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Analysis of near-surface tritium in materials by elastic recoil detection under MeV energy helium bombardment
475
Nuclear instruments and Methods in Physics Research B15 (1986) 475-480 N(~rtll-Holland, Amsterdam
AUALYSIS OF NEAR-SURFACE TRITIUM IN MATERIALS UUDER MeV ENERGY HELIUM BOMBARDMENT J. \. SAWICKI,
H.H. PLATTNER,
Arm~ie Energy of Canada Limited,
I.V. MITCHELL
Chcdk Rwer N&ear
Laboratories,
BY ELASTIC
RECOIL
DETECTION
and J. GALLANT Chalk Rwer, Onturio, Canada. KOJ lJ0
The method of elastic recoil detection, using 1.5-2.2 MeV energy 4He beams, has been used for the first time to detect tritium. Test samples were fabricated by thermal sorption of tritium into thin (5 ng cm-*) Ti films and by implantation of low energy (10 keV) HT’ ions into amorphized silicon and oxide targets. Distributions of surface and subsurface tritium has been profiled to a depth of a few tenths of a micron, with a depth resolution of 10 to 20 nm and a sensitivity better than lOi atoms cm-‘.
1. ln~oductjon Elastic recoil detection (ERD), using heavy projeztiles such as 12C or 35C1, has proven to be a useful alternative to nuclear reaction analysis and to secondary ion mass spectrometry for profiling hydrogen in solids [I --31. The ERD method has been extended to the profiling of both H and D under low energy He ion bombardment [4-91; more recently, Mills et al. have exploited the good spectral separation of the H and D profiles that this method provides, in a diffusion study of a deuterated polymer in its hydrogenated analogue [1*3]. To the present, no application of ERD to the analysis of near-surface tritium has been reported, the conventional ion beam analytical approach being based upon p + T nuclear reaction yields [11,12]. Our interest in evaluating ERD for tritium profiling is motivated by t\s’o present requirements, the first for a method of characterizing tritium behaviour at the first wall and at the surface of lithium-bearing ceramics now being consi
2. Experiment Methods distributions
for producing stable surface or near-surface of tritium parallel those that have been
adopted in hydrogen and deuterium studies. In particular, since deuterium content now can be determined absolutely through either low energy nuclear reaction [15] or ERD methods [9], deuterium specimens also have been prepared to guide the tritium study. (Both NRA and ERD techniques allow for discrimination against contaminant hydrogen). Deuterium samples have been prepared by thermal sorption of deuterium gas at 300°C in thin titanium films ‘(1.5-100 pg cmW2), deposited by vacuum evaporation onto oxidized ahuninium backings. Depending on the conditions of films fabrication and on film thickness, the deuterium to hydrogen ratio in these deutcride samples varied from 1 to 10. Secondary ion mass spectrometry indicates that such films can develop a 1 pg cm-2 oxide cover layer 1161. The stability of the Ti-D films under the analyzing helium beam was tested at E, = 2 MeV and a beam current of 20 nA to a total accumulated charge of 400 PC. A 10% loss of deuterium in the bombarded area was observed in the first 100 PC, dropping to about 2% per 100 PC thereafter. The films were found to be stable upon exposure to air during a several months period. Tritration of titanium films has been carried out under similar experimental conditions but using an apparatus suitable for safe handling of radioactive tritium gas. The thickness of Ti films used for tritiation was 5 a part of the sample was pg cmW2. After tritiation, coated with 40 ttg cme2 layer of gold. As in the case of Ti-D films, the losses of tritium under the beam were about 0.1% per 1 PC. A second set of samples was prepared by low-energy ion implantation of deuterium and tritium using the CRNL 70 kV isotope separator. Several targets made of titanium, amorphized silicon and oxidized metal targets (ZrO,, Ta,O,) have been examined as possible hosts, the latter obtained by anodic oxidation to produce layers with thicknesses of about 100 nm. To reduce the IX. ELASTIC RECOIL DETECTION
