Tadao Iwata

Fine structure of Wigner energy release spectrum in neutron irradiated graphite

Stored energy release spectra have been measured using the linear-rise method for pyrolytic graphite samples, neutron-irradiated to a fluence of 4 × 1017 n/cm2 at about 80°C. Seven heating rates were chosen in the range of 1°C/min to 100°C/min. Total stored energy release is (8±1)J/g. The spectra are composed of three peaks. The reactions of three peaks follow first-order kinetics, and their respective frequency factors and average activation energies are 2.2 × 1012 s−1 and 1.34 eV; 8.5 × 1012 s−1 and 1.50 eV; and 1.5 ×1014 s−1 and 1.78 eV. The initial activation energy spectrum is approximated by the Gaussian distribution. The fraction of each peak varies with the heating rate. The mechanism of the Wigner energy release is discussed on the basis of these kinetic results.

Atomic Displacements by Electron Irradiation in Pyrolytic Graphite

The anisotropic threshold energy for atomic displacements and the displacement cascade process in electron-irradiated pyrolytic graphite are studied in the energy range from 0.12 to 1.0 MeV at three different temperatures by observing the a -axis electrical resistivity increases. The threshold energy T d is represented as \(T_{\text{d}}(\varPsi){=}A\cos^{2}\varPsi+B\sin^{2}\varPsi+C(1-\cos 4\varPsi){\equiv}(A,B,C)\) in eV, where \(\varPsi\) is the angle between the c -axis and the displacement direction. In the 6 and 80 K irradiations T d is given as a set of (23, 30, 0) and (31, 30, -2)±2 eV, and the cascade obeys the Harrison-Seitz Replacement model for \(\varTheta{=}0\)°, but obeys the Kinchin-Pease model for \(\varTheta{=}90\)°, where \(\varTheta\) is the angle between the c -axis and the electron direction. In the 285 K irradiations T d is either (28, 42, 0)±2 eV, or a set of (28, 42, 0) and (32, 42, -1)±2 eV, and the cascade obeys the Harrison-Seitz (no replacement) model for \(\varTheta{=}0\)°, 30°...

Effect of electron excitation on radiation damage in fce metals

Abstract Defect production, radiation annealing and defect recovery are studied in several fcc metals (Al, Cu, Ni, Ag and Pt) irradiated with low-energy (∼ 1 MeV) and high-energy (∼ 100 MeV) ions. Irradiation of the metals with strong electron-lattice interaction (Al, Ni and Pt) by ∼ 100 MeV ions causes an anomalous reduction, or even a complete disappearance of stage-I recovery. This experimental result shows that the energy transferred from excited electrons to lattice atoms through the electron-lattice interaction contributes to the annihilation of single interstitials. This effect is also observed in Ni as a large cross section for radiation annealing, and a decrease of the damage efficiency. On the other hand, in Cu and Ag thin foils, we find that lattice defects are produced not only through elastic interactions, but also through a process strongly associated with electron excitation. In the latter process, the defect production cross section is proportional to Se1.7 in Cu and Se1.5 in Ag. The nearly quadratic dependence of the cross section on Se suggests that the mutual Coulomb repulsion of ions positively charged by electron excitation causes the defect production.

Thermal Resistivity Changes in Electron-Irradiated Pyrolytic Graphite

Changes in the a-axis and the c-axis thermal resistivity of pyrolytic graphite caused by electron irradiation at 82 K and by subsequent isochronal pulse annealing have been measured. The additive thermal resistivity at 78.5 K increases as 9.6×102N0.85f and (7.9±0.6)×103N0.75f cm deg/W in the a-axis and the c-axis directions, respectively, in the region of 2 ppm<Nf<70 ppm, where Nf is the concentration of nearly isolated Frenkel defects produced by irradiation. Annealing of the additive thermal resistivity occurs around 100 K and above about 220 K. The characteristics of annealing stages are different in some important respects from those obtained by Goggin and Reynolds. It is inferred that most interstitials do not recombine with vacancies below about 220 K.

