Shin-ichi Uehara

A multicounter goniometer for gamma-gamma angular correlation measurements

Abstract A multicounter goniometer for gamma-gamma angular correlation measurements was newly designed and constructed. It consists of 12 photomultipliers with NaI(Tl) crystals placed radially at intervals of 30° and a 12 × 12 coincidence matrix circuit which follows 12 linear amplifiers and 2 × 12 single channel pulse height analyzers. Coincidence counts at 11 angles between 30° and 330° are taken simultaneously for each of the 12 counters, so that with this method angular correlation measurements can be made within a time reduced by a factor 12 × 11, as compared with the ordinary method.

Gamma-Gamma Angular Correlations in the 5.8 min 51Ti and the 5.1 min 66Cu

Angular correlations of the 609–320 keV gamma ray cascade in 51 V and of the 833–1039 keV gamma ray cascade in 66 Zn have been measured using a multicounter goniometer. In the case of 51 V the correlation function is W (θ)=1+(0.075±0.024) P 2 (cos θ)-(0.029±0.028) P 4 (cos θ), which is compatible with δ 609 =0.14 or -6.5 for the spin sequence of 3/2–5/2–7/2. In the case of 66 Zn the correlation function is W (θ)=1+(0.30±0.05) P 2 (cos θ)+(0.21±0.07) P 4 (cos θ), which gives 1.57≤δ 833 ≤2.27 for the 2( D , Q )2( Q )0 sequence. The relative intensity ratio between the assumed crossover 1872 keV gamma ray and the 1039 keV gamma ray should be less than 5×10 -5 . A new 1333 keV gamma ray was found in 66 Zn. The relative intensity of it to the 1039 keV gamma ray was determined to be 4.3×10 -4 .

Excited Levels and γ-Transitions in 151Pm

A higher part of the level scheme of 151 Pm up to about 2 MeV has been investigated by measuring γ-ray singles and γ-γ coincidences. The energies and relative intensities of 218 γ-transitions with E γ >500 keV have been determined, and levels at 870.33, 1200.8, 1618.0, 1810.0, 1853.7, 1903.1 and 1933.9 keV have been introduced. The new level scheme has been discussed on the basis of the rotation-vibration coupling model.

Permeability of fault rocks from the Median Tectonic Line in Ohshika-mura, Nagano, Japan as studied by pressure-cycling tests

Abstract We describe laboratory measurements of the permeability of fault rocks and their host protoliths obtained from the Median Tectonic Line (MTL), the largest strike-slip fault in Japan. The measurements are made using a gas-medium apparatus that simulates in situ conditions. Samples of fault gouge, cataclastic mylonite, and protoliths were collected from the Kitagawa and Ankoh outcrops of the MTL and adjacent areas in Ohshika-mura, Nagano Prefecture, central Japan. Permeabilities of these samples were measured at room temperature under dry conditions, with nitrogen as the pore fluid. Most samples from the incohesive fault zone have a permeability ranging between 10−13 and 10−17 m2 (100–0.01 mD). These permeabilities are greater than those of cemented cataclasites and mylonites by more than two orders of magnitude at all effective pressures (Pe) up to 180 MPa. Clayey fault gouge material has a permeability as low as 10−19 m2 (0.1 µD) at high effective pressures, but such impermeable fault gouge does not constitute a continuous zone on the two outcrops we studied. Permeability of the incohesive fault rocks exhibits large hysteresis upon Pe cycling, compared with cataclasite and mylonite, because those cemented, cohesive fault rocks undergo much less inelastic deformation during the pressure cycling.

Local structure and valence of impurity ions in oxides by TDPAC and Mössbauer emission spectroscopy

The features of time-differential perturbed angular correlation (TDPAC) in studies on local chemical structures around impurity atoms are described in comparison with those of Mössbauer emission spectroscopy with four typical examples of recent applications to metal oxide systems, namely,99Rh→99Ru in α-Fe2O3 and YBa2Cu3O7−δ and117Cd→117In in MO (M=Mn, Co, Ni) and BaRu2/3Cd1/3O3.

The use of a radiactive beam of fission products for TDPAC experiments

In order to use a wide variety of radioactive nuclei as probes in material research, an on-line isotope separator (ISOL) for fission products has been utilized to produce and accelerate short-lived nuclei suitable for TDPAC experiments. The140Cs isotope (T1/2=63.7 s) from ISOL has been implanted into YBaCuO compounds to investigate the hyperfine field around a Ba atom. The140Ba-140La source of 3×104 decays/s is obtained for TDPAC experiments on the 329–487 keV γ-γ cascade in140Ce. Another candidate was examined to use the 345–94 keV cascade in the91Rb (T1/2=58 s) decay. The angular correlation coefficients, however, unfortunately turned out to be fairly small. A tentative result of theg-factor measurement has been given. The details of the present method have been described with emphasis on the application to material research.

The hyperfine magnetic field in Fe-Hf amorphous alloys at low temperatures

The time-differential perturbed angular correlation (TDPAC) technique was applied to study the hyperfine magnetic field in amorphous Fe-Hf alloys with different Hf contents at low temperatures. The Hf content was changed from 20 to 40 at% andthe TDPAC measurements were carried out in the temperature range from room temperature down to 18 K. Very strong perturbations were observed for the specimen of low Hf content at low temperatures due to a strong hyperfine magnetic field. The characteristic feature observed by the present study is that the temperature dependence of the perturbation for the Fe-30 at% Hf source is quite different from that for other sources, indicating a maximum around 80 K. Discussions have been given on the explanation of the experimental results in the light of magnetic properties of these alloys.

Dynamical observation of the structure change at the melting point of In metal by a time-differential perturbed angular correlation method

Abstract In order to study the structure change between solid and liquid in a metal, the time-differential perturbed angular correlation (TDPAC) experiment has been performed on In metal at the melting point utilizing radioactive 111 In probe atoms. The fraction of liquid in the solid state has been determined quantitatively as a function of time. The attenuation factors in the freezing process were found to be different from those in the melting process.

List mode acquisition method for time-differential perturbed angular correlations

Abstract A new data acquisition method for the time-differential perturbed angular correlation (TDPAC) experiment has been applied to the measurement of electric field gradient in In metal under melting. Combining a 4-detector goniometer with a multiparameter data acquisition system, the temperature of the sample and the time spectrum for each angle are stored event by event in a list mode. The fraction of liquid phase in solid state of In metal has been quantitatively determined as a function of time by this method and the “hysteresis” has been observed in the course of liquefaction. This technique can be applied to study hyperfine interactions by means of the TDPAC method in high frequency magnetic fields.

Structural Phase Transition of BaRu2/3M1/3O3 (M=Ca, Cd, and Sr) Studied by 117In(←117Cd) Time-Differential Perturbed-Angular-Correlation

Electric field gradients at 117 In(← 117 Cd) nuclei in BaRu 2/3 (M 0.97 116 Cd 0.03 ) 1/3 O 3 (M=Ca, Cd, and Sr) were measured in the temperature range between 4 and 773 K by time-differential perturbed-angular-correlation of the 90-345 keV cascade γ -rays. For each oxide, the quadrupole interaction frequency markedly increases below a specific temperature; the temperatures are between 4 and 77 K for M=Ca, and about 300 and 600 K for M=Cd and Sr, respectively. Combined with the results of the X-ray powder diffraction measurements, it is concluded that a hexagonal-monoclinic structural phase transition, associated with a deformation of MO 6 octahedra, takes place at the temperature.