Evaluation of gamma shielding parameters of bismuth oxide filled polymer composites

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 56, August 2018, pp Evaluation of gamma shielding parameters of bismuth oxide filled polymer composites M R Ambika a,b, N Nagaiah b * & S K Suman c a Department of Physics (S&H), P E S University, Bengaluru , India b Department of Physics, Bangalore University, Bengaluru , India c Radiation Safety Systems Division, BARC, Trombay, Mumbai , India Received 9 May 2018 The present work deals with the study of shielding ability of UP-Bi 2 O 3 polymer composites for gamma rays of energy MeV using a gamma ray spectrometer. The various shielding parameters such as linear and mass attenuation coefficients, half value and tenth value layer thickness, and mean free path have been evaluated for the composites. The attenuation coefficients of the composites have also been evaluated theoretically using the computer code XCOM and WinXCOM. The results show that, the shielding performance increases with increase in the filler concentration. The highest filled composite exhibits excellent shielding ability and is comparable to that of the conventional shielding materials. The theoretical and experimental values are in good agreement. Keywords: Attenuation coefficient, XCOM, Cesium-137, Density, Polymer composites, Radiation protection 1 Introduction The interaction of X/gamma rays with matter is significant in various fields such as agriculture, industries, biological/medical studies, nuclear science and space technology. Because of its wide applications, there is a need for a material that can absorb high energy radiations in order to avoid unwanted exposure. To develop and use an effective shielding material, the knowledge of some of the parameters such as linear attenuation coefficient, mass attenuation coefficient, half value layer thickness, effective atomic number, mean free path and electron density are very much essential. Likewise, several experimental and theoretical studies have been conducted over the globe to evaluate these parameters for various elements, mixtures/compounds and composite materials 1-9. The first modern systematic compilation on photon interaction cross sections and the attenuation coefficients was done by White in 1952 over the energy range of 10 kev 100 MeV, which was later published in the handbook of radiology, beta and gamma ray spectroscopy in the year of In 1958, Storm, Gilbert & Israel included all the elements with atomic number Z=1-100 by interpolation. Further, in 1965, these tables were updated by Davisson using *Corresponding author ( nagaiah@bub.ernet.in) the recent theoretical estimates. This motivated Hubbel to report the values of mass attenuation coefficients for 40 elements and 45 mixtures and compounds over the energy range 1 kev-20 MeV initially, which was then extended to all the elements Z=1-92 by him and Seltzer 10. By considering the experimental results of various researchers during 1960 s, Hubbell and Berger finally developed the compilation tool, computer program XCOM for various elements, compounds, mixtures for the energy ranging from 1 kev 100 GeV. This program provides total cross sections and attenuation coefficients as well as partial cross sections for the following processes; incoherent scattering, coherent scattering, photoelectric absorption, and pair production in the field of the atomic nucleus and atomic electrons. Gerward et al. converted this program to the windows version which is called as WinXCOM 3,11. In the present study, the authors have evaluated the attenuation parameters for unsaturated polyester based polymer composites filled with bismuth oxide experimentally using a scintillation gamma ray spectrometer. These were then compared with the ones calculated using the program XCOM/ WinXCOM. 2 Theory When a beam of monoenergetic gamma ray photons passes through a material medium, the

