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ERD technique with monochromatic neutrons as an effective tool for hydrogen-material systems study
Nuclear Instruments and Methods in Physics Research
NOMB
B 85 (1994) 803-807
North-Holland
Beam Interactions with Materials 8 Atoms
ERD technique with monochromatic for hydrogen-material systems study B.G. Skorodumov,
neutrons
as an effective tool
1.0. Yatsevich, V.G. Ulanov, E.V. Zhukovska, O.A. Zhukovsky
Institute of Nuclear Physics, Academy of Science RU., Ulugbek, 702132 Tashkent, Uzbekistan
Neutron elastic recoil detection (NERD) technique for hydrogen isotope concentration depth profiling is considered in comparison with IBA techniques. The use of 14 MeV neutrons instead of the usually applied accelerated ions provides a number of essential advantages which allow us to analyze the deep regions of the sample and to perform the investigations of some processes in H-M systems inaccessible for the ion methods. Examples of both analytical and scientific applications of the NERD technique are presented.
1. Introduction Among a variety of hydrogen determination techniques [l] the elastic recoil detection (ERD) ones, are able to analyze all hydrogen isotopes simultaneously [2,3]. The forward alpha-scattering technique (FAST) [4,5] is the nearest analog of the method presented here. It detects the emitted hydrogen recoils at 0” to the beam direction, whereas the choice of beam energy and sample thickness ensures that the He ions are stopped within the sample. This paper describes a method rather like the forward scattering ERD technique, but using 14 or 2.5 MeV monochromatic neutrons instead of ions. The main characteristics of ion and neutron techniques are compared and the ability of the NERD technique to carry out a number of untraditional studies is demonstrated.
source and the counter telescope: (1) samples of any kind (solid plates, pieces, powders, ingots, gases or liquids) for stationary H, D and T concentration depth profile measurements, (2) a gas diffusion cell, or (3) an electrolytic cell for direct study of hydrating-dehydrating kinetics. In cases (2) and (3), the diffusion gas flux through the membrane under study can be simultaneously measured.
3. Comparison between analytical characteristics and neutron methods
of ion
3.1. Depth profile calculation In order to convert the energy spectrum of the detected recoils N(E) into a concentration depth profile C(x) it is necessary to use a relationship of the type N(E)
2. Experiment
e)S-l
E mm
The details of the experimental technique have been described earlier [6]. It is based on a 1.50 keV deuteron accelerator design for 14 MeV and 2.5 MeV neutron production by the reactions of 3H(d, n)4He and *H(d, nj3He, respectively. The neutrons are incident onto the sample, and after their collision with hydrogen atoms the recoils emitted at an angle of near 0” to the neutron are detected by a telescope of three or two silicon detectors. Presently the technique is arranged in such a way that three modes of investigation can be performed by installing the following items between the neutron 0168-583X/94/$07.00 0 1994 - El sevier Science SSDZ 0168-583X(93)E0859-F
= const./E”“C(x(_!?‘))(r(EI(E’),
x(E')f(E, E’)
dE’.
(1)
For analysis with an ion beam, one has to know the reaction excitation function c+(E,, O), energy resolution function f(E, E') and stopping parameter s. The calculation of s is complicated [7] because stopping powers for both the projectile and recoil have to be taken into account. In our case the conversion of energy spectra to depth profiles is simpler. Indeed, at any point of the sample, neutrons interact with hydrogen atoms at the same energy and hence with the same cross section. Moreover, only one of the two interacting particles
B.V. All rights reserved
XIII. DEVELOPMENTS
804
B.G. Skorodumov et al. / Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 803-807
loses energy. This enables us to put u in Eq. (1) before t_he integral (or omit it using a reference sample), and S can be approximated by the stopping power S(E) of the sample material for a particle with the recorded energy E. Thus for neutron depth profiling we get N(E)
=const./%(~(E~))S-‘(E’)f(E, Gil
E’) dE’, (2)
where Eth is the threshold in the energy detection by the telescope and E, is the neutron energy multiplied by the kinematic factor k = 4m(m + l)-*, where m is the recoil mass. To extract c(x) from Eq. (2) a deconvolution procedure has to be made, but in a number of practical cases it can be obtained without significant distortions by using a very simple expression C(x)=A.N(E).S(E),
(3)
where x=R(E,)
-R(E).
