- Email: [email protected]
In situ study of ion-beam induced lattice damage in calcium fluoride crystals
Nuclear Instruments
and Methods in Physics
ResearchB l27/128 (1997) 591-595
EISEVIER In
situ study of ion-beam induced lattice damage in calcium fluoride crystals Ning Yu ’ , Michael Nastasi, Kurt E. Sickafus, Kazuhiro Yasuda *, Joseph R. Tesmer Muterials Science urul Technology Division, Los Alamos National Laboratory,
Los Alamos, NM 8754.5, USA
Abstract Lattice damage in calcium fluoride induced by 360 keV xenon ion irradiation was measured in situ using Rutherford backscattering and channeling (RBS/C) techniques. Calcium fluoride single crystals of (100) orientation were irradiated at - 115 and 35°C by Xe ion beams along a random direction 15” off the (100) axis. A 2 MeV He ion-beam from a 3 MV tandem accelerator was incident on the CaF, crystal along the (110) axis to determine the radiation damage. No amorphized layer was observed at both irradiation temperatures up to an irradiation dose of 2 X 1016 Xc/cm*. RBS/C measurements showed that lattice disorder in the irradiated layer saturated at 70% of the random level. Analysis of cross-sectional transmission electron microscopy (XTEM) revealed a damaged but single-crystalline CaFz layer. This study demonstrates the radiation tolerance of CaF, under displacive irradiation at low to ambient temperatures.
1. Introduction Radiation effects in ceramics are of practical interest due to their potential applications in fission and fusion reactors. Displacive irradiation can result in amorphization or damaged but crystalline states in irradiated materials. Naguib and Kelly [l] proposed a bonding criterion to predict whether ceramics remain crystalline or become amorphous under displacive radiation. It was demonstrated that ceramics with cubic structures [2,3] generally show the following trend. Highly ionic ceramics with ionicity greater than 0.59 tend to remain crystalline while low ionic ceramics with ionic&y less than 0.47 are readily amorphized under irradiation. Previous studies have shown that several ceramics with a fluorite-type structure are of high radiation resistance to amorphization, e.g., cubic-zirconia (ZrO,) [4,5], UO, [2,6-81, and CaF, [2,9]. These cubic ceramics all have ionicity greater than 0.59 (0.63 for ZrO,; 0.67 for UO,; and 0.89 for CaF,) and are expected to be radiation resistant based upon the ionicity criterion. Matzke and Whitton [2] were the first to irradiate CaFz at room temperature with 40 keV Xe ions and reflection electron diffraction showed no sign of amorphization. Aono et al. [9] irradiated CaFz single crystals at - 100, 25 and 100°C
’ Permanent address: Semiconductor Process and Device Center, Texas Instruments, Inc., MS 944, Dallas, Texas 75243, USA; email: [email protected]. * Permanent address: Department of Nuclear Engineering, Kyushu University, Hakozaki, Fukuoka 812-81, Japan. 0168-583X/97/$17.00 PII 80168-583X(96)01
0 1997 Elsevier Science B.V. All rights reserved 138-X
with 100 keV Tb ions. RBS/C measurements after irradiation showed that the irradiated CaF, remained crystalline up to a dose of 3 X 1016 Tb/cm’. Implanted Tb was also observed to occupy the substitutional sites of Ca and to produce luminescence as being in a Tb3’ state. The radiation resistance exhibited by Tb irradiation in CaFz at cryogenic temperatures may be altered by the presence of chemically reactive species implanted in the crystal. To minimize the chemical effects, chemically inert Xe species at 360 keV were chosen to irradiate CaF, single crystals in this study. Irradiation was done at - 115 and 35°C. Lattice damage was measured at the irradiation temperatures using in situ RBS/C techniques [lO,lll to eliminate possible annealing effects arising from analysis usually performed at room temperature. Radiation damage has been observed to increase monotonicly with increasing dose and to reach a saturated level equivalent to 70% of the random level of RBS yield. RBS/C measurements show no formation of an amorphized layer up to a dose of 2 X lOI Xe/cm2 at - 115°C. XTEM has further revealed the presence of damaged but crystalline layer.
