S. Ishino

In Situ Studies of the Effects of Ion Beams on Materials Using the Electron Microscope Ion Beam Interface

To investigate microstructural evolution by cascade damage produced by energetic heavy particles, we have built a facility which is capable of observing damaged structure introduced by heavy ions in situ in an electron microscope. A 400 kV Cockcroft-Walton type heavy ion accelerator with a Danfysik 911A heavy-ion source has been combined with a 200 kV electron microscope. Heavy ion beams of energies up to 400 keV can bombard the specimen in the microscope with an incident angle of 45 degrees to the electron beam axis. This paper describes the outline of the facility and some of the recent results mainly concerned with heavy radiation damage of materials which are relevant to fast breeder and fusion reactor development. For example, we have investigated microstructural evolution of SUS 316 stainless steel as a function of dose, dose rate and temperature. The topics will also include observation of short-lived clusters of point defects during irradiation in nickel and direct comparison of self-ion damage in aluminum with electron damage caused by an electron beam within the 200 kV microscope. Some of these results have been discussed, compared with cascade simulation and defect kinetics calculations. The experimental results may be useful to establish correlation between neutron and ion damage through microstructural evolution modelling.

Annealing of γ-ray irradiated N-type germanium

Abstract Isochronal and isothermal annealing above room temperature has been studied in antimony and arsenic doped γ-irradiated germanium. Three annealing stages are found at 370, 430 and 530°K with activation energies of 0·7–0·8 eV, 1·2–1·3 eV and 2·2 (As-doped)–2·5 (Sb-doped) eV, respectively. Impurity dependence of annealing behavior is found in all these stages. The results are discussed in terms of electronic and elastic interactions of defect centers with impurity atoms.

Time and temperature dependence of cascade induced defect production in in situ experiments and computer simulation

Abstract Understanding of the defect production and annihilation processes in a cascade is important in modelling of radiation damage for establishing irradiation correlation. In situ observation of heavy ion radiation damage has a great prospect in this respect. Time and temperature dependence of formation and annihilation of vacancy clusters in a cascade with a time resolution of 30 ms has been studied with a facility which comprises a heavy ion accelerator and an electron microscope. Formation and annihilation rates of defect clusters have been separately measured by this technique. The observed processes have been analysed by simple kinetic equations, taking into account the sink effect of surface and the defect clusters themselves together with the annihilation process due to thermal emission of vacancies from the defect clusters. Another tool to study time and temperature dependence of defect production in a cascade is computer simulation. Recent results of molecular dynamics calculations on the temperature dependence of cascade evolution are presented, including directional and temperature dependence of the lengths of replacement collision sequences, temperature dependence of the process to reach thermal equilibrium and so on. These results are discussed under general time frame of radiation damage evolution covering from 10 −15 to 10 9 s, and several important issues for the general understanding have been identified.

In-situ observation of cascade damage in gold under heavy ion irradiation at high temperature

Formation, stability and annihilation of vacancy clusters produced during irradiation with energetic heavy ions at high temperature have been studied using gold as a model material. The experiments were carried out using a 400 kV heavy ion accelerator/200 kV TEM linked facility. In-situ observation during Xe+ or Ar+ irradiations at temperatures between 290 and 670 K reveals instantaneous formation of defect clusters. These include vacancy loops and stacking fault tetrahedra (SFT) with some unresolved defects with dot contrast. At lower temperatures the clusters tend to form in groups, reflecting subcascade structure, while at 570 and 670 K, ungrouped clusters are dominant. Both vacancy loops and SFT have considerably shorter lifetimes during irradiation than in the absence of irradiation. Moreover, the lifetime spectrum has two components; a long component is related to SFT, while loops usually have shorter lifetimes. By interrupting irradiation at 570 and 670 K, defects annihilate and the process has two components. Since the SFT survive longer, the SFT/loop ratio is history dependent.

