Resonant Nuclear Reaction Analysis Research Articles

An experimental study of hydrogen implantation to minerals: Role of the solar wind as a source of water in terrestrial bodies

The solar wind is a possible source for hydrogen and other volatiles on planetary bodies. To better understand the role of the solar wind during the volatile acquisition of planetary materials, we conducted hydrogen implantation experiments on olivine, orthopyroxene, quartz single crystals. Depth profiles of hydrogen concentration after implantation are determined by the Nuclear Resonance Reaction Analysis. We find that energetic hydrogen particles penetrate into the sample and accumulate at a certain depth. The hydrogen concentration increases with the hydrogen fluence until a “saturation level” is attained. Hydrogen saturation level (e.g., ∼10–20 at.% in olivine, equivalent to 0.5–1.2 wt%) far exceeds the equilibrium solubility in the bulk crystal at a similar thermodynamic condition (∼10−22 wt%). The results of olivine show that the hydrogen penetration depth increases whereas the saturation level decreases (weakly) with the beam energy. Hydrogen saturation level also depends on the mineral species in the order: olivine > orthopyroxene > quartz. The experimental results can be applied to explain some observations on the high surface water content of some planetary bodies including Itokawa asteroid and the Moon. We also explore the possibility of hydrogenated dusts by the solar wind implantation as a source for water on terrestrial planets. We conclude that if all dusts were exposed to the solar wind and all implanted hydrogen were converted to water, then >10 ocean masses would have been acquired for Earth by ∼100 years. However, the main part of the proto-planetary disk was not exposed to the solar wind and dusts could have been hydrogenated only when they were far from the equatorial plane of the disk. We discuss a possible mechanism to transport the hydrogenated dusts to the disk interior via turbulent mixing. Also, our experimental results and the mass dependence of the particle energies in the solar wind suggest that the D/H ratio of the dusts exposed to the solar wind will be higher than the solar wind value.

An experimental study of proton implantation in olivine

Implantation of ions in minerals by high energy radiation is an important process in planetary and materials sciences. For example, the solar wind is a multi-ion flux that progressively modifies the composition and structure of near-surface domains in solar objects, like asteroids. A bombardment of a target by different elements like hydrogen (H) at various energies causes, among other things, the implantation of these particles in crystalline and amorphous materials. It is important to understand the mechanisms and features of this process (e.g., how much is implanted and retained), to constrain its contribution to the chemical budget of solar objects or for planning various material-science applications. Yet, there has been no detailed study on H implantation into olivine (e.g., the quantification of maximum retainable H), a major mineral in this context. We performed experiments on H implantation in San Carlos olivine at 10 and 20 keV with increasing fluences (up to 3×1018 at/cm2). Nanoscale H profiles that result from implantation were analyzed using Nuclear Resonance Reaction Analysis after each implantation to observe the evolution of the H distribution as a function of fluence. We observed that after a systematic growth of the characteristic, approximately Gaussian shaped, H profiles with increasing fluences, a maximum concentration at H ~ 20 at% is attained. The maximum concentration is approximately independent of ion energy, but the maximum penetration depth is a function of beam energy and is greater at higher energies. The shapes of the profiles as well as the maximum concentrations deviate from those predicted by currently available models and point to the need for direct experimental measurements. We compared the depth profiles with predictions by SRIM. Based on observations from this study, we were able to constrain the maximum retainable H in olivine as a function of ion energy.

Hydrogen Diffusion in Clinopyroxene at Low Temperatures (195°C–400°C) and Consequences for Subsurface Processes

