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The H(t, n)3He reaction for depth profiling of hydrogen by neutron time-of-flight
NUCLEAR
INSTRUMENTS
AND METHODS
149 ( 1 9 7 8 )
41-45 ;
(~)
NORTH-HOLLAND
PUBLISHING
CO.
THE H(t, n) 3 He REACTION FOR DEPTH PROFILING OF HYDROGEN BY NEUTRON TIME-OF-FLIGHT* J. C. DAVIS, H. W. LEFEVREt, C. H. POPPE
University of California, Lawrence Livermore Laboratory, Livermore, California 94550, U.S.A. D. M. DRAKE and L. R. VEESER
Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, U.S.A.
The usefulness of the H(t, n) reaction for depth profiling of hydrogen in solids was investigated using the pulsed T + beam from the Los Alamos vertical Van de Graaff accelerator. A 2.54 cm diameter by 2.54 cm thick stilbene neutron detector was placed at 0 °, 5.35 meters from the target. Pulse shape discrimination virtually eliminated gamma-ray background. Neutron spectra from targets of Ti and Till 2 with a nominal thickness of ten micrometer were compared with targets of SiO 2 , Si, C and Au to assess the importance of neutron backgrounds from triton reactions with the constituents of those targets. When the neutron spectra were transformed into profiles, the background corrected Till 2 spectrum showed a uniform hydrogen concentration over ten microns. A measurement of the neutron spectrum produced when bombarding an Au target on which (t, n) reactions are strongly inhibited by the Coulomb barrier gives a yield equivalent to a background concentration of 3 at.% of hydrogen in Ti. This background is due to reactions on hydrogen, hydrocarbons, or other light nuclei on or in beam line components. Similar measurements on other targets gave background values equivalent to hydrogen atomic concentrations in Ti of: SiO2-11 at.%, Si-7 at.%, C-22 at.%, Ti-5 at.%. Development of high sensitivity for depth profiling hydrogen with this technique appears possible only for high Z host materials and will require careful attention to beam line and vacuum system design.
1. Introduction Techniques for determining the depth distribution of hydrogen are required for investigation of a wide variety of problems in materials and metallurgical studies. Hydrogen absorbed, implanted, or produced by nuclear reactions can cause changes in the macroscopic properties of structural elements of fission or fusion reactors or chemical reactor vessels. Depth profiling techniques capable of studying the diffusion of hydrogen are needed to provide input data for the design of hydrogen isotope containment, transfer, and storage systems. A summary of methods currently used for hydrogen isotopes has been given recently1). Our previous work 2) using the T(p,n) and D(d, n) reactions for tritium and deuterium depth profiling via neutron time-of-flight demonstrated sensitivities down to the 1-10 ppm level, the capability to depth profile to tens of micrometer below the surface, and resolution limited by straggling of the probing beam for depth beyond N 1 ~zm. The technique is both nondestructive and rapid. An effort to investigate the usefulness of the H(t, n)
reaction for depth profiling hydrogen is worthwhile for several reasons. The ability to depth profile all the isotopes of hydrogen using the same technique, hence the same apparatus, would make possible studies of differential diffusion of hydrogen isotopes in systems of interest, e.g., neutron generator targets. Also, a technique using a nonresonant reaction and a light ion as the probing particle would be a useful complement to the welldeveloped helium and heavy ion profiling techniques which use mostly resonant (p, 7) reactions. This paper reports the first investigation of this new technique for hydrogen depth profiling.
Work performed under the auspices of the U.S. Department of Energy under Contract No. W-7405-Eng-48. t Permanent address: University of Oregon, Eugene, Oregon 97403, U.S.A.
Fig. 1. Schematic representation of hydrogen depth profiling measurement. The energy of neutrons produced by the H(t, n) reaction depends upon the triton energy, hence the depth x below the surface at which the reaction occurred.
Eo ~ TRITON
~ TITANIUM NEUTRON En
HYDROGEN
I. LIGHT E L E M E N T
PROFILING
42
J.C.
D A V I S et al.
