 NaI (Tl) detector with a Multi Channel Analyzer (MCA).The spectrometer communicates with the PC by Genie200 software. The experimental work consisted of two steps. In the first step, the attenuation was investigated by testing the rate of the penetrative radiation out of 3-axes of each sample. The average values and the standard deviations for the readings were calculated to determine the accuracy of the measurements and the homogeneity of the samples. In the second step the samples were cut into slabs with different thicknesses to define the attenuation curves and consequently to calculate the half value layer (HVL) for each sample. The linear attenuation coefficient μ l in cm -1 for these samples was experimentally determined using a narrow collimated mono- energetic beam of gamma-rays. These gamma-rays emitted from the decay of two gamma radioactive sources, Cs 137, of MeV and Co 60, of and MeV. The distance between detector and the radioactive source was set to 5cm. The schematic description of the experimental setup is shown in figure 1 [15-17]. sample between the detector and the source, from I and the incident photon Io for a thickness x of the absorber, the linear attenuation coefficients (μ) is given by the following formula [18, 20]: Fig. 2. Gamma Ray spectrum obtained from Co 60 source Fig. 1. Schematic view of the experimental setup The sorting, grading and determination of the grading limits for the raw aggregates were ordinarily conducted and the weight percentages of aggregates passing each sieve were attained. The various granule aggregate sizes for these concrete samples comprising combined different granule size aggregates were selected in a way that the aggregates of smaller granule sizes can easily arranged themselves interstitially in between aggregates of larger granule sizes during mixing process. This assures the reduction in the voids total volume within the concrete mixture. Calculations The background was subtracted from the initial intensity Io and the Intensity I of the transmitted beam. The density (ρ) is mass and volume dependant. The linear attenuation coefficient (μ) was determined by measuring the transmission of gamma-rays through the sample as a target of known thickness. Gamma ray spectrums for the Co 60 and Cs 137 sources are shown in figures 2 and 3 respectively. Figure 2 represents the two sharp peaks related to the Co 60 gamma source of the 1.17 and 1.33 MeV rays. The single sharp peak related to the Cs 137 source of MeV ray is shown in figure 2. The area under the curve of the photo peak spectrum is used to evaluate the intensity I of the transmitted beam. Evaluation of the I o which is the area under the photo peak is obtained without inserting any Fig. 3. Gamma ray spectrum obtained from Cs 137 source I = I o e -μx (1) where x = 5cm represents the thickness of each sample. The gamma-ray spectrum was obtained for a real time of 60s for each test which was rationally enough to gain an adequate high distribution pulse. Equation 1 is useful if two conditions are fulfilled; First, the photons in the incident beam are mono-energetic. Second, the beam must be tight. For wide beam source geometry or a thick shield, normally insert one more term to equation 1 named the build-up factor B is necessary. Attenuation coefficients were determined by graphical method, in which the slope of the fitted line to the plot of radiation transmission rate given by ln (Io/I) versus sample thickness x gives the total linear attenuation coefficient as shown in figure 4. The following formula is used to calculate the values of the standard deviations at 8 positions of each implemented sample: Where σ(μ) is the standard deviation at any position, μ is the linear attenuation coefficient at any position of the sample, μ is the average values of the linear attenuation coefficient and N is the number of the measured positions for each sample, in this study they were 8 positions. (2) REV. CHIM. (Bucharest) 64 No

3 Fig. 4. Linear attenuation coefficients from the measured I and I o values as a function of the sample thickness for the CS137 source Fig. 5. Average of linear attenuation coefficients with samples at energies for CS 137 and Co60 sources. Aggregate size of samples 1, 2, 3, and 4 is (5, 10, 15) mm respectively The half value level (HVL) for the samples of the combined aggregates of different granule sizes was calculated according the following formula: HVL = 0.693/μ (3) whereas HVL is the average amount of material needed to absorb 50% of all radiation, it is related to mean free path, however the mean free path (mpf) of a pencil beam of mono-energetic photons is the average distance that the photon travels between collisions with atoms of the absorbed material. It depends on the material and the energy of the photons, (mpf) was calculated using the following formula: mpf = μ -1 = ((μ/ρ)ρ) -1 (4) where; μ is the linear attenuation coefficient, μ/ρ is the mass attenuation coefficient and ρ is the density of the material. Results and discussions The average values of linear attenuation coefficient and their standard deviation and the half value level and mean free path for the four sets of concrete samples at three gamma-ray energies are given in table 1. Their radioactive sources were of 0.662MeV and MeV for Cs137 and Co60, respectively. It is clear from table 1 that the values of the standard deviations are small and varied from 0.01 to This gives good indication that the fabricated samples have good homogeneity and of even distributions of the dense components within the concrete cohesive mixture. The value for the fabricated samples was increased specially for samples no 4 which contains mixed aggregates of 3, 5 and 10 mm granule size. In fact, sample 4 produced attenuation which is better than other samples with about 13% at MeV for Radioactive source Cs 137. The linear attenuation coefficients μ of concrete containing different aggregate granule sizes was measured. The results for two radiation sources of Cs 137 of energy MeV and Co 60 of energies (1.17 and 1.33) MeV are displayed in figure 5, it is very obvious from the plots in this figure that the linear attenuation coefficients increased with the decrease in the source energy. The concrete samples of the mixed aggregates of granule sizes of 3, 5 and 10mm showed the highest linear attenuation coefficients as opposed to the other tested samples. This is attributed to their high close packing level. During mixing the small sizes arrange themselves in the Table 1 THE AVERAGE OF LINEAR ATTENUATION COEFFICIENT (cm -1 ) WITH THE STANDARD DEVIATION, HVL AND THE mfp FOR THE TESTED SAMPLES REV. CHIM. (Bucharest) 64 No

