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Dynamical behavior of helium bubbles in gold during irradiation with high-energy self-ions
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 455–457 www.elsevier.com/locate/nimb
Dynamical behavior of helium bubbles in gold during irradiation with high-energy self-ions K. Ono a
a,*
, M. Miyamoto a, K. Arakawa b, R.C. Birtcher
c
Department of Materials Science, Shimane University, 1060 Nishi-Kawatsu, Matsue 690-8504, Japan b UHV–Microscopy Center, Osaka University, Osaka 565-0871, Japan c MSD, Argonne National Laboratory, Argonne, IL 60439-4838, USA Available online 19 October 2005
Abstract The dynamical response of helium bubbles to irradiation with high-energy self-ions in pure gold has been studied by in situ electron microscopy. It is found that sporadic movement of small helium bubbles is induced in random directions within a few nm or less under the irradiation with 400 keV Au+ ions at room temperature, where bubbles are thermally immobile. Computer simulation indicates that these facts are consistent with the bubble movement interacting with the collision sub-cascades. 2005 Elsevier B.V. All rights reserved. PACS: 61.80.x; 61.80.Az; 61.82.Bg; 61.72.Ff Keywords: Helium bubble; Irradiation damage; Gold; Electron microscopy
1. Introduction The response of helium bubbles to irradiation with highenergy self-ions probes the dynamical effects of displacement damage and may give new insight into cascade damage. The resulting knowledge is essential for the development of fusion reactor materials. In early works on the motion of helium bubbles in gold, Willarts and Showmon [1] and Evans and van Veen [2] observed the bubble displacement during thermal annealing and estimated the bubble diffusivity, assuming Brownian type motion. Donnelly et al. [3] first observed the shrinkage, growth, motion and disappearance of helium bubbles under irradiation with high-energy Ar+ ions in gold at 500 K. This motion was described in terms of incorporation of a single He bubble into the melt zone of a displacement cascade. However, more precise observation of each bubble behavior, with finer dose steps, is necessary to more clearly elucidate cascade effects on He bubble evolution. *
Corresponding author. Tel.: +81 852 32 6403; fax: +81 852 32 6409. E-mail address: [email protected] (K. Ono).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.173
In the present work, an in situ TEM study has been made of He bubbles as a function of time during beam on and beam-off periods of irradiation with high-energy self-ions. This technique was successfully applied to Cu [4], where the intermittent rapid motion of the bubble was found. The main result is motion of He bubbles induced by displacement cascades in gold. 2. Experimental procedure Disk shaped poly-crystalline specimens of 99.999 at.% purity gold supplied by Tanaka–Kikinzoku group were annealed in ultra-high vacuum at 1150 K and then polished electrochemically for transmission electron microscopy (TEM). Small size helium bubbles were introduced into the specimen at around 600 K by irradiation with 10 keV helium ions in an electron microscope, to which a lowenergy ion accelerator is attached. The specimen was then warmed stepwise to higher temperatures and thermal motion of helium bubbles was examined. Dynamical response of helium bubbles to irradiation with 400 keV Au ions was examined by using the intermediate voltage
456
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 455–457
electron microscope (IVEM) facility at Argonne National Laboratory. The flux of Au ions beam was 1.1 · 1016 Au/ m2 s. The dynamical behavior of helium bubbles during the beam on or beam-off periods was monitored by TEM and recorded with a video system. 3. Results and discussion The location of helium bubbles formed by irradiation of gold with 10 keV He+ ions at 600 K should be described by the product of the projected ion range and damage deposition given by TRIM code [5], as similar with the case of other metal [6]. Fig. 1 shows the product distribution for helium bubbles and dpa-deposition calculated for 400 keV Au ions in gold. Bubbles should be located 5– 50 nm in the depth. The number density of the bubble measured from the projected TEM image is about 1 · 1016 m2. At temperatures higher than 900 K, Brownian type thermal motion of the helium bubble was demonstrated by a proportional relation between the mean square migration distance and time, as observed in the other metals [6,7]. The details will be shown elsewhere [8]. At room temperature, no bubble motion was observed during the beam-off periods. However, athermal bubble movement of a few nm or less was found under irradiation with 400 keV self-ions. In Fig. 2, a series of TV frames of the same area are shown, where the specimen was irradiated with 400 keV Au ions at 300 K for 0 s, 7 s and 14 s, respectively. In the image for time 0 s, clearly imaged bubbles are circled with the black line, and the circles are overlaid on the bubble images at 7 s and 14 s to display bubble movement during the irradiation. Bubbles judged to show clear movement are encircled with the dotted line in the photographs at 7 s and 14 s. About 20–30% of the bubbles moved in random directions during 7 s and 40–60% during 14 s. The movement is sporadic and the typical distance is less than 2–3 nm. These values are for clearly imaged bubbles. Some times, a bubbleÕs image disappeared because of a change of diffraction conditions due to dislocation loop formation or bending of the specimen. However, no true disappearance of a bubble during these times was observed. After irradiation to the order of 1 · 1019 ions/m2, bubbles tended to become large by absorption of cascade vacancies and then the bubble movement is reduced in contrast to the previous work [3], where more dynamical bubble behavior Au damage [arb. units]
He bubble
0
20
40 60 Depth [nm]
80
100
Fig. 1. Calculated depth distribution of helium bubbles produced by 10 keV He+ ions and damage deposition by 400 keV Au ions in Au, using the displacement threshold energy of 40 eV.
