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Depth profiles of 35 keV 3He ions in metals
592
Nuclear Instruments and Methods in Physics Research BlO/ll (1985) 592-595 North-Holland, Amsterdam
DEPTH PROFILES OF 35 keV 3He IONS IN METALS W.N. LENNARD, H. GEISSEL *, T.K. ALEXANDER, R. HILL, D.P. JACKSON, M.A. LONE and D. PHILLIPS AtomicEnergyof Canada Limited Research Company, Chafk River Nuclear Laboratories, Chalk River, Ontario, Canada K#
I&?
The depth profiles of 3He implanted at 35 keV energy into AI, Ti, V, Ni, Cu. Zn, Zr, Nb, Ag, Sn, Ta, W, Au and Bi targets have been measured using the thermal neutron reaction ‘He@, P)~H. The profiles obtained from the energy distribution of the emitted protons show a marked Z,osciIlation in both the most probable depth and depth straggling. The measured values are compared to Monte Car10computer simulation resuhs.
1. Inbroduction
2. Experimental
There is a strong t~~olo~~ need for data concerning the depth distribution of light ions in metals. This topic relates to the performance of the first wall of future controlled thermonuclear reactors (CTR’s) under intense bombardment by H, He and Li ions [l]. Depth profiles for saturated doses are important in the understanding of the recycling of plasma particles from the wall in such high temperature devices. The present data relate to undistorted profiles and room temperature implantation. Another application that requires knowledge of helium depth profiles in metal foils is the use of implanted He as a target in nuclear reaction studies. Several experimental techniques have been developed to measure depth profiles of light ions in metals: Rutherford backscattering [2], elastic recoil analysis [3] and nuclear reaction analysis [4,5]. These techniques have been applied to determine the depth distribution of light noble gases such as 3He and 4He. However, 3He is much easier to detect than 4He. Although both the 3He(d, 4He)p and ‘He@, P)~H reactions are sensitive probes of 3He, only the latter provides non-destructive analysis. A comparison of the (n, p) and (d, 4He) tech-
Polyc~s~~ne 0.1 mm thick foils of the elements Al, Ti, V, Ni, Cu, Zn, Zr, Nb, Ag, Sn, Ta, W, Au and Bi were implanted with 1.4 X 10” ‘He ions cm2 at an energy of 35 keV. The target foils were cleaned initially with trichloroethylene followed by an acetone rinse. The local vacuum during impl~tation was 0.1 mPa; the beam current during implantation was in the range 6-8 PA cme2, and the targets were held at room temperature. The implanted target foils were first analyzed for 3He content u sin g a 0.5 MeV energy deuteron beam. The total amount of trapped 3He was measured by counting the protons from the 3He(d, P)~H~ reaction whose cross section is known precisely [8]. The deuteron fluence was 2.5 X 1014 cme2 f a value negligible compared to the 3He doses. The depth profiles were subsequently measured using the 3He(n, P)~H thermal neutron reaction that has a cross section of 5.3 kb. A flux of 3 X 10’ neutrons cme2 s-* from the N4 external thermal neutron facility [9] at the Chalk River NRU reactor impinged on the targets as shown in fig. 1. A Si surface barrier detector (100 gm when fully depleted, 50 mm2 active area) was placed 10 cm from the target at 90’ to the neutron beam. Normally, we set the angle between the beam and the target surface, 8, to the value 8“. The Pb shielding and the magnetic field (15 mT) were effective in reducing background radiation in the detector. Only the central 20 mm* area of the detector was able to view the target. Furthermore, we reduced the detector bias voltage to limit its sensitive thickness to a value just exceeding the range of the protons without suffering decreased resolution. The initial energies of the proton and triton are 572.5 and 191.3 keV, respectively. A near-surface 3He target, fabricated by a glow
niques is given in ref. [4]. We have used the thermal neutron 3He(n, P)~H reaction to determine the most probable projected range and range straggling for 35 keV ‘He ions implanted into 14 metallic targets. The experimental data are compared to Monte Carlo computer simulation results (see ref. [6]), using well-defined input quantities. For this purpose, we have chosen to use the electronic stopping values of Ziegler [7] for He, and Moliere’s repulsive potential for treating the elastic (nuclear) collisions. We assume ‘He and 4He have the same stopping power at
the same velocity. * Preseat address: GSI, D-61 Darmstadt, FRG. 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
593
W.N. Lennard et al. / Depth profiles of 35 keV ‘He ions in metals
Pb SHIELD Si SURFACE BARRIER DETECTOR
-a
PROTONS, TRITONS
/rzrmm
-COLLIHATOR
I
3
Fig. 1. Schematic of the experimental setup for detecting ‘H and 3H particles (equal numbers of each) emitted from the thermal neutron 3He(n, P)~H reaction. The targets contained 3He ions implanted at 35 keV energy.
