In this paper are discussed the -whole aspects of the bispectrum of stationary time series: its definition, estimation, and application to signal processing. The bispectrum is defined as the two dimensional Fourier transform of the third order correlation function, and gives a description of the interrelation between the three frequency components at frequencies whose sum is equal to zero. The bispectrum Φ (ƒ1, ƒ2) can also be obtained as an averaged product of the outputs of three band-pass filters, with center frequencies ƒ1, ƒ2, and ƒ1+ƒ2, respectively. A simple procedure of derivation of the variance and other quantities necessary for designing an estima-tion procedure of a bispectrum is obtained through this physical interpretation. Applications of the bispectrum analysis to such problems as the analysis of EEG (electro-encephalogram), extraction of periodic signals submerged in noise, analysis of roughness of surface, and system identification are discussed and some examples are given. The examples show that the bispectrum can be very useful in many practical applications.
Permalloy films have been prepared on glass substrates by RF sputtering. Coercive force, resistivity and distributions of these characteristics on the substrate holder were investigated as functions of power density, sputtering argon gas pressure and amount of hydrogen in argon gas. The following results were obtained. (1) As power density increases, coercive force and resistivity decrease and uniformities of these characteristics are improved. (2) At optimum argon gas pressure, coercive force and resistivity have minimum values and improved uniformities of these characttristics are obtained. (3) As containing of 0.5_??_10mol% hydrogen in argon gas, low coercive force, low resistivity and good uniformities of these distributions are successfully obtained.
Aluminum nitride films (AINx) containing various amount of nitrogen were prepared by an R. F. reactive sputtering. The deposition rate, electrical resistivity, crystal structure and chemical composition were observed systematically on the sputtered films prepared under the nitrogen partial pressures ranging from 1×10-4 to 2.5×10-2 Torr in argon-nitrogen mixed gas (total pressure was 2.5×10-2 Torr). The deposition rate of the films was found to decrease drastically at a critical partial pressure of nitrogen, 4×10-3 Torr, while the resistivity of films began to increase gradually over the same critical pressure. X-ray diffraction measurement of the films prepared under various pressures of nitrogen was carried out. It was found that the structure of the films also began to change in the region of the critical pressure, namely, only the diffraction pattern from metallic Al was observed for the nitrogen partial pressures below 6×10-3 Torr, then the diffraction pattern from AIN began to appear over 6×10-3 Torr. Chemical compo-sition of the films determined by X-ray microanalysis showed the change corresponding to that of the crystal structure with the nitrogen pressure. On the basis of the experimental data, discussion was made on the change of deposition rate of the films with nitrogen pressure by applying a theoretical model for the sputtering process of target surface, and on the correlation between the electrical resistivity and the crystal structure of the sputtered films.
We Studied basic processes on thermal ionization by reflected shock waves with a pressure driven shock tube. Then we applied this ionization phenomenon to a crowbar method which short-circuits electric current of L-C circuits. Rather restricted shape of crowbar electrodes and use of ambient air in laboratory as a test gas did not decrease so much the insulating voltage of crowbar gaps. To breakdown the crowbar gaps by reflected shock waves with incident mach number of 7_??_16 in air at initial pressure of 5_??_250 μHg, electrodes voltage must be applied by 700_??_400 V and the deduced conductitivity of shocked plasma was 10-11_??_10-9_??_/cm. To crowbar the current successfully, thermal ionization played the most important role though the gas conditions crossed the Paschen's minimum just in shock fronts. Various improvements and another application of this type of experiments are proposed and shown briefly.
X-ray film emulsion consists of silver halide grains of various sizes, which have a large dispersion. If therefore, silver halide grains are considered to be bleached by reversal develop-ment from larger grains of higher sensitivity to smaller ones of lower sensitivity, it is to be expected that the image will be formed by the blackening of minute silver halide grains at the second exposure. Based on the assumption the image quality of reversal X-ray film was experimentally investigated. As the result of experiment, it became clear that reversal X-ray films have better granularity than negative X-ray films in the high exposure domain above a certain exposure time. They are also excellent for clinical purposes and detailed diagnostic views can be formed from them.
A Simple method is described for calibrating the counting efficiency of a neutron detector. First, the absolute efficiency is measured by the usual method of neutrons scattered at different angles from hydrogen, the incident energy being fixed. At the same time, the energy spec-trum (reference spectrum) is measured of neutrons scattered at a given angle from a certain scatterer (for example, Fe scatterer). Then, any variation of counting efficiency because of different experimental conditions may be found by comparing the energy spectrum of neutrons measured in the same way by using the Fe scatterer with the reference spectrum. An example is given for the case of 14. 1 MeV neutrons. This method makes it possible to save considerably the time and labor to calibrate the counting efficiency.