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A technique for measuring hydrogen concentration versus depth in solid samples
NUCLEAR
INSTRUMENTS
AND
METHODS
108
(1973) 67-7I; ©
NORTH-HOLLAND
PUBLISHING
CO.
A T E C H N I Q U E FOR MEASURING H Y D R O G E N C O N C E N T R A T I O N VERSUS D E P T H IN SOLID SAMPLES* D. A. LEICH and T. A. TOMBRELLO
California Institute of Technology, Pasadena, California, U.S.A. Received 21 November 1972 The resonance at 0.83 MeV (center-of-mass energy) in the nuclear reaction 1H(19F,ct7)160 has been used to measure hydrogen depth profiles in solids. Hydrogen concentrations have been measured to a depth of 0.4 #m both in samples implanted with 11.5-keV
protons and in several lunar samples. Further applications of this technique are discussed and several other resonant nuclear reactions are suggested for depth distribution studies of hydrogen and other elements.
1. Introduction
reaction 1H(19F, cty)160 to obtain direct hydrogen depth profiles on lunar samples. This technique is capable of producing much more quantitative depth profiles than those inferred from chemical etching results, thus providing an important tool in obtaining the historical record of the solar wind contained in lunar and meteoritic samples.
In order to determine whether the relatively high rare gas concentrations measured in lunar soils were due to implanted solar wind ions, several groups performed experiments to show that the rare gases were on the surfaces of the lunar soil particles rather than uniformly distributed throughout the volume of the grains. One method employed 1-3) was to measure the remaining rare gas concentrations in a number of similar grain fractions after removal of a surface layer by chemical etching. It was indeed found that the solar wind gases were localized on the surfaces of the grains; however, the implied gas layer thicknesses (up to a few micrometers) were significantly larger than the range of 1 keV/nucleon (solar wind) ions. To check this surprising result, we are using the resonant nuclear
2. Experimental 2.1. ANALYTICAL TECHNIQUE
The reaction 1 H ( t 9 F , 0C7)160shows a strong resonance at 0.83 MeV (center-of-mass energy), with a cross section of 0.5 barn and a width of about 5 keV, producing 6.1, 6.9 and 7.1-MeV 7-rays from the deexcitation of the residual 160 nucleus4'5). At energies different from the resonance energy by more than a few keV the cross section for the production of the high-energy 7-rays is negligible. Thus, if the surface of
* Supported in part by the National Science Foundation lGP-28027l.
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67
68
D. A. LEICH AND T. A. TOMBRELLO
a material containing hydrogen is irradiated with 19F ions at energies slightly greater than the resonance energy, the ~gF ions will be continuously slowed down until at a depth XR the resonance energy will be reached and the reaction will occur at a rate proportional to the hydrogen concentration in a thin layer at XR. At greater depths the ~9F energy will be below the resonance energy where the cross section is again negligible. Since the stopping power, d E / d x , is nearly independent of energy 6) (within ~ 2%) in the relevant range of 19F energies, the depth XR can be related to the ~9F beam energy Eo by the relation E o + x R ( d E / d x ) -- ER, where ER is the resonance energy in the laboratory or proton rest frame. Measuring the ~,-ray production rate as the 19F beam energy is varied gives a direct indication of hydrogen concentration as a function of depth in the target. The depth resolution is determined by the ratio of the resonance width to the stopping power and is about 200 A for typical target materials. A second strong resonance at 0.89 MeV (center-of-mass energy) limits the range of this particular reaction to a depth of about 0.4 #m, but unfolding techniques could be used to continue the profile to greater depths. 2.2. EXPERIMENTALAPPARATUS The Caltech tandem accelerator was used to provide a magnetically analyzed monoenergetic 19F4+ beam in the range of 16 to 18 MeV. Fig. 1 shows a diagram of the scattering chamber and pumping system designed to minimize contamination of the targets by water and oil vapors present in the beam line. Condensable vapors are trapped on the walls of a 30-cm long LN 2 cooled beam line baffle. The low conductance path provided by this baffle enables the getter-ion pumping system to maintain a pressure of about I 0 - 9 torr in the chamber with the beam on the target. All metal construction and metal-to-metal seals permit baking of the scattering chamber and pumping system to 300 °C to drive trapped gasses out of the walls. A rotating aluminum wheel provides mounts for 12 targets. A 7.6 cm × 7.6 cm NaI(T1) crystal detector placed outside the scattering chamber at about 30 ° scattering angle is used to detect ~-radiation. A 4 to 8 MeV counting window on the pulse-height spectrum includes all but the tail of the C o m p t o n distribution in the response of the NaI(TI) crystal to the three ~-rays. With the NaI(TI) crystal placed 7 cm from the target and using this energy window, the detected fraction of all 7-rays emitted is estimated to be about 0.016. An analyzed sample of the mineral chlorite (1.6% hydrogen) was used as a target to obtain this estimate of the detection efficiency.
