Kazuhiko Izui

Tracks of high energy heavy ions in solids

Abstract The observation of tracks of high energy heavy ions produced in some materials was made with electron microscope in order to clarify the governing factors and mechanisms for track formation. Irradiation experiments were performed with specified and well collimated ion beams of 100 MeV class obtained from the tandem accelerator. Target specimens were evaporated thin films of germanium, silicon and titanium, and crystals of MoO3 and MoS2. The present experiments showed that there were threshold values in the energy deposition of ion to specimens for the track formations and that the size of the tracks depended on the energies deposited into the specimens. The correlations between the sizes of the tracks and the energy depositions of incident ions were examined for the tracks produced in the evaporated films of germanium, and the results were interpreted in terms of a time dependent line source model of thermal spike. Even in such a high energy region, tracks produced through nuclear collision process were observed under some conditions.

Radiation damage in yttria-stabilized zirconia under Xe ion irradiation

Abstract Single crystals of yttria-stabilized zirconia were irradiated with 60 keV Xe ions in an electron microscope at several temperatures in the range 300–1473 K and the process of damage evolution was studied. Amorphization did not occur in the zirconia irradiated with 60 keV Xe ions up to the fluence of 1.8×1016 Xe/cm2 at all irradiation temperatures. In the specimen irradiated with Xe ions below 1073 K, defect clusters are formed first and then bubbles are formed. On the contrary, in the specimen irradiated above 1073 K bubbles are formed first and then defect clusters are formed. From the observation of bubble sizes and densities, temperature and dose dependence of bubble swelling were estimated. With increase in fluence or irradiation temperature the average size of the bubble increased resulting in greater values of swelling.

Fission Fragment Damage in Semiconductors and Ionic Crystals

Electron microscopic observation is carried out on fission fragment-irradiated germanium, silicon and magnesium oxide which have different grain size and thickness. Tracks of fission fragments are observed in evaporated films of germanium with about 10A grain size and of magnesium oxide with about 50A grain size, while no track is observed in other materials. A number of small spots are found to appear in large crystal films of germanium and silicon after the irradiation. It is found by electron diffraction study that there are a number of small disturbed or recrystalized regions in highly irradiated large crystals of germanium and silicon. The mechanism of thermal spike is discussed in some detail based on a estimate of the energy distribution of excited electrons and relaxation times for various collision processes of these electrons, and a new model which is applicable to non-metallic crystals is presented. Calculations of the temperature distribution in a spike region based on this model is found to g...

In-situ observation system of the dynamic process of structual changes during ion irradiation and its application to SiC and TiC crystals

Abstract A newly designed system of in-situ observation during ion irradiation and implantation was constructed by combining an electron microscope with an ion beam. The mass analyzed ion beam is introduced into the electron microscope in the horizontal direction and deflected by a large angle of 72° inside the specimen chamber so as to be incident nearly normally to the specimen surface maintained at temperatures up to 1350°C. The ion flux at the specimen position is so high, 6 × 10 18 ions m −2 s −1 , that the whole process from the initial stage to heavy irradiation of the fluence of 10 22 ions/m 2 can be recorded in an hour. The ion energy is 10 keV so that it is adequate for the study of dynamic reaction processes of ions implanted in thin films of about 100 nm in thickness. The results of applications of this system to SiC and TiC crystals are presented.

In situ EELS and TEM observation of silicon carbide irradiated with helium ions at low temperature and successively annealed

Abstract Structural and compositional changes in SiC crystal due to helium ion irradiation were examined by an electron microscope equipped with a thermal field-emission gun and electron energy-loss spectroscope. Amorphization was confirmed both by the shift of the plasmon loss peaks and the change of the carbon core loss peaks in EELS as well as by the change of electron diffraction patterns in SiC irradiated with 12 keV helium ions to a fluence above 6 × 10 19 ions/m 2 at 22 K. He 1s–2p transition peak (21.8–23.8 eV) was observed for SiC irradiated to a fluence above 1.8 × 10 21 ions/m 2 at 22 K. Recrystallization was found to occur during annealing above 1173 K after the irradiation.

