Interactions of 5–30 keV deuterons with a carbon surface

Measurements have been made of the temperature dependence of methane formation during the bombardment of carbon with hydrogen ions of varying energies. It has been found that the value of the temperature at which maximum methane production occurs varies with incident ion energy, in agreement with the prediction of our earlier theoretical model. Measurements have also been made of the release of previously trapped hydrogen in carbon using hydrogen and deuterium ion beams. The cross sections measured for this release as a function of energy have been used in the model for methane production and confirm that ion-induced release plays an important role in determining the methane production rate.     

Blistering of niobium due to 0.5 to 9 keV helium and hydrogen bombardment

Blistering of well-annealed niobium single crystals due to 0.5 to 9 keV helium and hydrogen ion bombardment at temperatures between ?110°C and 1000°C has been investigated by Rutherford backscattering (RIBS) in double alignment and with a scanning electron microscope (SEM). For He bombardment blistering was observed by RIBS in the temperature range investigated for all energies above 1 keV. The critical dose at which blisters first appear is about 1 to 2 × 10¹⁷ incident He ions per cm². It increases slightly with increasing ion energy and with decreasing target temperature. Blisters of 500 to 5000 Å in diameter were found. The depth at which the blisters develop increases from ≈ 180 Å for 1 keV to 1100 Å for 9 keV He ions. It is a factor of ≈ 3 larger than the theoretical mean range of the ions in amorphous material. Above ≈ 600°C grain boundaries develop extending also into the unbombarded region. For hydrogen bombardment no blistering could be observed at room temperature up to doses of 2 × 10¹⁹ ions per cm².     

Physical and chemical sputtering of graphite and SiC by hydrogen and helium in the energy range of 600 to 7500 eV

The erosion of pyrolytic graphite and silicon carbide due to the bombardment with monoenergetic hydrogen ions with energies of 600 to 7500 eV has been investigated in the temperature range of near room temperature to 750°C. The erosion yield of SiC is about 10^?2 and shows no pronounced temperature dependence. In contrast to SiC the erosion yield of pyrolytic graphite shows a maximum at a temperature of about 600°C. The ratio of the maximum erosion yield to that at room temperature depends on the energy of the hydrogen ions and increases from about 11 at 3000 eV to 32 at 670 eV. The production of CH₄ during the bombardment of the graphite has been found proportional to the erosion yield. When graphite was bombarded with He ions no hydrocarbon production and no temperature dependence of the erosion yield could be observed. The results are compared with values for the erosion yields of carbon by thermal atomic hydrogen taken from literature.     

Hydrogen and helium trapping in tungsten under single and sequential irradiations

The trapping of hydrogen and helium in polycrystalline tungsten irradiated with 500 eV He⁺, H⁺ and D⁺ ions, individually or sequentially, has been measured by thermal desorption spectroscopy. Specimens irradiated with 500 eV He⁺ at 300 K show three He release peaks in the vicinity of ∼500, ∼1000, and ∼1200 K. The helium is thought to form He vacancy complexes or bubbles. Increasing the specimen temperature to 700 K does not significantly affect the trapping behavior of He. Sequential He⁺-D⁺ irradiation at 300 K results in the elimination of He release above 800 K. Instead, both D and He were released in the range 400-800 K. This is interpreted as interstitial D and He released from the near surface. Sequential He⁺-D⁺ irradiation at 700 K resulted in a reduced single He peak at ∼1000 K with very little release observed below 800 K; no D was trapped for irradiations at 700 K. Sequential D⁺-He⁺ irradiations at 300 K show that He trapping occurs in much the same manner as for the He⁺-only case while D retention is reduced at the near surface. Sequential D⁺-He⁺ irradiations at 700 K indicate that pre-irradiation with D⁺ has little or no effect on the subsequent trapping behavior of He.     

Surface damage of metallic glasses by argon ion bombardment

The surface damage of metallic glasses Fe₄₀Ni₄₀P₁₄B₆, Fe₄₀Ni₃₈Mo₄B₁₈, Fe₄₀Ni₄₀B₂₀, Fe₈₀B₂₀ and Ni₆₄Zr₃₆ under argon ion bombardment at room temperature has been investigated. Blister formation was observed in the dose range of 1 × 10¹⁷ to 1 × 10¹⁸ ions/cm². At higher doses, blisters disappeared with concurrent roughening of the bombarded surface. Erosional features like cones and pyramids are not observed. Argon induced blisters also disappeared on room temperature aging without any further bombardment after blister formation. Post-irradiation annealing at high temperatures (673 and 873 K) resulted in blister formation, severe surface exfoliation and pin-hole formation. The precipitation of the implanted argon into bubbles is also observed.     