(4)
Here A can be determined in a measurement with the reference sample and R(E) is the tabulated range-energy relation for the given type of recoil in the sample material. 3.2. Detection limit Most ion methods are able to determine hydrogen contents at a detection limit of lo’--lo3 appm unless special conditions are arranged. The detection limits of NERD for the different hydrogen isotopes depend on the background level of the relevant nuclear reactions in the sample matrix. Fig. 1 represents the hydrogen detection limits for two samples (Ni and C) that are very different from the point of view of (n, p) background reactions. Limits were obtained from corresponding background spectra NJ E) using Eq. (31, with
40 “0 -
=
3
Ni(:lO)
0 Depth,/.~rn
Fig.
Hydrogen
C
0
detection limits for nickel and carbon samples.
where LY= 1 (reliability P(a) = 68%). Despite the fact that Ni is one of the few elements with a positive Q-value for the (n, p) reaction, a few at% of hydrogen might be easily determine in its near surface region. The absence of the (n, p) reaction contribution from carbon due to a very high negative Q-value (N 13 MeV) allows the detection limits to be less than 5 x lo* ppm up to a depth of 400 pm. The dominant part of the background comes from the *‘Si(n, p)“Al reaction in the first silicon GE-detector (proton peaks corresponding to the ground and a number of excited states of **Al can be clearly seen in the spectra). The detection limits for D and T are expected to be lower since both the (n, d) and (n, t) reactions have high negative Q-values for all elements. A better sensitivity for hydrogen might be obtained if a CO, gas filled proportional counter is used instead of the first silicon detector. Thus the ERD technique with 14 MeV neutrons is practically just as good in sensitivity as the technique with ions. Moreover, if one uses the neutrons from the D(d, n)3He reaction (whose energy is 2.5 MeV and does not exceed the energy thresholds of the (n-p, d, t) reactions for most of the elements), the detection limits for all hydrogen isotopes are estimated to be much lower due to the lower reaction background. 3.3. Speed of analysis The neutron technique is inferior to ion methods in this respect, because of the low intensity of monochromatic neutron sources. However, as hydrogen is very mobile in most of the species, the high stopping power of probing ions in the case of high beam-current values might cause a radiation-induced distortion in the initial hydrogen distribution by means of both local sample heating and radiation damage. Such ion beam induced desorption of hydrogen in the course of depth profiling by ERD using 350 keV 4He beam was recently observed to a different degree in almost all materials [8], which results not only in lowering of the total quantity of H but also in modification of the initial depth profile. In other words, if one wishes to perform a fast analysis, care must be taken when determining the maximum permissible beam current which still allows yet to obtain the real hydrogen distribution. The full absence of sample heating and minimum of radiation effects in the case of NERD allows us to perform the analysis without any precautions against possible distortions. Analysis of hydrogen-rich objects, such as Ti, Pd, Zr, hydrogen storage materials and those having organic composition is rather fast (a few minutes for one depth profile measurement). 3.4. Selectivity The possibility of simultaneous profiling of all hydrogen isotopes
concentration depth depends on the abil-
B.G. Skorodumou et al. /Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 803-807
80.5
ity of the detecting system to identify the type of recoils. Among various methods suggested for charged particle identification the AE-E method is the most simple one. However, it is effective with high energy recoils only. Among small accelerators, the neutron generator is the only one which at an accelerating voltage of 150 kV provides recoils of energies up to 14 MeV and gives a good ability of simultaneous H, D and T depth profiling. 3.5. Depth resolution and maximum
analyzable depth
In order to extract from Eq. (2) the real depth profile C(x), the resolution function f(E, E ‘) has to be known. That, in general, depends on a number of factors such as: (1) the energy spread of the incident particles; (2) the energy straggling of both incident and emitted particles in the sample material; (3) the effect of the finite angular aperture of the experimental setup; (4) the detector resolution coupled with the noise contribution from the electronics; (5) the energy spread due to multiple scattering, and (6) an additional energy broadening from straggling in the absorber foil, if used, in front of the detector. In our case point (2) and point (6) are partially excluded, but due to the low intensity of the neutron source, the geometric spread provides a dominant contribution to the energy resolution. In order to get the real resolution function and to achieve an optimum balance between registration efficiency and energy resolution, under the conditions of a “bad” geometry Monte Carlo simulation has been applied. A special program “Geometry” was developed (it will be published elsewhere) which considers both the neutron generation and the (n, charged particle) elastic scattering from any target into a counter telescope made of any number of detectors and diaphragms. Fig. 2 shows the comparison between experimental and calculated energy spectra obtained with a sandwich-type target made of five 30 pm CH,-films and four 160 km Al-foils. It is seen that the agreement is rather good if convolution with Gaussian-type broadening due to detectors (plus electronics) noise and straggling contribution calculated by means of the TRIM program [9] is performed. A residue discrepancy originates probably from the quite correct use of TRIM’s straggling under the conditions of a “bad” geometry and might be avoided by additional fitting using Gaussian broadening which is slightly dependent on energy. Maximum analyzable depth and depth resolution are contradictory. The methods possessing good depth resolution have small analyzable depths and vice versa. The ion methods based on small accelerators have their main advantage in the good depth resolution (tens-hundreds of angstroms) due to the large stopping power of the probing ions. But for the same reason their maximum probing depth does exceed a few pm.