Computer simulation of defect behavior under fusion irradiation environments

Abstract To simulate defect behavior under irradiation, three kinds of cascade-annealing calculations have been carried out in alpha-iron using the codes MARLOWE, DAIQUIRI and their modifications. They are 1. (1) cascade-annealing calculation with different masses of projectile, 2. (2) defect drifting near dislocations after cascade production and 3. (3) cascade-overlap calculation. The defect survival ratio is found to increase as decreasing mass of the projectile both after athermal close-pair recombination and after thermal annealing/ It is shown that at moderate temperatures vacancy clustering is enhanced near dislocations. Cascade-overlap is found to decrease the defect survivability. In addition, the role of helium in vacancy clustering has been calculated in aluminum lattices and its effect is found to depend strongly on temperature, interstitials and the mobility of small clusters. These results correspond well to the experimental data and will be helpful for correlating between fusion and simulation irradiations.

The Japanese experimental program on RTNS-II of DT-neutron irradiation of materials☆

Research project of D-T neutron radiation damage of materials with the rotating target neutron source RTNS-II of LLNL is explained with a brief explanation of the results obtained in the first year of the project. Wide variety of materials were irradiated at three temperatures; 25°C, 200°C and 400°C up to 1 × 1018n/cm2. From the result of the observations of defect structures, discussions are made on the conditions for the defect cluster formation from cascade damage, type of defects, defects made from sub-cascade damage, correlation between defects and disordered zones, and the roles of free interstitials released from cascades. Future research plans are briefly presented.

The interaction between interstitial impurities and substitutional solutes in vanadium

Abstract The nature of the interaction between interstitial impurities and substitutional solutes in vanadium is investigated experimentally by measuring the internal friction. The nature of the interaction can be explained in terms of the chemical affinity. Substitutional atoms, whose oxide or nitride formation energy is greater than that of vanadium act as attractive centers to oxygen or nitrogen. On the other hand, substitutional atoms with formation energy smaller than that of vanadium act as non-interactive or repulsive centers. Iron, copper, nickel, chromium, and manganese are non-interactive or repulsive to oxygen; iron, nickel, beryllium, copper, chromium, and manganese are non-interactive or repulsive to nitrogen. The region around the substitutional iron forbidden to oxygen is estimated to extend to the second nearest neighbour sites. Titanium, aluminum, and beryllium are attractive to oxygen and aluminum and titanium to nitrogen. Binding energies estimated are 0.26 ± 0.4 eV for titanium and oxygen, 0.14 eV for aluminum and oxygen, and 0.27 eV for beryllium and oxygen.

The effect of recoil energy spectrum on cascade structures and defect production efficiencies

Cascade defect structures produced by various ion irradiations have been examined in gold by in-situ TEM observations and computer simulations. The subcascade distribution is found to be highly sensitive to the incident ion mass and energy. By comparing the results of TEM observations and computer simulations, the subcascade energy is found to be about 20 keV and 10 keV in the case of Xe+ and Al+ ion irradiations, respectively. The number of subcascades is compared with the data for defect production efficiencies using the weighted average recoil energy as a correlation parameter. It is shown that the saturation of the efficiency at high recoil energies has a close relation to the cascade splitting into subcascades.

TEM-investigation of solute redistribution under irradiation

Abstract The solute redistribution processes under electron and ion irradiations have been observed with TEM in Ni-12.7at. % Si alloys. Surface precipitation of Ni3Si films are observed after electron irradiations. The defect production efficiencies are estimated from the film growth measurements. Disordering and solute segregation near dislocation loops are observed during ion irradiations.

The effect of cascade and helium on microstructural evolution under fusion irradiations

Abstract Cascade damage and high helium production rate are major characteristic features of fusion neutron radiation effects. Experimental as well as computational studies of these two factors which have been performed by the authors are presented and discussed from the standpoint of microstructural evolution. The experimental work has been performed using a facility comprising two small ion accelerators and an electron microscope, capable of observing cascade damage during heavy ion bombardment, and of carrying out dual beam irradiations to study the effect of simultaneous injection of helium atoms with displacement cascades. It has been shown that the evolving microstructures depend strongly on the nature of the cascade, type of materials, irradiation temperature, and amount and mode of helium implantation. The implication of these microstructural studies to the radiation effects relevant to fusion reactor design is discussed.