Measurement of Natural Radioactive Dust in the Atmosphere by Electric Precipitator

Abstract

A new automatic continuous recording system was set for a trial in order to measure the natural radioactive dust in the atmosphere. This recording system can measure not only the alpha-ray, but also the beta-ray at the same time. Sampling time can be arbitrarily adjusted to the half-life of radioactive elements.
As the aluminum tape reaches the d etector in five seconds after the collection of dust on it, so the apparatus can measure radioactive elements having such a short half-life as ⁸⁴Po²¹⁸.
The time function of the Radon daught e r collected on the aluminum tape is considered. It is possible to estimate the Radon amount from the value of ⁸⁴Po²¹⁸, provided that there exists an equilibrium between Radon and ⁸⁴Po²¹⁸. The radioactive equilibrium between ⁸⁴Po²¹⁸and its daughter products does not hold in the atmosphere owing to precipitation, diffusion and deposition. This non-equilibrium can be estimated from the decay curve of the alpha-ray.
High efficiency against submicron particles is required of collectors, because daughter products are attached to submicron particles. The electrostatic method can well trap particles of small sizes.
A field observation is done from May 16,1961, t o May 20 and from June 19 to June 24 at Tokyo, and from July 8,1961, to July 12at Karuizawa. Meteorological elements are measured and the dust monitor is used in these observations at the same time. The results show that vertical diffusion is predominant over the diurnal variation of the radioactivity. Air temperature and humidity are not direct causes of that variation. Dusts, particularly dusts larger than a micron in diameter, do not exert any influence on the radioactivity.