Keizo Yamasaki

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.

Trace Elements with Large Activation Cross Section in Concrete Materials in Japan

Amounts of trace elements with large activation cross section in concrete materials were measured to offer the basic data for developing of low activation concrete. From the measurements, the quantities of the activated radioactivities in biological shielding concrete were measured and evaluated for the clearance level. The average concentrations of 60Co, 152Eu and 134Cs formed in concrete were 21.9, 1.08 and 3.21 ppm, respectively. The combination of the concrete materials for the most lowering concentrations of 60Co, 152Eu and 134Cs was the limestone as aggregate and the white Portland cement produced in specific places. The most lowering concentrations of this limestone concrete were 0.16, 0.049 and 0.060 ppm, respectively. The limestone concrete was excellent as biological shielding concrete, because the neutron shielding effect was excellent a little compared with ordinary concrete. If this concrete used for biological shielding concrete, concrete waste will be able to handle as follows. Usage of this limestone low-activated concrete makes almost all concretes satisfy the clearance level for 60Co after 20 yr cooling from decommissioning. In respect of 152Eu, radioactivation quantity in the biological shielding concrete is reduced up to a half of the average value or less. With regard to 134Cs, all concrete satisfies the clearance level.

Applicability of the electrical low pressure impactor to size determination of aerosols attached to radon decay products

Aerosol size information is very important for the risk estimation of radon exposure. Recently, a cascade impactor called the electrical low pressure impactor (ELPI) was developed and commercialized. The impactor, which covers a wide diameter range from 0.03to10μm, gives number-weighted size distribution in real time by an electrical detection method. Prior to size measurements of the radon decay products being available, the effects of impaction substrates on size classification were studied. Tested substrate materials were stainless steel plate and aluminum foil, and those surfaces were as follows: (1) untreated, (2) covered with double sided adhesive tape, and (3) silicon grease coated. A large difference was observed among the measured size distributions. Compared with other sizing methods, it was found that the use of grease coated substrates did not interfere with the electrical measurements in the ELPI and also prevented particle rebound or redispersion. It was also confirmed that the coatings caus...

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.