Depth profiles of D and T in Metal-hydride films up to large depth

Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

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Depth profiles of D and T in Metal-hydride films up to large depth HongLiang Zhang a,b, Wei Ding b, Ranran Su a, Yang Zhang a, Liqun Shi a,⇑ a b

Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 20043, People’s Republic of China Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang 621000, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 30 June 2015 Received in revised form 17 November 2015 Accepted 17 November 2015 Available online xxxx Keywords: Depth profiling Nuclear reaction Proton backscattering Deuterium and tritium

a b s t r a c t In this paper, a method combining D(3He, p) 4He nuclear reaction and proton backscattering (PBS) was adopted to detect the depth profile of both D and T in TiDxTy/Mo film with thickness more than 5 lm. Different energies of 3He and proton beam, varied from 1.0 to 3.0 MeV and 1.5 to 3.8 MeV respectively, were used in order to achieve better depth resolution. With carefully varying incident energies, an optimum resolution of less than 0.5 lm for D and T distribution throughout the whole analyzed range could be achieved. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Metal hydrides are used in an increasingly broad range of applications and disciplines. A significant application is in the neutron generating devices in which the hydrides of transition or rare earth metals involving D and T are used as target materials bombarded by D ions. In this application, the exact determination of D and T concentration in the Ti films on the Mo substrates with thickness up to several lm’s is of great importance. Moreover, the plasmafacing materials such as molybdenum and tungsten in nuclear fusion research resulted in an increasing interest in quantitative depth profiling of deuterium up to very large depths [1–3]. A few methods have been developed to determine the depth profile of D or T in metal-hydride materials. One of the analysis techniques is combining elastic recoil detection (ERD) with a particles’ identifying system of DE E telescope detector which provides possibility for simultaneous profiling of all hydrogen isotopes in the thick hydride films [4]. However, very highenergy heavy ions and extremely high standard DE E identifying system for very wide-energy range of recoil particles are needed in order to achieve large analysis depth up to 5 lm, which is the subject to many analysis. Another technique is secondary ion mass spectroscopy (SIMS) which could achieve a hydrogen isotopes depth profile of up to about 100 lm. But it is time consuming and can’t provide quantitative results by itself [5,6]. It has to be

combined with the ion beam analysis results for surface layer which needs a really thick surface layer to achieve good accuracy. Proton backscattering technique has large detection depth up to 5 lm and good sensitivity for tritium detection when the incident energy is chosen higher than 2.7 MeV and in addition it is easier to perform. However in the case of co-existing of D and T in the materials, detecting depth of T is reduced greatly due to overlapping between their energy spectra. The analyzing depth may increase significantly by increasing energy, but at the expense of great decreasing in depth resolution. The nuclear reaction D(3He, p) 4He(NRA) is often used to determine the depth profile and the total content of deuterium atoms in the near-surface layer of solids [7]. Compared to elastic recoil detection analysis, the NRA method allows to analyze a larger depth range. What is more, it does not require grazing angles of the incident and exit beams (grazing angles are problematic for large or curved samples, such as whole tiles from nuclear fusion experiments). Furthermore, it is less sensitive to surface roughness and plural scattering, and results in smaller sample damage compared to heavy-ion ERD [8]. In this paper, a method combining D (3He, p) 4He nuclear reaction analysis and proton backscattering (PBS) was adopted in order to achieve quantification of D and T in Ti(D, T)/Mo film with thickness up to 5 lm. The PBS is used to measure the T concentration over large depth in principle, and depth distribution of Ti and Mo over whole film as well, and D (3He, p) 4He is used to determine D concentration.

⇑ Corresponding author. Tel.: +86 21 65642292. E-mail address: [email protected] (L. Shi). http://dx.doi.org/10.1016/j.nimb.2015.11.030 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: H. Zhang et al., Depth profiles of D and T in Metal-hydride films up to large depth, Nucl. Instr. Meth. B (2015), http://dx. doi.org/10.1016/j.nimb.2015.11.030