2 AMBIKA et al.: GAMMA SHIELDING PARAMETERS OF BISMUTH OXIDE FILLED POLYMER COMPOSITES 605 intensity of the photons is reduced due to absorption and scattering of photons. The three principal modes of gamma ray interaction are photoelectric effect, Compton effect and pair production and are energy dependent. At low energies upto 500 kev, photoelectric effect is dominant, between 100 kev to 10 MeV, Compton scattering is significant whereas pair production becomes dominant above 2 MeV. If I o and I are the intensity of incident and transmitted photons, then the decrease in intensity of photons is given by Lambert-Beer law: I = I e μ (1) Where, x is the thickness of the target medium with density ρ and µ is the linear attenuation coefficient. Mass attenuation coefficient of a material medium is given by: μ = μ ρ (2) In various shielding calculations and applications, various mixture of elements or compounds are frequently encountered. Hence, for a mixture or compound, the mass attenuation coefficient is given by: μ = w ( μ ρ ) (3) Where, w i is weight fraction of the i th constituent element and (µ/ρ) i is mass attenuation coefficient of the constituent element i. 3 Experimental Method Open mould cast technique was used to fabricate the polymer composite radiation shields. Bismuth oxide with different concentration (0, 10, 20, 30, 40 and 50%) was dispersed into the unsaturated polyester matrix using an electric blender and ultrasonicator. The sample was cured and post cured for setting and then used for gamma attenuation studies Scanning electron microscopy was used to study the distribution and dispersion of the filler particles within the polymer matrix. The density of the polymer composites was measured using Archimedes principle as per the ASTM standards (ASTM D ) using a calibrated single pan balance. Ethanol and chlorobenzene were used as immersion liquids for this purpose 15. Gamma attenuation studies were carried out using a narrow beam transmission geometry set up. The experimental set up consists of a 3 3 NaI(Tl) detector which is connected to a PC based MCA through succeeding electronic devices like preamplifier and spectroscopy amplifier 13,14,16. The energy resolution of the spectrometer was found to be 8% for MeV gamma rays. The linear attenuation coefficient µ for all the samples was measured using the gamma spectrum of each sample which was then used to evaluate the other shielding parameters such as mass attenuation coefficient µ m, half value and tenth value layer thickness (HVL and TVL) and mean free path 16,17 λ. The shielding parameters were also calculated theoretically using XCOM/WinXCOM. 4 Results and Discussion The gamma shielding ability of the polymer composites was evaluated by careful analysis of the gamma ray spectrum obtained for each thickness of the sample of different composition. The details of the samples are as given in Table 1. It is evident from the table that, the density of the composites increases with increase in the bismuth oxide content of the composite. The experimental values of various attenuation parameters are presented in Table 2. The probability of photon interacting with a material medium per unit path length is given by linear attenuation coefficient (µ) which depends on energy of the incident photon, density and atomic number of the material. The results reveal that, µ increases with increase in the filler concentration. µ is also found to increase with increase in the density of the composites as seen in Fig. 1. The highest filled composite UPB6 is found to exhibit the highest value of µ. Half value and tenth value layer thickness and mean free path are the parameters essential for the practical applications. HVL is the thickness of the material required to reduce the photon intensity to half of its initial intensity whereas, mean free path is the average distance that a gamma ray travels in the absorber before it suffers interaction. The HVL, TVL and λ for the composites are found to decrease with increase in the filler wt% which is the known fact and are in the Table 1 Details of the samples. Sample Wt % of Bi 2 O 3 Density (g/cc) UPB UPB UPB UPB UPB UPB

3 606 INDIAN J PURE & APPL PHYS, VOL. 56, AUGUST 2018 Table 2 Experimentally measured attenuation parameters for UP+Bi 2 O 3 polymer composites. Sample µ (cm -1 ) µ m (cm 2 /g) HVL (cm) TVL (cm) λ (cm) UPB UPB UPB UPB UPB UPB Fig. 1 Variation of Linear attenuation coefficient of Polymer composites. range from cm, cm and cm, respectively. Since, µ is density dependent as it is known, for the purpose of comparison, the mass attenuation coefficient was evaluated 16. This parameter quantifies the gamma-ray interaction probability of a material and the results are presented in Fig. 2. It is evident from Fig. 2 that, the mass attenuation coefficient increases with increase in the bismuth oxide content in the composite and is found to range from to cm 2 /g. The gamma shielding parameters which were calculated using XCOM/WinXCOM program are as given in Table 3. The trend of variation for µ and µ m is similar to that, as observed in the experimental results. In addition to this, two more parameters vize effective atomic number (Z eff ) and electron density (n e ) were also obtained as they are also essential in radiation therapy for calculating the dose 18,19. Figure 3 reveals that, both Z eff and n e are found to increase with increase in the filler concentration and are found to range from and electrons/cm 3, respectively, and of course they depend on the chemical content of the sample 18. The probability of interaction of gamma photons with the medium will be high if more number of Fig. 2 Variation of Mass attenuation coefficient of Polymer composites. electrons is available resulting in good absorption of photons 14,16. The theoretical and experimental results of µ and µ m with respect to filler wt % are in good agreement and are as shown in Figs 4 and 5, respectively. There is no such noticeable difference but, a slight difference being observed may be due to the problem of dispersion of the filler particles within the polymer matrix. This is confirmed through the SEM images Fig. 6(a-c). The pristine sample UPB1 as evidenced from Fig. 6(a), shows the presence of graded ridges whereas, Fig. 6(b,c) shows the uniform distribution and dispersion of filler particles in the filled composites. Besides, there exist agglomerations in the sample UPB6. As the concentration of the filler particles increases (say 50%), they tend to agglomerate forming bigger particles which in turn lowers the shielding performance 20. However, the polymer composites do exhibit excellent shielding ability for MeV gamma rays. Hence, they find their application in the field of radiation protection to protect both living and nonliving systems from the unwanted exposure to the high energetic and penetrating radiations such as X-rays and gamma rays.