2. Experimental procedure The basic features of neutron time-of-flight depth profiling have been described in some detail in previous publicationsZ3). The sample to be assayed is bombarded with nanosecond bursts of tritons at an energy above the threshold of the H(t, n) reaction (N3.0 MeV). As the tritons lose energy via ionization during penetration into the sample, neutrons are produced at different depths for triton energies between the incident beam energy and the threshold energy. A schematic of this process is shown in fig. 1. The resulting neutron energy spectrum is measured via neutron time-offlight from the target to an organic scintillator several meters away. From the neutron energy the energy of the triton initiating the reaction and hence the depth in the target at which the reaction occurred can be calculated. From the measured triton beam close and reaction cross section at that energy the number of hydrogen atoms present at the depth can be calculated. The hydrogen depth profile is thus obtained by unfolding the reaction cross section, reaction kinematics, and detector efficiency from the neutron spectrum. Detection of neutrons rather than charged particle reaction products has several advantages. As the mean free path for attenuation of MeV neutrons is several cm in most materials, samples or sample backings can be up to 1 cm in thickness before any corrections need be applied to the neutron spectra. As the neutrons do not lose energy exiting the sample, there is no contribution from reaction product straggling to the overall resolution. The products of other reaction channels (if any) do not introduce difficulties because no charged particles are detected and the organic scintillators used provide excellent discrimination against gamma radiation when pulse shape discrimination techniques are used. These latter conditions allow profiling in samples that are highly contaminated or radioactive. At some loss in depth resolution, the probing beam may be brought out of the accelerator vacuum through a thin foil to a sample in a helium atmosphere if the sample cannot be placed in the vacuum system. The present work was done using the triton beam from the Los Alamos vertical Van de Graaff accelerator at energies between 4 MeV and 5 MeV. A magnetic compression system was used to chop and bunch the triton beam, producing ~ 2 ns long bursts of tritons at intervals of 500 ns. The average beam current used was 0.5-1.0 #A. A capaci-
rive pickoff placed several cm before the final limiting aperture provided a timing signal indicating the arrival of each beam burst at the target. Neutrons were detected in a 2.5cm×2.5 cm stilbene crystal mounted on an RCA 8575 phototube. The detector was placed in a large shield and collimator assembly at 0° with respect to the direction of the triton beam. Flight paths used varied between 5 m and 7 m in the measurements. Efficiency of the detector was determined by setting an electronic pulse height bias at - 3 0 % of the pulse height from the 511 keV annihilation gamma from a 22Na source. Samples investigated were a hydrided 10tim layer of Ti on a 1 mm thick Cu backing and 1 mm thick amorphous S i O 2 wafer in which 4X 1017 H / c m 2 were implanted at 300 keV. Neutron spectra from tritons incident on unhydrided Ti and unimplanted SiO2 were also taken for background purposes. Targets of C, Si, and Au were also studied to provide information on backgrounds from typical beam line contaminants and components.
3. Sensitivity The absolute sensitivity of any depth profiling technique is determined by three factors: the cross section of the sampling reaction, backgrounds from other reactions occurring in the sample or on contaminants on surfaces in the vacuum system which contains the isotope to be profiled, and the maximum dose the sample can be given without modifying the profile to be determined. Laboratory differential cross sections at 0° for the H(t, n) and T(p, n) reactions are shown in fig. 2 for incident
~ . . . . ~ - H (t,n)
600
"C .~
400
o
~ 2oo b/-THRESHOLD / 0
I~
5
~ / - T
(p,n)
/ I
4
I
5
I
6
I
7
I
8
I
9
I
I0
l il
I
12
I
}:5
I
14
Et OR 3Ep (MeV)
Fig. 2. Laboratory differential cross sections at 0 ° for the T(p, n) and H(t, n) reactions.
