Kiichi Hojou

Consistent Theory of Sputtering of Solid Targets by Ion Bombardment Using Power Potential Law

Starting from a simple atomic model by Lindhard giving a power potential between ions and atoms as V(r)=z1z2e2an-1/nrn, we assumed that the first collision with a metal atom is of the weakly screened type (n=2) or Rutherford type (n=1) and that subsequent collisions of displaced atoms are similar to collisions of hard spheres (n=5). On this basis, consistent expressions were obtained for the maximum yield Smax with the corresponding energy Em for n=2, and the universal curve of yield-energy, which on the number of atms in the target materials per unit volume N, the effective atomic radius a and the displacement energy Ed. The results were in good agreement with experiments over the energy range from 50 eV to 500 keV.

Behaviour of implanted xenon in yttria-stabilised zirconia as inert matrix of a nuclear fuel

A zirconia-based fuel is studied for use of plutonium in light water reactors. Among the relevant properties for a nuclear fuel, efficient retention of fission products is required since the fuel matrix constitutes the first barrier against fission product release. To study the retention of xenon, its stopping power and its diffusion properties within (Er,Y,Pu,Zr)O2 potential inert matrix fuel (IMF) are investigated. Stopping and range of ions in matter (SRIM) calculations were carried out to estimate the average penetration depth of Xe ions as a function of their incident energy and of the material composition. To study its diffusion properties, Xe was implanted into yttria-stabilised zirconia (YSZ) to a depth of around 100 nm from the surface. After successive heat treatments to a maximum temperature of 1773 K, quantitative Xe depth profiles were determined by Rutherford backscattering. No profile modification by diffusion was observed. The behaviour of Xe is investigated at the subnanoscopic level and compared with the results obtained with zirconia samples implanted with Cs or I, as well as with Xe in UO2.

Design of neutron detector by using a novel superconductor MgB2

Abstract It is difficult to detect neutrons compared to charged particles. A novel metallic superconductor MgB 2 has a high T c of 40 K. It is well known that the 10 B has a huge cross-section for the nuclear reaction of 10 B( n , α ) 7 Li. It reaches 3800 barns for thermal neutrons. The Li particle has 0.84 MeV while α particle has 1.47 MeV. We propose to use the MgB 2 as a superconducting neutron detector. We design a prototype of a MgB 2 neutron detector from an MgB 2 film where the electron beam lithography is employed to fabricate a meandering MgB 2 stripe line of the 1-μm width. The hot spot caused by the nuclear reaction is responsible for detecting neutrons.

Formation of ‘environmentally friendly’ semiconductor (β-FeSi2) thin films prepared by ion beam sputter deposition (IBSD) method

Abstract ‘Environmentally friendly’ semiconductor β-FeSi 2 thin films have been prepared by the ion beam sputter deposition (IBSD) method on Si(100) substrate. The difference in crystallinity of the FeSi 2 obtained has been investigated using Fe and Fe–Si targets at various substrate temperatures. When the Fe target was used, highly (100)-oriented β-FeSi 2 films with relatively smooth surfaces were formed at 700°C. On the other hand, such films were scarcely observed when the Fe–Si target was used. Sharp edge grains were observed, which consisted of a mixture of α and β phases or α single phase. In the latter case, α-FeSi 2 is formed at T s above 600°C, which is lower than the conventional transformation temperature for the β→α phase.

Formation and migration of helium bubbles in Fe and Fe–9Cr ferritic alloy

Abstract Formation and migration of helium bubbles in high purity Fe and an Fe–9Cr ferritic alloy have been studied by in situ electron microscopy and scanning transmission electron microscope–electron energy loss spectrometry (STEM–EELS) analysis. The swelling under irradiation with 10 keV He + ions in the ferritic alloy was retarded in the low fluence region at 400 °C and was reduced at 600 °C even under heavy irradiation. These results suggest trapping effects of vacancies by Cr and its modulation with helium atoms in the bubble formation. Furthermore, it is found that the mean square of the bubble migration distance in both materials is proportional to time, which is a quantitative evidence of Brownian type motion and yields lower diffusivities of the bubbles in Fe–9Cr than in Fe. The retardation of the bubble mobility in Fe–9Cr should be caused by Cr segregation on the bubble surface which was revealed by STEM–EELS analysis.

