S. Yabushita

Tschebyscheff Polynomial Approximation Method of the Neutron‐Transport Equation

The Tschebyscheff polynomial approximation method of the neutron‐transport equation is developed. The relations between the relaxation constants in this approximation and the positive roots of TN+1(μ)=0 are derived. Using these relations, the necessary condition for reactor criticality is discussed. Application to Milnes problem leads to an explicit expression for extrapolated end point which is formally the same in the spherical‐harmonics method. Numerical comparison of this method with the spherical‐harmonics method verifies Conkies conclusion [W. R. Conkie, Nuclear Sci. and Eng. 6, 260 (1959)] that, for weak absorbers, the spherical‐harmonics method gives the values for extrapolated end point which is closer to the exact value than the TN method does while, for strong absorbers, the TN method gives closer value than the PL method does.

Simulation of grain-growth in the presence of second phase particles

On considere le cas ou les particules de seconde phase sont dispersees et ne forment pas des grains par elle-memes; on propose un modele applicable a ce cas

Periodicity and decay of craters over the past 600 myr

The data set of Grieve which provides diameters and ages of craters is analyzed to obtain periodicity of the formation rate and decay constant of craters. It is confirmed that large craters (D ⩾ 10 km) do not exhibit any periodicity while small ones appear to satisfy the Broadbent criterion for quantum (periodicity) hypothesis at P ≃ 29.5 myr. The result is consistent with a recent study of Bailey who showed that large craters are largely due to asteroids. Allowing for the decay of craters, an excess of 4 small craters within the nearest past is detected. In this sense, one may argue that the solar system is now in a moderate comet shower.

Radioactive heating and layered structure of cometary nuclei

Following the work of Whipple and Stefanik, radioactive heating by uranium, thorium and pottasium of a cometary nucleus is discussed. The assumed composition is that of interstellar medium. If thermal diffusivity is 10−4 cm2s−1, the central temperature of a nucleus with radius 10 km can be above 50 K, while if the thermal diffusivity is 5 × 10−3, the central temperature can be only 25 K or so. Volatile gases such as N2 and CO will flow toward the outer part of the nucleus and are lost in their first several approaches to the sun. This mechanism appears capable of explaining the depletion of N2 and CO relative to the interstellar abundance. It is argued that unfamiliar activity of comet Bowell could be explained by sublimation mainly of N2 and CO.

A spectroscopic study of the microorganism model of interstellar grains

The microorganism model of interstellar grains is investigated by spectroscopy from the infrared (IR), visible to the ultraviolet (UV) wave regions.E. coli, yeast and spores ofBacillus subtilis exhibit absorption bands at λ=3.1 and 9.7 μm; they also exhibit several absorptions at 6∼8 μm which are in agreement with the observed IS extinction curves.To obtain the extinction curves in the visible and UV regions, dry films of microorganism are prepared on a MgF2 plate or synthesized quartz plate and their spectra measured. In the wavelength region 190∼400 nm, conventional spectrophotometers are adopted for the measurement. The extinction curve of the film ofE. coli is similar to the observed IS curve.For the wave-range 100<λ<400 nm, a vacuum UV spectrometer is adopted to avoid absorptions due to O2 in the atmosphere. The extinction spectra by this method are in agreement with the result obtained by the conventional method where comparison is possible. The extinction curves ofE. coli and yeast are such that they incrase towards the short wavelength and exhibit a peak at λ-190 nm, which is different from the well-known IS peak at λ=220 nm. It remains to be seen whether interstellar low temperatures (10∼40 K) can shift the peak position in the extinction curve of biochemical materials.

The infrared and ultraviolet absorptions of micro-organisms and their relation to the Hoyle-Wickramashinghe hypothesis

Absorption of yeast andE. coli in the infrared and ultraviolet regions and that of diatomaceous soil in the infrared region have been obtained. Electron microscope photographs of aggregates ofE. coli have also been obtained. These results are discussed in relation to the Hoyle-Wickramasinghe hypothesis regarding the nature of interstellar grains.

Maximum non-gravitational acceleration due to out-gassing cometary nuclei

Maximum possible acceleration due to out-gassing from cometary nuclei is calculated for H2O and CO(N2) molecules. It is found that the maximum excess velocity at great distance is 0.18 km s−1 so that excess velocities less than this value are compatible with the non-gravitational acceleration due to non-symmetric out-gassing. On the other hand, Comet 1975q and comet 1955V have excess velocities 0.81 and 0.80 km s−1 respectively. These comets may be regarded as the candidates for possible interstellar comets.

Organic model of interstellar grains

Extinction efficiency of grains is calculated from the Mie formula on the premise that the grains are of organic composition. The optical constants adopted for the calculations are those ofE. coli, polystyrene and bovine albumin. The grain radiusa is assumed to obey a distribution of the formN(a) ∝a−α and the value of α is chosen so as to make the calculated extinction curve match the observed interstellar extinction curve. Although the calculated curve gives a reasonably good fit to the observed extinction curve for wavelength less than 2100 Å, at longer wavelength region agreement is poor. It is concluded that another component is required for the organic model to be viable.

Are periodicities in crater formations and mass

Periodicities in crater formation rate and mass-extinctions are reviewed. The former exhibits a period of 30 million yr, while the latter appear to have a periodicity at 26 myr. Results obtained earlier that small craters better satisfy the adopted criterion for statistical testing is shown due to the fact that there is a strong clustering of small craters in a recent past (<10 myr). On the basis of the dataset of craters compiled by Grieve, it is shown that there are several craters for which no mass extinctions correspond. The difference in the periods of the craters and of mass extinctions and the lack of mass extinctions that correspond to large craters appear to suggest that the two periodicities are not interrelated, and large impacts merely act as triggers for the mass-extinctions; the only exception being theK/T boundary.

ON THE POSSIBLE HAZARD ON THE MAJOR CITIES CAUSED BY ASTEROID IMPACT IN THE PACIFIC OCEAN - II

Earlier, it was pointed out that asteroids with diameters close to 200 m that hit the Pacific Ocean once in 104 y create large tsunami waves which could cause serious hazards to the cities and countries lying along the periphery. Here, a more detailed investigation is carried out in relation to tsunami hazards created by earthquakes. It is shown that the energy involved in the asteroid induced tsunami wave is some 300 times greater than large tsunami waves and 100 times greater than the Chile tsunami of 1960, the largest this century. Second, if the impact should take place in the Pacific south of Japan, so that the tsunami wave enters Tokyo Bay, the damage could cost 800 billion dollars, which is 6 times greater than the damage due to the Hanshin (Kōbe-Osaka) earthquake of 1995.