AbstractStudying diffusion of hydrogen in nominally anhydrous minerals, like clinopyroxene, at low temperatures is a challenging task due to experimental and analytical difficulties. In this study, to overcome these problems we have produced H concentration gradients in single crystals of natural diopsidic clinopyroxene by ion implantation and measured the nanoscale profiles before and after diffusion anneals using Nuclear Resonance Reaction Analysis. These steps allowed us to conduct experiments at temperatures between 195°C and 400°C. Obtained diffusion rates show a consistent Arrhenius relation DH= 5.47(±13.98) · 10−8 · exp (−115.64(±11.5) kJ mol−1/RT) m2s−1. Notably, our results lie well within the range of extrapolations from high temperature experiments (≥600°C) of previous studies. This implies that fast diffusion of hydrogen (compared to other elements) extends to low temperatures. We used these results in a non‐isothermal diffusion model that simulates the ascent of crystals (0.5, 1.0, and 2.0 mm) along two representative P‐T‐paths from 600°C to 100°C, to assess potential re‐equilibration of H contents in clinopyroxene at low temperatures. Our model highlights the need to carefully consider boundary conditions, which are a function of P‐T‐fO2, that control the concentration gradient at the crystal's rim. The results from this model help to assess, as a function of crystal size and cooling rate, when re‐equilibration must be considered.

Novel approach to study diffusion of hydrogen bearing species in silicate glasses at low temperatures

Diffusion of hydrogen bearing species in glasses plays a significant role in numerous applications in commercial as well as scientific domains. The investigation of diffusion of water in glasses at low temperatures led to experimental and analytical difficulties in the past. We present a new approach that lets us overcome these complications. Diffusion couples of An50Di50 glass (mol %, NBO/T = 0.67) were produced by coating anhydrous glass substrates with thin films of hydrated glass (~200 nm, ~2 wt% H2O) using pulsed laser deposition (PLD). Bonding the diffusant to the glass matrix of the thin film instead of using free water at the interface during experiments precludes other glass altering processes such as dissolution and precipitation. This allows us to confidently interpret the measured profiles to be a result of diffusion only. Nanoscale concentration profiles that result from diffusion at low temperatures on experimentally feasible time scales were measured with the Nuclear Resonance Reaction Analysis (NRRA, 1H(15N,αγ)12C). The non-destructive nature of NRRA enables us to observe and better understand the evolution of diffusion profiles with time within one sample. Evaluation of the sample quality by EPMA, SEM, optical microscopy, Rutherford backscattering spectroscopy (RBS), and NRRA was performed and confirmed the suitability of the samples for diffusion studies. Experiments at 1 atm in a box furnace and at 2 kbar in a CSPV (pressure medium = water) and an IHPV (pressure medium = Argon) prove that the diffusion couples can be used under various experimental conditions. We present diffusion profiles that were measured in experiments carried out in these devices and discuss the distinct features of each that result from different boundary conditions in the experiments.

Hydrogen induced lattice expansion and site occupation analyzed by ion beam methods

We study the absorption of H in ultrathin crystalline Fe/V stacks using elastic recoil detection (ERD) and resonant nuclear reaction analysis (NRA) combined with Rutherford backscattering spectrometry (RBS). Probing with the resonant 1H (15N,αγ) 12C nuclear reaction allows us to profile the hydrogen concentration with high depth resolution, while ERD permits us to quantify the integral hydrogen inventory while minimizing possible ion-beam induced hydrogen desorption. We perform angular scans in order to directly identify the interstitial site occupancy. A detailed analysis of channeling patterns permits us the deduction of crystal strain in as-deposited films as well as H-loaded targets. We observe tetragonal strain formation resulting in c/a = 1.06 and find qualitative indications for tetrahedral site occupation.

Control of germanium diffusion using low quantities of co-implanted silicon isotopes

The thermal diffusion of Ge implanted into SiO2 films growth on a Si substrate has been studied by nuclear analyses and μ-Raman spectroscopy with and without the presence of co-implanted 30Si and 29Si barriers, each located from both sides of the Ge implanted distribution. Combination of Rutherford backscattering spectroscopy and Resonant nuclear reaction analysis shows that, under thermal activation at 1100°C, implanted Ge diffuses differently toward the sample surface and the SiO2/Si interface due to the occurrence of Ge outgassing effects, as well as the non-homogenous distributions of the implanted ion species and the defects they have generated inside SiO2. A maximum local atom concentration of co-implanted silicon as low as ∼1.6 at. % is found to completely block the germanium diffusion in both directions, leading to the formation of Ge nanocrystals and Si/Ge aggregates evidenced by μ-Raman spectroscopy. In addition to highlighting the role of Si excess on the Ge trapping mechanism, such a result makes the nominal silicon oxide stoichiometry and composition two crucial parameters to stabilize Ge during high temperature annealing, which explains the strong discrepancies reported for the Ge thermal diffusion coefficient in the literature.