technique using a Nb sample as the cathode, was used for the absolute energy calibration. The negligible thickness of this calibration target (8 x 1015 3He atoms cme2) was confirmed by varying the angle B. The most probable depth for 3He in this target was measured also using the tilt technique and found to be 9 pg cmw2, a value that compared favourably to that expected based on the preparation conditions: i.e. 9 pg cmm2 is approximately the range of 1 keV ‘He ions in Nb [5]. The resolution obtained for 572 keV protons was 9 keV fwhm. The measured energy distribution of the protons (and tritons) was converted to a depth profile for the implanted 3He ions using ‘H stopping powers from Andersen and Ziegler [lo]. The ‘H spectra were not used to obtain quantitative depth values. First, there exist little stopping power data in the energy region of interest; second, low energy background in the vicinity of the 3H peak for the low dose glow discharge target made precise target surface location difficult.
I
I
I
,
I
bl PROTONS
discharge
3. Results anddiscussion Fig. 2a shows the depth distribution for 3He in Nb, deduced from the 3H energy spectrum and fig. 2b, from the ‘H energy spectrum. The proton peaks were fitted with a Gaussian with exponential tails on both the low and high energy sides, reflecting the energy loss straggling for the emergent protons and for the initially implanted 35 keV ‘He ions. The most probable values were obtained from the fitted parameters. The proton energy scale was converted to a target depth scale using dE/dx values from ref. [lo]. Corrections to the range and range straggling values (0.1% and 0.256, respec-
OEPTH lpg cms21
Fig. 2. (a) Depth distribution of ‘He ,deduced from the ‘H energy spectrum. The implanted fluence was 1.1 X10” ions cm-’ in Nb. (b) Depth distribution of 3He deduced from the ‘H energy spectrum for the same target as in (a).
tively) were made for the finite acceptance angle of the detector system. The particle energies corresponding to the target surface were obtained by reference to the glow discharge target, for which the proton spectrum was a symmetric Gaussian. We have measured most probable depth values for all samples with a retained dose - 1.1 X 10” cme2. We have chosen to determine the range straggling values from the fwhm of the peaks, after subtracting the wntributions of proton straggling (Bohr’s value) and detector resolution in quadrature. This choice seems reasonable since all the proton peaks were nearly symmetric. We have ignored the contribution of implanted 3He to the proton energy loss. The ‘He doses represented peak concentrations, n(‘He)/n(host), in the range 9 at.% to 21 at.%, where n represents a volume atomic density. Stopping power values for protons in 3He are much smaller than their values in the host material. We have used the ‘%(d, p)l’O nuclear reaction at 0.97 MeV deuteron energy to determine the near-surface oxygen concentration for the Al, Zn, Nb, Ta, W and Bi targets. The proton yields were compared to that from a V. ION IMPLANTATION..
.
W.N. Lennard et al. / Depth profiles of 35 keV ‘He ions in metals
594
Ta,O, standard of known thickness. If we assume (worst case) that all the oxygen is contained in a surface oxide, we find that the oxide layer thickness represented 5% of the 3He projected range except for Zn, where it was 15%. However, the 15% figure would lead to a correction to the ‘He range < 3%; we have therefore ignored the effect of the oxide since the depth distribution of the oxygen atoms was not measured. The ‘H energy spectrum shown in fig. 2a has been corrected for background by subtracting a smoothlyvarying function of particle energy fitted above and below the peak. The background is caused primarily by electrons produced in the sensitive volume of the detector via (II, y) reactions. Neither spectrum shows evidence for saturation in the peak region, which is observed for larger fluences. We conclude that the most probable depth values correspond to 3He range values for these samples. No diffusion effects have been reported for room temperature ‘He implants at doses 10’7cm-2 [11,12]. We have observed no difference in the most probable depth or fwhm for 3He implanted at 35 keV into Au samples for fluences from 1 to 4 X 1017 cme2, which is less than the saturation dose. A summary of the most probable range values measured for 35 keV 3He in y4 metals is given in table 1 and shown in fig. 3a. The estimated uncertainty for each datum is 5% excluding any contribution from uncertainties in the ‘H stopping power values. The reproducibility of our measurements was 2% deduced from 22 separate measurements for a Nb sample. The additional uncertainty is estimated from a study of the values deduced by fitting different functions to the proton peaks. The range straggling values are shown in fig. 3b, which have a similar structure to the range data. The uncertainties are 15% for these data. The results from Table 1 Range and range straggling values for 35 keV ‘He in metals. AU values are in cg cmm2. The calculated values are from Monte Carlo computer simulations using stopping power values from ref. [7] Target
RQP’ P
R*P
ARTpI
AR?
