Epoxy Resin Radiation Shielding

CHIN. PHYS. LETT. Vol. 29, No. 10 (2012) 108102

Effects of WO3 Particle Size in WO3 /Epoxy Resin Radiation Shielding Material

*

DONG Yu(董宇), CHANG Shu-Quan(常树全)** , ZHANG Hong-Xu(张红旭), REN Chao(任超), KANG Bin(康斌), DAI Ming-Zhu(代明珠), DAI Yao-Dong(戴耀东) College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016

(Received 11 May 2012) To verify the influence of the functional elements particular size for the radiation attenuation coefficients and mechanical properties radiation shielding material based on epoxy resin, we prepare two WO3 /E44 samples with different particular sizes of WO3 by a solidified forming approach. The linear attenuation coefficients of these samples are measured for 𝛾-ray photo energies of 59.6, 121.8, and 344.1 keV, etc. using narrow beam transmission geometry. It is found that the linear attenuation coefficients would increase with the decreasing particle size of the WO3 in the epoxy resin based radiation shielding material. The theoretical values of the linear attenuation coefficients and mass attenuation are calculated using WinXcom, and good agreements between the experimental data and the theoretical values are observed. From the studies of the obtained results, it is reported that from the shielding point of view the nano-WO3 is more effective than micro-WO3 in the epoxy resin based radiation shielding material.

PACS: 81.05.Lg, 21.60.Ka, 23.90.+w

DOI: 10.1088/0256-307X/29/10/108102

With a rapid development in science technology, nuclear technology is widely used in electricity generation, industry, and medical care, which have increased people’s probability to come into contact with different kinds of radiation.[1−4] Different radiation protection materials were developed to reduce the harm of radiation to the human body. Concrete and lead products have been widely used in fixed type nuclear reactors and accelerator protection; however, mobile nuclear power plants, nuclear waste transport and storage containers, and space vehicles require a particular weight and volume. Thus, lighter and more efficient materials of radiation protection have been required in these occasions. Due to the low density and the easy processing characteristics, a new type of material which is a polymer based compound material filled with radiopaque powder is now becoming more and more popular. Because of its good thermodynamic and structural properties, especially its good radiopaque ability, epoxy resin is widely used in these areas: circuit design, information technology, and space technology; generally it is used in making polymer compound materials.[5,6] Sayed (2003) studied the radiation shielding performance to gamma rays of ilmenite/epoxy resin.[7] Kamoshida (2004) developed a kind of epoxy resin based radiation shielding material which can be used in nuclear spent fuel storage containers, and it can resist from 150 to 200∘C.[8] Morioka (2007) invented a boron doped resin which can bear a 300∘C working temperature.[9] The epoxy resin based compound material is becoming a very promising ra-

diation protection material. According to the traditional radiation shielding theory, the attenuation effect of a shielding material is irrelevant to its microstructure, but mainly depends on factors of the type, energy of radiation, element composition and density of the material. However, the USA Radiation Shielding Technology Company (RST) (2001) modified the tantalum doped PVC and PE to make their electron cloud structure similar to heavy metal elements, thus it can be used to absorb radiation energy by the electron resonance effect and it is more efficient than the traditional radiation shielding materials.[10] Jaewoo et al. found that PVA with nano-B2 O3 was more efficient to attenuate the thermal neutrons than micro-B2 O3 .[11] On the other hand, there were different opinions upon how the ‘nano effect’ can prove the radiation shielding properties. In this Letter, we investigate the linear attenuation coefficients of an epoxy resin compound material in which nano-WO3 and micro-WO3 have been doped in the same proportion, and we use the WinXCom to calculate the depth dose and depth fluence of WO3 doped epoxy resin samples. Two cylindrical samples of compositions WO3 (1 µm and 50 nm)/E44 were prepared by a solidified forming approach. The crosslinking agent was used to modify the surface of WO3 powders so that it could link to the epoxy resign. The samples were prepared in the process: (1) 35 g WO3 was dissolved into acetone and 2 g crosslinking agent was added; (2) the mixture was ultrasonically dispersed at a temperature of 50∘C; (3) 35 g epoxy resin was added into the mixture and

* Supported by the National Natural Foundation of China (81071256, 81101146, 11105073), the National Science Foundation of Jiangsu Province (BK2012799, BK2011738, BK2011739), and Jiangsu Environmental Protection Research Funding (201151). ** Corresponding author. Email: [email protected] © 2012 Chinese Physical Society and IOP Publishing Ltd

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then agitated for 2 h at 80∘C; (4) then the mixture mixed with 2 g curing agent and agitated for 0.5 h; (5) finally, a steel mould (𝑟=66 mm) was prepared to contain the composite while it solidified at the temperature of 60∘C. The surface of two samples is shown in Fig. 1. The density and thickness of these two samples are listed in Table 1. WO3/E44 samples

Amplifier

NaI det MCA

HV

Pb shield

Sourse

Fig. 1. The experimental setup of the narrow beam transmission method.