Fig. 2. A series of images extracted from a video recording showing He bubble movement in Au during room temperature irradiation with 400 keV Au ions at 0 s, 7 s and 14 s, respectively. The flux of Au ions was 1/10 nm2 s.
was reported under the irradiated with high dose Ar+ ions at 500 K. To describe He bubble behavior, we consider the interaction of a bubble with a cascades zone, as the basic idea proposed [3]. The probability of a bubble to interact with a displacement cascade is the product of the cascade production rate in the region of the bubble and an appropriate interaction zone surrounding the bubble. The interaction rate is given by: 4 M ¼ pr3 N U=z; 3
ð1Þ
where r is the radius of the interaction volume, N is the number of the sub-cascades per ion in the bubble zone, U
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 455–457
6
Size [nm]
5 4 3 2 1 0 0
10
20 30 Depth [nm]
40
50
Fig. 3. Size (diameter of the same volume sphere) distribution of collision sub-cascades as a function of the specimen depth, which was calculation by the TRIM code [5].
the ion flux, and z the cascade zone thickness in the specimen. The number of the sub-cascades and their diameters were calculated by TRIM code [5] and the results are shown in Fig. 3, where the statistical standard deviation of the cascade size was assumed, being approximated with the same volume sphere. From Fig. 3, the average diameter of the sub-cascades larger than 2 nm (bubble size) is about 3.8 nm and the number per ion is 7. Therefore, Eq. (1) yields M = 0.05 s1 using r = 1.9 nm, N = 7, U = 1 · 1016 ions/m2 s and z = 40 nm. Sub-cascades were produced between the specimen surface and a 40 nm depth. Since the volume contains 70% of the bubbles, as known
457
from Fig. 1, about 3% of the bubble should interact with a cascade per second, which is in good agreement with the experimentally observed percents of moved bubbles. The observed motion distance is also consistent the average size of sub-cascade larger than 2 nm. These facts suggest that the bubbles interacting with sub-cascades are mobile in the cascade zone. The bubble movement may be induced by thermal spike effects [9,10]. The spike temperature and high density vacancies in the cascade could allow rearrangements allowing the bubble to move. References [1] L.E. Willarts, P.G. Showmon, Metall. Trans. 1 (1970) 2217. [2] J.H. Evans, A. van Veen, J. Nucl. Mater. 168 (1989) 12. [3] S.E. Donnelly, R.C. Birtcher, C. Templer, V. Vishnyakov, Phys. Rev. 52 (1995) 3970. [4] K. Ono, K. Arakawa, R.C. Birtcher, Nucl. Inst. and Meth. B 206 (2003) 114. [5] J.P. Biersack, L.G. Haggmark, Nucl. Inst. and Meth. 174 (1980) 257. [6] K. Ono, K. Arakawa, M. Oohashi, H. Kurata, K. Hojou, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 210. [7] K. Ono, K. Arakawa, K. Hojou, M. Oohashi, R.C. Birtcher, S.E. Donnelly, J. Elect. Microsco. 51 (2002) S245. [8] K. Ono, M. Miyamoto, K. Arakawa, R.C. Birtcher, Materia Japan, 2005, in press. [9] R.S. Averback, T. Diaz de la Rubia, H. Hsieh, R. Benedek, Nucl. Inst. and Meth. B 59/60 (1991) 709. [10] M. Ghaly, R.S. Averback, Phys. Rev. Lett. 72 (1994) 364.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 455–457 www.elsevier.com/locate/nimb
Dynamical behavior of helium bubbles in gold during irradiation with high-energy self-ions K. Ono a
a,*
, M. Miyamoto a, K. Arakawa b, R.C. Birtcher
c
Department of Materials Science, Shimane University, 1060 Nishi-Kawatsu, Matsue 690-8504, Japan b UHV–Microscopy Center, Osaka University, Osaka 565-0871, Japan c MSD, Argonne National Laboratory, Argonne, IL 60439-4838, USA Available online 19 October 2005
Abstract The dynamical response of helium bubbles to irradiation with high-energy self-ions in pure gold has been studied by in situ electron microscopy. It is found that sporadic movement of small helium bubbles is induced in random directions within a few nm or less under the irradiation with 400 keV Au+ ions at room temperature, where bubbles are thermally immobile. Computer simulation indicates that these facts are consistent with the bubble movement interacting with the collision sub-cascades. 2005 Elsevier B.V. All rights reserved. PACS: 61.80.x; 61.80.Az; 61.82.Bg; 61.72.Ff Keywords: Helium bubble; Irradiation damage; Gold; Electron microscopy