ENERGY.MeV
Fig. 2. Proton energy spectra obtained from (a) a target made of five 30 pm polyethylene and four 160 pm thick Al foils bombarded by 14 MeV neutrons; and (b) from a mylar Al mylar target with 2.5 MeV neutrons. The spectra demonstrate that the depth resolution with the 2.5 MeV neutrons is more than 10 times better but the statistics is weak due to the very low source intensity
Quite opposite, the neutron method normally has an absolute resolution of tens of urn at a maximum analyzable depth of hundreds of urn. This can be calculated from Eq. (4) at E = Et,,. In Pd it reaches 170 km and 350 p,rn for D and H, respectively, and 1000 km for H in C. Apart the large analyzable depth is also important in the case of hydrogen diffusion studies. The small accessible depth of ion methods restricts hydrogen diffusion studies to cases of low diffusion coefficients and to conditions when hydrogen was implanted or absorbed in a sample with highly polished surfaces. The high neutron penetrability along with the large depth of analysis allows one mount to between the neutron source and the detector any device for the direct study of hydrogen diffusion into the sample from gas, liquid or other volume in a wide range of thermodynamic conditions. XIII. DEVELOPMENTS
B.G. Skorodumov et al. / Nucl. Imtr. and Meth. in Phys. Rex B 85 (1994) 803-807
806 4. Application
examples
DEPTH,/m.
In order to verify the applicability of the technique for hydrogen isotope determination in thermonuclear reactor first-wall components, we measured a TiT target usable for neutron generation. This sample was made as a thin tritium saturated titanium layer (- 7 pm) on MO backing. The recoil energy spectrum obtained from this target is presented in Fig 3. The proton and deuteron peaks appearing together with the triton one indicate the presence of the corresponding hydrogen and deuterium admixtures in the tritium target. Unfortunately, while the method with 14 MeV neutrons while gives absolute hydrogen isotope contents, it is unable to provide any depth distribution, because the analyzed Ti-layer is too thin in comparison with the depth resolution. This disadvantage can be overcome by using 2.5 MeV neutrons (see Fig. 2b). Fig. 4 shows a proton energy spectrum obtained from a TiH, powder sample. It is in rather good agreement with the simulated spectrum obtained at C(x) = const. The depth profile in Fig. 4 was calculated by means of expression (3). Since the H-distribution in the powder is obviously uniform, the curves in Fig. 4 confirm the correctness of the applied conversions and indicate that for thick samples, instead of depth profile extraction from (21, the use of the simple expression (3) is quite enough. The NERD technique is very suitable to carry out test measurements on new organometallic or carboplastic materials. In Fig. 5 it is shown how the hydrogen escapes from organic impregnated carbon samples during the various stages of a special treatment in their fabrication. The 5 measurement take a total time of about half an hour. The use of NERD-technique for the direct study of diffusion of hydrogen isotopes in palladium has been demonstrated in [6,10]. Fig. 6 shows an example of hydrogen and deuterium depth profile time evolution that was observed in the course of isotope mixture
800 600 400 SOD],"" """""""""'-""
200 0 """""""""""'I
5.0
profile
600 r 5400 8
0 ENERGY,MeV
Fig. 4. Hydrogen depth profile in TiH, powder sample and comparison between experimental and simulated proton en-
ergy spectra. The fluctuations both in the measured simulated spectra are due to statistics.