4 Fig. 6. The average of linear attenuation coefficients of the tested samples as a function of the photon energies Fig. 8. The mean free path against the average of linear attenuation coefficient of various shields for MeV Fig. 7. The half values layers with the average of linear attenuation coefficient for various shields for MeV voids between large granules. This process reduces the pores and enhances the concrete density. Figure 6 shows the variation in the average linear attenuation coefficients μ (cm -1 ) against the photon energies of gamma rays of the tested samples. It can be seen that μ decreases as the photon energy is increased. The correlation between the average of linear attenuation coefficients and the photon energy is used to confirm the linearity. The HVLs are inversely proportional to the average omit of linear attenuation coefficient values. This is indicated clearly in figure 7. This figure demonstrates the relation between μ and the HVL. It is obvious that when the shielding material has large absorption potential of the incident radiation the recommended thickness of the shield will be substantially reduced. The disproportion between the mfp and the linear attenuation coefficient is shown in figure 8. It can be seen from this figure that the mfp decreased when the linear attenuation coefficients increased. This came in accord with the definition of the mfp, wherein the probability of many successive collisions means small mean free path and consequently gives large attenuation, it can be shown from this figure that the correlation coefficients have a strong linear relationship with (R 2 = 0.772) for concrete samples when Cs 137 of MeV emitted photon energy is used. Conclusions During this study, strict sieving, sizing and classification procedure for different crushed rock aggregates was implemented; the produced classified aggregates of different sizes were incorporated in concrete mix to fabricate four sample sets. The concrete samples underwent different radiation attenuation and homogenization tests at different energy levels. The concrete sample set of mixed aggregates of granule sizes of 3, 5 and 10 mm showed the highest linear attenuation coefficients as opposed to the other tested samples of single sized aggregates. This was attributed to the high close packing level of the mixed aggregates which consequently has led to the reduction of the porosity in the level within the concrete mix. Less porosity means higher density and better radiation attenuation performance. Denser concrete gives higher compressive stress which is another important parameter in constructions concrete requirements for anti radiation shielding purposes References 1.SUN, H., JAIN, R., NGUYEN, K., ZUCKERMAN, J., Clean Technologies and Environmental Policy, 12, 5, 2012, p ***, IAEA Treatment Machines For External Beam Radiotherapy. Chapter 5, IAEA Radiation Oncology Physics: A Handbook For Teachers And Students. International Atomic Energy Agency, Vienna, EAVES, G., Principles of Radiation Protection, 1964, Iliffe Books Ltd, London. 4.AKKURT, I., AKYILDIRIM, H., MAVI, B., KILINÇARSLAN, S., BASYIGIT, C., Ann. Nucl. Energy 37, 2010, p SINGH, K.J., SINGH, N., KAUNDAL, R.S., SINGH, K., Nuclear Instruments and Methods in Physics Research B, 266, 2008, p PAVLENKO, V.I., YASTREBINSKII, R.N., VORONOV, D.V., Journal of Engineering Physics and Thermophysics, 81, 2008, p MORTAZAVI, M.J., MOSLEH-SHIRAZI, M.A., BARADARAN- GHAHFAROKHI, M., SIAVASHPOUR, Z, FARSHADI, A., GHAFOORI, M., SHAHVAR, A., Iran. J. Radiat. Res., 8 (1), 2010, p MAKARIOUS, A.S., BASHTER, I.I., ABDO, A.E.S., AZIM, M.S.A., KANSOUH, W.A., Annals of Nuclear Energy, 23, 3, 1996, p BRANDT, A.M., Cement Wapno Beton, 18, 2, 2013, p TABACARU, C., CARLESCU, A., SANDU, A.V., PETCU, M.I., IACOMI, F., Rev. Chim.(Bucharest), 62, 4, 2011, p ALHAJALI, S., YOUSEF, S., KANBOUR, M., NAOUM, B., Radiation Protection Dosimetry, 154, 1, 2013, p MARIS, M., MARIS, D.A., JIPA, S., ZAHARESCU, T., GORGHIU, L.M., Rev. Chim. (Bucharest), 61, no. 3, 2010, p REV. CHIM. (Bucharest) 64 No

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