Kozo Atobe

F-Type Centers in Neutron-Irradiated AIN

The production of point defects by neutron irradiation and thermal decay in sintered AIN polycrystal are investigated. The absorption band at 370 nm is observed after reactor neutron irradiation to a dose of 1016 n/cm2 (E>0.1 MeV). The defect corresponding to the band is tentatively assigned as an F-type center from the optical absorption and electron spin resonance.

Thermoluminescence and F‐center annealing in alkaline‐earth fluoride crystals after reactor irradiation at low temperature

Thermoluminescence is related to the F‐center annealing stage in undoped alkaline‐earth fluoride (CaF2, SrF2, and BaF2) single crystals irradiated at 20 K with reactor neutrons. The temperature of glow peak maxima are classified in four groups (P‐1–4); P‐1: 110–130 K, P‐2: 134–152 K, P‐3: 183–200 K and P‐4: 201–245 K with a heating rate of 4 K/min. at temperatures ranging 77–300 K. These temperatures of peak maxima correspond well to decay stages of F‐centers. F‐centers play an important role as recombination centers in the TL process. The main TL emission spectrum of P‐1–4 peaks consists of the intrinsic luminescence as well as the case of the x‐ray emission spectrum at 77 K. It is revealed that these TL emission bands near 300 nm of P‐1, 2, and 3 peaks are due to the radiative recombination of (VK+F) and (H+F) reactions, respectively.

Electron Paramagnetic Center in Neutron-Irradiated AlN

Neutron-Irradiated AlN was investigated using electron paramagnetic resonance. The broad spectrum arising from the electrons trapped in N3- vacancies can be explained by the hyperfine interaction with the surrounding 27Al nuclei. The obtained parameters, the g-value and the hyperfine constant are g=2.007±0.001 and A=(10.9±0.5)×10-4 cm-1, respectively.

Point Defects in Cubic Boron Nitride after Neutron Irradiation

The production of point defects induced by reactor neutrons and the thermal behavior of defects in sintered cubic boron nitride are investigated using the optical absorption and electron spin resonance (ESR) methods. A strong structureless absorption over the visible region was observed after fast neutron irradiation to a dose of 5.3×1016 n/cm2 (E>0.1 MeV) at 25 K. This specimen also shows an ESR signal with g-value 2.006±0.001, which can be tentatively identified as an electron trapped in a nitrogen vacancy. On examination of the thermal decay of the signal, the activation energy for recovery of the defects was determined to be about 1.79 eV.

Lattice defects in thermoluminescent calcite

Abstract Optical absorption and luminescence in some natural calcites irradiated by reactor neutrons (15K, 360K) and/or α-rays (77K, 193K) are investigated for assingments of lattice defects. The 290 nm absorption band bleaches thermally at above 200K and emits luminescence having peak at 400 nm. These can be observed in calcites after energetic particle irradiations at low temperature.

Nature of Oxygen Donors and Radiation Defects in Oxygen-Doped Germanium

The nature of oxygen donors and radiation defects in oxygen-doped germanium were studied through measurements of the infrared absorption spectrum, deep level transient spectroscopy spectrum and carrier concentration. It is revealed that a new donor is not formed in oxygen-doped germanium. An A-center (interstitial oxygen-vacancy pair) forms a complex with a thermal donor in its anneallng stage at 60°C-140°C. The introduction rate of defects by 1.5 MeV electron irradiation was enhanced in thermal-donor-doped samples.

Superconductivity induced by irradiation in La2CuO4

Abstract Superconducting transition below a resistivity peak is observed in La 2 CuO 4 irradiated with low flux reactor radiation at 350 K. The transition temperature increases with increasing irradiation dose up to T c (onset) = 48.9 K at 4.8 ×10 16 n/cm 2 ( E > 0.1 MeV). The result is compared with previous irradiation experiments.

Property of Radiation-Induced Defects in Germanium Single Crystals

To study the property of defects produced in high-purity and p-type germanium single crystals by thermal neutron, fast neutron or electron irradiation, annealing experiments on irradiated samples were performed with electrical measurements and a deep-level transient spectroscopy technique. The annealing behavior of defects produced by the recoil energy of (n, γ) reaction in a high-purity sample irradiated with thermal neutrons indicates that interstitial arsenic atoms and interstitial gallium atoms are restored to lattice sites during annealing in the temperature range from 180° C to 240° C and that from 260° C to 340° C, respectively. The defects produced by neutron irradiation were annihilated by annealing for 20 min at 380° C. Two hole traps located at E v+0.32 eV and E v+0.53 eV were formed in a high-purity crystal by electron irradiation.

Radiation Defects in Thermal-Donor-Doped Silicon

The nature of radiation defects in thermal-donor-doped silicon crystals was studied by analyzing the deep level transient spectroscopy (DLTS) spectrum and carrier concentration. It is ascertained using the DLTS technique that the concentration of thermal donors decreased with annealing of A-centers. Namely, an A-center interacted with a thermal donor during annealing and both lost their electrical properties. The energy levels at Ec-0.17 eV, Ec-0.23 eV and Ec-0.43 eV were introduced in samples doped with group III atoms or group V atoms by irradiation. An Ec-0.27 eV level and Ec-0.21 eV level were generated during annealing at 240°C and 320°C, respectively. Sixty percent of the electrically active defects introduced by neutron irradiation were not annealed after 20 min at 400°C.

Changes of conductivity and superconductivity in LnBaCu oxides caused by neutron irradiation

Abstract In-pile measurements of resistance are continuously performed during reactor irradiation at 25 or 360 K for YBa2Cu3Ox, ErBa2Cu3Ox, LaBa2Cu3Ox and La2CuO4. The temperature dependence of resistance is also measured before and after the irradiations using low-temperature loop. Broadening of superconducting transition is observed at low fluences less than 1×1017 n/cm2. In non-superconducting LaBa2CuOx the zero-resistance temperatures are increased with the 360 K irradiations.