10 mm

Fig. 2. SEM images of the nano-WO3 sample without the linking agent. (a)

(c)

and the mass attenuation coefficient is calculated by the equation µm = ln(𝐼0 /𝐼)/𝑑𝜌. A gamma-ray spectrometer consisting of a NaI scintillation detector, an amplifier and a multi-channel analyzer has been used in this experiment. The detail of the experimental setup is shown in Fig. 1. In this experiment, the measurement results influence each other. The uncertainty of the linear attenuation coefficient consists of the errors of intensities 𝐼0 , 𝐼 and thickness. In this experiment, we only take the error of intensity 𝐼0 into consideration, so we use the equation 1 𝜎(𝜇) = √ . 𝑛 Figure 2 presents the SEM images of the nano-WO3 sample without the linking agent. Figure 3 displays the surfaces of the two samples. The micro-WO3 sample c shows green and the nano-WO3 sample d is darker. These two samples are used to perform the gamma-ray spectrometer experiment. The SEM images are shown in Fig. 3, where (a) is the micro-WO3 sample’s section and (b) is the nanoWO3 sample’s bending section. We can see that the two samples have a uniform dispersion of WO3 particles. On the surface of the particles, we can see a welldistributed clad compared with Fig. 2. Figure 3(a) proves that the crosslinking agent has already worked on the particles. The samples used were conductive coated before SEM because the epoxy resin has no electric conductive characteristic. 0.00

(d)

0.04

0.06

The linear attenuation coefficients of WO3 /E44 samples were measured for gamma rays of different energies which have been obtained from 241 Am, 137 Cs, 133 Ba, 60 Co and 152 Eu point sources, respectively. Counting of the photo peak of the samples is used to evaluate the intensity 𝐼 of the transmitted beam. 𝐼0 is the counting of the photo peak obtained without any samples between detector and source; the linear attenuation coefficients can be calculated with the equation: ln(𝐼0 /𝐼) 𝜇= , 𝑑

3500

Sample with linking agent Sample without linking agent 3000

2500

2000

1500 -1

Wave numbers (cm

829

Fig. 3. Surface of the two samples and the SEM of the sections.

1501

0.10 4000

100 nm

3426

0.08

1607

(b)

1080

1 mm

2842 2943 2990

Absorbance

0.02

1000

500

)

Fig. 4. FT-IR spectrum of nano-WO3 with and without a linking agent.

As shown in Fig. 4, 3426 cm−1 is the Si–O–H stretching vibration absorption peak; 2990 cm−1 and 1080 cm−1 are the absorption peaks of the KH-570 linking agent; 2943 cm−1 and 2842 cm−1 are the –CH3 stretching vibration absorption peaks; and 1607 cm−1 and 1510 cm−1 are the benzene ring vibration peaks. Compared with the sample without a linking agent, 1080 cm−1 appears to prove that the linking agent has already reacted on the particles (the peak of 2990 cm−1 is covered by the methylene asymmetric

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ties, which are attributed to the better dispersion and shorter relative gap of nano-WO3 particles.

stretching vibration peak). From Fig. 3(a), we can see the bending section which looks like a river pattern. In the process of the bending experiment, WO3 particles block where the crackle is heading to. The bending strength and tensile strength are listed in Table 2. It is obvious that the nano-WO3 sample has better mechanical proper-

Table 1. density and thickness of two samples Samples Micro-WO3 /E44 Nano-WO3 /E44

Thickness(mm) 8.708 8.700

Density (g/cm3 ) 2.18 2.29

Table 2. Mechanical properties of two different samples. Samples 1 2 3 4 5 Average

Bending strength(MPa) Nano-WO3 /E44 Micro-WO3 /E44 55.37 57.87 64.98 48.95 56.26 61.6 68.98 54.7 75.67 53.94 64.252 55.412

Tensile strength(MPa) Nano-WO3 /E44 Micro-WO3 /E44 21.43 12.9 19.8 17.71 20.83 15.06 22.91 11.41 24.36 12.45 21.866 13.906

Table 3. Linear and mass attenuation coefficients of the two samples. 𝑁0 , 𝑁1 , 𝑁2 are the counts of photo peak without sample, with micro-WO3 /E44 and nano-WO3 /E44. Source Co60 Eu152

Cs137 Am241 Ba133

Energy (keV) 1170 1330 122.8 334.1 778.6 964 1112 1407 663.8 59.6 355.9

𝑁0 44836 33140 51416 159952 43676 32770 37425 17504 60282 147923 152114

𝑁1 42057 30991 36689 127846 39188 29539 34609 16171 52259 21357 119948

41733 30696 36006 126080 38902 29360 34349 15948 51587 17431 118264

Table 4. Parameters of the WinXCom simulation. Constituent Weight (frac) 𝑍 Weight (frac) 𝑍 Weight (frac) WO3 0.5 H 0.03553 O 0.19751 6101(E44) 0.5 C 0.37047 W 0.39649

) -1

coefficient (m

Linear attenuation

250 Micro-WO3/E44

200

Nano-WO3/E44

150 100 50 0 0

𝜇/𝜌(cm2 /g)

𝜇(m−1 )

𝑁2

200 400 600 800 1000 1200 1400 1600 Energy (keV)

Fig. 5. The linear attenuation coefficient of the two samples.