416
J.A. Sawicki et al. / Analysis of near-surface tritium
implant depth of deuterium and tritium to within 30 nm of the surface, implantation was carried out at 60” incidence, using low energy (10 and 20 keV) D; and HT+ ions. Fluences were up to 10” Df ions cm-* and up to 1.5 X 1016 HT+ ions cm-‘. Bombarding currents were from 0.5 to 5 PA and implantations were carried out in a vacuum of 1.3 X lOem Pa (lo-” Torr). While D- and HT-implanted Ti and Si samples were stable both under beam and to ambient, some loss or redistribution of D and T was noticed in oxide targets. The measurement of the elastic recoil spectra was carried out using the CRNL 2.5 MV Van de Graaff accelerator and an ORTEC scattering chamber allowing for independent selection of range filters and angular positioning of two detectors for ERD and NRA measurements. A Faraday cup around the sample was kept at liquid nitrogen temperature in a vacuum of 7 X 10e5 Pa (5 x 10m7 Torr). Helium beam currents to target were in the range lo-30 nA. Elastic recoils were detected in a 100 pm surface barrier Si detector with a 0.5 x 3 or 2 x 3 mm aperture, placed at a distance of 10 cm from the sample, usually at an angle of 30” with respect to the beam direction and at a sample tilt angle of 75”. Mylar foils of three different thicknesses, 6, 8 and 10 pm, were available to filter out elastically scattered He ions. Energy losses of hydrogen isotope recoils were determined for these foils separately on a proton beam and were in agreement with data from the literature. Yields from D(3He, P)~H~, D(d, p)T and T(d, 4He)n reactions were determined in the same chamber, using conventional (backward) geometry and suitable filters to eliminate scattered beam. Reaction yields were calibrated through use of the “60(d, p,)“O reaction at 972 keV on a 100 nm Ta,O, film and through RBS measurements on a Si(Bi) implant standard. A similar setup for simultaneous ERD, RBS and NRA has also been installed in a UHV chamber.
below 1O-9 Pa (lo-.” Torr). Well separated from this signal are peaks associated with surface deuterium, D,$ (panels 1 and 3 of fig. I), deuterium at finite depth (panel 2) and surface tritium. As a consequence of the recoil kinematics and of differential energy losses in the absorber foil of deuterons and tritons, tritium and deuterium surface signals appear at essentially the same energy. For low energy accelerators, discrimination between D and T requires the use of nuclear reactions which are specific to each isotope as illustrated in fig. 2 or the use of momentum - or velocity - selective detection methods [2,3]. 3.2. Depth information Reference to panels 1 and 2 in fig. 1 indicates that depth information can be recovered by the ERD method. A displacement of the deuterium profile is observable for the implanted distribution (located at a depth of - 38 nm). Profile broadening is also evident. Energy straggling, kinematic broadening and instrumental resolution obscure the difference in profiles between adsorbed D (1.5 atomic layers thick) and the reacted D (- 10 nm thick). Depth information in the case of tritium has been extracted through a comparison of ERD profiles from bare and Au-coated surface T films. Fig. 3 shows profiles from a TipT film with and without a 40 pg cmm2 gold (20 nm) overlayer. The observed shift of the tritium peak in ERD spectra of uncovered and covered films equals about 90 keV, which agrees quite well with the combined energy losses calculated for 2 MeV a-particles (AE = 51 keV) and 1.5 MeV tritons (AE = 26 keV) in the gold layer. A resolution of 10 to 20 nm seems achievable. In the evaluation of the depth scale in the spectra, the energy losses for D and T recoils with an energy E were taken as equal to the stopping power for H with an energy E/2 and E/3, respectively. The broadening of the peak (from 60 keV to 110 keV fwhm) could be attributed to energy straggling both in the Au layer and in the range filter.