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

Hydrogen site location in ultrathin vanadium layers by N-15 nuclear reaction analysis

We present a method using resonant nuclear reaction analysis combined with optical transmission and heavy-ion Rutherford backscattering spectrometry to study the absorption of hydrogen in single crystalline thin vanadium films. Probing with the resonant 1H(15N,αγ)12C reaction allows for highly resolved hydrogen depth profiling, while measurements along the crystal axes render possible the direct identification of the interstitial site occupancy. First experiments were performed on thin vanadium hydrides in Fe(Cr)/V superlattices revealing differences in site occupancy.

Pure hydrogen references synthesized by low energy ion implantation into amorphous silicon

We developed a hydrogen standard by ion implantation in amorphous silicon, produced by implantation of 170 keV 28Si with 2·1015 at./cm2 in a silicon wafer (1 0 0) to obtain an amorphous depth of a few hundred of nanometers. The tilt and twist angles of the target were fixed to avoid channeling and the sample was cooled during the implantation. The amorphous silicon was then irradiated successively with hydrogen at 3.0 and 1.5 keV with doses 1.0 and 0.7·1017 at./cm2 respectively. The characteristics of the standards (retained dose, depth profile and stability under irradiation) were investigated by elastic recoil detection analysis (ERDA) and resonant nuclear reaction analysis (RNRA) with a 1H(15N,αγ)12C reaction at 6.385 MeV. These properties were compared with another previously developed standard (hydrogen implantation in monocrystalline silicon (1 0 0)). Results showed that, for the same implanted dose, the retained dose is about 55% larger in amorphous silicon. Furthermore, we showed these new standards are stable under beam irradiation for low current (10 nA/mm2) and could be used as IBA reference. Stability for higher currents (100 nA/mm2) showed a 10% release after an integrated charge about 2 mC. We showed that this release which occurs at the beginning of irradiation is due to desorption of the H-contamination layer on the surface. Implanted hydrogen remains stable under irradiation. We conclude that those standards are suitable target to study nuclear reactions of astrophysical interest, namely the 1H(13C,γ)14N reaction.

The influence of gas flow rate on the structural, mechanical, optical and wettability of diamond-like carbon thin films

In this research, diamond-like carbon (DLC) thin films were deposited on silicon substrates by radio-frequency plasma enhanced chemical vapor deposition method using gas mixture of CH4 and Ar. The effect of different CH4/Ar gas ratio on the structure, refractive index, transmission and hardness of the DLC thin films were investigated by means of Raman spectroscopy, ellipsometry, Fourier transform Infrared Spectroscopy and nano-indentation methods, respectively. Nuclear resonant reaction analysis was used to measure the amount of hydrogen and carbon in the thin films. Furthermore, wettability of the thin films was achieved by measuring of water contact angle (WCA). The results indicated that the structural properties of the diamond-like carbon thin films are strongly dependent on the composition of gas mixture. Based on ellipsometry results, refractive index of the thin films varied in the range of 1.89–2.06 at 550 nm. FTIR results determined that deposition of DLC thin films on silicon substrate led to an increase of the light transmission in IR region and these films have the potential to be used in silicon optics as the antireflective coatings in this region. Nano-indentation analysis showed that the thin films hardness changed in the range of 7.5–11 GPa. On the other hand hydrogen content and fraction of C‒H bonds in the samples increased by an increase in the gas ratio of CH4/Ar. Also, WCA measurements indicated that WCA for thin films with gas ratio of 3/7 is the most and equal to 79°.