Al Ti V Ni cu Zn Zr Nb
62 74 81 114 127 115 97 101 132 122 163 189 191 165
66 79 77 98 117 118 85 92 121 123 156 164 203 148
17 24 29 49 55 47 38 40 62 46 72 84 95 81
17 24 24 34 44 45 28 33 49 50 67 72 97 64
Ag Sn Ta W Au Bi
Oi--+-++ b)
IO0
Fig. 3. (a) Most probable depth, or range, for 35 keV 3He ions implanted into target materials, Z,, to a fluence of 1.1 X 10” cms2: 0 - experimental, X - Monte Carlo computer simulation values. (b) Range straggling for 35 keV 3He ions as a function of Z,: 0 - experimental, x - Monte Carlo computer simulation values.
the computer simulations (assuming amorphous media) are shown on the same figures and in table 1. Our results for both the range and range straggling indicate strong Z,-oscillations in electronic stopping for He (see ref. [7]). We note that the deviations between the most probable and mean range determined from the Monte Carlo results are small, indicating that their shapes are nearly Gaussian. For the 14 materials studied, the measured ranges are larger on average than those calculated by 5%. B&tiger et al. [13] have reported similar Z2-oscillations in 3He(d, 4He)p analysis for C, Al, Si, V, Ni and Zr. On average, our projected range (RJ and straggling values exceed theirs by 15% and 4%, respectively, for the four Z,-values studied in both works. Further, we note that the R,-value for 50 keV 3He in Ni measured by Biersack et al. [ll] using the 3He(n, P)~H technique also exceeds that measured [13] by the reaction 3He(d, 4He)p by 19%. The lack of agreement between results obtained by the two different profiling techniques is not understood.
595
W.N. L-ennardet al. / Depth profiles of 35 keV ‘He ions in metals 4. summary The behaviour of metals subjected to high fluences of energetic He ions comparable to those used in this study is of particular interest to fusion reactor technology [ 11. Another application of this knowledge relates to the use of He-implanted targets for the study of nuclear reactions. For example, the Doppler-shift attenuation method [15] has been used in conjunction with heliumimplanted targets, to deduce short lifetimes of recoiling, y-emitting nuclei formed by nuclear reactions between the helium target atoms and a beam of high velocity heavy ions. The same method has also been instrumental in deducing the amount of swelling caused by helium implantation [16]. A knowledge of the concentration profiles of the helium in the metal foil targets is essential for these studies. i We have measured the depth profiles of ‘He implanted at 35 keV energy into 14 metallic targets. We observed marked Z,-oscillations in both the range and range straggling that correlate well with the Z,-oscillations expected on the basis of electronic stopping. The measurements yield range and range straggling values that are in reasonable agreement with Monte Carlo computer simulations using He stopping power data from the Ziegler compilation [7]. The results suggest that the semi-empirical electronic stopping power tables for He are reliable at low energies, where there is a dearth of experimental results. Our measured values are systematically 15% larger than results obtained using the 3He(d, 4He)p reaction, an anomaly for which no explanation can be found at present. We are grateful for the technical assistance of O.M. Westcott and L. Milani throughout the course of these measurements. One of us (H.G.) acknowledges the receipt of a research grant from the Deutsche Forschungsgemeinschaft. This work was supported in part
by the Canadian FTP).