1. Introduction The response of helium bubbles to irradiation with highenergy self-ions probes the dynamical effects of displacement damage and may give new insight into cascade damage. The resulting knowledge is essential for the development of fusion reactor materials. In early works on the motion of helium bubbles in gold, Willarts and Showmon [1] and Evans and van Veen [2] observed the bubble displacement during thermal annealing and estimated the bubble diffusivity, assuming Brownian type motion. Donnelly et al. [3] first observed the shrinkage, growth, motion and disappearance of helium bubbles under irradiation with high-energy Ar+ ions in gold at 500 K. This motion was described in terms of incorporation of a single He bubble into the melt zone of a displacement cascade. However, more precise observation of each bubble behavior, with finer dose steps, is necessary to more clearly elucidate cascade effects on He bubble evolution. *
Corresponding author. Tel.: +81 852 32 6403; fax: +81 852 32 6409. E-mail address: [email protected] (K. Ono).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.173
In the present work, an in situ TEM study has been made of He bubbles as a function of time during beam on and beam-off periods of irradiation with high-energy self-ions. This technique was successfully applied to Cu [4], where the intermittent rapid motion of the bubble was found. The main result is motion of He bubbles induced by displacement cascades in gold. 2. Experimental procedure Disk shaped poly-crystalline specimens of 99.999 at.% purity gold supplied by Tanaka–Kikinzoku group were annealed in ultra-high vacuum at 1150 K and then polished electrochemically for transmission electron microscopy (TEM). Small size helium bubbles were introduced into the specimen at around 600 K by irradiation with 10 keV helium ions in an electron microscope, to which a lowenergy ion accelerator is attached. The specimen was then warmed stepwise to higher temperatures and thermal motion of helium bubbles was examined. Dynamical response of helium bubbles to irradiation with 400 keV Au ions was examined by using the intermediate voltage
456
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 455–457
electron microscope (IVEM) facility at Argonne National Laboratory. The flux of Au ions beam was 1.1 · 1016 Au/ m2 s. The dynamical behavior of helium bubbles during the beam on or beam-off periods was monitored by TEM and recorded with a video system. 3. Results and discussion The location of helium bubbles formed by irradiation of gold with 10 keV He+ ions at 600 K should be described by the product of the projected ion range and damage deposition given by TRIM code [5], as similar with the case of other metal [6]. Fig. 1 shows the product distribution for helium bubbles and dpa-deposition calculated for 400 keV Au ions in gold. Bubbles should be located 5– 50 nm in the depth. The number density of the bubble measured from the projected TEM image is about 1 · 1016 m2. At temperatures higher than 900 K, Brownian type thermal motion of the helium bubble was demonstrated by a proportional relation between the mean square migration distance and time, as observed in the other metals [6,7]. The details will be shown elsewhere [8]. At room temperature, no bubble motion was observed during the beam-off periods. However, athermal bubble movement of a few nm or less was found under irradiation with 400 keV self-ions. In Fig. 2, a series of TV frames of the same area are shown, where the specimen was irradiated with 400 keV Au ions at 300 K for 0 s, 7 s and 14 s, respectively. In the image for time 0 s, clearly imaged bubbles are circled with the black line, and the circles are overlaid on the bubble images at 7 s and 14 s to display bubble movement during the irradiation. Bubbles judged to show clear movement are encircled with the dotted line in the photographs at 7 s and 14 s. About 20–30% of the bubbles moved in random directions during 7 s and 40–60% during 14 s. The movement is sporadic and the typical distance is less than 2–3 nm. These values are for clearly imaged bubbles. Some times, a bubbleÕs image disappeared because of a change of diffraction conditions due to dislocation loop formation or bending of the specimen. However, no true disappearance of a bubble during these times was observed. After irradiation to the order of 1 · 1019 ions/m2, bubbles tended to become large by absorption of cascade vacancies and then the bubble movement is reduced in contrast to the previous work [3], where more dynamical bubble behavior Au damage [arb. units]
He bubble
0
20
40 60 Depth [nm]
80
100
Fig. 1. Calculated depth distribution of helium bubbles produced by 10 keV He+ ions and damage deposition by 400 keV Au ions in Au, using the displacement threshold energy of 40 eV.