2. Experiment CaF, single crystals in a dimension of 10 X 10 mm2 width and 0.3 mm thickness were used in this study. The samples of (100) orientation were polished to an optical finish. Prior to irradiation experiments, the front surfaces of the samples were carbon coated to a 20 nm thickness in order to avoid surface charging accumulated by ion beams.
IVa. SYNTHESIS
OF CERAMICS/INSULATORS
592
N. Yu et al./Nucl.
lnstr. andkfeth.
in Phys. Res. B 127/128
Ion irradiation and ion beam analysis were performed in the in situ ion beam facility at Los Alamos National Laboratory [lO,ll]. The sample mounted in the target chamber was irradiated with a 360 keV Xe*+ ion beam from a 200 kV ion implanter and analyzed with a 2 MeV He+ ion beam from a 3 MV tandem accelerator. The irradiation beam was scanned and incident at a 15” angle with respect to the sample normal (100) with minimized channeling effects during irradiation. RBS/C measurement of a He ion beam incident along the (110) axis of CaF, to a charge of 10 pC immediately followed each incremental dose of Xe irradiation. The sample temperature was maintained at 35 or - 115°C for both ion irradiation and analysis. The analytical beam of 2 mm diameter was incident in the center of irradiating beam spot of 7 mm diameter on the sample surface to ensure correct measurements of radiation damage. The irradiation was done at doses from 2 X lOI to 2 X 1016 Xc/cm* while the irradiation beam flux was maintained at m 1 X lOI Xe/cm’s. After irradiation to the highest dose, random RBS spectra were also taken while rocking the crystal off the (110) channeling direction. The projected range and the damage peak of 360 keV Xe ions at a 15” incident angle were estimated to be 99 and 68 nm, respectively, by the TRIM code [ 121 using a mass density of 3.18 g/cm3 for CaF,. The damage level near the damage peak was determined to be 30 dpa for a dose of 1 X 1OL6 Xc/cm*, assuming a displacement threshold energy of 40 eV for both Ca and F. The peak
concentration of implanted Xe was about 2 at.% for 1 X 1OL6Xe/cm2 in CaF,. The calcium fluoride samples irradiated at 35 or - 115°C were re-examined by RBS/C using a 2 MeV He ion beam along the (100) axis at room temperature two days after the in situ experiments. The bulk samples were then prepared into electron transparent cross-sectional specimens for XTEM examination.
3. Results and discussion Fig. 1 shows some of 18 in situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal following sequential 360 keV Xe irradiations at 35°C. The spectra are plotted as normalized yield obtained by normalizing the backscattering counts with the collected He charge and the detector solid angle. Minimum backscattering yield, defined as the RBS yield ratio of an aligned spectrum to a random spectrum, is 5% for the Ca signal near the surface prior to irradiation. This indicates good single crystallinity of the unit-radiated crystal. High dechanneling yields around the channel number 400 represent the buildup of radiation damage around a depth of 100 nm where the peak damage occurred. The near surface region ( < 50 nm) exhibited minimal damage for doses up to 7x10 I5 Xc/cm* while the peak damage steadily increased. Lattice damage started to build up in the initially defect-denuded
Energy 0.6
0.8
I :;
I
2 MeV HA+ - CaFa ..._ ... r;:..;.. )
200
250
300
(1997) 591-595
zone as the irradiation dose reached
(MeV)
1.0
1.2
1.4
I
I
I
+ 0, 35OC 1.5x1o16. 35Y! x 7x10’6, 3PC -.- 1.t3x1o’6, 35Y! .~.~..2xlO’s, 35°C. Random
350
400
450
:
Channel Fig. 1. In situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal following sequential 360 keV Xe irradiations at 35°C along with a random spectrum from the crystal irradiated to the highest dose.
