- Email: [email protected]
Effects of secondary electrons emitted from surroundings on defect formation in silica glass under γ-ray irradiation
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 250 (2006) 169–173 www.elsevier.com/locate/nimb
Effects of secondary electrons emitted from surroundings on defect formation in silica glass under c-ray irradiation S. Obata a, T. Yoshida a
a,*
, T. Tanabe b, C. Allen a, M. Okada c, Qiu Xu
c
Department of Nuclear Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Advanced Energy Engineering Science Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan c The Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan Available online 13 July 2006
Abstract We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in silica glass under c-ray irradiation. SiO2 (silica) glass plates and those sandwiched in a pair of various material disks (carbon, stainless steel or lead) were irradiated by c-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra of the glass plates after the irradiation showed three absorption bands peaked around 2 eV, 4 eV and 5.8 eV being assigned to color centers relate metal impurities (Al and Ge) and oxygen-deficient centers like E 0 center, respectively. All three bands were found to grow with c-ray irradiation dose and saturated at higher doses, and absorbance of the bands at the saturation for the sandwiched glass plates was higher than that for the bare glass plate. Moreover, the saturated absorbance was higher for the glass plate sandwiched with heavier materials. Employing Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process, enhanced energy deposition and numbers of secondary electrons and photons emitted from sandwiching material disks to a silica glass plate were calculated. The higher deposition energy correlates well to the higher saturated absorbance, indicating that the secondary electrons and photons emitted from the disks clearly enhanced the defect formation in the sandwiched silica glass plates. This suggests the existence of the dose effect above a critical does, i.e. the irradiation with higher dose will result in higher saturated absorbance. 2006 Elsevier B.V. All rights reserved. PACS: 78.40.q
1. Introduction Radiation damages in covalent bonding materials (semiconductors and insulators) have been investigated for their useful application in fission and fusion reactors. Irradiation of energetic ions, electrons, neutrons and photons (X-rays and c-rays) induces electron excitation and atomic displacement. In these materials, the electron excitation in radiation field plays an important role on degradation of optical and electric properties of those materials [1–7,10]. During the irradiation of high-energy photons, various surrounding materials around a specimen, i.e. a sample holders, con-
*
Corresponding author. Tel.: +81 52 789 5135; fax: +81 52 789 5137. E-mail address: [email protected] (T. Yoshida).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.102
tainer, etc., must emit secondary electrons and photons from their surfaces and could add some irradiation effects on the direct irradiation of the specimen. Nevertheless, the effects of the secondary electrons and photons coming from the surrounding materials have not been studied systematically. In order to demonstrate that the influence of the surrounding materials on the defect formation in the covalent bonding materials, we have irradiated SiO2 (silica) glass plate sandwiched in a pair of various materials (graphite, stainless steel or lead) by c-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined because irradiation effects are readily apparent via their optical signatures [8,9]. In addition, we have employed Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process.
170
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
plates of rectangles (10 · 10 · 1 mm3) and disks (13-mm diameter and 1-mm thickness) were cut from the glass, and their surfaces were polished to mirror finish. The silica glass plates were sandwiched in a pair of material disk made of graphite (C), stainless steel (SS) or lead (Pb). These silica glass plates were subjected to c-ray irradiation from all directions using a 60Co source at Nagoya University with an irradiation dose rate of 9.0 kGy/h and 3.3 kGy/h in air at room temperature. UV–vis spectra of silica glass plates before and after the irradiation were measured by a spectrophotometer (JASCO V550) for wavelength range of 190–900 nm. The MCNP version 4C2 developed by Los Alamos National Laboratory [11] was used for simulation of photon and electron transport in materials in order to obtain deposition energy in the silica glass plate, numbers and energies of secondary electron and photon emitted from sandwiching material disks to the silica glass plate.
