Helium Desorption Research Articles

Crystal orientation dependence of surface modification in molybdenum mirror irradiated with helium ions

The crystal orientation dependence on a surface damage in molybdenum mirror has been investigated under the irradiation with 3keV helium ions to a fluence of 1022ions/m2. A strong correlation between the surface roughening and the grain orientation was confirmed from SEM–EBSD analysis. It was found that significant surface roughening including crater-like depressions is formed on the grain surfaces with crystalline planes close to (100), while relatively smooth surface remains on grains with the other directions. In contrast, the reflectivity measurement of the single crystal molybdenum with a spectroscopic ellipsometry showed smaller reduction in (100) sample than in (110) and (111) samples. The smallest helium desorption in (100) sample was also detected with thermal desorption spectroscopy in comparison with them in (110) and (111) samples, which indicates (110) and (111) samples include the large amount of defects acting as effective trapping site for He atoms. A cross-sectional TEM observation actually showed a relatively large bubbles at sub-surface regions of (110) sample. These results indicate that the optical reflectivity should be affected by not only a top surface profile but also internal radiation damages such as high density of helium bubbles.

Thermal desorption of tritium and helium in aged titanium tritide films

Titanium films were deposited onto molybdenum substrates and then converted into titanium tritides (TiT1.5–1.8) films inside a tritiding apparatus loaded with pure tritium gas. Evolution of tritium and helium in the titanium tritide films over a period of four years was investigated using a thermal desorption technique, together with X-ray diffraction analysis. Results showed that desorption profiles of the tritium varied significantly with the evolution of He contents. Apart from the primary peak from tritium desorption located at a temperature between 610 and 840 K, another higher temperature tritium desorption peak (at ∼950 K) was observed, attributed to damages in the lattice structures induced by generation of 3He bubbles. Release of helium in the tritide film became significant after a long term aging process (i.e., after a few years). Depending on the amount of the 3He bubbles generated due to the decay of tritium, spectra of the thermal helium desorption showed five peaks in the range from room temperature to ∼1750 K, corresponding to different states of helium evolution during aging of the titanium tritide films. The amounts of helium desorption in different stages were estimated, and the dissociation energy of helium from different trap states as a function of the aging duration was obtained.

Effect of combined doping of Fe and C on the helium behavior in Al before and after aging

Effect of combined doping of Fe and C on the implanted-helium (10keV, 4.0×1021ions/m2) behavior in Al before and after aging for 1year at room temperature has been investigated by scanning electron microscope, transmission electron microscope and thermal helium desorption spectrometry (THDS). The combined doping of Fe and C has a pronounced influence on the surface blistering induced by He ion implantation, and the extent is dependent on the fluence of Fe and C doped. The lower fluence of Fe and higher fluence of C doped, the more resistive to the surface blistering in Al will be. Pipe-shaped structures of high density are produced in the matrix of He implanted Al after the combined doping of Fe and C and then aging at room temperature for 1year. The diameter of pipes increases with Fe fluence. The THDS results show that the thermal helium desorption shifts to the lower temperature range for the Fe, C and He implanted and aged Al samples. With the increase of C fluence, the thermal helium desorption in the lower temperature range will get more prominent. Therefore, aging has a considerable influence on the helium behaviors such as surface blistering, microstructure evolution and thermal desorption in Al, which can deteriorate the irradiation damages induced by He.

Tunneling effects in the kinetics of helium and hydrogen isotopes desorption from single-walled carbon nanotube bundles

The kinetics of desorption both helium isotopes and molecules of hydrogen and deuterium from open-ended or γ-irradiated single-walled carbon nanotube bundles was investigated in temperature range of 10–300 K. The gases desorption rates obey the Arrhenius law at high temperatures, deviate from it with temperature reduction and become constant at low temperatures. These results indicate the quantum nature of gas outflow from carbon nanotube bundles. We had deduced the crossover temperature below which the quantum corrections to the effective activation energy of desorption become significant. This temperature follows linear dependence against the inverse mass of gas molecule and is consistent with theoretical prediction.

Positron and thermal desorption studies on He ion implanted nuclear graphite

The positron beam Doppler Broadening (DB) and Thermal Helium Desorption Spectroscopy (THDS) techniques are applied to study the behavior of radiation induced point defects in IG-110 nuclear graphite. The defects are introduced by irradiation at room temperature with 200keV He+ at doses ranging from 1015 to 1017 He/cm2. In the thermal desorption spectroscopy, the release of He+ is observed between 500K and 800K. With increasing He+ implantation dose, the fraction of He+ desorbed decreases from 27% to 3%. In the DB-curves showing the S parameter values versus positron implantation depth, the derived vacancy type defect distribution are in accordance with those obtained by SRIM calculations. Subsequent annealing of the implanted samples in steps of 100K for 5 minutes up to 1200K shows a distinct decrease of the S parameter value to its reference value between 500K and 700K. This temperature interval corresponds to the literature values for single vacancy migration energy of 1-2eV.