DEPTH [ANGSTROMS) I000 2000 3000
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Fig. 2a. The ~'-ray yield versus bombarding energy are shown for an implanted fused silica sample and for an identical non-implanted (blank) sample. The energy scale shows the resonance energy (E~) subtracted from the tgF beam energy (E). The depth scale calculated from the energy loss is also shown. Error bars
show statistical uncertainties. 3. Results
3.1. IMPLANTEDSAMPLES In order to establish the sensitivity and reliability of this technique, implantation experiments have been carried out on crystalline quartz, fused silica, and Carich feldspar samples using 11.5-keV protons with doses of 1015-1016 protons/cm 2. The target materials were chosen for their chemical and physical similarity to the returned lunar samples. Targets were chemically cleaned and then baked in the scattering chamber to remove surface contamination before implantation. A magnetically analyzed proton beam with a flux of about 1012 s -1 cm -2 was obtained from a duoplasmatron ion source to implant a 0.5 cm 2 spot on each target. The scattering chamber was then transferred to the beam line where a beam of 101°F ions per second was directed into a 4-mm wide spot for analysis. Fig. 2a shows data for a typical implanted target and for a target which was not implanted but was otherwise identical. Fig. 2b shows an implantation profile obtained by subtracting the data in fig. 2a. 19F stopping powers, calculated for the particular target composition from values in Northcliffe 6) are used to determine the depth scale. The experimentally determined distributions show some deviation from the theoretical range distribution for 1 1.5-keV protons. A comparison between the measured and theoretical 7) mean range
69
MEASURING HYDROGEN CONCENTRATION DEPTH (/.,.g/cmz) 20 40 60 I
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bution, suggesting a c o n t i n u a t i o n o f the same process by which the range d i s t r i b u t i o n is distorted d u r i n g i m p l a n t a t i o n , a l t h o u g h at a slower rate. W i t h the exception o f this effect, the profiles are r e p r o d u c i b l e ; thus, there is no reason to d o u b t that the actual p r o t o n distribution is being measured.
i
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Fig. 2b. Implantation profile for the fused silica sample after the background has been subtracted. a n d full width at half m a x i m u m is shown in table 1. The differences may, in part, represent the effects o f diffusion d u r i n g the i m p l a n t a t i o n since there seems to be a weak inverse c o r r e l a t i o n between the observed widths and the i m p l a n t a t i o n ion currents, a l t h o u g h we have not used a wide e n o u g h range o f i m p l a n t a t i o n currents to check this t h o r o u g h l y . R e p e a t e d analyses o f a single i m p l a n t e d s a m p l e show g r a d u a l but consistent i n w a r d shifting and b r o a d e n i n g o f the distri-
H y d r o g e n depth profiles have been m e a s u r e d on several 2 3 mm-sized glass fragments a n d glass-coated rock fragments selected from the coarse fines o f the A p o l l o 11 and A p o l l o 15 missions. Because o f the small size o f these fragments, the t9F b e a m was collim a t e d to a 2-mm spot and the s a m p l e holders were masked with clean fused silica collimators. N o r m a l l y , at least 80% o f the 19F b e a m could be directed o n t o the lunar s a m p l e with the r e m a i n i n g b e a m fraction collected by the relatively hydrogen-free collimator. The fraction o f the b e a m hitting the sample was d e t e r m i n e d by masking the analyzed chlorite sample with a c o l l i m a t o r identical to those m o u n t e d with the lunar samples and noting the reduction in the 7-ray counting rate for the m a s k e d sample as c o m p a r e d to an u n m a s k e d chlorite target. D a t a were taken in 200 keV steps from a starting p o i n t near one end o f the 16.3-17.5 M e V energy range to the o p p o s i t e end and then back over the entire energy range in 100 keV steps. F o r the l u n a r samples the 19F b e a m current was reduced to ½ o f the current used on the i m p l a n t e d samples and d a t a runs were lengthened to 10 rain each.
TABLE 1 Most probable range (Rp), median range (RM) and full-width-at-half-maximum (fwhm) are given for samples implanted with l l.5 keV protons. Calculations from Schiott 7) predict a mean range of 34/~g/cm e for SiO2 and 35/tg/cm 2 for the feldspar composition, with fwhm's of 25 pg/cm 2 and 26/tg/cm 2, respectively.