1.3 MeV photon Metal disk
(a) 0.1 to 5 mm Metal disks 0.5 mm
1.3 MeV photon
(b) 0.1 to 2 mm Silica glass
1.3 MeV photon
(c) Silica glass sandwiched in the material disks Fig. 1. Schematics of irradiation geometries for MCNP simulation: (a) a material disk of 10 · 10 mm2 with a variable thickness is irradiated by 1.3 MeV c photons, (b) a silica glass plates of 10 · 10 mm2 with a variable thickness is sandwiched in a pair of material disks of 10 · 10 · 0.5t mm3 is irradiated by 1.3 MeV c photons and (c) a silica plate of 10 · 10 · 1t mm3 is sandwiched in a pair of material disks of 10 · 10 mm2 with a variable thickness is irradiated from all directions by 1.3 MeV c photons.
Deposited energy/Incident photon (keV)
1.4 Bare silica glass Silica glass sandwiched in Pb plates
1.2 1 0.8 0.6 0.4 0.2
(a) 0
0
0.2
2. Experimental procedure
0.4 0.6 Depth (mm)
0.8
1
Secondary electron yield/Incident photon (10-3)
10 8 6 4
8
6
4
2 C Al SS W Pb
2 0
Ratio of deposited energies
10
A low-OH fused silica glass (T-2030: OH < 1 ppm) manufactured by Toshiba Ceramics was used. The silica glass
0
1
2
3
4
5
(b) 0
6
Thickness of plates (mm)
Fig. 2. Net number of electrons escaped from various materials and various thicknesses calculated by the MCNP code.
0
0.2
0.4 0.6 Depth (mm)
0.8
1
Fig. 3. (a) Depth profiles of deposition energies for c-ray irradiated bare silica glass plates and those sandwiched with two Pb disks and (b) depth profiles of the ratio of the deposition energy for sandwiched glass plate against that for the bare glass plate calculated by the MCNP code.
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
3. Results 3.1. MCNP simulation Fig. 1(a) illustrates the irradiation geometry for the simulation: a material disk with a size of 10 · 10 mm2 with different thicknesses is irradiated in vacuum from one side of the material disk by 1.3-MeV c-ray photons (simulating photon energies from a 60Co source). Fig. 2 shows the results, i.e. the number of electrons generated and then escaped from the disk for various materials and thicknesses. With increasing the materials thickness of the disk, the number of escaping electrons first increases, reaches a maximum and decreases beyond a certain thickness. The increase is caused by the volumetric increase of Compton, photoelectron and electron knock-on events, while the decrease beyond a certain thickness is due to the self-shielding, i.e. the diminishing chance of electron escaping from the disk. For Pb and W disks, as high Z number and high electron density materials, the maximum appears around
0.5 mm. On the other hand, for the C and Al disks as low Z number and low electron density materials, the maximum is attained at rather thicker disks of around 2 mm. Because of escaping electrons and photons from a pair of Pb disks sandwiching a silica glass plate, for which geometry is given in Fig. 1(b), the deposited energy on the silica glass plate (normalized by its thickness) by cray irradiation are larger than that on a bare silica plate. The normalized deposited energies on the sandwiched silica glass plate decreased with increasing its thickness and above 2 mm the effect of the sandwiching disappears. Fig. 3(a) compares the depth distributions of the deposited energies in a silica glass plate sandwiched in a pair of Pb metal disks and that in a bare silica glass plate. Evidently, the deposited energy is always higher for the sandwiched silica glass plate, and the ratio of the deposited energy in the sandwiched glass plate to that in the bare glass plate is plotted against the distance from the silica glass surface facing to incident c-ray (Fig. 3(b)). 0.35
0.35
(a)
(b) 0.3
Diff. Absorbance (arb. unit)
Diff. Absorbance (arb. unit)
0.3 0.25 0.2
40000s 0.15
4000s 0.1
2000s 0.05
0.25 0.2
40000s 0.15 0.1
4000s 2000s
0.05
400s
400s 0
1
2
3
4
5
6
0
7
1
2
3
4
5
0.35
7
0.35
(c)
(d)
0.3
Diff. Absorbance (arb. unit)
0.3
0.25
40000s 0.2 0.15
4000s
0.1
2000s
0.05
0.25
40000s 0.2 0.15
4000s
0.1
2000s 0.05
400s 0
6
Energy (eV)
Energy (eV)
Diff. Absorbance (arb. unit)
171
1
2
3
4
Energy (eV)
5
6
400s 7
0
1
2
3
4
5
6
7
Energy (eV)
Fig. 4. Difference optical absorption spectra of c-ray irradiated silica glass plate (a) and those sandwiched in a pair of disks of carbon (b), stainless steel (c) and lead (d). The difference spectrum was obtained by subtracting the optical absorption spectrum of the unirradiated plate from that of its c-ray irradiated plate. The irradiation time were 400, 2000, 4000 and 40,000 s.