3. Results 3.3. Defection sensitivity 3.1. Tritium identification Characteristic energy spectra are shown in fig. 1 of (filtered) hydrogen isotope recoils produced by 2.2 MeV He ion bombardment of (top to bottom): a nickel surface saturated with adsorbed deuterium, an amorphized silicon film implanted with deuterium, a 5 pg cm-’ Ti layer reacted with deuterium and a 5 pg cm-’ Ti layer reacted with tritium, respectively. In all cases but one, a peak, H,, corresponding to surface protium. is observed. The exception is found for the [110] surface of a nickel single crystal which had been cleaned by sputtering and measured in situ in the LJHV chamber, where the hydrogen partial pressure was
The amount of tritium in the samples was determined from the T(d, 4He)n reaction yield, measured in backward (150°) scattering geometry at a deuteron energy of 600 keV. The differential cross section value was taken to be 34 mb sr-’ [17]. From the ERD yield we can then deduce values for the differential cross section for elastic recoil T(4He, 4He)T. A preliminary value of 200 mb sr-’ was obtained at 1.5 MeV He beam energy for a recoil angle of 30” (laboratory coordinates). The recoil probability has been mapped from 1.5 to 2.0 MeV He energy and over the angular range 20” to 45’, using a 5 pg cmp2 ‘K-T target. The variation is less than 20%. This cross section will be remeasured but it is
J.A. Sawicki et al. / Analysis of near-surjace
D on
trhum
477
[IIO] Ni
2 A,
0 -
P
a-Si(D1
+
&
i
IOJnm
1
Ti + II
a
d 10
. !
BI
J i
i
Ti + T
i
; a
b
t_
x10 t
Cl
T
0 O.r,
0.6
RECOIL ENERGY Fig. 1. ERD spectra measured obtained by chemisorption of charge Q = 15 PC); B - sample incidence angle 60”, projected into thin 5 kg cm-’ titanium ions-cm-‘) (Q = 10 PC).
0.8
ll0
(MeVl
with 2.2 MeV 4He beam and 8 pm mylar filter at 75” incidence and 30’ detection angle: A - sample 1.5 deuterium monolayers on [110] Ni surface carefully prepared at ultra high vacuum. (integrated obtained by ion implantation of 10 keV D: ions into amorphous silicon: dose 10” D ions cmP2. ion range below the surface 38 nm. (Q = 10 PC?); C - sample obtained by thermal sorption of deuterium film. (Q = 10 PC); D - sample
obtained
by thermal
sorption
of tritium
into similar
IX. ELASTIC
RECOIL
film ( - 10’” T
DETECTION
J. A. Sawicki et al. / Analysis
478
of neur -surface tritium
I SiKl) ;,
D(d,p)T
1, I
1:I A,
WIT)
T(d, a In
Bt
Ti + T
C, 200
300
CHANNEL
Fig. 2. Energy spectra of reaction D+ beam: A - silicon implanted cm-* titanium film.
products for D(d, p)T and T(d, 4He)n reactions measured at backward 150” geometry using 600 keV with 10” D+ ions cm- ‘., B - silicon implanted with 2 x lOI DT* ions cm-*; C - tritiated 5 pg
J.A. Sawckl
15 1 10
1
et al. / Anaiysls of near-surface
Au/Ti + T/Al
I I
0
I
I
,
I
I
,
I
0.5
I
’
1.0
RECOIL ENERGY (MeV) Fi;. 3. ERD spectra measured with 2.0 MeV 4He beam and 8 nnt mylar filter at 75” incidence and 30’ detection angle. (Q = 20 PC); top 5 pg cm-* tritiated titanium film; bottom - -ame film covered with 40 pg cm-’ gold film.