Fusion
Fuels Technology
Project (CF-
PI R. Behrisch and B.B. Kadomtsev,
Plasma Physics and Controlled Nuclear Fusion Research II (IAEA, Vienna, 1975) p. 229. PI R.S. Blewer, J. Nucl. Mat. 53 (1974) 268. 131 B. Terreault, M. Leroux, J.G. Martel, R. St.-Jacques, C. Brassard, C. Cardinal J. Chabbal, D. Dechenes, J.P. Labrie and J. I’Ecuyer, Adv. Chem. Ser. 158 (1976) 295. 141J.P. Biersack, D. Fink, R. Henkelmann and K. Miiler, Nucl. Instr. and Meth. 149 (1978) 93. [51 R. Behrisch, J. Bettiger, W. Eckstein, U. Littmark, J. Roth and B.M.U. Scherzer, Appl. Phys. Lett. 27 (1975) 199. 161 D.P. Jackson, J. Nucl. Mat. 93/94 (1980) 507. [71 J.F. Ziegler, Helium Stopping Powers and Ranges in All Elements (Pergamon, New York, 1977). PI W. MMler and F. Besenbacher, Nucl. Instr. and Meth. 168 (1980) 111. 191M.A. Lone, D.C. Santry and W.M. IngIis, Nucl. Instr. and Meth. 174 (1980) 521. WI H.H. Andersen and J.F. Ziegler, Hydrogen Stopping Powers and Ranges in All Elements (Pergamon, New York, 1977). 1111J.P. Biersack, D. Fink, R.A. Henkelmann and K. MtiIIer, J. Nucl. Mat. 85/86 (1979) 1165. WI D. Fink, J.P. Biersack, K. Tjan and V.K. Cheng, JNucl. Instr. and Meth. 194 (1982) 105. 1131 J. Bottiger, P.S. Jensen and U. Littmark, J. Appl. Phys. 49 (1978) 965. 1141 R. Behrisch, J. Bettiger, W. Eckstein, J. Roth and B.M.U. Schemer, J. Nucl. Mat. 56 (1975) 365. 1151 T.K. Alexander and J.S. Forster, Advances in Nuclear Physics, Vol. 10, eds. M. Baranger and E. Vogt (Plenum, New York, London, 1978) p. 197. WI T.K. Alexander, G.C. Ball, W.G. Davies and LV. Mitchell, J. Nucl. Mat. 96 (1981) 51.
V. ION IMPLANTATION..
.
Nuclear Instruments and Methods in Physics Research BlO/ll (1985) 592-595 North-Holland, Amsterdam
DEPTH PROFILES OF 35 keV 3He IONS IN METALS W.N. LENNARD, H. GEISSEL *, T.K. ALEXANDER, R. HILL, D.P. JACKSON, M.A. LONE and D. PHILLIPS AtomicEnergyof Canada Limited Research Company, Chafk River Nuclear Laboratories, Chalk River, Ontario, Canada K#
I&?
The depth profiles of 3He implanted at 35 keV energy into AI, Ti, V, Ni, Cu. Zn, Zr, Nb, Ag, Sn, Ta, W, Au and Bi targets have been measured using the thermal neutron reaction ‘He@, P)~H. The profiles obtained from the energy distribution of the emitted protons show a marked Z,osciIlation in both the most probable depth and depth straggling. The measured values are compared to Monte Car10computer simulation resuhs.