Investigations on the structural disordering of neutron-irradiated highly oriented pyrolytic graphite by X-ray diffraction and electron microscopy

Light and heavy neutron-irradiation damage of highly oriented pyrolytic graphite (HOPG) crystals was examined by means of X-ray diffraction and high-resolution high-voltage transmission electron microscopy (TEM). From the X-ray data analysis, it was found that there is an average increase of about 3% in the c-axis lattice parameter of the unit cell of graphite for lightly neutron-irradiated HOPG. However, the c-axis lattice parameter could not be estimated from the HOPG sample having the highest dose of neutron irradiation under the present investigation, because the X-ray profile was highly asymmetrical. This increase in the c-axis lattice parameter is attributed to lattice expansion due to the static displacement of atoms after neutron irradiation. Local structure analysis by TEM shows that the 0002 lattice spacing for the above-mentioned HOPG samples has been increased by up to 10% as a result of the neutron irradiation. This increase in c-axis lattice spacing can be ascribed to the fragmentation of the crystal lattice into nanocrystallites, breaking and bending of the 0002 straight lattice fringes, appearance of dislocation loops, and extra interstitial planes within the fragmented nanocrystallites. All these changes are a result of the static displacement of atoms after neutron irradiation.

Low Temperature Electron-Irradiation Damage and Recovery in Pyrolytic Graphite

The electrical resistivity changes by electron irradiation at 5 K and the isochronal annealing of resistivities in the temperature range 5-85 K (Stage I) in pyrolytic graphite are studied in order to get information on point defects. Both the a -axis and the c -axis resistivity changes by irradiation are explained by the ordinary transport theory with defect scattering. Small recoveries in Stage I are shown to be composed of four substages: I A (5-15 K); I B (15-45 K); I C (45-65 K); and I D (65-85 K). Effects of the irradiating electron energy, the radiation-doping and the graphitization temperature of samples on these substages are also studied. Substages I A and I B may be caused by the correlated rearrangement of close Frenkel pairs, where interstitial atoms form loose coupling with their own vacancies. The activation energy for I A is roughly estimated to be 0.027±0.004 eV. In I C the long-range free migration of interstitial atoms seems to occur.

Damage production and annealing in ion-irradiated FCC metals

Abstract Damage production, radiation annealing and stage I recovery in some FCC metals irradiated with ∼ 1 MeV and ∼ 100 MeV ions near 10 K are studied using electrical resistivity measurements. For ∼ 1 MeV light and heavy ion irradiations, the fraction of stage I recovery and the damage efficiency decrease with the PKA median energy. For ∼ 100 MeV heavy ion irradiations, an anomalous reduction of stage I recovery and a large cross-section for subthreshold recombination are found in Ni, and an enhancement of the damage efficiency is found in Cu; they are interpreted as due to the electron excitation by irradiating ions and the subsequent energy transfer from excited electrons to lattice atoms. Simultaneous differential equations describing the production and radiation annealing of two or more types of defects are solved, where the respective defect concentration is expressed as a function of fluence.

c-Axis Spacing Changes in Pyrolytic Graphite after Neutron Irradiation at 5 K

Change in the c -axis spacing in pyrolytic graphite after fast neutron irradiation at 5 K and subsequent isochronal pulse annealings have been measured in the range of 5∼900 K by X-ray diffraction method. The c -axis expansion was estimated to be Δ c 0 / c 0 =3.4 N i , where N i is the fractional concentration of interstitial atoms. This corresponds to a volume change of 3.3 atomic volume per interstitial atom. For annealings of the c -axis spacing, a small recovery below 80 K was followed by a large recovery at around 100 K, and above it a small continuous recovery occurred up to 900 K. The large recovery at around 100 K was interpreted by the formation of C 2 molecules and interstitial clusters. The present results were discussed comparing with the so far reported results of electrical resistivity, thermal resistivity and stored energy release measurements.

Characterization of Graphites by Positron Lifetimes

Positron lifetimes have been measured in different types of graphites such as glassy carbon, nuclear graphite, pyrolytic graphite and natural graphite. An analysis based on the trapping model shows that positrons preferentially monitor structural defects such as the internal surfaces between crystallites and surfaces around pores or voids. The lifetime of free positrons delocalized in the perfect graphite lattice is estimated to be 215?10 ps. The lifetime of 400?20 ps, observed in well-crystallized pyrolytic graphite and natural graphite, is interpreted as the lifetime of positrons trapped at the internal surfaces between crystallites. It is possible to characterize different types of graphites by the positron lifetime data.

Stored Energy Release in Electron-Irradiated Graphite

The stored energy release in electron-irradiated pyrolytic graphite was measured during annealing from around 80 K to 320 K by using a method of differential thermal analysis. For converting the recorded temperature difference into the energy release rate, the heat transfer coefficients and heat capacities of samples were measured as a function of temperature. A useful method of data analysis in differential calorimetry is presented; it gives the reproducible base line of stored energy release curves even if the heating rate is not strictly controlled. The energy release curves show peaks near 105, 150, 200, 250 and 295 K. The amount of the energy released around each peak is obtained for a pair of Frenkel defects produced by irradiation. The formation energy of a Frenkel defect and annealing of defects are discussed.