Observation of Lattice Defects in Graphite by Electron Microscopy, Part I

In Part I, lattice defects which can a priori exist in graphite crystals and be observed by means of electron microscopy, mainly dislocations, are reported. Four kinds of dislocations and their combination predicted from the crystal structure are observed through the diffraction contrast and the disturbances in the moire pattern such as the extra terminating half lines and the line shift. Their network arrangements and movement are also found. The line grating model reveals the relation between the line shift of the moire fringes and the Burgers vector of the glissile dislocations. New origins of extra terminating half lines are considered too.

EELS ANALYSIS OF SIC CRYSTALS UNDER HYDROGEN AND HELIUM DUAL-ION BEAM IRRADIATION

Abstract Electron microscopy and Electron energy loss spectroscopy (EELS) measurement were performed in SiC irradiated with three kinds of irradiation mode, namely, H2+ ions, He+ ions, and a dual beam of He+ and H2+ ions. Amorphization occurred in SiC at a damage density from 0.2 to 0.4 displacement per atom (dpa) irrespective of ion species. High resolution EELS revealed that a large amount of implanted hydrogen is contained in the form of hydrogen molecules and C–H compound in bubbles. It is inferred that some of hydrogen atoms implanted in SiC are trapped in both Si- and C-sites. Most of the helium atoms implanted are considered to exist in bubbles.

Ion irradiation and annealing effects in Al2O3 and MgAl2O4

Abstract In situ observation of structural changes of Al2O3 and MgAl2O4 induced by Xe and He ion irradiations and successive annealing were performed in an electron microscope equipped with ion accelerator. By Xe ion irradiation amorphization occurred in both the materials and bubbles were formed in MgAl2O4. By He ion irradiation bubbles were formed in both the materials, but amorphization was not observed to a fluence of 1.8 × 1021/m2 (11 dpa). By annealing after the irradiation, bubbles began to grow large by coalescence at 1000°C in Al2O3, and at 900°C in MgAl2O4, respectively. Amorphization began to recover in both materials at these temperatures. In the case of MgAl2O4 amorphous region recovers completely to polycrystalline state at 1000°C, and in the case of Al2O3 to original single crystal, respectively. Pit formation and exfoliation also occur in both materials at about 1100°C.

Bubbles in SiC crystals formed by helium ion irradiation at high temperatures

Abstract Helium bubbles were found to be formed in SiC crystals by irradiation with He+ ions at 1000 to 1200° C. The size of bubbles increased with increasing irradiation temperatures. The density of helium atoms in the bubbles was measured to be about 1028 atoms/m3 by EELS measurement in combination with electron microscopic observation in the same selected areas, and the internal pressure of the bubbles was estimated therefrom to be on the order of 108 Pa at room temperature.

Observation of Lattice Defects in Fission Fragment-irradiated Graphite

Some electron microscopic observations of large defects in graphite induced by fission fragment irradiation are shown. The observations were done on refined natural graphite powder mixed with uranium oxide powder in the ratio 10:1. The material was exposed to thermal neutrons in a reactor to total doses of the order of 10/sup 1//sup 6/ n/cm/sup 2/. Tracks of fission fragments from U/sup 2// sup 3//sup 5/ are sometimes observed as straight lines about 100 A wide accompanied by small dots. By tilting the specimen, their contrasts turn from black to white as the matrix changes from white to black. This indicates that the tracks are essentially the diffraction contrasts caused by variations in the Bragg conditions in the crystal and that these narrow linear regions have a crystallographic nature or orientation slightly different from the matrix, possibly exhibiting the thermal spike effect of fission fragments passing through the crystal. Dislocation lines are also partly due to absorbtion of interstitial atoms produced by the irradiation. (N.W.R.)