Masato Nishi

Paramagnetic Centers in Neutron-Irradiated Li2O

Four paramagnetic centers, i.e. F+, O-, H° and colloidal Li centers, were observed in neutron-irradiated Li2O by esr. The former two decay at 300°C and at 65°C, respectively. The concentration of H° centers increases by thermal annealing beyond 200°C. The electron density of F+ center on the nearest neighbor Li ions was obtained from hyperfine structure of the spectrum. The g values of O- centers are g⊥=2.0586 and g//=2.0015. The line shape of a resonance pattern of O- centers was calculated to fit to the experimental pattern.

Emission spectra in orthorhombic PbCl2 at low temperature

Abstract The emission spectra and its time decay in PbCl2 at low temperature are reported. Three emission bands lying at 3.81 (uv), 2.93 (blue) and 2.60 eV are found for the photo-excitation in the intrinsic absorption region, at 4.2 K. An exponential decay with decay time of 11.4 μs is observed for the uv emission. For the blue emission two kinds of time decay spectra are observed: a second order process below 10 K and an exponential decay above 14 K. The mechanisms of these emission are discussed by taking into account the above results.

ESR Study of Pb3+ Ions in Cubic PbF2 Crystals Irradiated with γ-Rays and Neutrons at Low Temperatures

The ESR spectra of Pb 3+ ions in cubic PbF 2 crystals produced by irradiation with γ-rays and neutrons at low temperatures have been studied. Besides a large hyperfine structure from 207 Pb nucleus a complex superhyperfine structure due to the interactions with surrounding F and Pb nuclei is observed. From Breit-Rabi diagram the hyperfine splitting constant A ( 207 Pb) can be determined; its reduction with regard to the free ion value is discussed. From the measured angular dependence of the superhyperfine structure the principal values A (F) // , A (F) ⊥ and a ( 207 Pb) of the SHF-tensors are obtained. This induced ESR can be bleached thermally at around 175 K.

Doppler Broadening Measurement of Positron Annihilation in Alkali Halides

Doppler broadening method of positron annihilation (DOBPA method) is developed, and applied to the study of electronic structures of alkali halides. Radiation from positron annihilation in 14 kinds of alkali halide single crystals were measured with a Ge(Li) detector system. Observed Doppler broadened spectra were unfolded by computer, and intrinsic distributions were obtained. Electron momentum distributions were extracted from these distributions. To study anisotropy of the momentum distribution, radiation was measured along the [110] direction as well as along the [100] direction. Anisotropy of the momentum distribution was observed except for lithium halides and cesium halides. The strongest anisotropy was found in RbBr.

Tritium Gas Target for T(d,n)He^4 Reaction

This paper reports a tritium gas target filled up to 6.5 atmospheres which has been used for the production of neutrons by the T(d, n)He4 reaction. Tritium gas is supplied from a uraniumtriteride (UT3) oven. Discussions are given of gas handling procedure and temperature dependence of decomposition pressure. This target tolerates a 1 µ amp. beam of 18 MeV. deuterons from I.N.S. (Tokyo) 160 cm cyclotron without foil damage and gas leakage for several days.

Doppler Broadening Measurement of Positron Annihilation in Rare Gas

Doppler broadening method of positron annihilation is applied to the study of electron momentum distributions in rare gases; He, Ne, Ar and Xe at 1 atm. In addition, the average energies of annihilation radiations for these gases are determined by means of the precision gamma-ray energy measurement. Two-quantum annihilation of positronium is not observed in our experimental results for rare gases.

ESR Study of Neutron-Irradiated LiCl Crystals

An ESR study of LiCl crystals irradiated with neutrons at about 20 K has been performed. In addition to the spectra due to the hydrogen and tritium atoms, those of the VK and Vn centers were observed at LNT. These hydrogen and tritium atoms, designated as Hs0 and Ts0, respectively, were assigned to the substitutional atoms. The hydrogen and tritium atoms associated with vacancies, named as H0 and T0, respectively, were observed in the crystals irradiated subsequently with 60Co γ-rays or tritium β--rays at LNT after the Hs0 and Ts0 atoms were annealed thermally. As a model for the Vn center, the Cl2- molecular ion associated with two nearest Li+ ion vacancies has been proposed from the analysis of the spin-Hamiltonian parameters.

Doppler Broadening Measurement of Positron Annihilation in Alkali Hydrides and Alkali Fluorides

Doppler broadening of the positron annihilation in LiH, NaH, LiF and NaF were measured with a Ge(Li) detector. The annihilation radiation spectra were observed up to higher momentum regions and an influence of the positive ions on the momentum distribution was investigated. It was shown that the distribution of LiF is broader than that of NaF, but the higher momentum regions of them are almost the same. The distribution of NaH is slightly narrower than that of LiH, but both distributions are narrower than those of LiF and NaF. The momentum distributions were decomposed into two or three components and the interference term was discussed.

Br2- Molecular Ions in Simple Cubic CsBr

Br 2 - molecular ions in simple cubic CsBr produced with 60 Co γ-rays at liquid nitrogen temperature have been studied by EPR. EPR spectra have been analyzed in terms of the usual spin-Hamiltonian on the assumption of axial symmetry of the molecular ions. The parameters are determined to be g // =1.9824, g ⊥ =2.171, A // =432.8 G and A ⊥ =73.0 G for the 81 Br 2 - molecular ion by using the value of 28.5 G for the nuclear quadrupole term. These molecular ions decayed thermally above 165 K.

Interstitial fluorine centers in the neutron-irradiated MgF2

Abstract The interstitial fluorine center is observed in a neutron-irradiated MgF 2 . It is F − 2 molecular ion; the hole trapped by the ion resides longer at one fluorine than at the other. Spin Hamiltonian parameters are determined from angular dependence of the ESR-spectrum. The center decays at about 550 K.