172
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
Thus, the MCNP simulation indicates that the enhancement of the radiation damage on the silica glass plate by sandwiching material disks clearly appears but it is limited to the region near the surface of the glass plates contacting to a material disks or to a thin glass plate. 3.2. Measurements of UV–vis optical absorption spectra The UV–vis optical absorbance for the sandwiched glass plates and for a bare glass plate is shown in Fig. 4 as difference absorbance spectra, i.e. the absorbance for the irradiated plate subtracted by that for the unirradiated one in order to emphasize the changes in UV–vis spectra by c-ray irradiation. All the different spectra have three
absorption bands at 2, 4 and 5.8 eV assigned to Al and Ge impurities-related centers and oxygen-deficient centers like E 0 center, respectively [7]. The absorbance of the 2and 5.8-eV bands rapidly increased with the irradiation time and reached to the nearly steady value after the prolonged irradiation, while the absorbance of the 4 eV band gradually decreased to the nearly steady state after attaining the maximum. We observed that the absorbance of the three bands is always higher for the sandwiched glass plates than that for the bare plate at the same irradiation time. Thus, we can conclude that defect production in a silica glass irradiated by c-rays is enhanced by penetrating secondary electrons and photons escaping from surrounding materials. 4. Discussion
Absorbance (mm-1)
0.15
0.1
(a) 0.05
0
Bare silica glass Silica glass sandwiched in C plates Silica glass sandwiched in SS plates Silica glass sandwiched in Pb plates
0
100 200 300 400 Calculated deposition energy (kGy)
500
Absorbance (mm-1)
0.16 0.12 0.08 0.04 0 0
(b) Bare silica glass Silica glass sandwiched in C plates Silica glass sandwiched in SS plates Silica glass sandwiched in Pb plates
100 200 300 400 Calculated deposition energy (kGy)
500
The enhancement of the absorbance by the sandwiching indicates the enhancement of defect formation by penetrating secondary electrons and photons emitted from the sandwiching materials. Therefore, the changing of the absorbance or the defect production rate could be correlated to the deposition energy. In Fig. 5, the observed absorbance for the above three bands in the UV–vis spectra is plotted against the total deposition energies calculated by the MCNP code for the geometry shown in Fig. 1(c). One can see two significant points: one is that the increasing rate with irradiation time became very similar with each other in Fig. 5(a)–(c); and the other is that the saturated absorbance increases as Z number of the sandwiching materials increases. In particular, the absorbance of the 5.8-eV band (Fig. 5(c)) shows this tendency most clearly. The similar increasing rates in the early stage of the irradiation evidences the defect formation is proportional to the energy deposition rates as expected. The different levels of the saturated absorbance may reflect the dose rate effect because of the different deposition energy rate. To confirm this, we have performed another c-ray irradiation of silica glass plates (without
Saturated absorbance of the 5.8 eV band (mm-1)
0.35
Absorbance (mm-1)
0.3
0.2
(c) Bare silica glass Silica glass sandwiched in C plates Silica glass sandwiched in SS plates Silica glass sandwiched in Pb plates
0.1
0 0
100 200 300 400 Calculated deposition energy (kGy)
500
Fig. 5. Changes in the absorbance of the bands at (a) 2, (b) 4 and (c) 5.8 eV, observed in the difference optical absorption spectra in Fig. 4, with calculated total energies deposited on silica glass plates.