Sample type
Proton implantation rate (10l° s-1)
Rp
RM
(/tg/cm2)
(/~g/cm2)
fwhm (#g/cm 2)
Crystalline quartz
50 95 95 125 125
35 39 34 24 39
35 39 37 35 44
23 47 41 35 45
Fused silica
40 95 125
50 35 35
45 33 41
47 35 59
Feldspar (Ca-rich)
95 125 125 125
33 34 40 34
43 38 44 33
45 46 50 35
70
D. A. L E I C H
A N D T. A. T O M B R E L L O TABLE 2
M e a s u r e d concentration o f h y d r o g e n in a 0 . 4 / t m surface layer is s h o w n for each o f the l u n a r soil fragments. Collection o f the I9F b e a m charge, estimation o f counting efficiency, a n d mobility o f the hydrogen c o m b i n e for an uncertainty o f -4-20% o f the m e a s u r e d value. Sample designation
10085,31-12 10085,31-9 10085,1 Glass f r a g m e n t 15413,5-2 15413,5-5 15533,4-1
Description
H y d r o g e n concentration ( x 1016 p r o t o n s / c m 2)
Brown glass fragment Brown glass f r a g m e n t
7 1
Brown glass f r a g m e n t Shocked, pyroxene-rich rock f r a g m e n t Shocked, light-colored plagioclase f r a g m e n t Glazed rock f r a g m e n t
2
Since the flux of solar wind protons is sufficient to saturate ( ~ 1017 protons/cm 2) an exposed surface in a very short time (N30 y), the turnover of the top soil layers should insure that each soil fragment be exposed long enough to reach saturation at some time in geologic history. Observation of micrometeorite impact pits on most surfaces of our fragments confirms the assumption that each of them had been exposed to the solar wind. The profiles obtained from our soil fragments show a wide variation in absolute hydrogen concentration (table 2), although the profile shapes are quite similar. Fig. 3 is typical of these profiles showing a significant concentration of hydrogen in a layer about 0.2-pro deep. From 0.2 pm to 0.4 pm there is a gradual decrease in hydrogen concentration with increasing depth. The residual concentration at 0.4-pro depth is typically 102o hydrogen atoms per cm a. Two surface chips from a glass-coated rock (15015, 39) showed a different profile. The predominant feature is a concentration of hydrogen (3 × 10~5 atoms/cm 2) within 100 A of the surface, and decreasing to a much lower level of concentration within the first 0.1 pm. The peak in the hydrogen profile at ~ 1000 A shown in fig. 3 is not present in these samples, probably because they had not been exposed to the solar wind. The two chips were both taken from the bottom surface of Rock 15015, and the lack of micrometeorite impact pits on the bottom surface of this rock indicates that it may never have been oriented to expose this side of the rock. Although the beam current density was comparable for the lunar sample analysis and the analysis of the implanted samples, the hydrogen in the lunar samples
2 0.4 0.5
appeared to be more mobile, at least in the outer 0.2 ~m. Consecutive measurements on the same sample give profiles with the same shape, but with a gradual decrease in hydrogen concentration, especially in the 0.2pm surface layer. Heavy radiation damage to depths of the order of 0.1/~m has been observed in some lunar samplesS), and may partially account for
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,o~o 20'00 3o~oo 4o'oo DEPTH (ANGSTROMS) Fig. 3. H y d r o g e n concentration versus depth in lunar soil fragment 10085,31-12. D a t a points are n u m b e r o f counts with statistical uncertainties plotted against the difference between b e a m energy (E) a n d resonance energy (ER). Hydrogen concentration a n d depth scales are calculated. The position o f the zero point on the h y d r o g e n concentration scale is determined by the b a c k g r o u n d counting rate.