Table 3 lists the result of the gamma-ray spectrum experiment used Co60 , Cs137 , Eu152 , Am241 and Ba133 as radiation sources. Because of the bubbles, which come out from the process of solidification of the two

Micro

Nano

Micro

Nano

7.34790 7.69914 38.7543 25.7289 12.4515 11.9203 8.98313 9.09622 16.4011 222.245 27.2820

8.24358 8.80559 40.9498 27.3514 13.3049 12.6299 9.85816 10.7006 17.9038 245.797 28.93228

0.0337 0.0353 0.1778 0.1180 0.0571 0.0546 0.0412 0.0417 0.0752 1.0196 0.1251

0.0359 0.0383 0.1783 0.1191 0.0579 0.0550 0.0429 0.0466 0.0779 1.0706 0.1260

samples, the error of density calculation arises. In order to avoid this kind of error, we also calculate the mass attenuation coefficient to measure the shielding capability of the two samples. It is obvious that the nano-WO3 /E44 sample has a higher linear attenuation coefficient and a higher mass attenuation coefficient, but there is no obvious regular pattern to show how it changes. We notice that the linear and mass attenuation coefficients at energy 1407 keV are higher than 1112 keV, which are caused by the error from the count of the photo peak. Around the energy of 59.6 keV, samples show the best radiation attenuation property. Both samples’ linear attenuation coefficients decrease with the increasing photon energies, as can be seen in Fig. 5. We also use the WinXCom to calculate the mass attenuation coefficient of WO3 /E44 theoretically. Table 4 is the setting of parameters in the WinXCom simulation. The result is compared with the data coming from the gamma-ray experiment, as shown in Fig. 6. In the WinXCom simulation program, we consider the mixture in an ideal dispersed state, but during the preparation of samples, the reunite and asymmetrical dispersion of the WO3 particles will result

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in an unexpected experimental error. Thus the mass attenuation coefficients of WO3 /E44 samples, calculated from the gamma-ray spectrum experiment are lower than the value of simulation. The mass attenuation coefficient of the sample has a remarkable jump around the energy of 69.5 keV, which attributes to the K absorption edge of W (69.508 keV). 5

3

2

Mass attenuation

coefficient (cm /g)

4

2 1 0 0

200

400

600

800 1000 1200 1400 1600

Energy (keV)

Fig. 6. The result of WinXCom simulation and mass attenuation coefficients calculated from the experimental data for the nano-WO3 sample and the micro-WO3 sample.

we cannot summarise how it changes with the increasing energy because this requires a large number of experiments to summary experience formula. From this work we can reach a conclusion that the smaller size of the WO3 particle can give the epoxy resin based radiation protection material a better mechanical property and a better shielding property in the gamma-ray radiation field. Because of the smaller size of the nano particle, the crackle is blocked more efficiently. However, there is no such formula summarised in this experiment, which is caused by the large number of effecting elements involved in sample preparation and photon transfer processing. However, a hypothesis can be put forward from the above experiments. In the range from 30 keV to 25 MeV, Compton scattering is regarded as the main means of photon energy attenuation. In terms of a nano particle, its relatively smaller particle clearance and larger electron cloud density bring a greater Compton scattering cross section, which increases the deposition speed of the photon energy during transfer processing. Thus the relative increasing rate of mass attenuation coefficients in Fig. 7 has a growth trend from 964 keV where the Compton scattering takes the place of the photoelectric effect to become the primary form of photon effect.

Relative increase rate (%)

20 18

Linear attenuation coefficient Mass attenuation coefficient

16

References

14 12 10 8 6 4 2 0 5

9

.6 1

2

2

.8 3

3

4

.1 3

5

5

.9 6

6

3

.8 7

7

8

.6

9

6

4 1

1

1

2 1

1

7

0 1

3

3

0 1

4

0

7

Energy (keV)

Fig. 7. Relative increasing rate of linear and mass attenuation coefficients.

Figure 7 shows the relevant increasing rate of linear and mass attenuation coefficients. The equation −𝜇mic 𝛿 = 𝜇nano ×100% is used to calculate the value. It 𝜇mic is clearly seen from this figure that the nano-WO3 /E44 sample has 1–18% increase of shielding capability, but

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