0.3 0.25 0.2 0.15 0.1 0.05 0
0
2 4 6 8 10 12 14 Deposition energy rate (Gy/s)
Fig. 6. Plots of the saturated absorbance of the 5.8 eV band against the deposition energy rate calculated by MCNP code for silica glass and those sandwiched in a pair of disks of carbon, stainless steel or lead.
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
173
the sandwiching material disks) with two different dose rates of 3.3 kGy/h and 9.0 kGy/h. The initial increasing rate of the absorbance for the 2-, 4- and 5.8-eV bands with the irradiation dose was similar to the previous results, while the values of the saturated absorbance for the two silica glasses after the prolonged c-ray irradiation were the same. The values of the saturated absorbance for 5.8-eV band, for example, are plotted against the normalized deposition energy rate calculated by the MCNP code for the similar geometry with the experiments, i.e. for a bare silica glass plate and for those sandwiched in a pairs of different material disks irradiated by c-rays from all directions (Fig. 6). We note that the saturated absorbance rises above approximately 8 Gy/s and linearly increases with the further increase of the deposition energy rate while it stays constant below 8 Gy/s. This strongly supports the appearance of dose effect. In order to confirm this dose effect directly, we need more intense c-ray irradiation than 8 Gy/s, which is the threshold value that appears in Fig. 6.
disks. The MCNP simulation confirms that the deposition energy is enhanced by secondary electrons and photons escaping from the surrounding materials. However, the effect is limited to the region near the surface and/or thin glasses. It was also revealed that the defect formation process in the silica glass plates is influenced by the dose rate effect, i.e. the irradiation dose rate that is above approximately 8 Gy/s the saturated defect density is increased proportionally to the dose rate, while below 8 Gy/s the saturated defect density stays constant. To confirm this we need higher dose rate irradiation and will do in near future.
5. Conclusion
References
We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in a silica glass under c-ray irradiation. The silica glass plate sandwiched in a pair of material disks made of carbon, stainless steel or lead was irradiated by c-rays and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra clearly demonstrated that the various types of defects such as Al and Ge impuritiesrelated centers and oxygen-deficient centers are produced by the c-ray irradiation. The defect production was enhanced in the silica glass plates sandwiched in a pair of the material disks because of the increase of the deposition energy in the glass plate due to the penetration of secondary electrons and photons emitted from the pair of material
[1] T. Yoshida, T. Tanabe, M. Watanabe, S. Takahara, S. Mizukami, F. Nucl. Mater. 329–333 (2004) 982. [2] S. Takahara, T. Yoshida, T. Tanabe, T. Ii, M. Hirano, M. Okada, Nucl. Instr. and Meth. B 218 (2004) 133. [3] C.D. Marshall, J.A. Speth, S.A. Payne, J. Non-Cryst. Solids 212 (1997) 59. [4] G.O. Sawakuchi, E. Okuno, Nucl. Instr. and Meth. B 218 (2004) 217. [5] Y. Kawaguchi, N. Kuzuu, J. Appl. Phys. 80 (1996) 10. [6] H.J. Fitting, T. Barfels, A.V. Czarnowski, A.N. Trukhin, Mater. Sci. Eng. B 71 (1999) 109. [7] S. Agnello, B. Boizot, J. Non-Cryst. Solids 322 (2003) 84. [8] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [9] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 899. [10] T. Tanabe, S. Tanaka, K. Yamaguchi, N. Otsuki, T. Iida, M. Yamawaki, J. Nucl. Mater. 212–215 (1994) 1050. [11] J.F. Briesmeister (Ed.), MCNPTM – A General Monte Carlo N-Particle Transport Code, Version 4C, LA-137090M, Los Alamos National Laboratory, USA, 2000.
Acknowledgement The authors are grateful to Professor S. Muto of the Department of Nuclear Engineering, Graduate School of Engineering, Nagoya University, for his helpful discussion.