MEASURING HYDROGEN
the increased hydrogen mobility. However, it may also be that the more mobile hydrogen is actually water of terrestrial origin. Epstein and Taylor 9) have shown that terrestrial water may exist in returned lunar soil samples in amounts comparable to the hydrogen content from solar wind implantation. It is worth noting in this regard that the two chips from rock 15015 were the only measured samples that had not been exposed extensively to the atmosphere. Although the unsealed teflon bag containing rock 15015 was exposed to the Apollo 15 cabin atmosphere, it was immediately stored under dry nitrogen on return and our samples were unpacked and transferred to the target chamber in dry nitrogen. However, when one of these chips was exposed to the atmosphere for 24 h and then rechecked, its hydrogen distribution was unchanged. A more detailed discussion of the hydrogen depth distribution measurements on lunar samples is the subject of a separate paper~°). 4. Discussion
Despite the implicit ambiguity in the interpretation of the origin of the observed hydrogen, the technique has been shown to provide an accurate and reproducible measurement of its distribution, limited only by the mobility of the hydrogen due to ~9F beam heating. Simply decreasing the beam current will help to control this problem. A new target chamber design will allow us to place the detector closer to the target and will allow more shielding against room background, which now becomes a problem at low beam current. Larger targets, and hence larger 19F beam sports will reduce target heating without lowering the counting rate. Cooling the target holder with liquid nitrogen is another alternative. Numerous possibilities suggest themselves as further applications of this technique. Low energy proton range distributions could be measured by implanting clean targets with monoenergetic protons and measuring the resulting depth profiles. Diffusion profiles could be measured to provide information which may be useful to better understand the processes of diffusion of gases in solids. The thickness and rate of growth of hydride or hydrated layers on surfaces can be measured easily and accurately. The usefulness of resonant nuclear reactions as a means to obtain depth distributions is by no means confined to the reaction aH(19F,~7)160. Another resonance which may be useful for studying proton penetration profiles is ~H(lSN,~)~2C at 0.40 MeV (center-of-mass energy)4'~t). The estimated depth
CONCENTRATION
71
resolution is about 50/~ and the maximum depth which can be studied is about 3/~m, representing a significant improvement over the resolution and range available using ~H(19F,0~7)160 a t 0.83 MeV. The reduction in counting rate by a factor of ten does not represent a serious problem except for the more mobile distributions. The same resonances in the inverse reactions 1 9 F ( p , ~ ) 1 6 0 and 15N(p,~7)11C could be used to study 19F and ~SN depth distributions. Distributions of 4He or l°B can be studied using the 1.08 MeV (c.m.) resonance in 4He(~°B,n)t3N (refs. 4, 12). This reaction is promising as a means to measure solar wind helium implanted in lunar samples. A 3.8-MeV I°B beam is necessary and should give neutron counting rates comparable to rates from 1 H ( 1 9 F , ~ ) 1 6 0 , with a 300/~ resolution. The best possibility for studying ~2C involves the 12C(3He, n)~40 reaction and the detection of the delayed 7 rays following the 140,/~+ decay. The yield will probably be down by a factor of ten from the present level, but the measurement should yield the absolute surface concentration of carbon and the width of the distribution. We would like to thank Prof. G. J. Wasserburg for permitting us to borrow lunar samples from his laboratory and Prof. S. Epstein for the use of his dry box. We also acknowledge our gratitude to Prof. D. S. Burnett for his advice and assistance in selecting and handling the lunar samples. References 1) p. Eberhardt, J. Geiss, H. Graf, N. Gr6gler, U. Kr/ihenbiihl, H. Schwaller, J. SchwarzmiiUer and A. Stettler, Proc. Apollo 11 Lunar Science Conf., vol. 2 (Pergamon Press, New York, 1970) p. 1037. 2) H. Hintenberger, H. W. Weber, H. Voshage, H. W~.nke, F. Begemann and F. Wlotzka, ibid., p. 1269. 3) T. Kirsten, O. Miiller, F. Steinbrunn and J. Z~hringer, ibia., p. 1331. 4) F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1. 5) F. Ajzenberg-Selove, Nucl. Phys. At90 (1972) 1. 6) L. C. Northcliffe, Ann. Rev. Nucl. Sci. 13 (1963) 67. 7) H. E. Schiott, Mat. Fys. Medd. Dan. Videnskab. Selskab 35, no. 9 (1966) 1. 8) j. p. Bibring, J. P. Duraud, L. Durrieu, C. Jouret, M. Maurette and R. Meunier, Science 175, no. 4023 (1972) 753. 9) S. Epstein and H. P. Taylor, Jr., Proc. Apollo 11 Lunar Science Conf., vol. 2 (Pergamon Press, New York, 1970) p. 1085. 10) D. A. Leich, T. A. Tombrello and D. S. Burnett, submitted for publication in Earth Planet. Sci. Letters. 11) F. Ajzenberg-Selove, Nucl. Phys. A166 (1971) 1. 1'>) F. Ajzenberg-Selove, Nucl. Phys. A152 (1970) 1.
INSTRUMENTS
AND
METHODS
108
(1973) 67-7I; ©
NORTH-HOLLAND
PUBLISHING
CO.
A T E C H N I Q U E FOR MEASURING H Y D R O G E N C O N C E N T R A T I O N VERSUS D E P T H IN SOLID SAMPLES* D. A. LEICH and T. A. TOMBRELLO
California Institute of Technology, Pasadena, California, U.S.A. Received 21 November 1972 The resonance at 0.83 MeV (center-of-mass energy) in the nuclear reaction 1H(19F,ct7)160 has been used to measure hydrogen depth profiles in solids. Hydrogen concentrations have been measured to a depth of 0.4 #m both in samples implanted with 11.5-keV
protons and in several lunar samples. Further applications of this technique are discussed and several other resonant nuclear reactions are suggested for depth distribution studies of hydrogen and other elements.