4 AMBIKA et al.: GAMMA SHIELDING PARAMETERS OF BISMUTH OXIDE FILLED POLYMER COMPOSITES 607 Table 3 Theoretically evaluated attenuation parameters for UP+Bi 2 O 3 polymer composites. Sample µ (cm -1 ) µ m (cm 2 /g) Z eff n e (electrons/cm 3 ) UPB UPB UPB UPB UPB UPB Fig. 3 Z eff & n e of the polymer composites. Fig. 6 (a-c) SEM images of UPB1, UPB4 and UPB6. Fig. 4 Theoretical & experimental - Linear attenuation coefficient of Polymer composites. Fig. 5 Theoretical & experimental Mass attenuation coefficient of Polymer composites. 5 Conclusions Bismuth oxide filled unsaturated polyester based polymer composites were fabricated successfully. The various shielding parameters were evaluated both theoretically and experimentally and some of the basic parameters were compared. The theoretical and experimental results are found to be in good agreement but slight difference is observed due to agglomeration of filler particles. The shielding efficiency of the composites increases with increase in the filler wt%. The highest filled, i.e., 50% filled composite exhibits excellent shielding ability with a half value layer thickness of 3.46 cm. Hence, the composites can be used for gamma shielding applications. Acknowledgement The authors are thankful to the DAE - Board of Research in Nuclear Sciences, BARC, Mumbai for granting financial assistance in terms of a research project under which the work was carried out (Sanction No. 2013/35/9/BRNS).

5 608 INDIAN J PURE & APPL PHYS, VOL. 56, AUGUST 2018 References 1 Abdel Rahman M A, Badawi E A, Abdel-Hady Y L & Kamel N, Nucl Instr Meth Phys Res Sec A, A447 (2000) Singh N, Singh K J, Singh K & Singh H, Nucl Instr Meth Phys Res Sec B, B225 (2004) Rao A S M, Narendra K, Rao K G K, Gopi K N & Murthy R K, Res J Phys Sci, 1 (2013) Abd El-Latif A A & Saeid K S, 10 th Radiation Physics & Protection Conference Proceedings, Egypt, Medhat M E & Singh V P, Radiat Eff Defects Solids, 169 (2014) Minxuan N, Xiaobin T, Hao C, Yun Z & Tuo C, Nucl Eng Technol, (2016) DOI.or/ /j.net Singh V P & Badiger N M, J Fusion Energy, 33 (2014) Gupta S & Sidhu G S, Int J Mod Eng Res, 3 (2013) Medhat M E, Mohammad A Z, Rahman S & Singh V P, Indian J Pure Appl Phys, 53 (2015) Hubbell J H, National Standard Reference Data System, NBS-29 US (1969) Gerward L, Guilbert N, Jensen K B & Levring H, Radiat Phys Chem, 71 (2004) Jose J P, Malhotra S K, Thomas S, Joseph K, Goda K & Sreekala M S, Polymer composites, (Wiley-VCH Verlag Gmb H & Co: Germany), Ambika M R, Nagaiah N & Suman S K, J Appl Polym Sci, 134 (2017) Harish V, Nagaiah N & Harish Kumar H G, Indian J Pure Appl Phys, 50 (2012) ASTM Standard D , ASTM International, West Conshohocken, Knoll G F, Radiation detection and measurement, (John Wiley & Sons Inc: New York), Singh K, Singh S, Singh S P, Mudahar G S & Dhaliwal A S, J Radiol Prot, 35 (2015) Annunziata M F L, Handbook of radioactivity analysis, (Elsevier: USA), Tursucu A, Onder P, Demir D & Oznuluer T, Int J Phys Sci, 8 (2013) Noor Azman N Z, Siddiqui S A & Low It M, Appl Phys A: Mater Sci Process, 110 (2013) 137.