H. Zhang et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

2. Experiment The measurements were carried out in a high vacuum chamber with pressure lower than 1  10 4 Pa. In order to achieve good resolution and large detecting depth for T in the film, the incident proton beams with varied energy range of 1.5–3.8 MeV were adopted and provided by the NEC 9SDH-2 2  3 MV tandem accelerator at Fudan University. The scattered protons were detected at a laboratory angle of 165° by an Au/Si surface barrier detector with a depletion thickness of 300 lm and an area of 50 mm2. A slit of the dimensions 3  4 mm at 80 mm distance defined a detector solid angle of 9.3  10 4 sr. The D(3He, p) 4He nuclear reaction was used to quantitate D depth profiling. The protons emitted from the D(3He, p) 4He were detected using another Au/Si surface barrier detector with an area of 300 mm2 and a depletion depth of 1.5 mm, located at 135° with a solid angle of 0.0148 sr. The rectangular detector slit of 20  2 mm was used to decrease kinematic energy spread. All measurements were performed at normal incidence. The beam spot size is 1mm in diameter. The incident beam currents were about 20 nA and 35 nA for PBS and NRA measurements, respectively. Depth resolution calculation for PBS and D(3He, p) 4He measurements were performed using the program Depth [9]. SIMNRA [10] program is adopted to simulate energy spectra for both PBS and D(3He, p) 4He measurements in order to obtain T and D depth profiles in the film respectively. Metal hydride of TiDxTy was prepared by conventional hydrogenation method. Titanium films with thickness of about 5 lm were firstly deposited onto molybdenum substrates by electron beam evaporation technique at 600 °C and a vacuum of 10 5 Pa. And then the deposited titanium films were transported to a tritiding facility and loaded with high-purity tritium–deuterium mixture gas at pressure of 2000 Pa and temperature of 400 °C to produce a hydride with a certain of initial composition of TiDxTy.

3. Results and discussion Fig. 1 shows, for TiDxTy/Mo film, proton backscattering energy spectra in different incident proton energies. It can be seen from Fig. 1 that the PBS measurement at lower incident proton energies can be only used to determine T and D concentrations in a very limited depth range, and thus higher proton energies are required in order to achieve T depth profile throughout the whole Ti film. The detecting depths and their dependence on depth resolutions

for D, T and Ti elements in different proton energies, for the TiDxTy/Mo with the film density of 3.4 g/cm2 are shown in Fig.2. On the other hand, since there exists a wide diffusion region in the interface between the substrate Mo and Ti film which is formed during the preparation of hydride film at higher temperature, the PBS measuring at lower energy is required to obtain Ti and Mo element distributions with higher depth resolution in the mixing region. However, as shown in Fig. 2, there are large differences in the detected depth among different atoms of the samples at a given energy, which significantly decreases for lighter atoms. The detecting depths for Ti, T, D at 1500 keV, for example, are about 10, 2.4 and 0.6 lm, Therefore, it is very difficult to achieve high precision for the distribution of D and T in the whole film by single energy PBS measurement, and also D depth distribution can be only extended to limited range under limited proton energy. Thus, another reaction for determining D concentration over whole Ti film such as D(3He, p) 4He is implored. The D(3He, p) 4He reaction exhibits low depth resolution near the surface but a higher one deeper in the sample. This is shown in Fig. 3. Using varying incident particle energy [1], therefore, from the 1000 to 2500 keV, a depth resolution better than 0.6 lm throughout the whole depth can be achieved. In the case of low incident energy, a good depth resolution is obtained both at the surface and deeper at the sample, while at the energy above 2.0 MeV the better resolution is deeper in the sample where the cross-section is in a resonance region. Decreasing slit width of the detector also increases depth resolution but with the cost of long analyzing time and decreasing sensitivity. Fig. 4 shows a 600

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Fig. 2. Dependence of detecting depth on depth resolution for D, T and Ti elements in different incident proton energies. The film density for TiDxTy/Mo using in the DEPTH program is 3.4 g/cm2.

Please cite this article in press as: H. Zhang et al., Depth profiles of D and T in Metal-hydride films up to large depth, Nucl. Instr. Meth. B (2015), http://dx. doi.org/10.1016/j.nimb.2015.11.030

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H. Zhang et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

(2) According to obtained T, Ti, Mo, the exact D concentration is obtained from the D(3He, p) 4He reaction with responding depth resolution at low 3He energy of 1000 keV. And then, the obtained D concentration is taken to step (1) to recheck above PBS simulation. The cross-section data from reference [8] was used in the simulation for D(3He, p) 4He reaction. (3) After finishing the simulating on T profile responding to the T energy spectrum without overlapping D by repeating steps (1) and (2), the incident proton energy is increased further to enlarge T detecting depth by using varied energy step width with ideal depth resolution until D and T spectra can be completely separated. In this process, the Ti and Mo profiles are still determined using the PBS spectrum at 1500 keV by substituting new D and T concentration obtained in each energy step, in order to keep relative better depth resolution. Once the resolution of T at new proton energy is worse than Ti at 1500 keV, the step for simulating PBS spectrum at (1) will be replaced by using the depth resolution of Ti.

