K.N. Kushita

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.

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.

In situ EELS observation of graphite structure modification due to hydrogen ion irradiation

Abstract Electron energy-loss spectroscopy (EELS) was applied to investigate the radiation damage of graphite. Graphite samples were irradiated with 10 KeV H+2 ions at ambient temperature in an electron microscope combined with an ion gun. The irradiation caused amorphization in graphite. This structural change brought about an energy-shift of plasmon peaks in EELS. The peak at 27.4 eV for crystalline graphite rapidly decreased when the sample was irradiated to a critical dose (7 × 1015 (H+) cm-2), and then became constant at 24.8 eV when irradiated up to 3.5 × 1016 (H+) cm-2. TRIM code calculations suggested that the onset and completion of amorphization process of graphite corresponded to about 0.05 and 0.25 dpa, respectively. It was also observed that the energy-loss near-edge structure (ELNES) of the CK edge was modified as the irradiation proceeded, implying a change of chemical structure of the graphite.

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.

Chemical behavior of tritium in neutron-irradiated Li3N

Abstract When the neutron-irradiated Li3N powder was heated at 1020 K under vacuum, almost all tritium was released from the solid within 30 min and more than 98% of the released tritium was in the chemical form of HT. The other tritiated species were HTO(1%) and CH3T(0.2%). No detectable amount of NH2T was released from the sample which had been degassed under vacuum at 920 K prior to the irradiation. The HT release temperature of Li3N was higher than that observed for LiAl. LiH and Li2C2 powders by about 200 K. The thermal release rates of HT were found to fit the first-order kinetics. The rate constant determined in the temperature range between 870 and 1020 K was k = 6.6 × 105exp (−158 × /RT) s−, where probable errors are ± 8.4 kJ mol−1 for the activation energy and ± 1.0 s−1 for the logarithmic pre-exponential term. From the experimental results, the thermal decomposition of Li3N has been suggested to play an important role in the tritium release process.

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.

Radiation damage of graphite due to hydrogen-ion bombardment at very low temperature

We have examined the radiation effects of hydrogen-ion bombardment at liquid nitrogen temperature on graphite, which is a candidate for plasma-facing material in fusion devices. The ion-irradiated graphite became amorphous as is the case in similar experiments at higher temperatures. This case was unique, however, in that the plasmon-loss peak energy of graphite in EELS spectra decreased and then increased again after reaching the lowest level, which corresponds to amorphous graphite. The final value of the plasmon-peak energy was that of crystalline graphite (about 27.4 eV), while electron diffraction patterns indicated that it was still amorphous. We have discussed the meaning of this unique phenomenon by comparing the CK edge profile for the graphite sample with the one for diamond.

Tritium diffusion in pseudo-monocrystal graphite, observed with a tritium imaging camera

Abstract Diffusion of tritium in pseudo-monocrystal graphite samples was observed using a tritium imaging camera. Samples implanted with tritium were annealed for hour long intervals at temperatures ranging from 375 to 1975 K and tritium images were photographically recorded after each anneal. Diffusion lengths of about 20 μm in the basal plane of the sample were recorded, implying a diffusion coefficient of about 2 × 1010 cm2/s at temperatures of about 1800 K. An unexpectedly large amount of tritium was retained in the near surface of the samples even after prolonged annealing to 1975 K.

Damage evolution in TiC crystals during hydrogen and helium dual-ion beam irradiation

Abstract The effect of He+ ion irradiation after H+2 ion pre-injection and of simultaneous 25 keV H+2 and 20 keV He+ ion irradiation at RT to 1423 K on the formation of bubbles in TiC crystals were examined. No amorphization occurred under these irradiation conditions. The formation and growth of bubbles produced by He+ ion irradiation is not enhanced by pre-injection of hydrogen atoms at RT. Bubbles are formed but do not grow appreciably by He+ ion, H+2 pre-injection and H+2+He+ dual-ion irradiation at room temperature. Remarkable growth and coalescence occurred during irradiation at high temperature of 1423 K.

In situ EELS observation of diamond during hydrogen-ion bombardment

Diamond samples were used to obtain basic data on radiation effects on carbon materials which are to be used for the first wall in a fusion reactor. The samples were irradiated with 10 keV H 2 + ions at room temperature in an analytical electron microscope and the irradiation effects were observed by TEM, electron diffraction and electron energy-loss spectroscopy (EELS). The crystalline diamond structure was destroyed by irradiation producing an amorphous carbon, which was confirmed by the energy shift of the plasmon peak in EELS as well as by the change of electron diffraction patterns. The amorphization process started at a fluence of about 5 × 10 16 H + /cm 2 and was completed at 3 × 10 17 H + /cm 2 , corresponding to 0.09 and 0.5 dpa, respectively. These damage levels were slightly higher than those for graphite. The different irradiation effects are discussed in relation with the different structure of diamond and graphite.