1. Introduction
reaction 1H(19F, cty)160 to obtain direct hydrogen depth profiles on lunar samples. This technique is capable of producing much more quantitative depth profiles than those inferred from chemical etching results, thus providing an important tool in obtaining the historical record of the solar wind contained in lunar and meteoritic samples.
In order to determine whether the relatively high rare gas concentrations measured in lunar soils were due to implanted solar wind ions, several groups performed experiments to show that the rare gases were on the surfaces of the lunar soil particles rather than uniformly distributed throughout the volume of the grains. One method employed 1-3) was to measure the remaining rare gas concentrations in a number of similar grain fractions after removal of a surface layer by chemical etching. It was indeed found that the solar wind gases were localized on the surfaces of the grains; however, the implied gas layer thicknesses (up to a few micrometers) were significantly larger than the range of 1 keV/nucleon (solar wind) ions. To check this surprising result, we are using the resonant nuclear
2. Experimental 2.1. ANALYTICAL TECHNIQUE
The reaction 1 H ( t 9 F , 0C7)160shows a strong resonance at 0.83 MeV (center-of-mass energy), with a cross section of 0.5 barn and a width of about 5 keV, producing 6.1, 6.9 and 7.1-MeV 7-rays from the deexcitation of the residual 160 nucleus4'5). At energies different from the resonance energy by more than a few keV the cross section for the production of the high-energy 7-rays is negligible. Thus, if the surface of
* Supported in part by the National Science Foundation lGP-28027l.
ACCELERATOR BEAM LINE
E
LM- [-
- -
GATE
IN-LINE TRAP i ~
LN2
~
~ I
L ~ ~
~
~
LN2
[. . . . . . . .
VERN!ER ~
ELECT~INDOW~TARGET WHEEL 1 CONNECTION~J~ROTARY
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DRIVE
~L--~NoI(TI)CRYSTAL PHOTOTUBE
~
'gF SEAM
SPOT
IUI~ I
I PULSE-HEIGHT !
ROUSING I ~MP [
I ANALYZER J
ROTARY TARGET WHEEL
Fig. 1. Schematic drawing of the target chamber vacuum system and detector electronics. The insert shows the arrangement of target mounts.
67
68
D. A. LEICH AND T. A. TOMBRELLO
a material containing hydrogen is irradiated with 19F ions at energies slightly greater than the resonance energy, the ~gF ions will be continuously slowed down until at a depth XR the resonance energy will be reached and the reaction will occur at a rate proportional to the hydrogen concentration in a thin layer at XR. At greater depths the ~9F energy will be below the resonance energy where the cross section is again negligible. Since the stopping power, d E / d x , is nearly independent of energy 6) (within ~ 2%) in the relevant range of 19F energies, the depth XR can be related to the ~9F beam energy Eo by the relation E o + x R ( d E / d x ) -- ER, where ER is the resonance energy in the laboratory or proton rest frame. Measuring the ~,-ray production rate as the 19F beam energy is varied gives a direct indication of hydrogen concentration as a function of depth in the target. The depth resolution is determined by the ratio of the resonance width to the stopping power and is about 200 A for typical target materials. A second strong resonance at 0.89 MeV (center-of-mass energy) limits the range of this particular reaction to a depth of about 0.4 #m, but unfolding techniques could be used to continue the profile to greater depths. 2.2. EXPERIMENTALAPPARATUS The Caltech tandem accelerator was used to provide a magnetically analyzed monoenergetic 19F4+ beam in the range of 16 to 18 MeV. Fig. 1 shows a diagram of the scattering chamber and pumping system designed to minimize contamination of the targets by water and oil vapors present in the beam line. Condensable vapors are trapped on the walls of a 30-cm long LN 2 cooled beam line baffle. The low conductance path provided by this baffle enables the getter-ion pumping system to maintain a pressure of about I 0 - 9 torr in the chamber with the beam on the target. All metal construction and metal-to-metal seals permit baking of the scattering chamber and pumping system to 300 °C to drive trapped gasses out of the walls. A rotating aluminum wheel provides mounts for 12 targets. A 7.6 cm × 7.6 cm NaI(T1) crystal detector placed outside the scattering chamber at about 30 ° scattering angle is used to detect ~-radiation. A 4 to 8 MeV counting window on the pulse-height spectrum includes all but the tail of the C o m p t o n distribution in the response of the NaI(TI) crystal to the three ~-rays. With the NaI(TI) crystal placed 7 cm from the target and using this energy window, the detected fraction of all 7-rays emitted is estimated to be about 0.016. An analyzed sample of the mineral chlorite (1.6% hydrogen) was used as a target to obtain this estimate of the detection efficiency.