Studies on synthesis of plasma fusion relevant tungsten dust particles and measurement of their hydrogen absorption properties

In this communication, we identify some thermal-plasma assisted synthesis techniques for the alternative production of fusion relevant tungsten dust particles in bulk amounts, to allow for the statistically meaningful study of their hydrogen absorption properties. Fast, single step synthesis of fine particles in the α-tungsten phase was demonstrated, using a high heat flux device for sizes in the micrometer region and an experimental nanoparticle reactor for dust with nanometer sizes. Production of both equilibrium polyhedral and nonequilibrium spherulitic crystal shapes was demonstrated, dust morphologies similar to which were identified before in existing tokamaks and divertor simulator systems. Their hydrogen retention characteristics were measured by the Nuclear Resonance Reaction Analysis technique, which was found to be orders of magnitude higher compared to the same metal in the bulk form. The mesoporous crystals generated during high power, high pressure synthesis conditions were assumed to be responsible for the unusually high gas retention of the nanoparticle sample. It was concluded that non-equilibrium tungsten dust morphologies would be dominant in the high power fusion machines in the future, which may aggravate the hydrogen isotope retention issues further.

Replacing NRA by fast GD-OES measurements as input to a model based prediction of hydrogen diffusion in a-Si

In order to predict hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) layers, Gerke et al.[1] proposed a model which requires just one nuclear resonant reaction analysis (NRA) and one Fourier transform infrared spectroscopy (FT-IR) as input. This contribution presents the possibility to substitute this single NRA measurement by a calibrated glow discharge optical emission spectroscopy (GD-OES) measurement. In the hydrogen concentration range of 2-30 at% relevant for a-Si:H layers there are currently no commercial calibration standards available, leading to the necessity of laboratory standards. Hydrogen depth profiles of a-Si:H layers exhibit plateaus of constant hydrogen concentration in a range large enough to qualify them as calibration standards. Therefore, six plasma enhanced physical vapor deposited (PECVD) a-Si:H layers with different hydrogen concentrations were prepared as laboratory standards. The absolute hydrogen concentration was determined using the NRA technique and the layer thickness was determined with a focused ion beam (FIB) in a scanning electron microscope (SEM). These results enabled a calibration of the GD-OES setup for a-Si:H, which successfully reproduced the NRA measurements of Gerke et al. [1].

Assessment of nitrogen incorporation in dilute GaAsN films using isotopically enriched molecular beam epitaxy and resonant nuclear reaction analysis

This paper describes the development of a new materials analysis technique enabled by modifying standard crystal growth methods for dilute nitride III–V semiconductors. The key to this method is the use of nitrogen gas enriched with the 15N isotope during material growth via molecular beam epitaxy, which allows enhanced detection of nitrogen using resonant nuclear reaction analysis (RNRA). Films of GaAs15N have been synthesized and examined using x-ray diffraction (XRD) and secondary ion mass spectroscopy. Accelerator-based ion beam analysis techniques, both RNRA and Rutherford backscattering spectrometry (including ion beam channeling configurations), were used to examine the films to obtain depth profiles of nitrogen, to assess their crystallinity, and to determine whether nitrogen is incorporated into substitutional or interstitial lattice sites. This method avoids possible artifacts from surface contamination by N and has confirmed that for the growth parameters investigated, the incorporation of 15N into the material corresponds with the expected enrichment of the feed gas, that the films have excellent crystallinity and uniform composition throughout their thickness, and that the nitrogen in GaAsN is predominantly incorporated substitutionally, in agreement with the lattice distortion observed by XRD.

