Nobuo Niimura

Short Range Structural Analysis of Lead Silicate Glasses by Pulsed Neutron Total Scattering

The static structure factors of lead silicate glasses (PbO) x (SiO 2 ) 1- x (0< x <0.66) have been measured over a wide range of the scattering vector Q (=4πsin θ/λ) up to 30 A -1 by means of the pulsed neutron total scattering. The peak separation of Pb–O and O–O pair in the radial distribution function was first observed, and the average coordination number of the nearest oxygen atoms around lead atom was determined to be about three. The deformation of SiO 4 tetrahedra was also observed as PbO concentration was increased. The detailed radial distribution analysis based on the pair distributions of the corresponding crystal structure strongly suggests the coexistence of different kinds of Pb–O bond with different distances in the glass system.

Partial Structure Factor of Amorphous Cu0.57Zr0.43 Alloy Determined by TOF Pulsed Neutron Diffraction

The partial structure factors S CuCu ( Q ), S CuZr ( Q ) and S ZrZr ( Q ) are determined consistently from the three total structure factors S ( Q ) for amorphous Cu 0.57 Zr 0.43 alloys with natural Cu, isotope Cu 63 and Cu 65 , which were observed by means of TOF pulsed neutron diffraction. The reduced partial radial distribution functions G i j ( r ) are also obtained by Fourier transforms of S i j ( Q ). The first peaks of G CuCu ( r ), G CuZr ( r ) and G ZrZr ( r ) appear at r =2.65, 2.80 and 3.15 A, respectively. The second peaks of both S i j ( Q ) and G i j ( r ) split into two subpeaks. This is interpreted as a consequence of tetrahedral close packing in amorphous alloys.

Coherent synchrotron radiation at submillimeter and millimeter wavelengths

Abstract To study the coherence effect of synchrotron radiation (SR), light emission from the Tohoku 300 MeV linac, whose energy and bunch length of electrons were 180 MeV and a few millimeters, was observed in the submillimeter and millimeter wavelength regions by using a grating spectrometer. The absolute intensity of ordinary SR in the visible region was also measured. In comparison with ordinary SR, the absolute intensity of SR in the millimeter wavelength region was enhanced by about a factor of 10 6 , which corresponded roughly to the number of electrons in one bunch. The intensity of enhanced SR showed the quadratic dependence on the beam current. The enhanced SR was strongly polarized in the electron orbital plane. The relation between the electron distribution in a bunch and the spectra of coherent SR is discussed.

Crystal Structure and Phase Transition of Hydrogen Chloride

Detailed structural studies of HCl single crystal were performed by X-ray and neutron diffraction experiments. A face-centered cubic lattice with a twelve-fold disordered orientation of HCl molecules was found only above 120 K. Below 120 K, the molecules form hydrogen bonded zigzag chains and a synthetic twin structure of an orthorhombic lattice with the (101) twin boundary is realized. No appreciable change of the crystal structure was seen in the diffraction experiments at the transition point 98 K. The mechanism of the phase transition can be explained by considering a fluctuation of chains and molecular flippings.

A low background neutron measuring system and its application to the detection of neutrons produced by the D2O electrolysis

Abstract A low background neutron measuring system has been constructed. The background neutron counting rate is 1.4 counts/h and the long term stability is within 0.1 counts/h over one year. The detection efficiency for fission neutrons from 252 Cf is 4.4%. The system is capable of measuring low rate neutron emission events with a half life of 10 20 yr. The rate of neutron production from copper and lead by cosmic-rays was measured in an underground laboratory. The system was used for an experimental test of neutron emission from the electrolysis of D 2 O. The results show no enhancement of neutron emission. No tritium production was observed.

Time of flight single crystal diffractometer using a position sensitive detector

Abstract A position sensitive thermal neutron detector system combined with a time of flight (TOF) single crystal diffractometer is described. The circuit of the electronics which is used to determine both the time of flight of the neutrons and the position of detection of the neutrons has been designed, constructed and tested on several neutron diffraction experiments. The measurements of the Bragg reflections of a KBr single crystal and the intensity of the diffuse scattering of Cu 3 Mn are demonstrated.

Neutron diffraction study on molecular libration in solid DCl

Neutron diffraction measurements of DCI using a single crystal revealed that such an anomalous transition as observed in the higher temperature cubic phase of HCl did not occur. Mean square angular displacements due to the molecular libration of DCl were obtained from anisotropic temperature factors of deuterium atom. Their temperature variation showed a rapid increase in the rate of the 60° molecular flipping with increasing temperature. The difference of the molecular motion between DCl and HCl was observed, which could not be explained by a simple harmonic oscillator model.

A New Cryostat for Neutron Diffraction Experiments

A special cryostat for neutron diffraction experiments was designed. Single crystals of substances which are gaseous at room temperature can be grown in the cryostat, and then the diffraction measurements are carried out. Various advantages of this device in comparison with a usual conventional-type cryostat are described.

Local structure of Mn atoms and spins in Ag–Mn spin‐glass alloys

We studied the atomic and spin correlations in Ag–Mn spin‐glass alloys by x‐ray and neutron diffuse scattering. The x‐ray scattering results from Ag‐13.3, Ag‐20.8, and Ag‐28.1 at.% Mn alloys show diffuse scattering due to atomic short‐range order (ASRO) with diffuse maximum at 1,0.5±δ,0 reciprocal‐lattice position, where δ depends on composition. The neutron scattering result from Ag‐20.8 at.% Mn shows sharp diffuse maxima at 1,0.5±δ’, 0 positions near the atomic ones. We derived values for the magnetic short‐range order (MSRO) parameters from both the x‐ray and neutron diffuse intensities and have shown that the two types of spin correlations coexisted in the spin‐glass state; one is the short‐range spin correlation formed on the ASRO structure of Mn atoms and the other is the longer‐range spin correlation with about two times longer than atomic one.

Data acquisition of neutron crystallography on tetragonal and triclinic forms of hen-egg-white (HEW) lysozyme with an elastically bent Si monochromator

Abstract An elastically bent silicon (EBSi) as a monochromator has been optimized for neutron diffractometers of biocrystallography. It was found that several stacked thin Si plates were easier to be bent much for the near focusing point and they increased neutron reflectivity by aligning the plates. Currently, an EBSi(1 1 1) monochromator system was equipped at a diffractometer (BIX-I). It took 50 days to collect about 12 000 reflections of hen-egg-white lysozyme. The minimum d -spacing was 2.1 A.