Influence of MeV helium implantation on deuterium retention in self-ion implanted tungsten

The influence of helium implanted with MeV energy on deuterium retention in tungsten damaged by self-ion implantation was investigated. Damage was introduced into the material using self-ion implantation. Two different cases—low (0.02 dpa (displacement per atom)) and high (0.89 dpa) W damage levels were investigated. Helium was introduced into the resulting damage zone using MeV ion implantation. Different He fluences were applied to investigate the He concentration dependence between 0 and 1000 ppm. D and He retention was investigated by ion beam analysis and thermal desorption spectroscopy. No helium desorption under thermal treatment up to 2000 K was observed. In both high and low damage level cases He does not affect the maximum D concentration within the damaged zone. For the 0.89 dpa and high He fluence samples a significant enhancement of D concentration behind the damaged zone was observed. In the lower damage case D transport within the damaged zone and further into the bulk was slowed down significantly even by lower He concentrations.

Spatially resolved stochastic cluster dynamics for radiation damage evolution in nanostructured metals

A spatially resolved stochastic cluster dynamics (SRSCD) model is introduced to describe radiation-induced defect evolution in metals. The stochastic nature of the method allows SRSCD to model more chemical species and more mobile defects than rate theory methods without loss of computational efficiency, while reaching larger timescales and simulating larger volumes than object-oriented kinetic Monte Carlo (OKMC) methods. To comprehend the capabilities of the method and access new understanding of defect evolution, SRSCD is used in three scenarios. In the first, the results of Frenkel pair implantation are found to match those of rate theory in both spatially homogeneous and spatially resolved media. Next, to study spatial resolution effects and correspondence to OKMC, the results of 20keV cascade implantation into copper is simulated and an acceptable match with OKMC is found. Finally the method is used to study the problem of helium desorption in thin iron foils. The model is compared with available experimental measures and is found to be in good agreement. The ability of SRSCD to include many mobile species of defects allows a detailed analysis of the mechanisms of helium release from the free surface of the iron foils. As a result new dominant mechanisms of helium release are discussed as well as their operating regimes.

Quantifying He-point defect interactions in Fe through coordinated experimental and modeling studies of He-ion implanted single-crystal Fe

Understanding the effects of helium on the microstructural evolution and mechanical properties of structural materials are among the most challenging issues in fusion materials research. In this work, we combine thermal helium desorption spectroscopy (THDS) with positron annihilation spectroscopy (PAS) and a spatially dependent cluster dynamics model to investigate the energetics of helium-point defect interactions in helium-implanted single-crystal iron. The combination of modeling and thermal desorption measurements allows identification of the binding energies of small He–V clusters, the migration energy of single vacancy and possible mechanisms (e.g., shrinkage of He3V2 clusters) responsible for measured Helium desorption peaks, and the effect of impurities (e.g., carbon) on these values. Furthermore, the model predicts the depth dependence of the helium and helium–vacancy clusters as a function of time and temperature during the thermal desorption measurement. Here, we report the THDS measurement results as a function of He implantation energy from 10 to 40keV at a fluence level of 1×1015He/cm2, along with selected PAS measurements. The experimental results are compared to the modeling predictions to evaluate the extent to which self-consistent values of the He-point defect binding and interaction energies and diffusivities can explain the data.

Radiation enhanced diffusion and redistribution of helium in LiNbO3

Effects of ionizing radiation on helium diffusion and desorption from LiNbO3 previously implanted with helium have been evaluated. The study has been performed for 45keV helium implanted samples for doses from 2×1015 up to 2×1017ions/cm2. The results show that the temperature for helium desorption decreases as the implantation dose increases. In order to study ionizing radiation effects, samples previously implanted with helium were irradiated with 1.8MeV electrons and the radiation-enhanced helium desorption was measured during irradiation at 500Gy/s. Ionizing-radiation-enhanced diffusion and redistribution of helium were observed to occur during electron irradiation.The helium-release behavior after 45keV implantation and after 1.5keV helium bombardment were compared.

Heavy-ion irradiation defect accumulation in ZrN characterized by TEM, GIXRD, nanoindentation, and helium desorption

A study on zirconium nitride was performed to assess the effect of radiation damage by heavy ions at cryogenic and elevated temperatures. Cross-sectional transmission electron microscopy, grazing incidence X-ray diffraction, nanoindentation, and helium desorption studies were used to assess the damage and its effects. Xenon and krypton were used as heavy ions at 300keV to displacement damage as high as 200dpa. Implants were cryogenic, 350°C, 580°C, and 800°C. Amorphization was not observed at low temperatures nor was bubble formation observed at elevated temperatures, however, defect migration was observed at elevated temperatures. Nanoindenter results showed the onset of defect saturation. Helium release studies were performed to show the effect of increasing damage by Xe to 40dpa.