DEPTH [ANGSTROMS) I000 2000 3000
0 I
I
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• IMPLANTED • BLANK
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Fig. 2a. The ~'-ray yield versus bombarding energy are shown for an implanted fused silica sample and for an identical non-implanted (blank) sample. The energy scale shows the resonance energy (E~) subtracted from the tgF beam energy (E). The depth scale calculated from the energy loss is also shown. Error bars
show statistical uncertainties. 3. Results
3.1. IMPLANTEDSAMPLES In order to establish the sensitivity and reliability of this technique, implantation experiments have been carried out on crystalline quartz, fused silica, and Carich feldspar samples using 11.5-keV protons with doses of 1015-1016 protons/cm 2. The target materials were chosen for their chemical and physical similarity to the returned lunar samples. Targets were chemically cleaned and then baked in the scattering chamber to remove surface contamination before implantation. A magnetically analyzed proton beam with a flux of about 1012 s -1 cm -2 was obtained from a duoplasmatron ion source to implant a 0.5 cm 2 spot on each target. The scattering chamber was then transferred to the beam line where a beam of 101°F ions per second was directed into a 4-mm wide spot for analysis. Fig. 2a shows data for a typical implanted target and for a target which was not implanted but was otherwise identical. Fig. 2b shows an implantation profile obtained by subtracting the data in fig. 2a. 19F stopping powers, calculated for the particular target composition from values in Northcliffe 6) are used to determine the depth scale. The experimentally determined distributions show some deviation from the theoretical range distribution for 1 1.5-keV protons. A comparison between the measured and theoretical 7) mean range
69
MEASURING HYDROGEN CONCENTRATION DEPTH (/.,.g/cmz) 20 40 60 I
E41 (b)
Q o
©: Z 0 I---
I
I
8O
I00
I
I-
{{{{{
3.2. LUNAR SAMPLES
{
Q:2, Z 0 Z (C, 0 Z w (D 0 t22 Q >-
r 0
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T
o
,
bution, suggesting a c o n t i n u a t i o n o f the same process by which the range d i s t r i b u t i o n is distorted d u r i n g i m p l a n t a t i o n , a l t h o u g h at a slower rate. W i t h the exception o f this effect, the profiles are r e p r o d u c i b l e ; thus, there is no reason to d o u b t that the actual p r o t o n distribution is being measured.
i
i
iooo 2ooo 3ooo DEPTH (ANGSTROMS)
4ooo
Fig. 2b. Implantation profile for the fused silica sample after the background has been subtracted. a n d full width at half m a x i m u m is shown in table 1. The differences may, in part, represent the effects o f diffusion d u r i n g the i m p l a n t a t i o n since there seems to be a weak inverse c o r r e l a t i o n between the observed widths and the i m p l a n t a t i o n ion currents, a l t h o u g h we have not used a wide e n o u g h range o f i m p l a n t a t i o n currents to check this t h o r o u g h l y . R e p e a t e d analyses o f a single i m p l a n t e d s a m p l e show g r a d u a l but consistent i n w a r d shifting and b r o a d e n i n g o f the distri-
H y d r o g e n depth profiles have been m e a s u r e d on several 2 3 mm-sized glass fragments a n d glass-coated rock fragments selected from the coarse fines o f the A p o l l o 11 and A p o l l o 15 missions. Because o f the small size o f these fragments, the t9F b e a m was collim a t e d to a 2-mm spot and the s a m p l e holders were masked with clean fused silica collimators. N o r m a l l y , at least 80% o f the 19F b e a m could be directed o n t o the lunar s a m p l e with the r e m a i n i n g b e a m fraction collected by the relatively hydrogen-free collimator. The fraction o f the b e a m hitting the sample was d e t e r m i n e d by masking the analyzed chlorite sample with a c o l l i m a t o r identical to those m o u n t e d with the lunar samples and noting the reduction in the 7-ray counting rate for the m a s k e d sample as c o m p a r e d to an u n m a s k e d chlorite target. D a t a were taken in 200 keV steps from a starting p o i n t near one end o f the 16.3-17.5 M e V energy range to the o p p o s i t e end and then back over the entire energy range in 100 keV steps. F o r the l u n a r samples the 19F b e a m current was reduced to ½ o f the current used on the i m p l a n t e d samples and d a t a runs were lengthened to 10 rain each.