THE
H ( t , n)3He
particle energies giving the same energy in the center of mass system; under this condition the depth resolution obtained with the two reactions is comparable. As the H(t, n) reaction yield is 4 - 6 times larger than that of the T(p, n) reaction and a sensitivity of 1-10 ppm for tritium in metal has been demonstrated for profiling with the latter reaction, an equivalent sensitivity might be obtained for hydrogen profiling. Such high sensitivity generally will not be achieved because of the two sources of background neutrons. Hydrocarbon contaminants and water in quantities of a few # g / c m 2 are present on surfaces of vacuum systems using diffusion pumps or turbopumps; collimating slits and apertures of most beam transport systems contain embedded hydrogen resulting from normal operation with proton beams. Neutrons produced via the H(t, n) reaction on these hydrogen atoms may partially overlap the time spectrum of neutrons from H(t, n) reactions occurring in the sample being assayed. Finally, the Qvalues for (t, n) reactions are generally positive. For light isotopes such as C, O, Si, Ti, etc., the Coulomb barrier is low, resulting in non-negligible reaction probabilities. Neutrons from these reactions may overlap those from the sample. Effects of the dose delivered to the sample are small because for probing beams of a few MeV in energy the beam stops well beyond the 10-20 # m range of interest in most studies. The implanted hydrogen isotopes have had no effect upon the distributions studied in work to date. However, a 1/.tA T + beam deposits - 1 4 mCi of tritium per hour; this presents a minor radiological hazard but will require changing limiting apertures in the beam line if good sensitivity is to be obtained in subsequent T(p, n) profiling. ,
i
,
i.o E _ _m z o
-
=,
i
~
i
uOVE~ALL , RESOLUTION • /
/../~.
,
,
,
~ 1 1 ~ / / / ~ / j I ~
i
~ ~-
~ / ,.~._~_I ~TR AGe LI NG
/ --
.1
//"
o 0.5 ¢o ~ ~-
/ "~
_// O.O
O
/
,,.--SPECTROMETER RESOLUTION
/--INCIDENT BEAM SPREAD f I
I
I
2
.3 4 5 6 DEPTH IN Ti (/xrn)
I
Z
I
r
J
I
7
8
9
IO
Fig. 3. Depth resolution in # m as a f u n c t i o n o f depth in Ti. The various c o m p o n e n t s of the overall resolution are discussed in the text.
43
REACTION
4. Resolution Depth resolution obtained with this method of profiling has three major components: energy spread of the incident beam, straggling of that beam as it penetrates the sample, and the time resolution of the neutron time-of-flight spectrometer. These three contributions and the overall resolution are shown in fig. 3 for typical conditions for depth profiling hydrogen in titanium; all values are full-width-at-half-maximum. Energy of the probing triton beam is set at 5 MeV; its energy spread is 5 keV. Time resolution of the spectrometer (the combination of charged particle burst width, detector time resolution, and electronic resolution) is assumed to be 1.5 ns. A neutron flight path of 10 m was used in the calculation of neutron energy resolution. These conditions are slightly better than obtained in the present work. Resolution is calculated at each depth in the sample by converting all energy spreads to an equivalent thickness using the electronic stopping power for the triton energy at that depth. Within N 2 ~m of the front surface of the sample the overall depth resolution is limited by the spectrometer time resolution. No improvement in time-of-flight spectrometry is anticipated which would make the resolution comparable to that which can be obtained from heavy ion probes using (p, ~) reactions. Beyond ~ 3 # m in the sample, however, straggling of the probe beam dominates the overall resolution. For profiling beyond this depth, use of the H(t, n) reaction may be preferable to heavy ion techniques because much less energy is deposited in the hydrogen containing portion of the sample. At 25 # m and 50 # m in Ti, the depth resolution is 1.5/~m and 2 #m, respectively. 5. Results Neutron spectra for the Till17 and Ti targets are shown in fig. 4. These spectra were obtained with 120/.tC integrated triton current. The hydrogen depth profile calculated from these spectra by subtracting background and removing the effects of cross section and detector efficiency variation with energy is shown in fig. 5. The hydrogen concentration and the profile shape observed are consistent with those seen in diffusion loaded targets containing deuterium and tritium. In fig. 6 neutron spectra from tritons on targets of C, Si, SiO2, Ti, and Au are shown: the spectrum from the TiHt, 7 target has been repeated for I. L I G H T
ELEMENT
PROFILING
44
J . c . DAVIS et al.