and
sorption-desorption using a gas diffusion cell with a 500 pm Pd-membrane. It is clearly seen that the G-P interphase boundary behavior is distinctly different for hydrating and dehydrating processes, but it is almost the same for the two hydrogen isotopes. The ability of the technique to directly study of hydrogen diffusivity and solubility dependence on isotope mass during electrochemical saturation of Pdmembranes is presented in Fig. 7, where rather large isotopic effect can be clearly seen.
5. Summary The NERD method has been shown to be a useful tool to profile hydrogen isotopes in various samples not depending on their state (gas, liquid, solid), size, form and surface quality. Peculiarities of the technique, such
r
PO
45
67
Fig. 3. Energy
0
spectrum
*
10
ii
Ia
is
*4
EnoroY,mo”
from a titanium-tritium
target.
L
Fig. 5. Hydrogen depth profiles in carbon fabrics impregnated by an organic compound. Numbers l-5 reflect the process of material carbonization.
B.G. Skorodumou et al. /Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 803-807
807
allow one to obtain perfectly new direct information, not available before on hydrogen isotopes behavior in matter. The potential of NERD technique for scientific applications is far from being exhausted. An optimal choice of neutron energy and geometry of experimental set-up as well as of the thicknesses and types of the detectors (gas, silicon, position sensitive) provide good resources in: (i) lowering of the detection limits for all hydrogen isotopes: (ii) increasing of the probing depth and depth resolution improvement: (iii) three-dimensional depth profiling. The analytical characteristics of the technique will be essentially improved using a high intensity neutron source.
Acknowledgements Fig. 6. Hydrogen and deuterium depth profiles in process of sorption-desorption of isotope mixture in 500 km Pd membrane at ambient temperature using a gas diffusion cell.
The authors are indebted to A. Sevakin and Zh. Saidmuradov for ensuring good performance of the neutron generator NG-150, and to Ja.S. Abdullaeva, S.P. Kiseleva and V.Sh. Yalyshev for technical assistance and to J.F. Ziegler for kind assignment of the TRIM 91 program..
as large analyzable depth and simultaneous depth profiling of all hydrogen isotopes together with the possibility to use special cells (gas or electrolytic) as targets References
1 Fig. 7. Deuterium (1) and hydrogen (2) depth profiles obtained in the stationary process of D,O plus 15% Hz0 mixture in a cell with a 100 pm Pd membrane cathode. Right-hand slopes of the depth profiles reflect the difference in absolute depth resolution for water and for palladium. The maximum analyzable depths are = 350 p,rn and = 150 pm, respectively.
[l] P.K. Khabibullaev and B.G. Skorodumov, Determination of Hydrogen in Material. Nuclear Physics Methods, vol. 117 (Springer, 1989). [2] J. L’Ecuyer, C. Brassard, C. Cardinal, J. Chabbal, L. Deschence, J.P. Labric, B. Terreault, J.G. Martel and R.St. Jacques, J. Appl. Phys. 47 (1976) 881. [3] I.P. Chernov, J.P. Matusevich and V.P. Cosyr, Atom. Ener. 41 (1976) 51. [4] L.S. Wielunski, R.E. Benenson and W.A. Lanford, Nucl. Instr. and Meth. 218 (1983) 120. [5] J. Tirira, P. Trocellier and J.P. Frontier, Nucl. Instr. and Meth. B 50 (1990) 135. [6] B.G. Skorodumov and 1.0. Yatsevich, Nucl Instr. and Meth. B 64 (1992) 338. [7] B.L. Doyle and D.K. Price, Nucl. Instr. and Meth. B 36 (1988) 301. [8] G.G. Ross and I. Richard, Nucl. Instr. and Meth. B 64 (1992) 603. [9] J.F. Ziegler, TRIM-91 version. [lo] B.G. Skorodumov, 1.0. Yatsevich and O.A. Zhukovsky, these Proceedings (IBA-11, Balatonfilred, Hungary, 19931, Nucl. Instr. and Meth. B 85 (1994) 301.