Surface chemical states and oxidation resistivity of ‘ecologically friendly’ semiconductor (β-FeSi2) thin films

Abstract Surface chemical states and oxidation resistivity of the ‘ecologically friendly’ semiconductor β-FeSi 2 have been investigated. Previously, we studied β-FeSi 2 thin films prepared by the ion-beam sputter deposition method (IBSD) on an Si(100) substrate. Through these studies, it was observed that the oxidation of the formed FeSi 2 does not proceed so much even in the air atmosphere compared with elemental Si or Fe or other compound semiconductors. In the present study, the obtained films were analyzed by synchrotron–radiation X-ray photoelectron spectroscopy (SR-XPS) in order to obtain chemical states and depth profiles of films non-destructively. From the XPS spectra, the results indicated FeSi 2 films have strong oxidation resistivity. The XPS spectra reveal that oxidation resistivity is caused by a very thin SiO 2 layer covering the formed β-FeSi 2 . The results of cross sectional observation by transmission electron microscope and surface morphological analysis by scanning electron microscope are consistent with the SR-XPS results.

Irradiation effects on MgB2 bulk samples and formation of columnar defects in high-Tc superconductor

Abstract Irradiation effects on MgB2 were studied to investigate the pinning property of this new material. It is pointed out that flux pinning at grain boundaries is important for MgB2 bulk samples. We confirmed this with electron irradiation. Degradation of inter-grain coupling by electron irradiation resulted in degradation of pinning properties. Heavy ion irradiation was also accomplished on MgB2. Columnar defects introduced by the irradiation improve pinning in higher field regions. Concerning the formation of columnar defects in high-Tc superconductors, we conducted a systematic analysis of Bi2212 single crystal with different irradiation ions. Applying the time dependent line source model to our results, only a third of the electronic losses contributed to the formation of columnar defects.

Structural change in graphite under electron irradiation at low temperatures

Changes in crystallographic and electronic structure of electron-irradiated graphite were examined at low temperatures by means of transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The long range order within the basal planes at the low temperatures was lost more quickly than at room temperature, which hardly affected the continuous changes in electronic and short range structures. This was mainly caused by the faster fragmentation of the crystal into pieces with the local structure maintained. The basal plane buckling and lattice dilation in the c-direction, similar to the previous results obtained at room temperature, suggest the local formation of non-hexagonal atomic rings, on the analogy of fullerenes, by the displacement damage incorporated with electronic excitations.

Structural changes induced by helium ion irradiation in silicon carbide crystals

Electron microscopic observations were performed on defect structures in polycrystalline SiC crystals irradiated with 30 keV He ions to fluences from 1014 to 1017 ions/cm2. A number of defect clusters were observed which were hexagonal in shape when observed from the c-axis direction but were seen to be of linear form when viewed from a direction parallel to the c-plane. These clusters increased in density with increasing irradiation fluence, and were considered to be formed by the agglomeration of irradiation-produced point defects or the injected helium atoms on the c-planes. Amorphization was observed for a fluence above a critical value of 7 × 1015 ions/cm2 which corresponds to about one dpa for the 30 keV helium ions incident at random orientations on a SiC crystal. Irradiations of this critical fluence were unable to cause amorphization in the ion channeling directions. Heavy irradiations at the high temperature of 1000 K resulted in a number of fine crystallites, that is, recrystallized zones.

Formation and migration of helium bubbles in Fe-16Cr-17Ni austenitic alloy at high temperature

Abstract The formation and migration of helium bubbles in Fe–16Cr–17Ni austenitic model alloy at temperatures 400–1250°C have been studied by in situ electron microscopy, energy dispersive spectrometry (EDS) and EELS analysis. The nucleation of bubbles under 10 keV He+ ion irradiation was dominant at 400°C, while the growth was dominant above 600°C. It was revealed that the mean square of the migration distance was proportional to time, which is quantitative evidence of Brownian type motion and yielded diffusivities from 10−18 to 10−20 m2/s at 1185°C, depending on the diameter from 1 to 3 nm. These facts suggest that the mobility of helium–vacancy complexes or bubbles is an important factor governing the formation process of helium bubbles. The analysis by scanning transmission electron microscope-electron energy loss spectrometry (STEM-EELS) elucidated Ni precipitation even around small helium bubbles.