apparent that it exceeds the Rutherford cross section by approximately a factor four (ERD cross sections for H and D are also higher than the Rutherford cross sections, see refs. [7-91). The value of the T recoil cross section determined by us is also higher than expected from inverse reaction the cross section 4He(T, T)4He data [18,19]. altxady
4. Discussion Even in the simple form in which we have applied it here, the ERD methods shows some promise for tritium analysis. The advantage is apparent for surface or nearsurface distributions of tritium, free of deuterium. A detection sensitivity better than lOI T atoms cm-’ is indicated for ERD in 2 MeV He energy range with lo-20 nm depth resolution, down to a depth of 200-300 nm where H recoil signals produce a spectral background. The depth resolution of the ERD method compares favourably with the results of earlier works. By measuring neutron-time-of-flight for the T(p, n)3He reaction, Davies and Anderson [ll] have profiled tritium in titanium over the first 10 l.trn of depth with a resolution of 0.6 pm and up to 50 km with a resolution of 1.5 l.trn. Okuda et al. [13,14] were able to probe about 2 pm in
479
tritium
titanium with the resolution about 0.2 l.trn, by T(d, 4He)n reaction at E, = 500-700 keV and 120” scattering geometry. It should be noted that Caterini et al. [20] have demonstrated considerable improvement in the depth resolution available through use of the T(d, 4He)n reaction in forward detection geometry. Over a probing depth of 1 l.trn a depth resolution of 0.1 l.trn is realized. The probing depth and the sensitivity of tritium analysis by 4He-ERD technique could be markedly increased at higher bombarding energy, in particular near E, = 5.1 MeV; i.e. in the region corresponding to narrow resonance level in 7Li [21,22]. ERD may prove useful in cases where simultaneous mapping of H and T is required; its relevance as a diagnostic for methods of producing thin, stable, planar T sources is under study. ERD also shows promise for tritium trapping and release studies in materials that cannot be studied readily by NRA methods, owing to intense reaction product backgrounds. This is a problem for D and 3He beam analyses of lithium-bearing ceramics such as Li ,O which are candidate materials for fusion breeder blankets. The evaluation of the ERD method is continuing. The authors express their thanks to T.E. Jackman, W.N. Lennard and J.A. Davies for useful comments and assistance and to O.M. Westcott for providing the implants. One of us (J.A.S.) wishes to acknowledge the support provided by AECL through a visiting scientist appointment. This work was performed as part of the Fusion Breeder Blanket Program, jointly funded by the Canadian Fusion Fuels Technology Project and Atomic Energy of Canada Limited Research Company.
References
VI J.F. Ziegler and 25 others, Nucl. Instr. and Meth. 149 (1978) 19.
121 J. L’Ecuyer,
C. Brassard, C. Cardinal, J. Chabbal, L. Deschenes, J.P. Labrie, B. Terreault, J.G. Martel and R.G. St. Jacques, J. Appl. Phys. 47 (1976) 381. [31J. L’Ecuyer, C. Brassard, C. Cardinal and B. Terreault, Nucl. Instr. and Meth. 149 (1978) 271. [41 B.L. Doyle and P.S. Peercy, Appl. Phys. Lett. 34 (1979) 811. [51 B.L. Doyle and P.S. Peercy, Proc. Workshop on the Analysis of Hydrogen in Solids, Albuquerque, (Jan. 23-25, 1979). [61A. Turos and 0. Meyer, Nucl. Instr. and Meth. B4 (1984) 92. [71 L.S. Wielunski, R.E. Benenson and W.A. Lanford, Nucl. Instr. and Meth. 218 (1983) 120. [8] F. Besenbacher, I. Stensgaard and P. Vase, these Proceedings (IBA ‘85) Nucl. Instr. and Meth. B15 (1986) 459. [9] S. Nagata, S. Yamagauchi, Y. Fujino, Y. Hori, N. Sugiyama and K. Kamada, Nucl. Instr. and Meth. B6 (1985) 533. IX. ELASTIC
RECOIL
DETECTION
480 [lo] [ll] (121 [13] [14] [15]
J.A. Sawicki et al. /Analysis P.J. Mills, P.F.. Green, C.J. Palmstrom, J.W. Mayer and E.J. Kramer, Appl. Phys. Lett. 45 (1984) 957. J.C. Davis and J.D. Anderson, J. Vat. Sci. Technol. 12 (1975) 358. Ion Beam Handbook, eds., J.W. Mayer and E. Rimini, (Academic Press, New York, 1979). S. Okuda, R. Taniguchi, M. Fujishiro, Y. Satoh and E. Hiraoka, J. Appl. Phys. 54 (1984) 6790. S. Okuda, R. Taniguchi, M. Fujishiro. Y. Satoh and E. Hiraoka, J. Nucl. Mater. 128/129 (1984) 725. J.A. Davies and P.R. Norton, Nucl. Instr. and Meth. 168 (1980) 611.