Fig. 3. Depth resolution for D in the Ti film by the D(3He, p) 4He reaction at different incident 3He energies.

When performing simulation of PBS spectrum of TiDxTy/Mo sample, the spectrum of Mo substrate at low channels could not be fitted well with present available simulation program owing to the multiple scattering effect. Therefore, the Mo background from the experimental spectrum at the low channel area was subtracted to obtain the element spectrum of D and T, and thus to facilitate computer simulation. To ensure the accuracy of the subtraction, this was solved by subtracting an experimental spectrum of a TiHx/Mo sample, measured under the same experimental conditions. Fig. 5 shows the measured backscattering spectrum in 2.0 MeV protons by using above treatment. The cross sections for D(p, p)D and T(p, p)T in the simulation come from references [11,12]. Thus, the simulation for energy spectra of all the elements in the film can be performed. Combining PBS and NRA spectra by the program SIMNRA, the obtained depth profiles of D and T in the Ti film is shown in Fig. 6. It should be noted that the depth resolutions of D and T in deep region near the interface, i.e., diffusion region, for NRA and PBS analysis are all poor as shown in Figs. 2 and 3. The depth step shown in the Fig. 6 in near-interface region is concordant with the simulation of 1500 keV PBS spectrum for Ti and Mo compositions, which is less than the depth resolution value of D and T in this region. Therefore, relatively large errors exist in simulated values

typical energy spectra of scattered 3He and reacted protons from the D(3He, p) 4He at incident energy of 2.0 MeV. The high-yield peak between channels 700 and 800 is caused by the protons produced from the D(3He, p) 4He reaction. The low-yield D peak around channel 450 is due to the nuclear reaction between 3He and T which is 3He + T ? 4He + D. The broaden peak in channels 150–350 is caused by the high energy protons from D(3He, p) 4 He reaction which can come through the Mo foil before the detector. The narrow peak in channel about 100 was not clear for now. Compared with the spectra of other deuterated samples, there always is a peak around this channel in each spectrum. Maybe it is a reaction product between 3He and purity in the film. However, the existence of this peak has no influence on our concerning problems. The strong peak in channels below 100 corresponds to backscattered 3He. The general steps for determining D and T in the sample by combining the PBS and NRA reactions are as follows: (1) PBS spectrum at low energy of 1500 keV is first used to determine T, Ti, Mo concentration distribution and a rough D concentration at given depth step being equal to T depth resolution (since D spectrum is overlapped with T spectrum of low energy part, simulated D spectrum is not exact).

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Please cite this article in press as: H. Zhang et al., Depth profiles of D and T in Metal-hydride films up to large depth, Nucl. Instr. Meth. B (2015), http://dx. doi.org/10.1016/j.nimb.2015.11.030

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H. Zhang et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

4. Summary

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The exact determination of concentration on D and T in metalhydride materials is an important issue. Single D(3He, p) 4He nuclear reaction or proton backscattering is hard to achieve D or T depth profile, or is confined to a certain of detecting depth for T measuring by PBS under limited energy. We combine D(3He, p) 4 He nuclear reaction and proton backscattering to detect the depth profile of both D and T in TiDxTy/Mo film with thickness up to large depth. Varying incident beam energies of 3He or proton can improve greatly depth resolution of D or T in the film, especially for D measurements by D(3He, p) 4He reaction. The depth resolution, which becomes worse with increasing the detecting depth, can be better than 0.5 lm for D and T distribution throughout the whole analyzed range.

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Fig. 5. The backscattering spectrum of TiDxTy/Mo using 2.0 MeV proton at 165°. The solid curve is simulation results. The D and T element Spectra are obtained by subtracting the Mo background yields.

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The authors thank all the staffs of the ion beam laboratory in Fudan University for their help. Our work was supported by the National Nature Science Foundation of China under Grant No. 91126019 and 11375046.

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of D and T over the diffusion region. It can be found from the Fig. 6 that the concentrations of D and T keep almost constant before 50,000 atoms/cm2 and then decrease greatly after that. This is very likely due to the diffusion of substrate Mo atoms into the Ti film which causes the decrease of H isotopes concentration.

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Please cite this article in press as: H. Zhang et al., Depth profiles of D and T in Metal-hydride films up to large depth, Nucl. Instr. Meth. B (2015), http://dx. doi.org/10.1016/j.nimb.2015.11.030