TABLE 1 Most probable range (Rp), median range (RM) and full-width-at-half-maximum (fwhm) are given for samples implanted with l l.5 keV protons. Calculations from Schiott 7) predict a mean range of 34/~g/cm e for SiO2 and 35/tg/cm 2 for the feldspar composition, with fwhm's of 25 pg/cm 2 and 26/tg/cm 2, respectively.
Sample type
Proton implantation rate (10l° s-1)
Rp
RM
(/tg/cm2)
(/~g/cm2)
fwhm (#g/cm 2)
Crystalline quartz
50 95 95 125 125
35 39 34 24 39
35 39 37 35 44
23 47 41 35 45
Fused silica
40 95 125
50 35 35
45 33 41
47 35 59
Feldspar (Ca-rich)
95 125 125 125
33 34 40 34
43 38 44 33
45 46 50 35
70
D. A. L E I C H
A N D T. A. T O M B R E L L O TABLE 2
M e a s u r e d concentration o f h y d r o g e n in a 0 . 4 / t m surface layer is s h o w n for each o f the l u n a r soil fragments. Collection o f the I9F b e a m charge, estimation o f counting efficiency, a n d mobility o f the hydrogen c o m b i n e for an uncertainty o f -4-20% o f the m e a s u r e d value. Sample designation
10085,31-12 10085,31-9 10085,1 Glass f r a g m e n t 15413,5-2 15413,5-5 15533,4-1
Description
H y d r o g e n concentration ( x 1016 p r o t o n s / c m 2)
Brown glass fragment Brown glass f r a g m e n t
7 1
Brown glass f r a g m e n t Shocked, pyroxene-rich rock f r a g m e n t Shocked, light-colored plagioclase f r a g m e n t Glazed rock f r a g m e n t
2
Since the flux of solar wind protons is sufficient to saturate ( ~ 1017 protons/cm 2) an exposed surface in a very short time (N30 y), the turnover of the top soil layers should insure that each soil fragment be exposed long enough to reach saturation at some time in geologic history. Observation of micrometeorite impact pits on most surfaces of our fragments confirms the assumption that each of them had been exposed to the solar wind. The profiles obtained from our soil fragments show a wide variation in absolute hydrogen concentration (table 2), although the profile shapes are quite similar. Fig. 3 is typical of these profiles showing a significant concentration of hydrogen in a layer about 0.2-pro deep. From 0.2 pm to 0.4 pm there is a gradual decrease in hydrogen concentration with increasing depth. The residual concentration at 0.4-pro depth is typically 102o hydrogen atoms per cm a. Two surface chips from a glass-coated rock (15015, 39) showed a different profile. The predominant feature is a concentration of hydrogen (3 × 10~5 atoms/cm 2) within 100 A of the surface, and decreasing to a much lower level of concentration within the first 0.1 pm. The peak in the hydrogen profile at ~ 1000 A shown in fig. 3 is not present in these samples, probably because they had not been exposed to the solar wind. The two chips were both taken from the bottom surface of Rock 15015, and the lack of micrometeorite impact pits on the bottom surface of this rock indicates that it may never have been oriented to expose this side of the rock. Although the beam current density was comparable for the lunar sample analysis and the analysis of the implanted samples, the hydrogen in the lunar samples
2 0.4 0.5
appeared to be more mobile, at least in the outer 0.2 ~m. Consecutive measurements on the same sample give profiles with the same shape, but with a gradual decrease in hydrogen concentration, especially in the 0.2pm surface layer. Heavy radiation damage to depths of the order of 0.1/~m has been observed in some lunar samplesS), and may partially account for
I
r
-2o0
o
i
E-ER(keY) 200 400 600
T
I
I
800 ]
I000 ]
10085,BI-12
15
o~
v
z
o B.
io~
I-I1: w n
o
i
},lii
8
i
o
"I-
,o~o 20'00 3o~oo 4o'oo DEPTH (ANGSTROMS) Fig. 3. H y d r o g e n concentration versus depth in lunar soil fragment 10085,31-12. D a t a points are n u m b e r o f counts with statistical uncertainties plotted against the difference between b e a m energy (E) a n d resonance energy (ER). Hydrogen concentration a n d depth scales are calculated. The position o f the zero point on the h y d r o g e n concentration scale is determined by the b a c k g r o u n d counting rate.