of near-surface
tntium
[16] J.I. Gallant and P. Dmytrenko, Nucl. Instr. and Meth. BlO/ll (1985) 382. [17] S.J. Bame and J.E. Perry, Jr., Phys. Rev. 107 (1957) 1616. [18] R.C. Allen and N. Jarmie, Phys. Rev. 111 (1958) 1129. [19] L.S. Chuang, Nucl. Phys. Al74 (1971) 399. [20] M. Caterini. D.A. Thompson, P.T. Wan, and J.A. Sawicki. these Proceedings (IBA’85) Nucl. Instr. and Meth. [21] M. Ivanovitch. P.G. Young and G.G. Ohlsen, Nucl. Phys. All0 (1968) 441. [22] R.J. Spiger and T.A. Tombrello. Phys. Rev. 163 (1967) 964.
Nuclear instruments and Methods in Physics Research B15 (1986) 475-480 N(~rtll-Holland, Amsterdam
AUALYSIS OF NEAR-SURFACE TRITIUM IN MATERIALS UUDER MeV ENERGY HELIUM BOMBARDMENT J. \. SAWICKI,
H.H. PLATTNER,
Arm~ie Energy of Canada Limited,
I.V. MITCHELL
Chcdk Rwer N&ear
Laboratories,
BY ELASTIC
RECOIL
DETECTION
and J. GALLANT Chalk Rwer, Onturio, Canada. KOJ lJ0
The method of elastic recoil detection, using 1.5-2.2 MeV energy 4He beams, has been used for the first time to detect tritium. Test samples were fabricated by thermal sorption of tritium into thin (5 ng cm-*) Ti films and by implantation of low energy (10 keV) HT’ ions into amorphized silicon and oxide targets. Distributions of surface and subsurface tritium has been profiled to a depth of a few tenths of a micron, with a depth resolution of 10 to 20 nm and a sensitivity better than lOi atoms cm-‘.