Roughening of silicon (1 0 0) surface during low energy Cs ion bombardment

Nowadays, the use of low energy ion bombardment in secondary ion mass spectrometry (SIMS) is a mandatory step to obtain high depth resolution for the characterization of ultra shallow junctions. However, increment of roughness during ion bombardment leads to the degradation of depth resolution. The evolution of surface roughening and ripple formation on silicon at ambient temperature under 1 keV Cs⁺ ion bombardment with and without sample rotation has been studied by means of atomic force microscopy.Ripple formation with a perpendicular orientation with respect to the Cs⁺ beam direction has been detected, and their wavelength and correlation length have been monitored as a function of the experimental conditions. Roughness and wavelength increased with increasing ion fluence, while variations of ion flux showed little effect. The effect of sample rotation during ion bombardment led to a critical reduction of the surface roughness and disappearance of ripples.     

Measurement of chemical sputtering yields of various types of carbon

Measurements of the chemical sputtering during the bombardment of pyrolytic graphite, isotropic carbon and glassy carbon with 0.1–6 keV hydrogen ions have been made in the temperature rarige of room temperature to 700°C. The maximum production rate occurs at 1 keV for the incident energy and 525°C for the target temperature in all types of carbon. Energy and temperature dependences of chemical sputtering of carbon are not affected by the structures of the carbon. The reason is ascribed to radiation damage of the surface of the carbon. The dose dependence of the methane production rate was influenced by the hydrogen concentration in a target prior to bombardment, but the steady rate was obtained after the target was bombarded with protons at a dose of more than $\text{1 × 10}{}^{18}\text{H}{}^{\mathrm{+}}\text{/cm}{}^2$.     

Erosion yield of Ti from TiC-deposited graphite by hydrogen ion bombardment at high temperatures

Erosion yields of Ti atoms from a TiC-deposited graphite by bombardment with 1 keV hydrogen ion beam of various current densities at 900°C have been investigated by means of the Rutherford backscattering (RBS) technique. It has been observed that the sputtering yields for Ti atoms at 900°C are almost zero below a critical ion flux of 1 × 10¹⁵/cm²·s, compared with the sputtering yield of Ti atoms at room temperature which has been measured to be 1 × 10⁻²atoms/ion. No sputtering of Ti atoms observed at 900°C is explained in terms of self-sustaining coating of the TiC surface with segregated carbon layer. The condition for the self-sustaining coating is discussed.     

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Surface erosion in pyrolytic graphite by 100 keV ⁴He⁺ and 200 keV H⁺₂ ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10¹⁷ to 5 × 10¹⁸ particles/cm². Although the surface is eroded at 1 × 10¹⁷ particles/cm² in helium bombardment, it is not eroded so heavily even at 5 × 10¹⁷ particles/cm² in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10¹⁸ particles/cm², and a cone structure appears at 5 × 10¹⁸ particles/cm², which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10¹⁸ particles/cm², many circular blisters are formed which are sputtered off at 5 × 10¹⁸ particles/cm². The surface roughness of the target also affects the erosion.     

Specific low temperature release of Xe from irradiated MOX fuel

A particular low temperature behaviour of the ¹³¹Xe isotope was observed during release studies of fission gases from MOX fuel samples irradiated at 44.5 GWd/tHM. A reproducible release peak, representing 2.7% of the total release of the only ¹³¹Xe, was observed at ∼1000 K, the rest of the release curve being essentially identical for all the other xenon isotopes. The integral isotopic composition of the different xenon isotopes is in very good agreement with the inventory calculated using ORIGEN-2. The presence of this particular release is explained by the relation between the thermal diffusion and decay properties of the various iodine radioisotopes decaying all into xenon.     