1. Inbroduction
2. Experimental
There is a strong t~~olo~~ need for data concerning the depth distribution of light ions in metals. This topic relates to the performance of the first wall of future controlled thermonuclear reactors (CTR’s) under intense bombardment by H, He and Li ions [l]. Depth profiles for saturated doses are important in the understanding of the recycling of plasma particles from the wall in such high temperature devices. The present data relate to undistorted profiles and room temperature implantation. Another application that requires knowledge of helium depth profiles in metal foils is the use of implanted He as a target in nuclear reaction studies. Several experimental techniques have been developed to measure depth profiles of light ions in metals: Rutherford backscattering [2], elastic recoil analysis [3] and nuclear reaction analysis [4,5]. These techniques have been applied to determine the depth distribution of light noble gases such as 3He and 4He. However, 3He is much easier to detect than 4He. Although both the 3He(d, 4He)p and ‘He@, P)~H reactions are sensitive probes of 3He, only the latter provides non-destructive analysis. A comparison of the (n, p) and (d, 4He) tech-
Polyc~s~~ne 0.1 mm thick foils of the elements Al, Ti, V, Ni, Cu, Zn, Zr, Nb, Ag, Sn, Ta, W, Au and Bi were implanted with 1.4 X 10” ‘He ions cm2 at an energy of 35 keV. The target foils were cleaned initially with trichloroethylene followed by an acetone rinse. The local vacuum during impl~tation was 0.1 mPa; the beam current during implantation was in the range 6-8 PA cme2, and the targets were held at room temperature. The implanted target foils were first analyzed for 3He content u sin g a 0.5 MeV energy deuteron beam. The total amount of trapped 3He was measured by counting the protons from the 3He(d, P)~H~ reaction whose cross section is known precisely [8]. The deuteron fluence was 2.5 X 1014 cme2 f a value negligible compared to the 3He doses. The depth profiles were subsequently measured using the 3He(n, P)~H thermal neutron reaction that has a cross section of 5.3 kb. A flux of 3 X 10’ neutrons cme2 s-* from the N4 external thermal neutron facility [9] at the Chalk River NRU reactor impinged on the targets as shown in fig. 1. A Si surface barrier detector (100 gm when fully depleted, 50 mm2 active area) was placed 10 cm from the target at 90’ to the neutron beam. Normally, we set the angle between the beam and the target surface, 8, to the value 8“. The Pb shielding and the magnetic field (15 mT) were effective in reducing background radiation in the detector. Only the central 20 mm* area of the detector was able to view the target. Furthermore, we reduced the detector bias voltage to limit its sensitive thickness to a value just exceeding the range of the protons without suffering decreased resolution. The initial energies of the proton and triton are 572.5 and 191.3 keV, respectively. A near-surface 3He target, fabricated by a glow
niques is given in ref. [4]. We have used the thermal neutron 3He(n, P)~H reaction to determine the most probable projected range and range straggling for 35 keV ‘He ions implanted into 14 metallic targets. The experimental data are compared to Monte Carlo computer simulation results (see ref. [6]), using well-defined input quantities. For this purpose, we have chosen to use the electronic stopping values of Ziegler [7] for He, and Moliere’s repulsive potential for treating the elastic (nuclear) collisions. We assume ‘He and 4He have the same stopping power at
the same velocity. * Preseat address: GSI, D-61 Darmstadt, FRG. 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
593
W.N. Lennard et al. / Depth profiles of 35 keV ‘He ions in metals
Pb SHIELD Si SURFACE BARRIER DETECTOR
-a
PROTONS, TRITONS
/rzrmm
-COLLIHATOR
I
3
Fig. 1. Schematic of the experimental setup for detecting ‘H and 3H particles (equal numbers of each) emitted from the thermal neutron 3He(n, P)~H reaction. The targets contained 3He ions implanted at 35 keV energy.
technique using a Nb sample as the cathode, was used for the absolute energy calibration. The negligible thickness of this calibration target (8 x 1015 3He atoms cme2) was confirmed by varying the angle B. The most probable depth for 3He in this target was measured also using the tilt technique and found to be 9 pg cmw2, a value that compared favourably to that expected based on the preparation conditions: i.e. 9 pg cmm2 is approximately the range of 1 keV ‘He ions in Nb [5]. The resolution obtained for 572 keV protons was 9 keV fwhm. The measured energy distribution of the protons (and tritons) was converted to a depth profile for the implanted 3He ions using ‘H stopping powers from Andersen and Ziegler [lo]. The ‘H spectra were not used to obtain quantitative depth values. First, there exist little stopping power data in the energy region of interest; second, low energy background in the vicinity of the 3H peak for the low dose glow discharge target made precise target surface location difficult.
I
I
I
,
I
bl PROTONS
discharge
3. Results anddiscussion Fig. 2a shows the depth distribution for 3He in Nb, deduced from the 3H energy spectrum and fig. 2b, from the ‘H energy spectrum. The proton peaks were fitted with a Gaussian with exponential tails on both the low and high energy sides, reflecting the energy loss straggling for the emergent protons and for the initially implanted 35 keV ‘He ions. The most probable values were obtained from the fitted parameters. The proton energy scale was converted to a target depth scale using dE/dx values from ref. [lo]. Corrections to the range and range straggling values (0.1% and 0.256, respec-
OEPTH lpg cms21
Fig. 2. (a) Depth distribution of ‘He ,deduced from the ‘H energy spectrum. The implanted fluence was 1.1 X10” ions cm-’ in Nb. (b) Depth distribution of 3He deduced from the ‘H energy spectrum for the same target as in (a).