XIII. DEVELOPMENTS
NOMB
B 85 (1994) 803-807
North-Holland
Beam Interactions with Materials 8 Atoms
ERD technique with monochromatic for hydrogen-material systems study B.G. Skorodumov,
neutrons
as an effective tool
1.0. Yatsevich, V.G. Ulanov, E.V. Zhukovska, O.A. Zhukovsky
Institute of Nuclear Physics, Academy of Science RU., Ulugbek, 702132 Tashkent, Uzbekistan
Neutron elastic recoil detection (NERD) technique for hydrogen isotope concentration depth profiling is considered in comparison with IBA techniques. The use of 14 MeV neutrons instead of the usually applied accelerated ions provides a number of essential advantages which allow us to analyze the deep regions of the sample and to perform the investigations of some processes in H-M systems inaccessible for the ion methods. Examples of both analytical and scientific applications of the NERD technique are presented.
1. Introduction Among a variety of hydrogen determination techniques [l] the elastic recoil detection (ERD) ones, are able to analyze all hydrogen isotopes simultaneously [2,3]. The forward alpha-scattering technique (FAST) [4,5] is the nearest analog of the method presented here. It detects the emitted hydrogen recoils at 0” to the beam direction, whereas the choice of beam energy and sample thickness ensures that the He ions are stopped within the sample. This paper describes a method rather like the forward scattering ERD technique, but using 14 or 2.5 MeV monochromatic neutrons instead of ions. The main characteristics of ion and neutron techniques are compared and the ability of the NERD technique to carry out a number of untraditional studies is demonstrated.
source and the counter telescope: (1) samples of any kind (solid plates, pieces, powders, ingots, gases or liquids) for stationary H, D and T concentration depth profile measurements, (2) a gas diffusion cell, or (3) an electrolytic cell for direct study of hydrating-dehydrating kinetics. In cases (2) and (3), the diffusion gas flux through the membrane under study can be simultaneously measured.
3. Comparison between analytical characteristics and neutron methods
of ion
3.1. Depth profile calculation In order to convert the energy spectrum of the detected recoils N(E) into a concentration depth profile C(x) it is necessary to use a relationship of the type N(E)
2. Experiment
e)S-l
E mm
The details of the experimental technique have been described earlier [6]. It is based on a 1.50 keV deuteron accelerator design for 14 MeV and 2.5 MeV neutron production by the reactions of 3H(d, n)4He and *H(d, nj3He, respectively. The neutrons are incident onto the sample, and after their collision with hydrogen atoms the recoils emitted at an angle of near 0” to the neutron are detected by a telescope of three or two silicon detectors. Presently the technique is arranged in such a way that three modes of investigation can be performed by installing the following items between the neutron 0168-583X/94/$07.00 0 1994 - El sevier Science SSDZ 0168-583X(93)E0859-F
= const./E”“C(x(_!?‘))(r(EI(E’),
x(E')f(E, E’)
dE’.
(1)
For analysis with an ion beam, one has to know the reaction excitation function c+(E,, O), energy resolution function f(E, E') and stopping parameter s. The calculation of s is complicated [7] because stopping powers for both the projectile and recoil have to be taken into account. In our case the conversion of energy spectra to depth profiles is simpler. Indeed, at any point of the sample, neutrons interact with hydrogen atoms at the same energy and hence with the same cross section. Moreover, only one of the two interacting particles
B.V. All rights reserved
XIII. DEVELOPMENTS
804
B.G. Skorodumov et al. / Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 803-807
loses energy. This enables us to put u in Eq. (1) before t_he integral (or omit it using a reference sample), and S can be approximated by the stopping power S(E) of the sample material for a particle with the recorded energy E. Thus for neutron depth profiling we get N(E)
=const./%(~(E~))S-‘(E’)f(E, Gil
E’) dE’, (2)
where Eth is the threshold in the energy detection by the telescope and E, is the neutron energy multiplied by the kinematic factor k = 4m(m + l)-*, where m is the recoil mass. To extract c(x) from Eq. (2) a deconvolution procedure has to be made, but in a number of practical cases it can be obtained without significant distortions by using a very simple expression C(x)=A.N(E).S(E),
(3)
where x=R(E,)
-R(E).
