Michikuni Shimo

Development of an apparatus for measuring ground exhalation rates of 222Rn and 220Rn

A field apparatus with a large area ZnS(Ag) scintillator (30 cm × 40 cm) was developed for measuring simultaneously the exhalation rates of 222Rn and 220Rn from the surface of the ground. The system is based on alpha particles emitted from 222Rn, 218Po, 214Po, 220Rn, and 216Po nuclei floating in the air and deposited on the mylar foil or fringe (skirt), which covers the surface of ground. The alpha particles are subsquently counted at 1-min intervals for about 10 min. Setting a detection limit equal to three times the standard deviation of the background count rate, the detection limit of this system corresponds to 140 mBq m−2 s−1 for 220Rn and 6.8 mBq m−2 s−1 for 222Rn. This detection limit of 222Rn, which decreases with time, corresponds to a 30-min sampling.

Measurement of small ions and condensation nuclei over the sea near the land

SummaryTo study the ionization equilibrium, simultancous measurements of the concentration of small ions (n), concentration of condensation nuclei (Z), and the diffusion coefficient of condensation nuclei (D) have been carried out over the sea. If we take the variation ofD into consideration, the correlation among them was found to be well expressed by the simple formula;q=βn Z. The dependence of the effective attachment coefficient of small ions (β) upon the size of nuclei (2r) were also studied, and β was found to correlate well withD orr. Slight differences of the dependence of β uponD orr over the sea from that over the land seem to be well explained by the difference of charged states of nuclei over the sea and land.ZusammenfassungZur Untersuchung des Ionisationsgleichgewichts wurden gleichzeitige Messungen der Konzentration schneller Ionen (n), der Konzentration der Kondensationskerne (Z) und des Diffusions-Koeffizienten der Kondensationskerne (D) über dem Meere ausgefürt. Unter Berücksichtigung des Diffusions-Koeffizienten (D) konnte der Zusammenhang zwischen den beiden anderen Grössen recht gut durch eine einfache Formel ausgedrückt werden:q=βn Z. Die Abhängigkeit des wirksamen Anlagerungskoeffizienten für schnelle Ionen (β) von der Grösse der Kondensationskerne (2r) wurde ebenfalls untersucht, und es wurde gefunden, dass β recht gut mitD oderr korreliert. Kleinere Unterschiede in der Abhängigkeit von β vonD oderr, welche bei einem Vergleich der Messungen über See und über Land festgestellt wurden, liessen sich gut durch den Unterschied der geladenen Zustände der Kondensationskerne über See und Land erklären.

On some properties of 222Rn short-lived decay products in air.

Simultaneous measurements were made of such properties as the fraction of charged and uncharged atoms, the balance of radioactive equilibrium between 222Rn and its daughters, and the concentration of aerosol particles and their mean radii in tunnel air. It became clear that the behavior of 222Rn decay products in tunnel air could be expressed well by equations based on a simple model, taking the following into account: the attachment of free atoms to aerosol particles, the deposition of radioactive particles on the tunnel wall, emission of alpha recoils from aerosol particles and the surface of the tunnel wall, and radioactive decay. In addition, the effective attachment coefficient of an observed RaA-atom was found to agree well with that calculated. The results obtained should facilitate in the future estimation of the relation between 222Rn daughters and the lung dose to the population.

Measurements of the effective attachment coefficient between RaA ions and condensation nuclei

SummaryTo clarify the interaction between RaA ions and condensation nuclei, simultaneous measurements of the concentration of RaA ions (nA), radon-222, and condensation nuclei (Z) were carried out at several stations. In some occasions, the diffusion coefficients of nuclei (D) were also measured. It was found that the correlation among them may be well expressed by the simple formula;qA = βA nA Z. The correlation between βA andD (or radius of nuclei) was also obtained.The effective attachment coefficient of RaA ions was compared with that of small ordinary ions, and it was presumed that about one fourth of RaA atoms formed on the decay of radon-222 is positively charged, and the rest is neutral.

A mysterious spot on the outdoor concentration of radon isotopes

Abstract Temporal and spatial distribution of 222 Rn and 220 Rn and their progeny concentrations in the open atmosphere, 222 Rn and 220 Rn exhalation rates, radioactivities in soil and underground water, including some meteorological factors, were measured at a spot of high 222 Rn and 220 Rn concentrations to investigate the source and occurrence mechanism. Hourly mean concentrations of 222 Rn and 220 Rn during August 1993 were about 230 Bq m −3 and 130 Bq m −3 at 1 m above ground surface. Diurnal variations were high at daytime and low at nighttime, which gives an inverse variation compared with the typical variation in the normal environment. 222 Rn concentrations varied quickly in a few minutes and reached a maximum of about 14 kBq m −3 . The maximum daily mean 222 Rn concentration at 5 cm above ground surface was 11 kBq m −3 . Concentrations of 218 Po, 214 Pb, 214 Bi, and 220 Rn progeny were 8∼61 Bqm −3 , 1∼7 Bqm −3 , 0.5∼5 Bq m −3 , and 0.02∼0.14 Bq m −3 , respectively. The equilibrium factor for 222 Rn was estimated to be 0.01∼0.2. The main source of the mysterious spot of high 222 Rn and 220 Rn concentrations was a small but deep channel between the basement of the building and the surrounding soil, and the extremely localized weather condition.