Effect of grain microstructure on thermal helium desorption from pure iron

Experiments were carried out to evidence the role played by grain boundaries on He/Fe interactions. Three microstructures involving grain sizes from 2 to 100μm in average diameter were irradiated with 8-keV 4He+ ions at 298K and helium desorption was then studied during constant rate heating by thermal desorption spectroscopy (TDS) from room temperature to 1330K. The two larger microstructures (40 and 100μm) could be characterised with the same multiple desorption groups, combining helium involved in small and large clusters as well as in bubbles. The finer microstructural scale (2μm) was peculiar due to the existence of a broad split of desorption peak (at 856 and 917K). Such a peak is ascribed to the movement of grain boundaries during recrystallization which drains helium towards surface. The high density of grain boundaries in the finer microstructure also explains the thermal shift and the larger desorption at low temperatures. As a first step towards the quantitative assessment of this effect, cluster dynamics simulation have been performed in order to give the quantity of helium trapped in grain boundaries for the different microstructures.

New geochronometer for the direct isotopic dating of native platinum minerals (190Pt-4He method)

A new method of isotope geochronology was proposed for dating native platinum minerals on the basis of the α-decay of the natural isotope 190Pt. The analysis of the thermal desorption of helium in the crystal lattice of native metals, including platinum, allowed us to predict a very high thermal stability (retentivity) of radiogenic 4He in native platinum minerals up to their melting temperatures. In order to validate the proposed 190Pt-4He method, direct isotopic dating was performed for isoferroplatinum from the Galmoenan dunite-clinopyroxenite and Kondyor alkaline ultramafic massifs. The results of dating obtained by this method for primary ore platinum from the Galmoenan Massif (70 ± 5 Ma) are consistent with geological observations and mean Sm-Nd and Rb-Sr isotopic age estimates. The 190Pt-4He age obtained for placer isoferroplatinum from the Kondyor Massif (112 ± 7 Ma) also agrees with geological observations and is close to the K-Ar and Rb-Sr ages of koswites (phlogopite-magnetite pyroxenites, gabbros, nepheline syenites, and metasomatic rocks after dunites). Our experimental data demonstrated that the 190Pt-4He method is a promising tool for dating native platinum minerals.

Helium retention and early stages of helium-vacancy complexes formation in low energy helium-implanted tungsten

Tungsten has been selected as the material of the divertor of the ITER fusion reactor. In operation, tungsten will be submitted to high alpha particles bombardment. The consequence of helium implantation is a major issue for the reliability of tungsten components. The aim of the study was to investigate the behavior of helium implanted in tungsten at low energy and low flux. 320eV Helium ions were introduced by plasma immersion at the flux of 2.5×1018ion/m−2/s−1. The helium behavior was investigated by Nuclear Reaction Analysis and the evolution of the tungsten lattice by Positron Annihilation Spectroscopy (PAS). Helium-implanted tungsten exhibits a low retention rate (13.6% at 9.4×1019Hem−2) which decreases with the implantation fluence. The desorption of helium starts at low temperature (<400K). SEM analysis after annealing over 973K shows sparse pores probably due to bubbles opening at the surface. The creation of helium-filled defects in the near surface layer (0.5 to ∼20nm) was followed by PAS. A low level of damages was introduced by 12MeV proton irradiation, prior to He introduction and allowed to examine the influence of pre-existing defects on the helium trapping. The PAS results suggest that the early stage of the formation of helium-filled vacancy clusters does not require the presence of pre-existing vacancy and thus proceed from the trap mutation phenomenon.

First-principles calculations of helium and neon desorption from cavities in silicon

Combining density functional theory, the nudged elastic band technique, and the ultradense fluid model, we investigated the desorption process of He and Ne in silicon. Our results show that the internal surfaces of gas-filled bubbles are not a limiting factor during desorption experiments, since the surface reconstruction opens diffusion paths easier than in the bulk. We show that the vibrational contribution to the energy of helium in the bulk has to be considered in order to determine realistic pressures in the bubbles, when comparing experiments and simulations. At the maximum of desorption, an average pressure of 1–2 GPa is computed.

Thermal Annealing Behavior of Helium in Ti Films Deposited by Magnetron Sputtering

Helium contents of up to 30at.% are prepared in sputter-deposited Ti films. Isochronal annealing behaviors of helium including the depth profiles and the evolution of helium bubbles in the films at different temperatures are examined by ion beam analysis including Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA), as well as thermal helium desorption spectroscopy (THDS). It is found that the energy spreading induced by structural inhomogeneities in the spectra of RBS and ERDA as well as the increment in the width of spectra occurs, which corresponds to the change of stopping cross-section of helium atoms in the Ti film due to the change of physical-state of helium in the evolution of helium bubble. The ion beam analysis on the helium evolution is consistent with the THDS measurement. Ion beam technique opens interesting possibilities in the characterizing on the growth of helium bubbles.