Enhanced hydrogen trapping due to He ion damage

Single crystals and rolled foil of Mo with and without predamage by 11 or 18 keV⁴ He at room temperature have been injected with 8 or 16 keV H and D isotopes. The H depth profiles and the total D retention were measured by nuclear microanalysis techniques. A strong enhancement in the trapping of hydrogen isotopes after He predamage over Mo without predamage is observed and the H depth profiles scale with the He ion energy. Cold work increases the D trapping in the absence of He damage but after 3 × 10¹⁵ He/cm² predamage little difference remains in the total trapping for single crystals and rolled foil. The release of the D upon annealing due to detrapping occurs primarily between 100 and 450°C. A similar fractional reduction in trapping is observed for elevated temperature D injection as is found in the anneal to corresponding temperatures of room temperature injected samples.     

Chemical sputtering of graphite by H+ ions

Methane production during graphite bombardment by H⁺ ions is observed. The temperature dependence of the graphite sputtering yield is obtained for the temperature range 100–1400°C. A simple model of chemical sputtering of graphite under H⁺ ions bombardment is presented.     

Observation and analysis of damage structure in Al and Al/Mg (N4) alloy after irradiation with 100 and 400 keV aluminium ions

A comparative TEM study has been made of ion irradiation damage structure in pure aluminium, commercial aluminium (grade 1100) and in a modified N4 (Al/2.95% Mg) alloy of the type used in the construction of the calandria of the Winfrith prototype SGHW Reactor. Atom displacements equivalent to many years neutron irradiation were simulated by bombardment with 100 and 400 keV Al⁺ ions to doses of up to 200 dpa at temperatures between 30 and 250°C. Dynamic observations of damage formation were made during irradiation with 100 keV ions in a linked heavy-ion accelerator/200 keV electron microscope, and further results were obtained by 400 keV Al⁺ ion bombardment in a Cockcroft-Walton accelerator. Dislocation structure and voids were seen in aluminium irradiated with 100 and 400 keV A1⁺ at temperatures between 30 and 250°C. Void swelling of 8.7% at 104 dpa was a maximum at 1̃50°C in type 1100 aluminium. No voids were found at temperatures μ 250°C. No voids were seen in the Al/Mg N4 alloy after bombardments up to 200 dpa with 100 keV A1⁺, and 150 dpa with 400 keV Al⁺ at temperatures between 50 and 170°C. The void-resistant property is consistent with observations in the USA of neutron-irradiated 5052 Al alloy which has a similar magnesium content to the modified N4 alloy. The 1100 alloy and N4 results have been analysed using the rate theory of swelling. The absence of voids in the N4 alloy indicates an effective vacancy annihilation mechanism, which possibly occurs at small precipitates formed during irradiation.     

Isotopic Effect in Ion-Induced Re-Emission of Hydrogen Implanted into Graphite

The re-emission of H and D atoms, implanted into graphite, induced by 1.5 MeV He⁺bombardment has been studied by means of the elastic recoil detection (ERD) technique by changing the initial peak concentration of H and D, the temperature and the He⁺ ion flux. The experimental re-emission profiles were analyzed solving analytically the mass balance equations. The whole re-emission profiles are shown to be excellently reproduced in terms of the ion- induced detrapping (σd), trapping (or retrapping) (Σ_T ) and local molecular recombination (k) between an activated (free) hydrogen atom and a trapped one and that the effective detrapping cross sections Σ_dK/Σ_T for H and D are determined as the best fitting parameter to be 2.9xl0-¹⁸ and 7.7x 10-¹⁹ cm², respectively. It is also shown from the experimental data on little dependence of the re-emission profile on the He⁺ ion flux that the local molecular recombination plays a major role in the ion-induced re-emission. Furthermore, the difference between the effective detrapping cross sections for H and D is reasonably explained in terms of isotopic difference in the values of K for H and D.     

Real-time X-ray studies of the growth of Mo-seeded Si nanodots by low-energy ion bombardment

Silicon surface evolution during room temperature low-energy (300, 500 and 1000 eV) normal incidence Ar⁺ ion bombardment in the presence of Mo seed atoms has been studied with real-time grazing-incidence small-angle X-ray scattering and ex situ atomic force microscopy. When a small amount of Mo atoms was supplied to the Si surface during ion bombardment, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (“dots”) coarsened with time until they reached a constant spatial wavelength. The longer length scale corrugations associated with kinetic roughening, however, continued to grow in amplitude during bombardment. The overall roughness is dominated by different corrugations at different times in the kinetic evolution, showing a complex behavior. The evolution of the kinetic roughening can be described by the Family-Vicsek scaling hypothesis, but measured scaling exponents are not in agreement with those of existing models.     