Collision cascades enhanced hydrogen redistribution in cobalt implanted hydrogenated diamond-like carbon films

Hydrogenated diamond-like carbon films produced by C3H6 deposition at 5kV and implanted at room temperature with 30keV Co atoms to 12at.% show not only a bimodal distribution of Co atoms but also a massive redistribution of hydrogen in the films. Resonant nuclear reaction analysis was used to measure the hydrogen depth profiles (15N-method). Depletion of hydrogen near the surface was measured to be as low as 7at.% followed by hydrogen accumulation from 27 to 35at.%. A model is proposed considering the thermal energy deposited by collision cascade for thermal insulators. In this model, sufficient energy is provided for dissociated hydrogen to diffuse out of the sample from the surface and diffuse into the sample towards the interface which is however limited by the range of the incoming Co ions. At a hydrogen concentration of ∼35at.%, the concentration gradient of the mobile unbounded hydrogen atoms is neutralised effectively stopping diffusion towards the interface. The results point towards new routes of controlling the composition and distribution of elements at the nanoscale within a base matrix without using any heat treatment methods. Exploring these opportunities can lead to a new horizon of materials and device engineering needed for enabling advanced technologies and applications.

Model based prediction of the trap limited diffusion of hydrogen in post‐hydrogenated amorphous silicon

The diffusion of hydrogen within an hydrogenated amorphous silicon (a‐Si:H) layer is based on a trap limited process. Therefore, the diffusion becomes a self‐limiting process with a decreasing diffusion velocity for increasing hydrogen content. In consequence, there is a strong demand for accurate experimental determination of the hydrogen distribution. Nuclear resonant reaction analysis (NRRA) offers the possibility of a non‐destructive measurement of the hydrogen distribution in condensed matter like a‐Si:H thin films. However, the availability of a particle accelerator for NRR‐analysis is limited and the related costs are high. In comparison, Fourier transform infrared spectroscopy (FTIR) is also a common method to determine the total hydrogen content of an a‐Si:H layer. FTIR spectrometers are practical table‐top units but lack spatial resolution. In this study, an approach is discussed that greatly reduces the need for complex and expensive NRR‐analysis. A model based prediction of hydrogen depth profiles based on a single NRRA measurement and further FTIR measurements enables to investigate the trap limited hydrogen diffusion within a‐Si:H. The model is validated by hydrogen diffusion experiments during the post‐hydrogenation of hydrogen‐free sputtered a‐Si. The model based prediction of hydrogen depth profiles in a‐Si:H allows more precise design of experiments, prevents misinterpretations, avoids unnecessary NRRA measurements and thus saves time and expense. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

About Nuclear Resonant Reaction Analysis for Hydrogen Investigations in Amorphous Silicon

Nuclear Resonant Reaction Analysis (NRRA) is a common method detecting the nearsurface hydrogen distribution of a sample in a depth of up to a few microns. The mass density and related stopping power of a hydrogenated amorphous silicon (a-Si:H) layer depends on the hydrogen content. Correct hydrogen depth profiles are calculated considering the effective ion beam stopping crosssection as well as the related stopping power of the investigated film. The consideration of the local hydrogen concentration is important to avoid misinterpretations of the hydrogen distribution regarding profile depth. Therefore, stopping powers have to be considered carefully when interpreting hydrogen depth profiles especially of films that exhibit variations in hydrogen concentration. Moreover, further investigations like morphology dependent changes of the effective stoppingcross section are not possible without correctly calculated hydrogen depth profiles. Here the correct way is presented how to consider the embedded hydrogen of an a-Si:H layer when calculating absolute depth information of a NRRA measured hydrogen depth profile.

Influence of post-hydrogenation upon electrical, optical and structural properties of hydrogen-less sputter-deposited amorphous silicon