Ti H
1.7
(x I15)
J I/J Z Z < :I= ¢J
..-..". Ti H
03 I"Z
1.7 ..... ...,',
:..,.
..."
_J W Z Z
..,,..'"
O O
C
.' 7
I
Si 0 2 .-" Ti ' ................... :.:::i..,....,......,..............f...........-...' ......'"-'."....... ::.,.,,,,.,,.,.:,,,.I.,..,,.....,,
400
450
500
CHANNEL
.'%~.. ..4...
Z 0 cJ
NUMBER
.---~---.""' A,, /: "~i..._.......
Neutron time-of-flight spectra for 4.5 MeV tritons incident on targets of TiHL7 and Ti. Neutrons from H ( t , n ) reactions occurring at the surface of the 1 0 / z m t h i c k T i l l 1 . 7 layer are a t ~ c h a n n e l 4 8 0 . Fig.
4.
comparison. All spectra were taken for the same 120/~C integrated change. For the Au target, the Coulomb barrier is high enough to inhibit all (t, n) reactions on Au itself: neutrons in this spectrum come from hydrocarbons and low-Z contaminants on beam line surfaces and the target itself. A surface contaminant peak is observable in this spectrum; when repeated spectra were taken on the same target, the height of this peak grew steadily. At a depth of a few /,zm into the sample, the observed background is equivalent to a 3 at. % hydrogen concentration. Neutron spectra obtained for the other targets are a sum of (t, n) neutrons from the reaction on the target sample and from reactions on contam.~
70
z_o
60
niz aJ O
50
.., .'" "..... "..~......,..........'..'.'....
4O
Z O
o
NUMBER
Fig. 6. Neutron spectra from C, Si, SiO 2 , Ti and Au compared with the spectrum from Till1. 7 . Details of the comparison are given in the text.
inants, etc. Hydrogen surface peaks are observable in some of the spectra. No implantation peak could be observed in the spectrum from SiO2. The neutron spectra are consistent with positive Q values and lower Coulomb barriers for reactions on these light nuclei. Equivalent hydrogen atom concentrations which would be inferred from background neutrons in the portion of the spectra in which neutrons from the H(t, n) reaction are kinematically allowed are: C - 2 2 at. %, Si - 7 at. %, SiO2-11 at. %, T i - 5 at. %. Study of the relaxation of a hydrogen implantation peak would be difficult if not impossible in these samples because of the structure of the neutron spectra from the other (t, n) reactions.
30
o 20
o
.-.'..". '.. r i -5-2.-I
I 0
I
I
I
I
I
I
I
I
f
I 2_ 5 4. 5 6 7 8 91011 DEPTH
"1 '.1 ....
12
(bLm)
Depth profile of hydrogen in Till].7 calculated from the spectra of f i g . 4. Fig.
CHANNEL
5.
6. Conclusions The depth profiling of hydrogen by the H(t, n) reaction has been shown to be feasible. The technique is non-resonant, i.e., allowing a simulatenous assay of the entire depth to be sampled, and allows thick or backed targets to be assayed. The former advantage significantly reduces the probability of error due to surface contamination. Disadvantages of the technique are the radiological
THE H ( t , n)3He R E A C T I O N
and neutronic consequences of the acceleration of tritium and the large backgrounds associated with (t, n) reactions on nuclei other than hydrogen. Development of sensitivity equivalent to 1-10 ppm is not possible; a sensitivity of perhaps 1000 ppm for profiling hydrogen could be obtained in high-Z materials if adequate care were taken in the details of beam line construction. The authors would like to express their appreciation to G. J. Thomas and W. Bauer of Sandia
45
Laboratories, Livermore for preparation of the implanted SiO2 samples.
References 1) j. Boettinger, S. T. Picraux and N. Rud, Ion beam surface layer analysis (Plenum Press, New York, 1976). 2) H. W. Lefevre, J. D. Anderson and J. C. Davis, Proc. 4th Conf. on Scientific and industrial applications of small accelerators, Denton (1976) p. 225. 3) j. C. Overly and H. W. Lefevre, Radiation effects on solid surfaces, Adv. in Chem. Series, no. 158 (1976) p. 282.