(4)
Here A can be determined in a measurement with the reference sample and R(E) is the tabulated range-energy relation for the given type of recoil in the sample material. 3.2. Detection limit Most ion methods are able to determine hydrogen contents at a detection limit of lo’--lo3 appm unless special conditions are arranged. The detection limits of NERD for the different hydrogen isotopes depend on the background level of the relevant nuclear reactions in the sample matrix. Fig. 1 represents the hydrogen detection limits for two samples (Ni and C) that are very different from the point of view of (n, p) background reactions. Limits were obtained from corresponding background spectra NJ E) using Eq. (31, with
40 “0 -
=
3
Ni(:lO)
0 Depth,/.~rn
Fig.
Hydrogen
C
0
detection limits for nickel and carbon samples.
where LY= 1 (reliability P(a) = 68%). Despite the fact that Ni is one of the few elements with a positive Q-value for the (n, p) reaction, a few at% of hydrogen might be easily determine in its near surface region. The absence of the (n, p) reaction contribution from carbon due to a very high negative Q-value (N 13 MeV) allows the detection limits to be less than 5 x lo* ppm up to a depth of 400 pm. The dominant part of the background comes from the *‘Si(n, p)“Al reaction in the first silicon GE-detector (proton peaks corresponding to the ground and a number of excited states of **Al can be clearly seen in the spectra). The detection limits for D and T are expected to be lower since both the (n, d) and (n, t) reactions have high negative Q-values for all elements. A better sensitivity for hydrogen might be obtained if a CO, gas filled proportional counter is used instead of the first silicon detector. Thus the ERD technique with 14 MeV neutrons is practically just as good in sensitivity as the technique with ions. Moreover, if one uses the neutrons from the D(d, n)3He reaction (whose energy is 2.5 MeV and does not exceed the energy thresholds of the (n-p, d, t) reactions for most of the elements), the detection limits for all hydrogen isotopes are estimated to be much lower due to the lower reaction background. 3.3. Speed of analysis The neutron technique is inferior to ion methods in this respect, because of the low intensity of monochromatic neutron sources. However, as hydrogen is very mobile in most of the species, the high stopping power of probing ions in the case of high beam-current values might cause a radiation-induced distortion in the initial hydrogen distribution by means of both local sample heating and radiation damage. Such ion beam induced desorption of hydrogen in the course of depth profiling by ERD using 350 keV 4He beam was recently observed to a different degree in almost all materials [8], which results not only in lowering of the total quantity of H but also in modification of the initial depth profile. In other words, if one wishes to perform a fast analysis, care must be taken when determining the maximum permissible beam current which still allows yet to obtain the real hydrogen distribution. The full absence of sample heating and minimum of radiation effects in the case of NERD allows us to perform the analysis without any precautions against possible distortions. Analysis of hydrogen-rich objects, such as Ti, Pd, Zr, hydrogen storage materials and those having organic composition is rather fast (a few minutes for one depth profile measurement). 3.4. Selectivity The possibility of simultaneous profiling of all hydrogen isotopes
concentration depth depends on the abil-
B.G. Skorodumou et al. /Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 803-807
80.5
ity of the detecting system to identify the type of recoils. Among various methods suggested for charged particle identification the AE-E method is the most simple one. However, it is effective with high energy recoils only. Among small accelerators, the neutron generator is the only one which at an accelerating voltage of 150 kV provides recoils of energies up to 14 MeV and gives a good ability of simultaneous H, D and T depth profiling. 3.5. Depth resolution and maximum
analyzable depth
In order to extract from Eq. (2) the real depth profile C(x), the resolution function f(E, E ‘) has to be known. That, in general, depends on a number of factors such as: (1) the energy spread of the incident particles; (2) the energy straggling of both incident and emitted particles in the sample material; (3) the effect of the finite angular aperture of the experimental setup; (4) the detector resolution coupled with the noise contribution from the electronics; (5) the energy spread due to multiple scattering, and (6) an additional energy broadening from straggling in the absorber foil, if used, in front of the detector. In our case point (2) and point (6) are partially excluded, but due to the low intensity of the neutron source, the geometric spread provides a dominant contribution to the energy resolution. In order to get the real resolution function and to achieve an optimum balance between registration efficiency and energy resolution, under the conditions of a “bad” geometry Monte Carlo simulation has been applied. A special program “Geometry” was developed (it will be published elsewhere) which considers both the neutron generation and the (n, charged particle) elastic scattering from any target into a counter telescope made of any number of detectors and diaphragms. Fig. 2 shows the comparison between experimental and calculated energy spectra obtained with a sandwich-type target made of five 30 pm CH,-films and four 160 km Al-foils. It is seen that the agreement is rather good if convolution with Gaussian-type broadening due to detectors (plus electronics) noise and straggling contribution calculated by means of the TRIM program [9] is performed. A residue discrepancy originates probably from the quite correct use of TRIM’s straggling under the conditions of a “bad” geometry and might be avoided by additional fitting using Gaussian broadening which is slightly dependent on energy. Maximum analyzable depth and depth resolution are contradictory. The methods possessing good depth resolution have small analyzable depths and vice versa. The ion methods based on small accelerators have their main advantage in the good depth resolution (tens-hundreds of angstroms) due to the large stopping power of the probing ions. But for the same reason their maximum probing depth does exceed a few pm.