tively) were made for the finite acceptance angle of the detector system. The particle energies corresponding to the target surface were obtained by reference to the glow discharge target, for which the proton spectrum was a symmetric Gaussian. We have measured most probable depth values for all samples with a retained dose - 1.1 X 10” cme2. We have chosen to determine the range straggling values from the fwhm of the peaks, after subtracting the wntributions of proton straggling (Bohr’s value) and detector resolution in quadrature. This choice seems reasonable since all the proton peaks were nearly symmetric. We have ignored the contribution of implanted 3He to the proton energy loss. The ‘He doses represented peak concentrations, n(‘He)/n(host), in the range 9 at.% to 21 at.%, where n represents a volume atomic density. Stopping power values for protons in 3He are much smaller than their values in the host material. We have used the ‘%(d, p)l’O nuclear reaction at 0.97 MeV deuteron energy to determine the near-surface oxygen concentration for the Al, Zn, Nb, Ta, W and Bi targets. The proton yields were compared to that from a V. ION IMPLANTATION..
.
W.N. Lennard et al. / Depth profiles of 35 keV ‘He ions in metals
594
Ta,O, standard of known thickness. If we assume (worst case) that all the oxygen is contained in a surface oxide, we find that the oxide layer thickness represented 5% of the 3He projected range except for Zn, where it was 15%. However, the 15% figure would lead to a correction to the ‘He range < 3%; we have therefore ignored the effect of the oxide since the depth distribution of the oxygen atoms was not measured. The ‘H energy spectrum shown in fig. 2a has been corrected for background by subtracting a smoothlyvarying function of particle energy fitted above and below the peak. The background is caused primarily by electrons produced in the sensitive volume of the detector via (II, y) reactions. Neither spectrum shows evidence for saturation in the peak region, which is observed for larger fluences. We conclude that the most probable depth values correspond to 3He range values for these samples. No diffusion effects have been reported for room temperature ‘He implants at doses 10’7cm-2 [11,12]. We have observed no difference in the most probable depth or fwhm for 3He implanted at 35 keV into Au samples for fluences from 1 to 4 X 1017 cme2, which is less than the saturation dose. A summary of the most probable range values measured for 35 keV 3He in y4 metals is given in table 1 and shown in fig. 3a. The estimated uncertainty for each datum is 5% excluding any contribution from uncertainties in the ‘H stopping power values. The reproducibility of our measurements was 2% deduced from 22 separate measurements for a Nb sample. The additional uncertainty is estimated from a study of the values deduced by fitting different functions to the proton peaks. The range straggling values are shown in fig. 3b, which have a similar structure to the range data. The uncertainties are 15% for these data. The results from Table 1 Range and range straggling values for 35 keV ‘He in metals. AU values are in cg cmm2. The calculated values are from Monte Carlo computer simulations using stopping power values from ref. [7] Target
RQP’ P
R*P
ARTpI
AR?
Al Ti V Ni cu Zn Zr Nb
62 74 81 114 127 115 97 101 132 122 163 189 191 165
66 79 77 98 117 118 85 92 121 123 156 164 203 148
17 24 29 49 55 47 38 40 62 46 72 84 95 81
17 24 24 34 44 45 28 33 49 50 67 72 97 64
Ag Sn Ta W Au Bi
Oi--+-++ b)
IO0
Fig. 3. (a) Most probable depth, or range, for 35 keV 3He ions implanted into target materials, Z,, to a fluence of 1.1 X 10” cms2: 0 - experimental, X - Monte Carlo computer simulation values. (b) Range straggling for 35 keV 3He ions as a function of Z,: 0 - experimental, x - Monte Carlo computer simulation values.
the computer simulations (assuming amorphous media) are shown on the same figures and in table 1. Our results for both the range and range straggling indicate strong Z,-oscillations in electronic stopping for He (see ref. [7]). We note that the deviations between the most probable and mean range determined from the Monte Carlo results are small, indicating that their shapes are nearly Gaussian. For the 14 materials studied, the measured ranges are larger on average than those calculated by 5%. B&tiger et al. [13] have reported similar Z2-oscillations in 3He(d, 4He)p analysis for C, Al, Si, V, Ni and Zr. On average, our projected range (RJ and straggling values exceed theirs by 15% and 4%, respectively, for the four Z,-values studied in both works. Further, we note that the R,-value for 50 keV 3He in Ni measured by Biersack et al. [ll] using the 3He(n, P)~H technique also exceeds that measured [13] by the reaction 3He(d, 4He)p by 19%. The lack of agreement between results obtained by the two different profiling techniques is not understood.