N. Yu et al./Nucl.
1 x 1Or6 Xc/cm’ (spectrum not shown in Fig. 1). The average lattice damage over the entire irradiated layer approached 60% of the random level after irradiation to 1.6 X lOI Xc/cm*. Fig. 2 shows several in situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal after sequential 360 keV Xe irradiations at - 115°C. The minimum yield of Ca (4.5%) from the unirradiated crystal at - 115°C is lower than that (5%) measured at 35°C. This is due to reduced lattice thermal vibrations. Similar to the case of irradiation at room temperature shown in Fig. 1, lattice damage first built up around the depth of 100 nm and grew steadily with increasing dose. Appreciable lattice disorder near the surface was also observed after irradiation to 7 X 10” Xc/cm’. The amount of damage in both peak damage and near surface regions increased monotonically with irradiation dose. Finally, lattice damage over the entire irradiated layer reached a saturation level of 70% of the random level in Fig. 2 at a dose of 2 X lOI Xc/cm’. No continuous amorphous layer was observed to this dose. The implanted Xe atoms are apparent from the RBS spectra in Fig. 2 and its concentration increases with the dose. The Xe depth profile roughly follows a Gaussian distribution. It is interesting to note that the aligned spectrum of Xe is almost identical to the random one for the crystal irradiated to 2 X 10 l6 Xc/cm’. This feature was also observed at lower irradiation doses. It clearly indicates that implanted Xe is on interstitial positions rather than
360 keV ti+
0.8
1.0
0.0
1.0
0.5
1.5
2.0
Dose (1O’6 Xe/cm2)
Fig. 3. The minimum yield of Ca from the peak damaged region as a function of Xe irradiation dose for irradiation at 35°C.
lattice sites. This observation differs from the report 191of Tb irradiation in CaF2 where 50-100% of implanted Tb was found to be substitutional. Fig. 3 shows minimum yield of Ca integrated over the peak damaged region around 100 nm as a function of Xe dose for irradiation at 35°C. The kinetic curve of lattice damage shows a rapid rise in the minimum yield from 5% to 40% as the dose increases to 1.5 X 1015 Xe/cm2. The lattice damage increases from 40% to 60% at much slower rates with increasing dose to 1 X 1016 Xc/cm’. The amount of lattice disorder approaches a saturation level of 70% of the random level in Fig. 3 for a dose of 2 X lOI
(MeV)
1.2
1.4
1.6
-115’C,
300
- CaFz at 351:
0.8 8
Energy 0.6
593
Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 591-595
400
500
1.8
Random
600
Channel Fig. 2. In situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal foIlowing sequential 360 keV Xe irradiations at - 115°C. along with a random spectrum from the crystal irradiated to the highest dose.
IVa. SYNTHESIS OF CERAMICS/INSULATORS
594
N. Yu et al./Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 591-595
0.6 15, (
1
2 y_:..
Energy
(MeV)
1.0
1.2
0.8
MeVHd-
.:+A .,.._ : ‘.._:,, .,,,
I
car,
I
+ Unirradiated. -2x10” -1WC. ~Unirradiated.
1.4
I
I
Aligned AlIgned Random
Channel Fig. 4. Ex situ RBS/C spectra of a 2 MeV He ion-beam incident along the (100) axis of CaF* crystals before and after the irradiations of 360 keV Xe to 2X lOI Xc/cm* at - 1lS”C, along with a random spectrum from the uninadiated CaF,.