1. ln~oductjon Elastic recoil detection (ERD), using heavy projeztiles such as 12C or 35C1, has proven to be a useful alternative to nuclear reaction analysis and to secondary ion mass spectrometry for profiling hydrogen in solids [I --31. The ERD method has been extended to the profiling of both H and D under low energy He ion bombardment [4-91; more recently, Mills et al. have exploited the good spectral separation of the H and D profiles that this method provides, in a diffusion study of a deuterated polymer in its hydrogenated analogue [1*3]. To the present, no application of ERD to the analysis of near-surface tritium has been reported, the conventional ion beam analytical approach being based upon p + T nuclear reaction yields [11,12]. Our interest in evaluating ERD for tritium profiling is motivated by t\s’o present requirements, the first for a method of characterizing tritium behaviour at the first wall and at the surface of lithium-bearing ceramics now being consi
2. Experiment Methods distributions
for producing stable surface or near-surface of tritium parallel those that have been
adopted in hydrogen and deuterium studies. In particular, since deuterium content now can be determined absolutely through either low energy nuclear reaction [15] or ERD methods [9], deuterium specimens also have been prepared to guide the tritium study. (Both NRA and ERD techniques allow for discrimination against contaminant hydrogen). Deuterium samples have been prepared by thermal sorption of deuterium gas at 300°C in thin titanium films ‘(1.5-100 pg cmW2), deposited by vacuum evaporation onto oxidized ahuninium backings. Depending on the conditions of films fabrication and on film thickness, the deuterium to hydrogen ratio in these deutcride samples varied from 1 to 10. Secondary ion mass spectrometry indicates that such films can develop a 1 pg cm-2 oxide cover layer 1161. The stability of the Ti-D films under the analyzing helium beam was tested at E, = 2 MeV and a beam current of 20 nA to a total accumulated charge of 400 PC. A 10% loss of deuterium in the bombarded area was observed in the first 100 PC, dropping to about 2% per 100 PC thereafter. The films were found to be stable upon exposure to air during a several months period. Tritration of titanium films has been carried out under similar experimental conditions but using an apparatus suitable for safe handling of radioactive tritium gas. The thickness of Ti films used for tritiation was 5 a part of the sample was pg cmW2. After tritiation, coated with 40 ttg cme2 layer of gold. As in the case of Ti-D films, the losses of tritium under the beam were about 0.1% per 1 PC. A second set of samples was prepared by low-energy ion implantation of deuterium and tritium using the CRNL 70 kV isotope separator. Several targets made of titanium, amorphized silicon and oxidized metal targets (ZrO,, Ta,O,) have been examined as possible hosts, the latter obtained by anodic oxidation to produce layers with thicknesses of about 100 nm. To reduce the IX. ELASTIC RECOIL DETECTION
416
J.A. Sawicki et al. / Analysis of near-surface tritium
implant depth of deuterium and tritium to within 30 nm of the surface, implantation was carried out at 60” incidence, using low energy (10 and 20 keV) D; and HT+ ions. Fluences were up to 10” Df ions cm-* and up to 1.5 X 1016 HT+ ions cm-‘. Bombarding currents were from 0.5 to 5 PA and implantations were carried out in a vacuum of 1.3 X lOem Pa (lo-” Torr). While D- and HT-implanted Ti and Si samples were stable both under beam and to ambient, some loss or redistribution of D and T was noticed in oxide targets. The measurement of the elastic recoil spectra was carried out using the CRNL 2.5 MV Van de Graaff accelerator and an ORTEC scattering chamber allowing for independent selection of range filters and angular positioning of two detectors for ERD and NRA measurements. A Faraday cup around the sample was kept at liquid nitrogen temperature in a vacuum of 7 X 10e5 Pa (5 x 10m7 Torr). Helium beam currents to target were in the range lo-30 nA. Elastic recoils were detected in a 100 pm surface barrier Si detector with a 0.5 x 3 or 2 x 3 mm aperture, placed at a distance of 10 cm from the sample, usually at an angle of 30” with respect to the beam direction and at a sample tilt angle of 75”. Mylar foils of three different thicknesses, 6, 8 and 10 pm, were available to filter out elastically scattered He ions. Energy losses of hydrogen isotope recoils were determined for these foils separately on a proton beam and were in agreement with data from the literature. Yields from D(3He, P)~H~, D(d, p)T and T(d, 4He)n reactions were determined in the same chamber, using conventional (backward) geometry and suitable filters to eliminate scattered beam. Reaction yields were calibrated through use of the “60(d, p,)“O reaction at 972 keV on a 100 nm Ta,O, film and through RBS measurements on a Si(Bi) implant standard. A similar setup for simultaneous ERD, RBS and NRA has also been installed in a UHV chamber.
