Supplementary Report on the Radioactivity of Coal-Measures in Some Coalfields of Kyûshû

Abstract

In my last report on the radioactivity of coal and attendant rocks in the Saga Coalfield(Vol. 6(4)of this magazine), the relatively high radioactivity of all things in the vicinity of a rhyolite intrusion in the mine of the Meiji Saga Coal Mine was noted. But, as the measurements were performed in an existing gallery, it is suspected that the results maybe due to the direct influence of the rhyolite, though there may be practically no influence of the accumulating β-ray sources on account of the full ventilation of the mine. The writer, then, directed to take and send the hand-samples of the coal. and rocks at the points where the counts were taken to ascertain whether or not these things by themselves have relatively high radioactivity compared with those of other places. The measurements were done in Tokyo, and the results showed that radioactivity of these samples, measured 12 days after they were taken, also increased according to the nearness to the rhyolite intrusion(Tab.1), as those of matters in the original places, while the samples from other parts of the mine where the igneous rock is absent showed the normal low counts(Tab.2). It is, therefore, concluded that the radioactive influence of the rhyolite intrusion, permeates the coal and walirocks in its vicinity, and that the influence is greater near the intrusion, though it is not yet certain whether the it is limited to the mine wall or is more wide-spread.
The intrusion of basic volcanic rock such as basalt, on the other hand, seems to have no such influence on the coal-measures, according to the recent survey in the Akaike Coal Mine Fukuoka Pref.(the data are omitted). This is to be expected from the low counts of basalt itself both in the original rock-body and in hand-samples(Tab.3). Moreover, it is to be noted that the radioactivity of basalt differs according to its surroundings. As shown in Tab. 3, radioactivity is extremely low where basalt is intruded in the coal seams, while it is relatively high where it is in contact with shale. Basalt in this area seems to be very low in its radioactivity, but may be somewhat higher than coal.