Xc/cm’. Similar kinetics of radiation damage accumulation were also observed from the in situ experiment performed at - 115°C. The in situ kinetic results resemble the ex situ ones [9] for Tb irradiation in CaF,. However, a lower damage saturation level (50%) was observed for Tb irradiation to a dose of 3 X 1Or6‘I’b/cm’ in CaF, [9]. This difference may be a consequence of the role of the implanted species and annealing effects at room temperature, which will be discussed below. The irradiated samples were unloaded from the irradiation stage after the in situ experiments and stored at room temperature for two days before repeating the RBS/C analysis. Fig. 4 shows ex situ RBS/C spectra of a 2 MeV He ion-beam incident along the (100) axis of CaF, crystal before and after the irradiation of 360 keV Xe at - 115°C to 2 X 1O’6 Xc/cm’. Lattice damage is dominant at the depth of 100 nm with less damage near the surface. The damage peak is coincident with the peak of implanted Xe. The average minimum yield near the surface ( < 50 nm> is 55%, as compared to 70% in Fig. 2. This suggests that room temperature annealing plays a role in removing part of the initial lattice damage near the surface. In contrast, the lattice damage in the vicinity of the implanted Xe remains relatively stable in the presence of room temperature annealing. The single-crystallinity of the irradiated calcium fluoride layer to a dose of 2 X lOi Xc/cm* has been further confirmed by XTEM. Microdiffraction patterns taken from the irradiated layer and the substrate zone showed identical features. However, the microstructure in the irradiated region is complex and a more detailed study is required. Also, microstructural evolution occurs during TEM charac-
terization in the electron microscope at room temperature. This is consistent with the report of Hobbs [13] that electron irradiation can result in Ca metal colloid formation due to radiolysis effects. Weber et al. [14] also observed metal colloid formation in CaF, under 5.8 MeV alpha-particle bombardment above 100°C. The possibility of colloid formation in Xe-irradiated CaF, is not clear at this stage. Further studies are required to understand the relationship between ionization and displacive processes in damage accumulation in CaF, and other materials with high ionicity. This study demonstrates that the defect-denuded zone near the CaF, surface is not stable under high dose Xe irradiation (> 1 X lOi Xc/cm*). The denuded zone observed at doses below 7 X lOi Xc/cm* disappears at a dose of 2 X lOI Xc/cm* (Fig. 2). This is in contrast to ZrO,, in which case the surface acts as a more effective defect sink under Xe irradiation [5]. Furthermore, the study suggests that although lattice disorder near the surface in CaF, is present immediately following high dose irradiation, it can be partially removed by room temperature annealing. Also, this study shows that the irradiation behavior of CaF, for Xe ions is similar to that for Tb ions although Tb is chemically reactive and has a tendency to occupy the Ca lattice site. These results along with previous reports [2,9] are consistent with the ionicity criterion for the prediction of the radiation resistance.
4. Conclusions Lattice damage in calcium fluoride crystals induced by 360 keV Xe ion irradiation was measured in situ using RBS/C techniques. No amorphized layer was observed at both irradiation temperatures up to an irradiation dose of 2 x lOI Xc/cm*. RBS/C measurements showed that lattice disorder in the irradiated layer saturated at 70% of the random level. XTEM analysis following the in situ experiment confirmed the presence of a single-crystalline and damaged CaF, layer on an undamaged substrate. This study demonstrates the radiation tolerance of CaF, under displacive irradiation at low to ambient temperatures.
Acknowledgements We are grateful to Caleb Evans and Mark Hollander for technical assistance with Xe ion irradiations and to William J. Weber of Pacific Northwest Laboratory for valuable discussions on electron irradiation of CaF,. All ion beam work was performed at the Los Alamos Ion Beam Materials Laboratory. This research was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences. Work was also partially supported by 17.
N. Yu et al./Nucl.
Instr. and Meth. in Phys. Res. B 127/128
References [I] H.M. Naguib and R. Kelly, Radiat. Eff. 25 (19751 1. [2] Hj. Matzke and J.L. Whitton, Can. J. Phys. 44 (19661995. [3] Hj. Matzke, Radiat. Eff. 64 (1982) 3. [4] E.L. Fleischer, M.G. Norton, M.A. Zaleski, W. Hertl, C.B. Carter and J.W. Mayer, J. Mater. Res. 6 (1991) 1905. 151 N. Yu, K.E. Sickatiis, P. Kodali and M. Nastasi, J. Nucl. Mater. 244 (1997) 266. [6] W.J. Weber, J. Nucl. Mater. 98 (1981) 206. [7] A. Turos, Hj. Matzke and S. Kwiatkowski, Phys. Rev. I&t. 65 (1990) 1215. [8] Hj. Matzke, A. Turos and G. Linker, Nucl. lnstr. and Meth. B 91 (1994) 294.