Amorphous silicon (a-Si) is common in the production of technical devices and can be deposited by several techniques. In this study intrinsic and doped, hydrogen-less amorphous silicon films are RF magnetron sputter deposited and post-hydrogenated in a remote hydrogen plasma reactor at a temperature of 370°C. Secondary ion mass spectrometry of a boron doped (p) a-Si layer shows that the concentration of dopants in the sputtered layer becomes the same as present in the sputter-target. Improved surface passivation of phosphorous doped 5Ωcm, FZ, (n) c-Si can be achieved by post-hydrogenation yielding a minority carrier lifetime of ~360μs finding an optimum for ~40nm thin films, deposited at 325°C. This relatively low minority carrier lifetime indicates high disorder of the hydrogen-less sputter deposited amorphous network. Post-hydrogenation leads to a decrease of the number of localized states within the band gap. Optical band gaps (Taucs gab as well as E04) can be determined to ~1.88eV after post-hydrogenation. High resolution transmission electron microscopy and optical Raman investigations show that the sputtered layers are amorphous and stay like this during post-hydrogenation. As a consequence of the missing hydrogen during deposition, sputtered a-Si forms a rough surface compared to CVD a-Si. Atomic force microscopy points out that the roughness decreases by up to 25% during post-hydrogenation. Nuclear resonant reaction analysis permits the investigation of hydrogen depth profiles and allows determining the diffusion coefficients of several post-hydrogenated samples from of a model developed within this work. A dependency of diffusion coefficients on the duration of post-hydrogenation indicates trapping diffusion as the main diffusion mechanism. Additional Fourier transform infrared spectroscopy measurements show that hardly any interstitial hydrogen exists in the post-hydrogenated a-Si layers. The results of this study open the way for further hydrogen diffusion experiments which require an initially unhydrogenated drain layer.

Morphology and Hydrogen in Passivating Amorphous Silicon Layers

Hydrogenated intrinsic amorphous silicon ((i)°a-Si:H) can be grown by plasma-enhanced chemical vapor deposition with a non-columnar or columnar morphology. Nuclear resonant reaction analysis and corresponding effective stopping cross section analysis indicate a dependency of hydrogen effusion on the morphology of the (i)°a-Si:H layer as well as the doping type and concentration of the c-Si wafer. The doping type of the c-Si wafer also affects the growth of the amorphous network. It is found that for moderately doped p-type c-Si a non-columnar (i)°a-Si:H layer yields a significantly better and more stable passivation already during thermal anneal and illumination, while for passivating n-type c-Si a columnar layer is recommended. Passivating lowly doped c-Si by (i)°a-Si:H is not dependent on morphology. Combining different (i)°a-Si:H morphologies to a multi-layer stack improves the quality of surface passivation. Hydrogen embedded in a well passivating but hydrogen-permeable columnar layer supports good surface passivation when covered by a non-columnar layer, featuring a fast growing layer acting as a hydrogen barrier and enhancing surface passivation quality.

Depth profiling Cr in surface layers by 52Cr(p,γ)53Mn nuclear resonance reaction analysis

A method for depth profiling chromium in the surface and near surface regions of materials using the resonance at 1,005 keV in 52Cr(p,γ)53Mn nuclear reaction is presented. The detection sensitivity, depth resolution and probing depth of the resonance in Si are determined to be about 3 at.%, 25 nm and 2.5 µm respectively from the excitation function of the reaction constructed in 0.90–1.2 MeV proton energy region by measuring 378 keV prompt γ-rays from 53Mn nuclei. The reaction is interference free. These features make the approach attractive for profiling chromium in mid as well as high Z matrices.

Hydrogen accumulation in nanostructured as compared to the coarse-grained tungsten

We report on the influence of sample microstructure and of irradiation conditions on the H behaviour in Tungsten (W). For this purpose, commercial coarse grained (CGW) and nanostructured W (NW) samples were implanted with (i) H at room temperature (RT), (ii) sequentially with C and H at RT, and (iii) simultaneously (co-implanted) with C and H at RT. To study the possible effect of implantation temperature on H behaviour, a CGW sample and a NW sample were sequentially implanted with C at RT and with H at 673K. The H and C implantation fluence was 5×1020m−2 and the implantation energies were 160keV for H and 650keV for C which are above the displacement damage threshold. Scanning electron microscopy images show that nanostructured samples consist of columns with an average diameter of about 100nm. These nanocolumns are stable under the studied implantations conditions. Moreover, surface modification is absent in all studied samples. X-ray diffraction data illustrate that all samples are mono-phase (α-W phase) and that none of the implantations led to the appearance of secondary phases. Resonant nuclear reaction analysis data show that the H retention in NW samples is larger than in CGW and that synergistic effect has a significant influence on the H retention in CGW samples but not in NW samples.