ENERGY.MeV
Fig. 2. Proton energy spectra obtained from (a) a target made of five 30 pm polyethylene and four 160 pm thick Al foils bombarded by 14 MeV neutrons; and (b) from a mylar Al mylar target with 2.5 MeV neutrons. The spectra demonstrate that the depth resolution with the 2.5 MeV neutrons is more than 10 times better but the statistics is weak due to the very low source intensity
Quite opposite, the neutron method normally has an absolute resolution of tens of urn at a maximum analyzable depth of hundreds of urn. This can be calculated from Eq. (4) at E = Et,,. In Pd it reaches 170 km and 350 p,rn for D and H, respectively, and 1000 km for H in C. Apart the large analyzable depth is also important in the case of hydrogen diffusion studies. The small accessible depth of ion methods restricts hydrogen diffusion studies to cases of low diffusion coefficients and to conditions when hydrogen was implanted or absorbed in a sample with highly polished surfaces. The high neutron penetrability along with the large depth of analysis allows one mount to between the neutron source and the detector any device for the direct study of hydrogen diffusion into the sample from gas, liquid or other volume in a wide range of thermodynamic conditions. XIII. DEVELOPMENTS
B.G. Skorodumov et al. / Nucl. Imtr. and Meth. in Phys. Rex B 85 (1994) 803-807
806 4. Application
examples
DEPTH,/m.
In order to verify the applicability of the technique for hydrogen isotope determination in thermonuclear reactor first-wall components, we measured a TiT target usable for neutron generation. This sample was made as a thin tritium saturated titanium layer (- 7 pm) on MO backing. The recoil energy spectrum obtained from this target is presented in Fig 3. The proton and deuteron peaks appearing together with the triton one indicate the presence of the corresponding hydrogen and deuterium admixtures in the tritium target. Unfortunately, while the method with 14 MeV neutrons while gives absolute hydrogen isotope contents, it is unable to provide any depth distribution, because the analyzed Ti-layer is too thin in comparison with the depth resolution. This disadvantage can be overcome by using 2.5 MeV neutrons (see Fig. 2b). Fig. 4 shows a proton energy spectrum obtained from a TiH, powder sample. It is in rather good agreement with the simulated spectrum obtained at C(x) = const. The depth profile in Fig. 4 was calculated by means of expression (3). Since the H-distribution in the powder is obviously uniform, the curves in Fig. 4 confirm the correctness of the applied conversions and indicate that for thick samples, instead of depth profile extraction from (21, the use of the simple expression (3) is quite enough. The NERD technique is very suitable to carry out test measurements on new organometallic or carboplastic materials. In Fig. 5 it is shown how the hydrogen escapes from organic impregnated carbon samples during the various stages of a special treatment in their fabrication. The 5 measurement take a total time of about half an hour. The use of NERD-technique for the direct study of diffusion of hydrogen isotopes in palladium has been demonstrated in [6,10]. Fig. 6 shows an example of hydrogen and deuterium depth profile time evolution that was observed in the course of isotope mixture
800 600 400 SOD],"" """""""""'-""
200 0 """""""""""'I
5.0
profile
600 r 5400 8
0 ENERGY,MeV
Fig. 4. Hydrogen depth profile in TiH, powder sample and comparison between experimental and simulated proton en-
ergy spectra. The fluctuations both in the measured simulated spectra are due to statistics.
