Studies on Fall-out Particles, (I)

Abstract

Highly radioactive fall-out particles were found throughout Japan, resulting from the series of large scale nuclear test explosions carried out by U.S.S.R. during September and October, 1961. Fig. 1 shows the change of radioactivity in air-borne dusts since the beginning of these explosions and Plates 1 (a)-(h) and Table 1 show autoradiographically the changing aspect of fall-out particles in the atmosphere during this period. Many highly radioactive particles were collected and isolated from among numerous non-radioactive dust particles after elaborate procedures. Photomicrographs of them were taken and are shown in Plates 3-24 and Table 2. Plates designated by (P) were taken focussing at the perimeter of particles, showing the shape of the particle contour clearly, but the front side only vaguely. Plates designated by (F) and (M) were taken in order to show the change in depth of the surface state of particles, focussing at the front side of particles and at the middle part between the front side and the perimeter, respectively. We observed microscopically more than fifty highly radioactive particles. Most of them were quite, or nearly, spherical. There were, however, a few particles which deviated appreciably from sphere, as shown in Plates 13 and 16. The surface of some particles was irregular and creased. Several particles were coated with colorless amorphous materials like frosting, as shown in Plates 10 and 11. These materials were assumed to be non-radioactive because they had the same appearance as that of non-radioactive dust particles which were found numerously in air-borne dusts. Several particles had a few small particles of the same color attached on their surface, as shown in Plates 8-11. Almost all of the particles were yellowish, or reddish brown, however, there were some colorless, golden, or black particles. All the highly radioactive particles that we could isolate from among dust particles seemed to be so hard that only an impact of a needle could not crack them into two or more particles. It was found that radioactivity is roughly proportional to volume, as shown in Fig. 2. It seems that particles of light color have larger specific activity than those of dense color and that their conditions of particle formation were different from each other.