(1997) 591-59.5
595
[9] K. Aono, M. Kumagai, M. Iwaki, Y. Aoyagi and S. Namba, Nucl. Instr. and Meth. B 80/81 (1993) 1226. [lo] N. Yu, M. Nastasi, T.E. Levine, J.R. Tesmer, M.G. Hollander, C.R. Evans and C.J. Maggiore, Nucl. Instr. and Meth. B 99 (1995) 566. [ll] N. Yu, T.E. Levine, K.E. Sickafus, M. Nastasi, J.N. Mitchell, C.J. Maggiore, C.R. Evans, M.G. Hollander, J.R. Tesmer, W.J. Weber and J.W. Mayer, Nucl. Instr. and Meth. B 118 (1996) 766. [12] J.F. Ziegler, J.P. Biersack and U. L&mark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [13] L.W. Hobbs, J. Phys. 37 (1976) C7-3. 1141 W.J. Weber, G.J. Exarhos and L.M. Wang, MRS Symp. Proc. 373 (1995) 311.
IVa. SYNTHESIS OF CERAMICS/INSULATORS
and Methods in Physics
ResearchB l27/128 (1997) 591-595
EISEVIER In
situ study of ion-beam induced lattice damage in calcium fluoride crystals Ning Yu ’ , Michael Nastasi, Kurt E. Sickafus, Kazuhiro Yasuda *, Joseph R. Tesmer Muterials Science urul Technology Division, Los Alamos National Laboratory,
Los Alamos, NM 8754.5, USA
Abstract Lattice damage in calcium fluoride induced by 360 keV xenon ion irradiation was measured in situ using Rutherford backscattering and channeling (RBS/C) techniques. Calcium fluoride single crystals of (100) orientation were irradiated at - 115 and 35°C by Xe ion beams along a random direction 15” off the (100) axis. A 2 MeV He ion-beam from a 3 MV tandem accelerator was incident on the CaF, crystal along the (110) axis to determine the radiation damage. No amorphized layer was observed at both irradiation temperatures up to an irradiation dose of 2 X 1016 Xc/cm*. RBS/C measurements showed that lattice disorder in the irradiated layer saturated at 70% of the random level. Analysis of cross-sectional transmission electron microscopy (XTEM) revealed a damaged but single-crystalline CaFz layer. This study demonstrates the radiation tolerance of CaF, under displacive irradiation at low to ambient temperatures.
1. Introduction Radiation effects in ceramics are of practical interest due to their potential applications in fission and fusion reactors. Displacive irradiation can result in amorphization or damaged but crystalline states in irradiated materials. Naguib and Kelly [l] proposed a bonding criterion to predict whether ceramics remain crystalline or become amorphous under displacive radiation. It was demonstrated that ceramics with cubic structures [2,3] generally show the following trend. Highly ionic ceramics with ionicity greater than 0.59 tend to remain crystalline while low ionic ceramics with ionic&y less than 0.47 are readily amorphized under irradiation. Previous studies have shown that several ceramics with a fluorite-type structure are of high radiation resistance to amorphization, e.g., cubic-zirconia (ZrO,) [4,5], UO, [2,6-81, and CaF, [2,9]. These cubic ceramics all have ionicity greater than 0.59 (0.63 for ZrO,; 0.67 for UO,; and 0.89 for CaF,) and are expected to be radiation resistant based upon the ionicity criterion. Matzke and Whitton [2] were the first to irradiate CaFz at room temperature with 40 keV Xe ions and reflection electron diffraction showed no sign of amorphization. Aono et al. [9] irradiated CaFz single crystals at - 100, 25 and 100°C
’ Permanent address: Semiconductor Process and Device Center, Texas Instruments, Inc., MS 944, Dallas, Texas 75243, USA; email: [email protected]. * Permanent address: Department of Nuclear Engineering, Kyushu University, Hakozaki, Fukuoka 812-81, Japan. 0168-583X/97/$17.00 PII 80168-583X(96)01
0 1997 Elsevier Science B.V. All rights reserved 138-X
with 100 keV Tb ions. RBS/C measurements after irradiation showed that the irradiated CaF, remained crystalline up to a dose of 3 X 1016 Tb/cm’. Implanted Tb was also observed to occupy the substitutional sites of Ca and to produce luminescence as being in a Tb3’ state. The radiation resistance exhibited by Tb irradiation in CaFz at cryogenic temperatures may be altered by the presence of chemically reactive species implanted in the crystal. To minimize the chemical effects, chemically inert Xe species at 360 keV were chosen to irradiate CaF, single crystals in this study. Irradiation was done at - 115 and 35°C. Lattice damage was measured at the irradiation temperatures using in situ RBS/C techniques [lO,lll to eliminate possible annealing effects arising from analysis usually performed at room temperature. Radiation damage has been observed to increase monotonicly with increasing dose and to reach a saturated level equivalent to 70% of the random level of RBS yield. RBS/C measurements show no formation of an amorphized layer up to a dose of 2 X lOI Xe/cm2 at - 115°C. XTEM has further revealed the presence of damaged but crystalline layer.
