P.K. Tseng

High‐precision harmonic magnetic‐field measurement and analysis using a fixed angle Hall probe

A special theoretical concept involving the harmonic magnetic‐field measurement and analysis method using a fixed angle Hall probe was developed at the Synchrotron Radiation Research Center (SRRC) for measuring as well as analyzing the triple bending achromat magnets of the storage ring. The method can be used to simulate the harmonic magnetic‐field calculation of the two‐dimensional (2D) ‘‘magnet’’ program. This method has a quite simple mechanism. The Hall plate was fixed at a 0° angle with respect to the horizontal plane of magnet to measure only the vertical field. Additionally, the 2D nonlinear least‐square fitting was used to fit the mapping data so as to find the normal and skew multipole field strength. If necessary, the angle of Hall probe could be rotated 90° precisely for measuring the horizontal field. Consequently, the skew dipole field could be obtained. A real‐time Hall‐probe automatic magnetic‐field measurement system was also developed for fulfilling the concept of the harmonic field meas...

Temperature dependence of positron annihilation in KBr single crystals

Abstract Positron lifetimes and peak counts of 2-photon angular correlations in KBr single crystals have been measured as a function of temperature in the range between 20°C and 400°C. Results are discussed with current existing models.

Temperature dependence of positron lifetimes in mercury

Abstract Marked increase in positron lifetime was observed in mercury around the melting point. Below the melting point the quenched samples have lifetimes consistently closer to the liquid values than the slow cooling samples.

Study of magnetic properties of Y3AlxFe5-xO12 by Mössbauer effect

Abstract The garnets Y3Fe5O12 and Y3Al1.25Fe3.75O12 were studied by means of the Mossbauer effect. The longitudinal relaxation rates of both a- and d-site ferric ions were obtained for 300 K ⩽ T ⩽ TN. It was suggested that the dipole-dipole interaction plays the dominant role in the relaxation.

Detection of a change in the oxidation state on aluminum surface using angular correlation of positron annihilation radiation

Abstract Annealed pure Al oxidized in air at various temperatures, 20, 430 and 480°C, was investigated by positron angular correlation experiments. It was found that the intensity of the low-momentum component (parabola portion) increased dramatically for the sample oxidized at 480°C as compared with those oxidized at 430 or 20°C. The results were explained as due to the interaction of a positron with anhydrous γ-Al 2 O 3 which was formed at an oxidation temperature of ≈ 480°C but not at 430°C or less. It was concluded that it is possible to use the positron method as a means of NDE to evaluate the types of oxide states formed on Al.

Positronium formation in xenon gas near the critical point

Abstract The amount of positronium formation in xenon gas has been determined from the intensity of the long lived component, I3, in the positron decay spectrum. I3 remains essentially constant for temperatures T > Tc + 1 along the critical isochore. It decreases from the constant value for T Tc − ΔT. These variations of the positron formation are interpreted as due to scattering of the slowing down positrons by the increasing critical fluctuations which occur when T is near Tc.

Angular correlation of positron annihilation in KBr

Abstract The angular distribution of two photons from the positron annihilation in the pure KBr crystal and in the colored KBr crystals are measured. The effects of F-centers on the positron annihilation in KBr crystal are analyzed.

Microprocessor‐based multiscaler for angular correlation of annihilation radiation experiment

A microprocessor‐based multiscaler system for angular correlation of annihilation radiation (ACAR) experiments is reported. This system is capable of registering three 64‐channel multiscaling spectra simultaneously in order to satisfy special requirements of ACAR experiments. The system is low cost and simple and can be modified for other special applications by minor changes in the software in the microprocessor.