Nuclear Instruments and Methods in Physics Research B 250 (2006) 169–173 www.elsevier.com/locate/nimb
Effects of secondary electrons emitted from surroundings on defect formation in silica glass under c-ray irradiation S. Obata a, T. Yoshida a
a,*
, T. Tanabe b, C. Allen a, M. Okada c, Qiu Xu
c
Department of Nuclear Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Advanced Energy Engineering Science Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan c The Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan Available online 13 July 2006
Abstract We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in silica glass under c-ray irradiation. SiO2 (silica) glass plates and those sandwiched in a pair of various material disks (carbon, stainless steel or lead) were irradiated by c-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra of the glass plates after the irradiation showed three absorption bands peaked around 2 eV, 4 eV and 5.8 eV being assigned to color centers relate metal impurities (Al and Ge) and oxygen-deficient centers like E 0 center, respectively. All three bands were found to grow with c-ray irradiation dose and saturated at higher doses, and absorbance of the bands at the saturation for the sandwiched glass plates was higher than that for the bare glass plate. Moreover, the saturated absorbance was higher for the glass plate sandwiched with heavier materials. Employing Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process, enhanced energy deposition and numbers of secondary electrons and photons emitted from sandwiching material disks to a silica glass plate were calculated. The higher deposition energy correlates well to the higher saturated absorbance, indicating that the secondary electrons and photons emitted from the disks clearly enhanced the defect formation in the sandwiched silica glass plates. This suggests the existence of the dose effect above a critical does, i.e. the irradiation with higher dose will result in higher saturated absorbance. 2006 Elsevier B.V. All rights reserved. PACS: 78.40.q
1. Introduction Radiation damages in covalent bonding materials (semiconductors and insulators) have been investigated for their useful application in fission and fusion reactors. Irradiation of energetic ions, electrons, neutrons and photons (X-rays and c-rays) induces electron excitation and atomic displacement. In these materials, the electron excitation in radiation field plays an important role on degradation of optical and electric properties of those materials [1–7,10]. During the irradiation of high-energy photons, various surrounding materials around a specimen, i.e. a sample holders, con-
*
Corresponding author. Tel.: +81 52 789 5135; fax: +81 52 789 5137. E-mail address: [email protected] (T. Yoshida).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.102
tainer, etc., must emit secondary electrons and photons from their surfaces and could add some irradiation effects on the direct irradiation of the specimen. Nevertheless, the effects of the secondary electrons and photons coming from the surrounding materials have not been studied systematically. In order to demonstrate that the influence of the surrounding materials on the defect formation in the covalent bonding materials, we have irradiated SiO2 (silica) glass plate sandwiched in a pair of various materials (graphite, stainless steel or lead) by c-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined because irradiation effects are readily apparent via their optical signatures [8,9]. In addition, we have employed Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process.
170
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
plates of rectangles (10 · 10 · 1 mm3) and disks (13-mm diameter and 1-mm thickness) were cut from the glass, and their surfaces were polished to mirror finish. The silica glass plates were sandwiched in a pair of material disk made of graphite (C), stainless steel (SS) or lead (Pb). These silica glass plates were subjected to c-ray irradiation from all directions using a 60Co source at Nagoya University with an irradiation dose rate of 9.0 kGy/h and 3.3 kGy/h in air at room temperature. UV–vis spectra of silica glass plates before and after the irradiation were measured by a spectrophotometer (JASCO V550) for wavelength range of 190–900 nm. The MCNP version 4C2 developed by Los Alamos National Laboratory [11] was used for simulation of photon and electron transport in materials in order to obtain deposition energy in the silica glass plate, numbers and energies of secondary electron and photon emitted from sandwiching material disks to the silica glass plate.