2. Experiment CaF, single crystals in a dimension of 10 X 10 mm2 width and 0.3 mm thickness were used in this study. The samples of (100) orientation were polished to an optical finish. Prior to irradiation experiments, the front surfaces of the samples were carbon coated to a 20 nm thickness in order to avoid surface charging accumulated by ion beams.
IVa. SYNTHESIS
OF CERAMICS/INSULATORS
592
N. Yu et al./Nucl.
lnstr. andkfeth.
in Phys. Res. B 127/128
Ion irradiation and ion beam analysis were performed in the in situ ion beam facility at Los Alamos National Laboratory [lO,ll]. The sample mounted in the target chamber was irradiated with a 360 keV Xe*+ ion beam from a 200 kV ion implanter and analyzed with a 2 MeV He+ ion beam from a 3 MV tandem accelerator. The irradiation beam was scanned and incident at a 15” angle with respect to the sample normal (100) with minimized channeling effects during irradiation. RBS/C measurement of a He ion beam incident along the (110) axis of CaF, to a charge of 10 pC immediately followed each incremental dose of Xe irradiation. The sample temperature was maintained at 35 or - 115°C for both ion irradiation and analysis. The analytical beam of 2 mm diameter was incident in the center of irradiating beam spot of 7 mm diameter on the sample surface to ensure correct measurements of radiation damage. The irradiation was done at doses from 2 X lOI to 2 X 1016 Xc/cm* while the irradiation beam flux was maintained at m 1 X lOI Xe/cm’s. After irradiation to the highest dose, random RBS spectra were also taken while rocking the crystal off the (110) channeling direction. The projected range and the damage peak of 360 keV Xe ions at a 15” incident angle were estimated to be 99 and 68 nm, respectively, by the TRIM code [ 121 using a mass density of 3.18 g/cm3 for CaF,. The damage level near the damage peak was determined to be 30 dpa for a dose of 1 X 1OL6 Xc/cm*, assuming a displacement threshold energy of 40 eV for both Ca and F. The peak
concentration of implanted Xe was about 2 at.% for 1 X 1OL6Xe/cm2 in CaF,. The calcium fluoride samples irradiated at 35 or - 115°C were re-examined by RBS/C using a 2 MeV He ion beam along the (100) axis at room temperature two days after the in situ experiments. The bulk samples were then prepared into electron transparent cross-sectional specimens for XTEM examination.
3. Results and discussion Fig. 1 shows some of 18 in situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal following sequential 360 keV Xe irradiations at 35°C. The spectra are plotted as normalized yield obtained by normalizing the backscattering counts with the collected He charge and the detector solid angle. Minimum backscattering yield, defined as the RBS yield ratio of an aligned spectrum to a random spectrum, is 5% for the Ca signal near the surface prior to irradiation. This indicates good single crystallinity of the unit-radiated crystal. High dechanneling yields around the channel number 400 represent the buildup of radiation damage around a depth of 100 nm where the peak damage occurred. The near surface region ( < 50 nm) exhibited minimal damage for doses up to 7x10 I5 Xc/cm* while the peak damage steadily increased. Lattice damage started to build up in the initially defect-denuded
Energy 0.6
0.8
I :;
I
2 MeV HA+ - CaFa ..._ ... r;:..;.. )
200
250
300
(1997) 591-595
zone as the irradiation dose reached
(MeV)
1.0
1.2
1.4
I
I
I
+ 0, 35OC 1.5x1o16. 35Y! x 7x10’6, 3PC -.- 1.t3x1o’6, 35Y! .~.~..2xlO’s, 35°C. Random
350
400
450
:
Channel Fig. 1. In situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal following sequential 360 keV Xe irradiations at 35°C along with a random spectrum from the crystal irradiated to the highest dose.