Investigation on Thermal Release Behavior of Helium-Charged Copper Films by DC Magnetron Sputtering

Helium atoms are introduced into Cu films at room temperature by direct current (DC) magnetron sputtering in a He/Ar mixed atmosphere. The doped helium atoms are distributed evenly in the film and the content can be easily controlled by changing the process parameters. The structure of Cu films with trapped helium was investigated by X-ray diffraction (XRD) technology. With increasing helium irradiation flux, the lattice spacing and width of diffraction peaks increased due to helium effects, corresponding to the increase of finite and infinite size defects in the film. The shape of thermal desorption spectrum (TDS) and the number of peaks strongly depended on the amount of helium introduced into Cu. With increase of helium content, helium release temperature decreases. At the same amount of helium, the peak temperature became higher with increase of heating rate and from this we can obtain a picture which could calculate the activation energy of helium desorption.

Stability and dissolution of helium–vacancy complexes in vanadium solid

We report the energetics, stability, and diffusion behavior of helium (He), vacancies (V), and helium–vacancy complex clusters He n V m ( n, m = 0–4) in vanadium solid from first-principles calculations. For He, vacancy site is more energetically favorable than tetrahedral interstitial by ∼0.74 eV, while hydrogen always prefers to stay in tetrahedral sites in vanadium. He exhibits a low migration energy (0.06 eV) and can be easily trapped in vacancy. A nearly linear relationship between formation energy and the number of He or vacancy is obtained for He or vacancy clusters, and the weak binding energies of He clusters indicate that He clusters themselves are unstable. The binding energies and dissociation energies of He and vacancy to helium–vacancy complex clusters are computed and compared well with the experimental observation from helium desorption spectra. The cluster stability depends on He content. Finally, He diffusion coefficients are predicted to be (1.07–1.27) × 10 −8 m 2 s −1 at typical temperatures of 600–800 K. We thus propose that He aggregation via vacancy trapping should be the main mechanism for He bubble formation.

Thermal desorption of helium from reactor steel

Thermal desorption was used to study the behavioral particulars of the ion-implanted at 20 and 650°C helium in reactor ferrite-martensite EP-450, Eurofer-97, dispersion-hardened EP-450, Eurofer-97, and EK-164 austenitic steel. It is established that compared with the thermal desorption spectrum of EP-450 and Euroffer-97 steel the temperature interval for helium release from dispersion-hardened EP-450 and Eurofer 97 steels is wider for helium implantation at room temperature and especially at 650°C. In steel hardened by disperse oxides Y2O3, a substantial amount of helium is released at high temperatures after the main maximum in the thermal desorption spectrum as a result of the formation at the incoherent particle–matrix boundary of bubbles with high bonding energy with particles. In contrast to the ferrite-martensite steel irradiated in the temper state, the thermal desorption spectrum of EK-164 austenitic steel is more complex because of the multiple stage helium release due to the structural defects introduced by cold deformation.

In situ probing of helium desorption from individual nanobubbles under electron irradiation

The understanding of the mechanisms of helium bubble formation and evolution in materials requires the quantitative determination of several key quantities such as the helium density in the bubbles. Helium nanobubbles of about 16 nm in diameter were created in silicon by helium implantation at high fluence and subsequent annealing. Individual nanobubbles were analyzed by spatially resolved Electron Energy-loss Spectroscopy (EELS). We report on the in situ probing of helium desorption from the nanobubbles under electron irradiation. This opens new perspectives for a more accurate determination of the helium density through spatially resolved EELS.

Thermal desorption of helium from defects in nickel

Helium atoms, introduced into materials by helium plasma or generated by the ( n, α) nuclear reaction, have a strong tendency to accumulate at trapping sites such as vacancy clusters and dislocations. In this paper, the effects of dislocations, single vacancies and vacancy clusters on the retention and desorption of helium atoms in nickel were studied. Low energy (0.1–0.15 keV) helium atoms were implanted in nickel with vacancies or dislocations without causing any displacement damage. He atoms, interstitial-type dislocation loops, and vacancy clusters were also introduced with irradiation damage by 5.0 keV helium ions. Helium thermal desorption peaks from dislocations, helium-vacancy clusters and helium bubbles were obtained by thermal desorption spectroscopy at 940 K, in the range from 900 to 1370 K, and at 1500 K, respectively. In addition, a thermally quasi-stable state was found for helium-vacancy clusters.