1.3 MeV photon Metal disk
(a) 0.1 to 5 mm Metal disks 0.5 mm
1.3 MeV photon
(b) 0.1 to 2 mm Silica glass
1.3 MeV photon
(c) Silica glass sandwiched in the material disks Fig. 1. Schematics of irradiation geometries for MCNP simulation: (a) a material disk of 10 · 10 mm2 with a variable thickness is irradiated by 1.3 MeV c photons, (b) a silica glass plates of 10 · 10 mm2 with a variable thickness is sandwiched in a pair of material disks of 10 · 10 · 0.5t mm3 is irradiated by 1.3 MeV c photons and (c) a silica plate of 10 · 10 · 1t mm3 is sandwiched in a pair of material disks of 10 · 10 mm2 with a variable thickness is irradiated from all directions by 1.3 MeV c photons.
Deposited energy/Incident photon (keV)
1.4 Bare silica glass Silica glass sandwiched in Pb plates
1.2 1 0.8 0.6 0.4 0.2
(a) 0
0
0.2
2. Experimental procedure
0.4 0.6 Depth (mm)
0.8
1
Secondary electron yield/Incident photon (10-3)
10 8 6 4
8
6
4
2 C Al SS W Pb
2 0
Ratio of deposited energies
10
A low-OH fused silica glass (T-2030: OH < 1 ppm) manufactured by Toshiba Ceramics was used. The silica glass
0
1
2
3
4
5
(b) 0
6
Thickness of plates (mm)
Fig. 2. Net number of electrons escaped from various materials and various thicknesses calculated by the MCNP code.
0
0.2
0.4 0.6 Depth (mm)
0.8
1
Fig. 3. (a) Depth profiles of deposition energies for c-ray irradiated bare silica glass plates and those sandwiched with two Pb disks and (b) depth profiles of the ratio of the deposition energy for sandwiched glass plate against that for the bare glass plate calculated by the MCNP code.
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
3. Results 3.1. MCNP simulation Fig. 1(a) illustrates the irradiation geometry for the simulation: a material disk with a size of 10 · 10 mm2 with different thicknesses is irradiated in vacuum from one side of the material disk by 1.3-MeV c-ray photons (simulating photon energies from a 60Co source). Fig. 2 shows the results, i.e. the number of electrons generated and then escaped from the disk for various materials and thicknesses. With increasing the materials thickness of the disk, the number of escaping electrons first increases, reaches a maximum and decreases beyond a certain thickness. The increase is caused by the volumetric increase of Compton, photoelectron and electron knock-on events, while the decrease beyond a certain thickness is due to the self-shielding, i.e. the diminishing chance of electron escaping from the disk. For Pb and W disks, as high Z number and high electron density materials, the maximum appears around
0.5 mm. On the other hand, for the C and Al disks as low Z number and low electron density materials, the maximum is attained at rather thicker disks of around 2 mm. Because of escaping electrons and photons from a pair of Pb disks sandwiching a silica glass plate, for which geometry is given in Fig. 1(b), the deposited energy on the silica glass plate (normalized by its thickness) by cray irradiation are larger than that on a bare silica plate. The normalized deposited energies on the sandwiched silica glass plate decreased with increasing its thickness and above 2 mm the effect of the sandwiching disappears. Fig. 3(a) compares the depth distributions of the deposited energies in a silica glass plate sandwiched in a pair of Pb metal disks and that in a bare silica glass plate. Evidently, the deposited energy is always higher for the sandwiched silica glass plate, and the ratio of the deposited energy in the sandwiched glass plate to that in the bare glass plate is plotted against the distance from the silica glass surface facing to incident c-ray (Fig. 3(b)). 0.35
0.35
(a)
(b) 0.3
Diff. Absorbance (arb. unit)
Diff. Absorbance (arb. unit)
0.3 0.25 0.2
40000s 0.15
4000s 0.1
2000s 0.05
0.25 0.2
40000s 0.15 0.1
4000s 2000s
0.05
400s
400s 0
1
2
3
4
5
6
0
7
1
2
3
4
5
0.35
7
0.35
(c)
(d)
0.3
Diff. Absorbance (arb. unit)
0.3
0.25
40000s 0.2 0.15
4000s
0.1
2000s
0.05
0.25
40000s 0.2 0.15
4000s
0.1
2000s 0.05
400s 0
6
Energy (eV)
Energy (eV)
Diff. Absorbance (arb. unit)
171
1
2
3
4
Energy (eV)
5
6
400s 7
0
1
2
3
4
5
6
7
Energy (eV)
Fig. 4. Difference optical absorption spectra of c-ray irradiated silica glass plate (a) and those sandwiched in a pair of disks of carbon (b), stainless steel (c) and lead (d). The difference spectrum was obtained by subtracting the optical absorption spectrum of the unirradiated plate from that of its c-ray irradiated plate. The irradiation time were 400, 2000, 4000 and 40,000 s.