Fig. 2. A series of images extracted from a video recording showing He bubble movement in Au during room temperature irradiation with 400 keV Au ions at 0 s, 7 s and 14 s, respectively. The flux of Au ions was 1/10 nm2 s.
was reported under the irradiated with high dose Ar+ ions at 500 K. To describe He bubble behavior, we consider the interaction of a bubble with a cascades zone, as the basic idea proposed [3]. The probability of a bubble to interact with a displacement cascade is the product of the cascade production rate in the region of the bubble and an appropriate interaction zone surrounding the bubble. The interaction rate is given by: 4 M ¼ pr3 N U=z; 3
ð1Þ
where r is the radius of the interaction volume, N is the number of the sub-cascades per ion in the bubble zone, U
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 455–457
6
Size [nm]
5 4 3 2 1 0 0
10
20 30 Depth [nm]
40
50
Fig. 3. Size (diameter of the same volume sphere) distribution of collision sub-cascades as a function of the specimen depth, which was calculation by the TRIM code [5].
the ion flux, and z the cascade zone thickness in the specimen. The number of the sub-cascades and their diameters were calculated by TRIM code [5] and the results are shown in Fig. 3, where the statistical standard deviation of the cascade size was assumed, being approximated with the same volume sphere. From Fig. 3, the average diameter of the sub-cascades larger than 2 nm (bubble size) is about 3.8 nm and the number per ion is 7. Therefore, Eq. (1) yields M = 0.05 s1 using r = 1.9 nm, N = 7, U = 1 · 1016 ions/m2 s and z = 40 nm. Sub-cascades were produced between the specimen surface and a 40 nm depth. Since the volume contains 70% of the bubbles, as known
457
from Fig. 1, about 3% of the bubble should interact with a cascade per second, which is in good agreement with the experimentally observed percents of moved bubbles. The observed motion distance is also consistent the average size of sub-cascade larger than 2 nm. These facts suggest that the bubbles interacting with sub-cascades are mobile in the cascade zone. The bubble movement may be induced by thermal spike effects [9,10]. The spike temperature and high density vacancies in the cascade could allow rearrangements allowing the bubble to move. References [1] L.E. Willarts, P.G. Showmon, Metall. Trans. 1 (1970) 2217. [2] J.H. Evans, A. van Veen, J. Nucl. Mater. 168 (1989) 12. [3] S.E. Donnelly, R.C. Birtcher, C. Templer, V. Vishnyakov, Phys. Rev. 52 (1995) 3970. [4] K. Ono, K. Arakawa, R.C. Birtcher, Nucl. Inst. and Meth. B 206 (2003) 114. [5] J.P. Biersack, L.G. Haggmark, Nucl. Inst. and Meth. 174 (1980) 257. [6] K. Ono, K. Arakawa, M. Oohashi, H. Kurata, K. Hojou, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 210. [7] K. Ono, K. Arakawa, K. Hojou, M. Oohashi, R.C. Birtcher, S.E. Donnelly, J. Elect. Microsco. 51 (2002) S245. [8] K. Ono, M. Miyamoto, K. Arakawa, R.C. Birtcher, Materia Japan, 2005, in press. [9] R.S. Averback, T. Diaz de la Rubia, H. Hsieh, R. Benedek, Nucl. Inst. and Meth. B 59/60 (1991) 709. [10] M. Ghaly, R.S. Averback, Phys. Rev. Lett. 72 (1994) 364.