N. Yu et al./Nucl.
1 x 1Or6 Xc/cm’ (spectrum not shown in Fig. 1). The average lattice damage over the entire irradiated layer approached 60% of the random level after irradiation to 1.6 X lOI Xc/cm*. Fig. 2 shows several in situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal after sequential 360 keV Xe irradiations at - 115°C. The minimum yield of Ca (4.5%) from the unirradiated crystal at - 115°C is lower than that (5%) measured at 35°C. This is due to reduced lattice thermal vibrations. Similar to the case of irradiation at room temperature shown in Fig. 1, lattice damage first built up around the depth of 100 nm and grew steadily with increasing dose. Appreciable lattice disorder near the surface was also observed after irradiation to 7 X 10” Xc/cm’. The amount of damage in both peak damage and near surface regions increased monotonically with irradiation dose. Finally, lattice damage over the entire irradiated layer reached a saturation level of 70% of the random level in Fig. 2 at a dose of 2 X lOI Xc/cm’. No continuous amorphous layer was observed to this dose. The implanted Xe atoms are apparent from the RBS spectra in Fig. 2 and its concentration increases with the dose. The Xe depth profile roughly follows a Gaussian distribution. It is interesting to note that the aligned spectrum of Xe is almost identical to the random one for the crystal irradiated to 2 X 10 l6 Xc/cm’. This feature was also observed at lower irradiation doses. It clearly indicates that implanted Xe is on interstitial positions rather than
360 keV ti+
0.8
1.0
0.0
1.0
0.5
1.5
2.0
Dose (1O’6 Xe/cm2)
Fig. 3. The minimum yield of Ca from the peak damaged region as a function of Xe irradiation dose for irradiation at 35°C.
lattice sites. This observation differs from the report 191of Tb irradiation in CaF2 where 50-100% of implanted Tb was found to be substitutional. Fig. 3 shows minimum yield of Ca integrated over the peak damaged region around 100 nm as a function of Xe dose for irradiation at 35°C. The kinetic curve of lattice damage shows a rapid rise in the minimum yield from 5% to 40% as the dose increases to 1.5 X 1015 Xe/cm2. The lattice damage increases from 40% to 60% at much slower rates with increasing dose to 1 X 1016 Xc/cm’. The amount of lattice disorder approaches a saturation level of 70% of the random level in Fig. 3 for a dose of 2 X lOI
(MeV)
1.2
1.4
1.6
-115’C,
300
- CaFz at 351:
0.8 8
Energy 0.6
593
Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 591-595
400
500
1.8
Random
600
Channel Fig. 2. In situ RBS/C spectra of a 2 MeV He ion-beam incident along the (110) axis of a CaF, crystal foIlowing sequential 360 keV Xe irradiations at - 115°C. along with a random spectrum from the crystal irradiated to the highest dose.
IVa. SYNTHESIS OF CERAMICS/INSULATORS
594
N. Yu et al./Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 591-595
0.6 15, (
1
2 y_:..
Energy
(MeV)
1.0
1.2
0.8
MeVHd-
.:+A .,.._ : ‘.._:,, .,,,
I
car,
I
+ Unirradiated. -2x10” -1WC. ~Unirradiated.
1.4
I
I
Aligned AlIgned Random
Channel Fig. 4. Ex situ RBS/C spectra of a 2 MeV He ion-beam incident along the (100) axis of CaF* crystals before and after the irradiations of 360 keV Xe to 2X lOI Xc/cm* at - 1lS”C, along with a random spectrum from the uninadiated CaF,.