I. LIGHT E L E M E N T
PROFILING
INSTRUMENTS
AND METHODS
149 ( 1 9 7 8 )
41-45 ;
(~)
NORTH-HOLLAND
PUBLISHING
CO.
THE H(t, n) 3 He REACTION FOR DEPTH PROFILING OF HYDROGEN BY NEUTRON TIME-OF-FLIGHT* J. C. DAVIS, H. W. LEFEVREt, C. H. POPPE
University of California, Lawrence Livermore Laboratory, Livermore, California 94550, U.S.A. D. M. DRAKE and L. R. VEESER
Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, U.S.A.
The usefulness of the H(t, n) reaction for depth profiling of hydrogen in solids was investigated using the pulsed T + beam from the Los Alamos vertical Van de Graaff accelerator. A 2.54 cm diameter by 2.54 cm thick stilbene neutron detector was placed at 0 °, 5.35 meters from the target. Pulse shape discrimination virtually eliminated gamma-ray background. Neutron spectra from targets of Ti and Till 2 with a nominal thickness of ten micrometer were compared with targets of SiO 2 , Si, C and Au to assess the importance of neutron backgrounds from triton reactions with the constituents of those targets. When the neutron spectra were transformed into profiles, the background corrected Till 2 spectrum showed a uniform hydrogen concentration over ten microns. A measurement of the neutron spectrum produced when bombarding an Au target on which (t, n) reactions are strongly inhibited by the Coulomb barrier gives a yield equivalent to a background concentration of 3 at.% of hydrogen in Ti. This background is due to reactions on hydrogen, hydrocarbons, or other light nuclei on or in beam line components. Similar measurements on other targets gave background values equivalent to hydrogen atomic concentrations in Ti of: SiO2-11 at.%, Si-7 at.%, C-22 at.%, Ti-5 at.%. Development of high sensitivity for depth profiling hydrogen with this technique appears possible only for high Z host materials and will require careful attention to beam line and vacuum system design.
1. Introduction Techniques for determining the depth distribution of hydrogen are required for investigation of a wide variety of problems in materials and metallurgical studies. Hydrogen absorbed, implanted, or produced by nuclear reactions can cause changes in the macroscopic properties of structural elements of fission or fusion reactors or chemical reactor vessels. Depth profiling techniques capable of studying the diffusion of hydrogen are needed to provide input data for the design of hydrogen isotope containment, transfer, and storage systems. A summary of methods currently used for hydrogen isotopes has been given recently1). Our previous work 2) using the T(p,n) and D(d, n) reactions for tritium and deuterium depth profiling via neutron time-of-flight demonstrated sensitivities down to the 1-10 ppm level, the capability to depth profile to tens of micrometer below the surface, and resolution limited by straggling of the probing beam for depth beyond N 1 ~zm. The technique is both nondestructive and rapid. An effort to investigate the usefulness of the H(t, n)
reaction for depth profiling hydrogen is worthwhile for several reasons. The ability to depth profile all the isotopes of hydrogen using the same technique, hence the same apparatus, would make possible studies of differential diffusion of hydrogen isotopes in systems of interest, e.g., neutron generator targets. Also, a technique using a nonresonant reaction and a light ion as the probing particle would be a useful complement to the welldeveloped helium and heavy ion profiling techniques which use mostly resonant (p, 7) reactions. This paper reports the first investigation of this new technique for hydrogen depth profiling.
Work performed under the auspices of the U.S. Department of Energy under Contract No. W-7405-Eng-48. t Permanent address: University of Oregon, Eugene, Oregon 97403, U.S.A.
Fig. 1. Schematic representation of hydrogen depth profiling measurement. The energy of neutrons produced by the H(t, n) reaction depends upon the triton energy, hence the depth x below the surface at which the reaction occurred.
Eo ~ TRITON
~ TITANIUM NEUTRON En
HYDROGEN
I. LIGHT E L E M E N T
PROFILING
42
J.C.
D A V I S et al.
