H. Otsu

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.

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.

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.

In-situ observation of damage evolution in SiC crystals during He+ and H2+ dual-ion beam irradiation

Using the new system of in-situ observation during dual-ion beam irradiation recently developed at JAERI, effects of simultaneous 12 keV He + and 15 keV H 2 + ion irradiation at room temperature on the formation of bubbles in SiC crystals were examined. It was found that hydrogen atoms gave quite a different effect on the formation of helium bubbles according to whether they were pre-injected or simultaneously injected with the helium atoms. Bubble formation is enhanced by pre-injection of hydrogen atoms, whereas it is little influenced by simultaneous injection of hydrogen atoms and helium atoms.

In-situ observation of damage evolution in TiC crystals during helium ion irradiation

Abstract In-situ observations were performed on bubble formation and growth in TiC during 20 keV helium ion irradiation over the wide range of irradiation temperatures from 12 to 1523 K. No amorphization occurred over this temperature range. The bubble densities and sizes were almost independent of irradiation temperatures from 12 to 1273 K. Remarkable growth and coalescence occurred during irradiation at high temperature above 1423 K and during annealing above 1373 K after irradiation.

In situ observation of damage evolution in TiC during hydrogen and deuterium ion irradiation at low temperatures

Abstract The processes of damage evolution in TiC crystals irradiated with 25 keV H2+ ions at 12 K and 28 K and 25 keV D2+ ions at 18 K and the effect of annealing after the irradiation were examined by in situ observation with an electron microscope equipped with an ion accelerator. Amorphization was confirmed in TiC irradiated with H2+ ions to a fluence of about 1 × 1021 H2+/m2 at 12 K, while no amorphization occurred in TiC irradiated with D2+ ions to a fluence of 4.5 × 1021 D2+/m2 at 18 K. No amorphization in D2+ irradiation is considered to be due to the difficulty of producing chemical bonding species which suppress the recovery of irradiation induced defects in comparison to H2+ irradiation.

In-situ observation of the dynamic behavior of bubbles in aluminum during 10 keV H2+ ion irradiation and successive annealing

The behavior of bubbles in aluminum during 10 keV H2+ ion irradiation and successive annealing was investigated by in-situ observation in an electron microscope. Bubbles were formed by irradiation at 300 K, but no bubbles were observed at 113 and 373 K. After irradiation at 113 K, bubbles were found to be formed during annealing from 113 to 300 K. During annealing from 300 to 498 K, smaller bubbles began to shrink and disappear at a lower temperature in comparison with larger bubbles. From these experiments, the activation energy for bubble shrinkage was found to be about 1 eV, and the binding energy of a vacancy to a bubble was inferred to be about 0.4 eV.

In situ observation of the damage evolution in ß-SiC crystals during helium and hydrogen dual-ion irradiations

Abstract In situ observations were performed on the bubble formation in s-SiC crystals during three kinds of ion irradiations: (a) single helium ion irradiation, (b) hydrogen ion pre-irradiation followed by helium ion irradiation, and (c) simultaneous irradiation of helium and hydrogen ions. The bubble formation and growth in s-SiC, at the same level of irradiation fluences, were remarkably depressed in the case of simultaneous helium- and hydrogen-ion irradiations compared to the other two cases. The effect of simultaneous ion irradiation on the bubble formation is discussed with respect to the diffusion of the incident ions and the number of nucleation sites for helium bubbles.