172
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
Thus, the MCNP simulation indicates that the enhancement of the radiation damage on the silica glass plate by sandwiching material disks clearly appears but it is limited to the region near the surface of the glass plates contacting to a material disks or to a thin glass plate. 3.2. Measurements of UV–vis optical absorption spectra The UV–vis optical absorbance for the sandwiched glass plates and for a bare glass plate is shown in Fig. 4 as difference absorbance spectra, i.e. the absorbance for the irradiated plate subtracted by that for the unirradiated one in order to emphasize the changes in UV–vis spectra by c-ray irradiation. All the different spectra have three
absorption bands at 2, 4 and 5.8 eV assigned to Al and Ge impurities-related centers and oxygen-deficient centers like E 0 center, respectively [7]. The absorbance of the 2and 5.8-eV bands rapidly increased with the irradiation time and reached to the nearly steady value after the prolonged irradiation, while the absorbance of the 4 eV band gradually decreased to the nearly steady state after attaining the maximum. We observed that the absorbance of the three bands is always higher for the sandwiched glass plates than that for the bare plate at the same irradiation time. Thus, we can conclude that defect production in a silica glass irradiated by c-rays is enhanced by penetrating secondary electrons and photons escaping from surrounding materials. 4. Discussion
Absorbance (mm-1)
0.15
0.1
(a) 0.05
0
Bare silica glass Silica glass sandwiched in C plates Silica glass sandwiched in SS plates Silica glass sandwiched in Pb plates
0
100 200 300 400 Calculated deposition energy (kGy)
500
Absorbance (mm-1)
0.16 0.12 0.08 0.04 0 0
(b) Bare silica glass Silica glass sandwiched in C plates Silica glass sandwiched in SS plates Silica glass sandwiched in Pb plates
100 200 300 400 Calculated deposition energy (kGy)
500
The enhancement of the absorbance by the sandwiching indicates the enhancement of defect formation by penetrating secondary electrons and photons emitted from the sandwiching materials. Therefore, the changing of the absorbance or the defect production rate could be correlated to the deposition energy. In Fig. 5, the observed absorbance for the above three bands in the UV–vis spectra is plotted against the total deposition energies calculated by the MCNP code for the geometry shown in Fig. 1(c). One can see two significant points: one is that the increasing rate with irradiation time became very similar with each other in Fig. 5(a)–(c); and the other is that the saturated absorbance increases as Z number of the sandwiching materials increases. In particular, the absorbance of the 5.8-eV band (Fig. 5(c)) shows this tendency most clearly. The similar increasing rates in the early stage of the irradiation evidences the defect formation is proportional to the energy deposition rates as expected. The different levels of the saturated absorbance may reflect the dose rate effect because of the different deposition energy rate. To confirm this, we have performed another c-ray irradiation of silica glass plates (without
Saturated absorbance of the 5.8 eV band (mm-1)
0.35
Absorbance (mm-1)
0.3
0.2
(c) Bare silica glass Silica glass sandwiched in C plates Silica glass sandwiched in SS plates Silica glass sandwiched in Pb plates
0.1
0 0
100 200 300 400 Calculated deposition energy (kGy)
500
Fig. 5. Changes in the absorbance of the bands at (a) 2, (b) 4 and (c) 5.8 eV, observed in the difference optical absorption spectra in Fig. 4, with calculated total energies deposited on silica glass plates.