2. Experimental procedure The basic features of neutron time-of-flight depth profiling have been described in some detail in previous publicationsZ3). The sample to be assayed is bombarded with nanosecond bursts of tritons at an energy above the threshold of the H(t, n) reaction (N3.0 MeV). As the tritons lose energy via ionization during penetration into the sample, neutrons are produced at different depths for triton energies between the incident beam energy and the threshold energy. A schematic of this process is shown in fig. 1. The resulting neutron energy spectrum is measured via neutron time-offlight from the target to an organic scintillator several meters away. From the neutron energy the energy of the triton initiating the reaction and hence the depth in the target at which the reaction occurred can be calculated. From the measured triton beam close and reaction cross section at that energy the number of hydrogen atoms present at the depth can be calculated. The hydrogen depth profile is thus obtained by unfolding the reaction cross section, reaction kinematics, and detector efficiency from the neutron spectrum. Detection of neutrons rather than charged particle reaction products has several advantages. As the mean free path for attenuation of MeV neutrons is several cm in most materials, samples or sample backings can be up to 1 cm in thickness before any corrections need be applied to the neutron spectra. As the neutrons do not lose energy exiting the sample, there is no contribution from reaction product straggling to the overall resolution. The products of other reaction channels (if any) do not introduce difficulties because no charged particles are detected and the organic scintillators used provide excellent discrimination against gamma radiation when pulse shape discrimination techniques are used. These latter conditions allow profiling in samples that are highly contaminated or radioactive. At some loss in depth resolution, the probing beam may be brought out of the accelerator vacuum through a thin foil to a sample in a helium atmosphere if the sample cannot be placed in the vacuum system. The present work was done using the triton beam from the Los Alamos vertical Van de Graaff accelerator at energies between 4 MeV and 5 MeV. A magnetic compression system was used to chop and bunch the triton beam, producing ~ 2 ns long bursts of tritons at intervals of 500 ns. The average beam current used was 0.5-1.0 #A. A capaci-
rive pickoff placed several cm before the final limiting aperture provided a timing signal indicating the arrival of each beam burst at the target. Neutrons were detected in a 2.5cm×2.5 cm stilbene crystal mounted on an RCA 8575 phototube. The detector was placed in a large shield and collimator assembly at 0° with respect to the direction of the triton beam. Flight paths used varied between 5 m and 7 m in the measurements. Efficiency of the detector was determined by setting an electronic pulse height bias at - 3 0 % of the pulse height from the 511 keV annihilation gamma from a 22Na source. Samples investigated were a hydrided 10tim layer of Ti on a 1 mm thick Cu backing and 1 mm thick amorphous S i O 2 wafer in which 4X 1017 H / c m 2 were implanted at 300 keV. Neutron spectra from tritons incident on unhydrided Ti and unimplanted SiO2 were also taken for background purposes. Targets of C, Si, and Au were also studied to provide information on backgrounds from typical beam line contaminants and components.
3. Sensitivity The absolute sensitivity of any depth profiling technique is determined by three factors: the cross section of the sampling reaction, backgrounds from other reactions occurring in the sample or on contaminants on surfaces in the vacuum system which contains the isotope to be profiled, and the maximum dose the sample can be given without modifying the profile to be determined. Laboratory differential cross sections at 0° for the H(t, n) and T(p, n) reactions are shown in fig. 2 for incident
~ . . . . ~ - H (t,n)
600
"C .~
400
o
~ 2oo b/-THRESHOLD / 0
I~
5
~ / - T
(p,n)
/ I
4
I
5
I
6
I
7
I
8
I
9
I
I0
l il
I
12
I
}:5
I
14
Et OR 3Ep (MeV)
Fig. 2. Laboratory differential cross sections at 0 ° for the T(p, n) and H(t, n) reactions.