and
sorption-desorption using a gas diffusion cell with a 500 pm Pd-membrane. It is clearly seen that the G-P interphase boundary behavior is distinctly different for hydrating and dehydrating processes, but it is almost the same for the two hydrogen isotopes. The ability of the technique to directly study of hydrogen diffusivity and solubility dependence on isotope mass during electrochemical saturation of Pdmembranes is presented in Fig. 7, where rather large isotopic effect can be clearly seen.
5. Summary The NERD method has been shown to be a useful tool to profile hydrogen isotopes in various samples not depending on their state (gas, liquid, solid), size, form and surface quality. Peculiarities of the technique, such
r
PO
45
67
Fig. 3. Energy
0
spectrum
*
10
ii
Ia
is
*4
EnoroY,mo”
from a titanium-tritium
target.
L
Fig. 5. Hydrogen depth profiles in carbon fabrics impregnated by an organic compound. Numbers l-5 reflect the process of material carbonization.
B.G. Skorodumou et al. /Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 803-807
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allow one to obtain perfectly new direct information, not available before on hydrogen isotopes behavior in matter. The potential of NERD technique for scientific applications is far from being exhausted. An optimal choice of neutron energy and geometry of experimental set-up as well as of the thicknesses and types of the detectors (gas, silicon, position sensitive) provide good resources in: (i) lowering of the detection limits for all hydrogen isotopes: (ii) increasing of the probing depth and depth resolution improvement: (iii) three-dimensional depth profiling. The analytical characteristics of the technique will be essentially improved using a high intensity neutron source.
Acknowledgements Fig. 6. Hydrogen and deuterium depth profiles in process of sorption-desorption of isotope mixture in 500 km Pd membrane at ambient temperature using a gas diffusion cell.
The authors are indebted to A. Sevakin and Zh. Saidmuradov for ensuring good performance of the neutron generator NG-150, and to Ja.S. Abdullaeva, S.P. Kiseleva and V.Sh. Yalyshev for technical assistance and to J.F. Ziegler for kind assignment of the TRIM 91 program..
as large analyzable depth and simultaneous depth profiling of all hydrogen isotopes together with the possibility to use special cells (gas or electrolytic) as targets References
1 Fig. 7. Deuterium (1) and hydrogen (2) depth profiles obtained in the stationary process of D,O plus 15% Hz0 mixture in a cell with a 100 pm Pd membrane cathode. Right-hand slopes of the depth profiles reflect the difference in absolute depth resolution for water and for palladium. The maximum analyzable depths are = 350 p,rn and = 150 pm, respectively.
[l] P.K. Khabibullaev and B.G. Skorodumov, Determination of Hydrogen in Material. Nuclear Physics Methods, vol. 117 (Springer, 1989). [2] J. L’Ecuyer, C. Brassard, C. Cardinal, J. Chabbal, L. Deschence, J.P. Labric, B. Terreault, J.G. Martel and R.St. Jacques, J. Appl. Phys. 47 (1976) 881. [3] I.P. Chernov, J.P. Matusevich and V.P. Cosyr, Atom. Ener. 41 (1976) 51. [4] L.S. Wielunski, R.E. Benenson and W.A. Lanford, Nucl. Instr. and Meth. 218 (1983) 120. [5] J. Tirira, P. Trocellier and J.P. Frontier, Nucl. Instr. and Meth. B 50 (1990) 135. [6] B.G. Skorodumov and 1.0. Yatsevich, Nucl Instr. and Meth. B 64 (1992) 338. [7] B.L. Doyle and D.K. Price, Nucl. Instr. and Meth. B 36 (1988) 301. [8] G.G. Ross and I. Richard, Nucl. Instr. and Meth. B 64 (1992) 603. [9] J.F. Ziegler, TRIM-91 version. [lo] B.G. Skorodumov, 1.0. Yatsevich and O.A. Zhukovsky, these Proceedings (IBA-11, Balatonfilred, Hungary, 19931, Nucl. Instr. and Meth. B 85 (1994) 301.
XIII. DEVELOPMENTS