MEASURING HYDROGEN
the increased hydrogen mobility. However, it may also be that the more mobile hydrogen is actually water of terrestrial origin. Epstein and Taylor 9) have shown that terrestrial water may exist in returned lunar soil samples in amounts comparable to the hydrogen content from solar wind implantation. It is worth noting in this regard that the two chips from rock 15015 were the only measured samples that had not been exposed extensively to the atmosphere. Although the unsealed teflon bag containing rock 15015 was exposed to the Apollo 15 cabin atmosphere, it was immediately stored under dry nitrogen on return and our samples were unpacked and transferred to the target chamber in dry nitrogen. However, when one of these chips was exposed to the atmosphere for 24 h and then rechecked, its hydrogen distribution was unchanged. A more detailed discussion of the hydrogen depth distribution measurements on lunar samples is the subject of a separate paper~°). 4. Discussion
Despite the implicit ambiguity in the interpretation of the origin of the observed hydrogen, the technique has been shown to provide an accurate and reproducible measurement of its distribution, limited only by the mobility of the hydrogen due to ~9F beam heating. Simply decreasing the beam current will help to control this problem. A new target chamber design will allow us to place the detector closer to the target and will allow more shielding against room background, which now becomes a problem at low beam current. Larger targets, and hence larger 19F beam sports will reduce target heating without lowering the counting rate. Cooling the target holder with liquid nitrogen is another alternative. Numerous possibilities suggest themselves as further applications of this technique. Low energy proton range distributions could be measured by implanting clean targets with monoenergetic protons and measuring the resulting depth profiles. Diffusion profiles could be measured to provide information which may be useful to better understand the processes of diffusion of gases in solids. The thickness and rate of growth of hydride or hydrated layers on surfaces can be measured easily and accurately. The usefulness of resonant nuclear reactions as a means to obtain depth distributions is by no means confined to the reaction aH(19F,~7)160. Another resonance which may be useful for studying proton penetration profiles is ~H(lSN,~)~2C at 0.40 MeV (center-of-mass energy)4'~t). The estimated depth
CONCENTRATION
71
resolution is about 50/~ and the maximum depth which can be studied is about 3/~m, representing a significant improvement over the resolution and range available using ~H(19F,0~7)160 a t 0.83 MeV. The reduction in counting rate by a factor of ten does not represent a serious problem except for the more mobile distributions. The same resonances in the inverse reactions 1 9 F ( p , ~ ) 1 6 0 and 15N(p,~7)11C could be used to study 19F and ~SN depth distributions. Distributions of 4He or l°B can be studied using the 1.08 MeV (c.m.) resonance in 4He(~°B,n)t3N (refs. 4, 12). This reaction is promising as a means to measure solar wind helium implanted in lunar samples. A 3.8-MeV I°B beam is necessary and should give neutron counting rates comparable to rates from 1 H ( 1 9 F , ~ ) 1 6 0 , with a 300/~ resolution. The best possibility for studying ~2C involves the 12C(3He, n)~40 reaction and the detection of the delayed 7 rays following the 140,/~+ decay. The yield will probably be down by a factor of ten from the present level, but the measurement should yield the absolute surface concentration of carbon and the width of the distribution. We would like to thank Prof. G. J. Wasserburg for permitting us to borrow lunar samples from his laboratory and Prof. S. Epstein for the use of his dry box. We also acknowledge our gratitude to Prof. D. S. Burnett for his advice and assistance in selecting and handling the lunar samples. References 1) p. Eberhardt, J. Geiss, H. Graf, N. Gr6gler, U. Kr/ihenbiihl, H. Schwaller, J. SchwarzmiiUer and A. Stettler, Proc. Apollo 11 Lunar Science Conf., vol. 2 (Pergamon Press, New York, 1970) p. 1037. 2) H. Hintenberger, H. W. Weber, H. Voshage, H. W~.nke, F. Begemann and F. Wlotzka, ibid., p. 1269. 3) T. Kirsten, O. Miiller, F. Steinbrunn and J. Z~hringer, ibia., p. 1331. 4) F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1. 5) F. Ajzenberg-Selove, Nucl. Phys. At90 (1972) 1. 6) L. C. Northcliffe, Ann. Rev. Nucl. Sci. 13 (1963) 67. 7) H. E. Schiott, Mat. Fys. Medd. Dan. Videnskab. Selskab 35, no. 9 (1966) 1. 8) j. p. Bibring, J. P. Duraud, L. Durrieu, C. Jouret, M. Maurette and R. Meunier, Science 175, no. 4023 (1972) 753. 9) S. Epstein and H. P. Taylor, Jr., Proc. Apollo 11 Lunar Science Conf., vol. 2 (Pergamon Press, New York, 1970) p. 1085. 10) D. A. Leich, T. A. Tombrello and D. S. Burnett, submitted for publication in Earth Planet. Sci. Letters. 11) F. Ajzenberg-Selove, Nucl. Phys. A166 (1971) 1. 1'>) F. Ajzenberg-Selove, Nucl. Phys. A152 (1970) 1.