THE
H ( t , n)3He
particle energies giving the same energy in the center of mass system; under this condition the depth resolution obtained with the two reactions is comparable. As the H(t, n) reaction yield is 4 - 6 times larger than that of the T(p, n) reaction and a sensitivity of 1-10 ppm for tritium in metal has been demonstrated for profiling with the latter reaction, an equivalent sensitivity might be obtained for hydrogen profiling. Such high sensitivity generally will not be achieved because of the two sources of background neutrons. Hydrocarbon contaminants and water in quantities of a few # g / c m 2 are present on surfaces of vacuum systems using diffusion pumps or turbopumps; collimating slits and apertures of most beam transport systems contain embedded hydrogen resulting from normal operation with proton beams. Neutrons produced via the H(t, n) reaction on these hydrogen atoms may partially overlap the time spectrum of neutrons from H(t, n) reactions occurring in the sample being assayed. Finally, the Qvalues for (t, n) reactions are generally positive. For light isotopes such as C, O, Si, Ti, etc., the Coulomb barrier is low, resulting in non-negligible reaction probabilities. Neutrons from these reactions may overlap those from the sample. Effects of the dose delivered to the sample are small because for probing beams of a few MeV in energy the beam stops well beyond the 10-20 # m range of interest in most studies. The implanted hydrogen isotopes have had no effect upon the distributions studied in work to date. However, a 1/.tA T + beam deposits - 1 4 mCi of tritium per hour; this presents a minor radiological hazard but will require changing limiting apertures in the beam line if good sensitivity is to be obtained in subsequent T(p, n) profiling. ,
i
,
i.o E _ _m z o
-
=,
i
~
i
uOVE~ALL , RESOLUTION • /
/../~.
,
,
,
~ 1 1 ~ / / / ~ / j I ~
i
~ ~-
~ / ,.~._~_I ~TR AGe LI NG
/ --
.1
//"
o 0.5 ¢o ~ ~-
/ "~
_// O.O
O
/
,,.--SPECTROMETER RESOLUTION
/--INCIDENT BEAM SPREAD f I
I
I
2
.3 4 5 6 DEPTH IN Ti (/xrn)
I
Z
I
r
J
I
7
8
9
IO
Fig. 3. Depth resolution in # m as a f u n c t i o n o f depth in Ti. The various c o m p o n e n t s of the overall resolution are discussed in the text.
43
REACTION
4. Resolution Depth resolution obtained with this method of profiling has three major components: energy spread of the incident beam, straggling of that beam as it penetrates the sample, and the time resolution of the neutron time-of-flight spectrometer. These three contributions and the overall resolution are shown in fig. 3 for typical conditions for depth profiling hydrogen in titanium; all values are full-width-at-half-maximum. Energy of the probing triton beam is set at 5 MeV; its energy spread is 5 keV. Time resolution of the spectrometer (the combination of charged particle burst width, detector time resolution, and electronic resolution) is assumed to be 1.5 ns. A neutron flight path of 10 m was used in the calculation of neutron energy resolution. These conditions are slightly better than obtained in the present work. Resolution is calculated at each depth in the sample by converting all energy spreads to an equivalent thickness using the electronic stopping power for the triton energy at that depth. Within N 2 ~m of the front surface of the sample the overall depth resolution is limited by the spectrometer time resolution. No improvement in time-of-flight spectrometry is anticipated which would make the resolution comparable to that which can be obtained from heavy ion probes using (p, ~) reactions. Beyond ~ 3 # m in the sample, however, straggling of the probe beam dominates the overall resolution. For profiling beyond this depth, use of the H(t, n) reaction may be preferable to heavy ion techniques because much less energy is deposited in the hydrogen containing portion of the sample. At 25 # m and 50 # m in Ti, the depth resolution is 1.5/~m and 2 #m, respectively. 5. Results Neutron spectra for the Till17 and Ti targets are shown in fig. 4. These spectra were obtained with 120/.tC integrated triton current. The hydrogen depth profile calculated from these spectra by subtracting background and removing the effects of cross section and detector efficiency variation with energy is shown in fig. 5. The hydrogen concentration and the profile shape observed are consistent with those seen in diffusion loaded targets containing deuterium and tritium. In fig. 6 neutron spectra from tritons on targets of C, Si, SiO2, Ti, and Au are shown: the spectrum from the TiHt, 7 target has been repeated for I. L I G H T
ELEMENT
PROFILING
44
J . c . DAVIS et al.
Ti H
1.7
(x I15)
J I/J Z Z < :I= ¢J
..-..". Ti H
03 I"Z
1.7 ..... ...,',
:..,.