Experimental study of swelling of irradiated solid methane during annealing

Solid methane is still widely in use at pulsed neutron sources due to its excellent neutronic performance (IPNS, KENS, Second Target Station at ISIS), notwithstanding poor radiation properties. One of the specific problems is radiolytic hydrogen gas pressure on the walls of a methane chamber during annealing of methane. In this paper results of an experimental study of this phenomenon under fast neutron irradiation with the help of a specially made low temperature irradiation rig at the IBR-2 pulsed reactor are presented. The peak pressure on the wall of the experimental capsule during heating of a sample irradiated at 23-35 K appears to have a maximum of 2.7 MPa at an absorbed dose 20 MGy and then falls down with higher doses. The pressure always reached its peak value at the temperature range 72-79 K. Generally, three phases of methane swelling during heating can be distinguished, each characterized by a proper rate and intensity.     

The collapse of defect cascades to dislocation loops

We describe a number of experiments that we have recently performed to investigate the collapse of defect cascades to dislocation loops. This important ion and neutron irradiation phenomenon has been studied with in situ ion bombardment using the High Voltage Electron Microscope-Ion Accelerator Facility at Argonne National Laboratory in Cu₃Au, Cu, and Fe at temperatures of 30 and 300 K and in Ni at 30, 300 and 600 K. These experiments have demonstrated that individual defect cascades collapse to dislocation loops athermally at 30 K in some materials (Ni, Cu and Cu₃Au), while in Fe overlapping of cascades is necessary to produce dislocation loops. A slight sensitivity to the irradiation temperature is demonstrated in Cu₃Au and Fe, and a strong dependence on the irradiation temperature is seen in Ni. This phenomenon of cascade collapse to dislocation loops in metals at 30 K provides an understanding for previous neutron irradiation data. The more detailed dependencies of the collapse probability on material, temperature, bombarding ion dose, ion energy and ion mass contribute much information to a thermal spike model of the collision cascade which we will describe.     

Reaction of atomic hydrogen with chemisorbed species: C2H4 on W

Atomic hydrogen produced in controlled thermonuclear reactors is capable of very rapid reaction with both the material of the first wall and with adsorbed species. We have been investigating the reactions of thermal atomic hydrogen with ethylene adsorbed on a tungsten film. When adsorbed at room temperature ethylene reacts to yield gaseous hydrogen and a carbonaceous residue which reacts with incident atomic hydrogen to yield methane along with minor amounts of ethane. This product distribution, however, depends on the chemical nature of the adsorbed species. Adsorption at 178K, a temperature at which the ethylene molecule is stable, inverts this distribution with ethane now being the major product. Analysis of the system dynamics shows that the rate of production of each product is proportional to the product of the steady state increase in the product pressure and the pumping time constant of the vacuum system. For the case of methane production following room temperature adsorption of ethylene, the reaction probability for the production of methane for the reaction C + 4H → CH₄is ~0.001 and arguments are given for a larger value.     

Stable absolute calibration standards for hydrogen profile analysis using nuclear resonance techniques

Samples of pyrolytic carbon, CH_x (x ≈ 0.03) from pyrolysis of methane, and amorphous silicon, SiH_x (x ≈ 0.2) from plasma deposition of silane, are shown to be stable during ion bombardment and vacuum storage. They are therefore well suited as calibration standards for hydrogen profile analysis using nuclear resonance reactions such as ¹H(¹⁵N, αγ)¹²C. Due to its high hydrogen content and the possibility of absolute calibration by vacuum extraction, tantalum hydride, TaH_x (x ≈ 0.5) has been used for the absolute calibration of these standards.     

Study of the temperature evolution of defect agglomerates in neutron irradiated molybdenum single crystals

Small angle neutron scattering as a function of temperature, differential thermal analysis, electrical resistivity and transmission electron microscopy studies have been performed in low rate neutron irradiated single crystalline molybdenum, at room temperature, for checking the evolution of the defects agglomerates in the temperature interval between room temperature and 1200 K. The onset of vacancies mobility was found to happen in temperatures within the stage III of recovery. At around 550 K, the agglomerates of vacancies achieve the largest size, as determined from the Guinier approximation for spherical particles. In addition, the decrease of the vacancy concentration together with the dissolution of the agglomerates at temperatures higher than around 920 K was observed, which produce the release of internal stresses in the structure.