Xc/cm’. Similar kinetics of radiation damage accumulation were also observed from the in situ experiment performed at - 115°C. The in situ kinetic results resemble the ex situ ones [9] for Tb irradiation in CaF,. However, a lower damage saturation level (50%) was observed for Tb irradiation to a dose of 3 X 1Or6‘I’b/cm’ in CaF, [9]. This difference may be a consequence of the role of the implanted species and annealing effects at room temperature, which will be discussed below. The irradiated samples were unloaded from the irradiation stage after the in situ experiments and stored at room temperature for two days before repeating the RBS/C analysis. Fig. 4 shows ex situ RBS/C spectra of a 2 MeV He ion-beam incident along the (100) axis of CaF, crystal before and after the irradiation of 360 keV Xe at - 115°C to 2 X 1O’6 Xc/cm’. Lattice damage is dominant at the depth of 100 nm with less damage near the surface. The damage peak is coincident with the peak of implanted Xe. The average minimum yield near the surface ( < 50 nm> is 55%, as compared to 70% in Fig. 2. This suggests that room temperature annealing plays a role in removing part of the initial lattice damage near the surface. In contrast, the lattice damage in the vicinity of the implanted Xe remains relatively stable in the presence of room temperature annealing. The single-crystallinity of the irradiated calcium fluoride layer to a dose of 2 X lOi Xc/cm* has been further confirmed by XTEM. Microdiffraction patterns taken from the irradiated layer and the substrate zone showed identical features. However, the microstructure in the irradiated region is complex and a more detailed study is required. Also, microstructural evolution occurs during TEM charac-
terization in the electron microscope at room temperature. This is consistent with the report of Hobbs [13] that electron irradiation can result in Ca metal colloid formation due to radiolysis effects. Weber et al. [14] also observed metal colloid formation in CaF, under 5.8 MeV alpha-particle bombardment above 100°C. The possibility of colloid formation in Xe-irradiated CaF, is not clear at this stage. Further studies are required to understand the relationship between ionization and displacive processes in damage accumulation in CaF, and other materials with high ionicity. This study demonstrates that the defect-denuded zone near the CaF, surface is not stable under high dose Xe irradiation (> 1 X lOi Xc/cm*). The denuded zone observed at doses below 7 X lOi Xc/cm* disappears at a dose of 2 X lOI Xc/cm* (Fig. 2). This is in contrast to ZrO,, in which case the surface acts as a more effective defect sink under Xe irradiation [5]. Furthermore, the study suggests that although lattice disorder near the surface in CaF, is present immediately following high dose irradiation, it can be partially removed by room temperature annealing. Also, this study shows that the irradiation behavior of CaF, for Xe ions is similar to that for Tb ions although Tb is chemically reactive and has a tendency to occupy the Ca lattice site. These results along with previous reports [2,9] are consistent with the ionicity criterion for the prediction of the radiation resistance.
4. Conclusions Lattice damage in calcium fluoride crystals induced by 360 keV Xe ion irradiation was measured in situ using RBS/C techniques. No amorphized layer was observed at both irradiation temperatures up to an irradiation dose of 2 x lOI Xc/cm*. RBS/C measurements showed that lattice disorder in the irradiated layer saturated at 70% of the random level. XTEM analysis following the in situ experiment confirmed the presence of a single-crystalline and damaged CaF, layer on an undamaged substrate. This study demonstrates the radiation tolerance of CaF, under displacive irradiation at low to ambient temperatures.
Acknowledgements We are grateful to Caleb Evans and Mark Hollander for technical assistance with Xe ion irradiations and to William J. Weber of Pacific Northwest Laboratory for valuable discussions on electron irradiation of CaF,. All ion beam work was performed at the Los Alamos Ion Beam Materials Laboratory. This research was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences. Work was also partially supported by 17.
N. Yu et al./Nucl.
Instr. and Meth. in Phys. Res. B 127/128
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