..."
_J W Z Z
..,,..'"
O O
C
.' 7
I
Si 0 2 .-" Ti ' ................... :.:::i..,....,......,..............f...........-...' ......'"-'."....... ::.,.,,,,.,,.,.:,,,.I.,..,,.....,,
400
450
500
CHANNEL
.'%~.. ..4...
Z 0 cJ
NUMBER
.---~---.""' A,, /: "~i..._.......
Neutron time-of-flight spectra for 4.5 MeV tritons incident on targets of TiHL7 and Ti. Neutrons from H ( t , n ) reactions occurring at the surface of the 1 0 / z m t h i c k T i l l 1 . 7 layer are a t ~ c h a n n e l 4 8 0 . Fig.
4.
comparison. All spectra were taken for the same 120/~C integrated change. For the Au target, the Coulomb barrier is high enough to inhibit all (t, n) reactions on Au itself: neutrons in this spectrum come from hydrocarbons and low-Z contaminants on beam line surfaces and the target itself. A surface contaminant peak is observable in this spectrum; when repeated spectra were taken on the same target, the height of this peak grew steadily. At a depth of a few /,zm into the sample, the observed background is equivalent to a 3 at. % hydrogen concentration. Neutron spectra obtained for the other targets are a sum of (t, n) neutrons from the reaction on the target sample and from reactions on contam.~
70
z_o
60
niz aJ O
50
.., .'" "..... "..~......,..........'..'.'....
4O
Z O
o
NUMBER
Fig. 6. Neutron spectra from C, Si, SiO 2 , Ti and Au compared with the spectrum from Till1. 7 . Details of the comparison are given in the text.
inants, etc. Hydrogen surface peaks are observable in some of the spectra. No implantation peak could be observed in the spectrum from SiO2. The neutron spectra are consistent with positive Q values and lower Coulomb barriers for reactions on these light nuclei. Equivalent hydrogen atom concentrations which would be inferred from background neutrons in the portion of the spectra in which neutrons from the H(t, n) reaction are kinematically allowed are: C - 2 2 at. %, Si - 7 at. %, SiO2-11 at. %, T i - 5 at. %. Study of the relaxation of a hydrogen implantation peak would be difficult if not impossible in these samples because of the structure of the neutron spectra from the other (t, n) reactions.
30
o 20
o
.-.'..". '.. r i -5-2.-I
I 0
I
I
I
I
I
I
I
I
f
I 2_ 5 4. 5 6 7 8 91011 DEPTH
"1 '.1 ....
12
(bLm)
Depth profile of hydrogen in Till].7 calculated from the spectra of f i g . 4. Fig.
CHANNEL
5.
6. Conclusions The depth profiling of hydrogen by the H(t, n) reaction has been shown to be feasible. The technique is non-resonant, i.e., allowing a simulatenous assay of the entire depth to be sampled, and allows thick or backed targets to be assayed. The former advantage significantly reduces the probability of error due to surface contamination. Disadvantages of the technique are the radiological
THE H ( t , n)3He R E A C T I O N
and neutronic consequences of the acceleration of tritium and the large backgrounds associated with (t, n) reactions on nuclei other than hydrogen. Development of sensitivity equivalent to 1-10 ppm is not possible; a sensitivity of perhaps 1000 ppm for profiling hydrogen could be obtained in high-Z materials if adequate care were taken in the details of beam line construction. The authors would like to express their appreciation to G. J. Thomas and W. Bauer of Sandia
45
Laboratories, Livermore for preparation of the implanted SiO2 samples.
References 1) j. Boettinger, S. T. Picraux and N. Rud, Ion beam surface layer analysis (Plenum Press, New York, 1976). 2) H. W. Lefevre, J. D. Anderson and J. C. Davis, Proc. 4th Conf. on Scientific and industrial applications of small accelerators, Denton (1976) p. 225. 3) j. C. Overly and H. W. Lefevre, Radiation effects on solid surfaces, Adv. in Chem. Series, no. 158 (1976) p. 282.
I. LIGHT E L E M E N T
PROFILING