595
W.N. L-ennardet al. / Depth profiles of 35 keV ‘He ions in metals 4. summary The behaviour of metals subjected to high fluences of energetic He ions comparable to those used in this study is of particular interest to fusion reactor technology [ 11. Another application of this knowledge relates to the use of He-implanted targets for the study of nuclear reactions. For example, the Doppler-shift attenuation method [15] has been used in conjunction with heliumimplanted targets, to deduce short lifetimes of recoiling, y-emitting nuclei formed by nuclear reactions between the helium target atoms and a beam of high velocity heavy ions. The same method has also been instrumental in deducing the amount of swelling caused by helium implantation [16]. A knowledge of the concentration profiles of the helium in the metal foil targets is essential for these studies. i We have measured the depth profiles of ‘He implanted at 35 keV energy into 14 metallic targets. We observed marked Z,-oscillations in both the range and range straggling that correlate well with the Z,-oscillations expected on the basis of electronic stopping. The measurements yield range and range straggling values that are in reasonable agreement with Monte Carlo computer simulations using He stopping power data from the Ziegler compilation [7]. The results suggest that the semi-empirical electronic stopping power tables for He are reliable at low energies, where there is a dearth of experimental results. Our measured values are systematically 15% larger than results obtained using the 3He(d, 4He)p reaction, an anomaly for which no explanation can be found at present. We are grateful for the technical assistance of O.M. Westcott and L. Milani throughout the course of these measurements. One of us (H.G.) acknowledges the receipt of a research grant from the Deutsche Forschungsgemeinschaft. This work was supported in part
by the Canadian FTP).
Fusion
Fuels Technology
Project (CF-
PI R. Behrisch and B.B. Kadomtsev,
Plasma Physics and Controlled Nuclear Fusion Research II (IAEA, Vienna, 1975) p. 229. PI R.S. Blewer, J. Nucl. Mat. 53 (1974) 268. 131 B. Terreault, M. Leroux, J.G. Martel, R. St.-Jacques, C. Brassard, C. Cardinal J. Chabbal, D. Dechenes, J.P. Labrie and J. I’Ecuyer, Adv. Chem. Ser. 158 (1976) 295. 141J.P. Biersack, D. Fink, R. Henkelmann and K. Miiler, Nucl. Instr. and Meth. 149 (1978) 93. [51 R. Behrisch, J. Bettiger, W. Eckstein, U. Littmark, J. Roth and B.M.U. Scherzer, Appl. Phys. Lett. 27 (1975) 199. 161 D.P. Jackson, J. Nucl. Mat. 93/94 (1980) 507. [71 J.F. Ziegler, Helium Stopping Powers and Ranges in All Elements (Pergamon, New York, 1977). PI W. MMler and F. Besenbacher, Nucl. Instr. and Meth. 168 (1980) 111. 191M.A. Lone, D.C. Santry and W.M. IngIis, Nucl. Instr. and Meth. 174 (1980) 521. WI H.H. Andersen and J.F. Ziegler, Hydrogen Stopping Powers and Ranges in All Elements (Pergamon, New York, 1977). 1111J.P. Biersack, D. Fink, R.A. Henkelmann and K. MtiIIer, J. Nucl. Mat. 85/86 (1979) 1165. WI D. Fink, J.P. Biersack, K. Tjan and V.K. Cheng, JNucl. Instr. and Meth. 194 (1982) 105. 1131 J. Bottiger, P.S. Jensen and U. Littmark, J. Appl. Phys. 49 (1978) 965. 1141 R. Behrisch, J. Bettiger, W. Eckstein, J. Roth and B.M.U. Schemer, J. Nucl. Mat. 56 (1975) 365. 1151 T.K. Alexander and J.S. Forster, Advances in Nuclear Physics, Vol. 10, eds. M. Baranger and E. Vogt (Plenum, New York, London, 1978) p. 197. WI T.K. Alexander, G.C. Ball, W.G. Davies and LV. Mitchell, J. Nucl. Mat. 96 (1981) 51.
V. ION IMPLANTATION..
.