below 1O-9 Pa (lo-.” Torr). Well separated from this signal are peaks associated with surface deuterium, D,$ (panels 1 and 3 of fig. I), deuterium at finite depth (panel 2) and surface tritium. As a consequence of the recoil kinematics and of differential energy losses in the absorber foil of deuterons and tritons, tritium and deuterium surface signals appear at essentially the same energy. For low energy accelerators, discrimination between D and T requires the use of nuclear reactions which are specific to each isotope as illustrated in fig. 2 or the use of momentum - or velocity - selective detection methods [2,3]. 3.2. Depth information Reference to panels 1 and 2 in fig. 1 indicates that depth information can be recovered by the ERD method. A displacement of the deuterium profile is observable for the implanted distribution (located at a depth of - 38 nm). Profile broadening is also evident. Energy straggling, kinematic broadening and instrumental resolution obscure the difference in profiles between adsorbed D (1.5 atomic layers thick) and the reacted D (- 10 nm thick). Depth information in the case of tritium has been extracted through a comparison of ERD profiles from bare and Au-coated surface T films. Fig. 3 shows profiles from a TipT film with and without a 40 pg cmm2 gold (20 nm) overlayer. The observed shift of the tritium peak in ERD spectra of uncovered and covered films equals about 90 keV, which agrees quite well with the combined energy losses calculated for 2 MeV a-particles (AE = 51 keV) and 1.5 MeV tritons (AE = 26 keV) in the gold layer. A resolution of 10 to 20 nm seems achievable. In the evaluation of the depth scale in the spectra, the energy losses for D and T recoils with an energy E were taken as equal to the stopping power for H with an energy E/2 and E/3, respectively. The broadening of the peak (from 60 keV to 110 keV fwhm) could be attributed to energy straggling both in the Au layer and in the range filter.
3. Results 3.3. Defection sensitivity 3.1. Tritium identification Characteristic energy spectra are shown in fig. 1 of (filtered) hydrogen isotope recoils produced by 2.2 MeV He ion bombardment of (top to bottom): a nickel surface saturated with adsorbed deuterium, an amorphized silicon film implanted with deuterium, a 5 pg cm-’ Ti layer reacted with deuterium and a 5 pg cm-’ Ti layer reacted with tritium, respectively. In all cases but one, a peak, H,, corresponding to surface protium. is observed. The exception is found for the [110] surface of a nickel single crystal which had been cleaned by sputtering and measured in situ in the LJHV chamber, where the hydrogen partial pressure was
The amount of tritium in the samples was determined from the T(d, 4He)n reaction yield, measured in backward (150°) scattering geometry at a deuteron energy of 600 keV. The differential cross section value was taken to be 34 mb sr-’ [17]. From the ERD yield we can then deduce values for the differential cross section for elastic recoil T(4He, 4He)T. A preliminary value of 200 mb sr-’ was obtained at 1.5 MeV He beam energy for a recoil angle of 30” (laboratory coordinates). The recoil probability has been mapped from 1.5 to 2.0 MeV He energy and over the angular range 20” to 45’, using a 5 pg cmp2 ‘K-T target. The variation is less than 20%. This cross section will be remeasured but it is
J.A. Sawicki et al. / Analysis of near-surjace
D on
trhum
477
[IIO] Ni
2 A,
0 -
P
a-Si(D1
+
&
i
IOJnm
1
Ti + II
a
d 10
. !
BI
J i
i
Ti + T
i
; a
b
t_
x10 t
Cl
T
0 O.r,
0.6
RECOIL ENERGY Fig. 1. ERD spectra measured obtained by chemisorption of charge Q = 15 PC); B - sample incidence angle 60”, projected into thin 5 kg cm-’ titanium ions-cm-‘) (Q = 10 PC).