0.3 0.25 0.2 0.15 0.1 0.05 0
0
2 4 6 8 10 12 14 Deposition energy rate (Gy/s)
Fig. 6. Plots of the saturated absorbance of the 5.8 eV band against the deposition energy rate calculated by MCNP code for silica glass and those sandwiched in a pair of disks of carbon, stainless steel or lead.
S. Obata et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 169–173
173
the sandwiching material disks) with two different dose rates of 3.3 kGy/h and 9.0 kGy/h. The initial increasing rate of the absorbance for the 2-, 4- and 5.8-eV bands with the irradiation dose was similar to the previous results, while the values of the saturated absorbance for the two silica glasses after the prolonged c-ray irradiation were the same. The values of the saturated absorbance for 5.8-eV band, for example, are plotted against the normalized deposition energy rate calculated by the MCNP code for the similar geometry with the experiments, i.e. for a bare silica glass plate and for those sandwiched in a pairs of different material disks irradiated by c-rays from all directions (Fig. 6). We note that the saturated absorbance rises above approximately 8 Gy/s and linearly increases with the further increase of the deposition energy rate while it stays constant below 8 Gy/s. This strongly supports the appearance of dose effect. In order to confirm this dose effect directly, we need more intense c-ray irradiation than 8 Gy/s, which is the threshold value that appears in Fig. 6.
disks. The MCNP simulation confirms that the deposition energy is enhanced by secondary electrons and photons escaping from the surrounding materials. However, the effect is limited to the region near the surface and/or thin glasses. It was also revealed that the defect formation process in the silica glass plates is influenced by the dose rate effect, i.e. the irradiation dose rate that is above approximately 8 Gy/s the saturated defect density is increased proportionally to the dose rate, while below 8 Gy/s the saturated defect density stays constant. To confirm this we need higher dose rate irradiation and will do in near future.
5. Conclusion
References
We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in a silica glass under c-ray irradiation. The silica glass plate sandwiched in a pair of material disks made of carbon, stainless steel or lead was irradiated by c-rays and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra clearly demonstrated that the various types of defects such as Al and Ge impuritiesrelated centers and oxygen-deficient centers are produced by the c-ray irradiation. The defect production was enhanced in the silica glass plates sandwiched in a pair of the material disks because of the increase of the deposition energy in the glass plate due to the penetration of secondary electrons and photons emitted from the pair of material
[1] T. Yoshida, T. Tanabe, M. Watanabe, S. Takahara, S. Mizukami, F. Nucl. Mater. 329–333 (2004) 982. [2] S. Takahara, T. Yoshida, T. Tanabe, T. Ii, M. Hirano, M. Okada, Nucl. Instr. and Meth. B 218 (2004) 133. [3] C.D. Marshall, J.A. Speth, S.A. Payne, J. Non-Cryst. Solids 212 (1997) 59. [4] G.O. Sawakuchi, E. Okuno, Nucl. Instr. and Meth. B 218 (2004) 217. [5] Y. Kawaguchi, N. Kuzuu, J. Appl. Phys. 80 (1996) 10. [6] H.J. Fitting, T. Barfels, A.V. Czarnowski, A.N. Trukhin, Mater. Sci. Eng. B 71 (1999) 109. [7] S. Agnello, B. Boizot, J. Non-Cryst. Solids 322 (2003) 84. [8] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [9] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 899. [10] T. Tanabe, S. Tanaka, K. Yamaguchi, N. Otsuki, T. Iida, M. Yamawaki, J. Nucl. Mater. 212–215 (1994) 1050. [11] J.F. Briesmeister (Ed.), MCNPTM – A General Monte Carlo N-Particle Transport Code, Version 4C, LA-137090M, Los Alamos National Laboratory, USA, 2000.
Acknowledgement The authors are grateful to Professor S. Muto of the Department of Nuclear Engineering, Graduate School of Engineering, Nagoya University, for his helpful discussion.