0.8
ll0
(MeVl
with 2.2 MeV 4He beam and 8 pm mylar filter at 75” incidence and 30’ detection angle: A - sample 1.5 deuterium monolayers on [110] Ni surface carefully prepared at ultra high vacuum. (integrated obtained by ion implantation of 10 keV D: ions into amorphous silicon: dose 10” D ions cmP2. ion range below the surface 38 nm. (Q = 10 PC?); C - sample obtained by thermal sorption of deuterium film. (Q = 10 PC); D - sample
obtained
by thermal
sorption
of tritium
into similar
IX. ELASTIC
RECOIL
film ( - 10’” T
DETECTION
J. A. Sawicki et al. / Analysis
478
of neur -surface tritium
I SiKl) ;,
D(d,p)T
1, I
1:I A,
WIT)
T(d, a In
Bt
Ti + T
C, 200
300
CHANNEL
Fig. 2. Energy spectra of reaction D+ beam: A - silicon implanted cm-* titanium film.
products for D(d, p)T and T(d, 4He)n reactions measured at backward 150” geometry using 600 keV with 10” D+ ions cm- ‘., B - silicon implanted with 2 x lOI DT* ions cm-*; C - tritiated 5 pg
J.A. Sawckl
15 1 10
1
et al. / Anaiysls of near-surface
Au/Ti + T/Al
I I
0
I
I
,
I
I
,
I
0.5
I
’
1.0
RECOIL ENERGY (MeV) Fi;. 3. ERD spectra measured with 2.0 MeV 4He beam and 8 nnt mylar filter at 75” incidence and 30’ detection angle. (Q = 20 PC); top 5 pg cm-* tritiated titanium film; bottom - -ame film covered with 40 pg cm-’ gold film.
apparent that it exceeds the Rutherford cross section by approximately a factor four (ERD cross sections for H and D are also higher than the Rutherford cross sections, see refs. [7-91). The value of the T recoil cross section determined by us is also higher than expected from inverse reaction the cross section 4He(T, T)4He data [18,19]. altxady
4. Discussion Even in the simple form in which we have applied it here, the ERD methods shows some promise for tritium analysis. The advantage is apparent for surface or nearsurface distributions of tritium, free of deuterium. A detection sensitivity better than lOI T atoms cm-’ is indicated for ERD in 2 MeV He energy range with lo-20 nm depth resolution, down to a depth of 200-300 nm where H recoil signals produce a spectral background. The depth resolution of the ERD method compares favourably with the results of earlier works. By measuring neutron-time-of-flight for the T(p, n)3He reaction, Davies and Anderson [ll] have profiled tritium in titanium over the first 10 l.trn of depth with a resolution of 0.6 pm and up to 50 km with a resolution of 1.5 l.trn. Okuda et al. [13,14] were able to probe about 2 pm in
479
tritium
titanium with the resolution about 0.2 l.trn, by T(d, 4He)n reaction at E, = 500-700 keV and 120” scattering geometry. It should be noted that Caterini et al. [20] have demonstrated considerable improvement in the depth resolution available through use of the T(d, 4He)n reaction in forward detection geometry. Over a probing depth of 1 l.trn a depth resolution of 0.1 l.trn is realized. The probing depth and the sensitivity of tritium analysis by 4He-ERD technique could be markedly increased at higher bombarding energy, in particular near E, = 5.1 MeV; i.e. in the region corresponding to narrow resonance level in 7Li [21,22]. ERD may prove useful in cases where simultaneous mapping of H and T is required; its relevance as a diagnostic for methods of producing thin, stable, planar T sources is under study. ERD also shows promise for tritium trapping and release studies in materials that cannot be studied readily by NRA methods, owing to intense reaction product backgrounds. This is a problem for D and 3He beam analyses of lithium-bearing ceramics such as Li ,O which are candidate materials for fusion breeder blankets. The evaluation of the ERD method is continuing. The authors express their thanks to T.E. Jackman, W.N. Lennard and J.A. Davies for useful comments and assistance and to O.M. Westcott for providing the implants. One of us (J.A.S.) wishes to acknowledge the support provided by AECL through a visiting scientist appointment. This work was performed as part of the Fusion Breeder Blanket Program, jointly funded by the Canadian Fusion Fuels Technology Project and Atomic Energy of Canada Limited Research Company.
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