Sadayoshi Kobayashi

Spatial distribution of thoron and radon concentrations in the indoor air of a traditional Japanese wooden house.

A radon-thoron discriminative passive dosimeter has been developed that can estimate both radon (222Rn) and thoron (220Rn) concentrations at the same time. Two polycarbonate films are installed as solid-state nuclear track detectors in the dosimeter housing. One film registers alpha tracks originating from predominantly thoron and its progeny together with a small contribution from radon, and the other film registers alpha tracks originating from radon and its progeny together with a negligible contribution from thoron. The lower detection limit is estimated to be 2.9 Bq m-3 for the radon concentration and 9.0 Bq m-3 for the thoron concentration for 2 mo exposure. Preliminary measurements from 1991-1992, in a traditional Japanese wooden house located in Kyoto, indicated that the indoor thoron concentration increases exponentially as the interior mud (or plaster-coated) wall is approached. A soil-based plaster commonly used in Japanese wooden houses to fill walls (or as a surface coating on the walls) is the probable source of the indoor thoron. Since thoron is not measured by the usual radon measurements, and the majority of Japanese houses are made of wood, attention should be paid to indoor thoron and its decay products, which may give a significant fraction of the total natural radiation exposure to the general public.

Characterization of Japanese wooden houses with enhanced radon and thoron concentrations.

Indoor thoron and radon concentrations were surveyed in the 42 rooms of 21 houses in Hiroshima Prefecture with a radon-thoron discriminative dosimeter in 1992. The survey estimated the average indoor thoron concentration to be 84.7 +/- 15.6 Bq m-3 and that of radon to be 25.6 +/- 1.1 Bq m-3 at 20 cm from the interior wall. The results indicated the potential risk of indoor thoron in the Japanese style wooden houses. Multivariate regression analysis among some factors which characterize each of the houses examined and concentrations of radon and thoron suggested that enhanced concentration of indoor thoron was often observed in a typical Japanese style room with interior soil wall and tatami flooring. Characterization of the rooms with enhanced levels of thoron showed that mud-based plaster wall material was probably the source.

Influence of air flow on the behavior of thoron and its progeny in a traditional Japanese house.

Air flow influence on the spatial distribution of thoron (220Rn) concentration in a typical Japanese traditional house was investigated at various indoor air flow levels. The effect of air flow on the behavior of both thoron and radon progeny were examined simultaneously. Measurements were carried out by using two types of passive monitors, the radon-thoron discriminative monitor and the Radtrak monitor. Thoron and radon progeny were measured by filter grab sampling with ZnS scintillation counting. Under static condition, a horizontal distribution with greatly varied thoron concentrations was found as reported by previous studies. Under turbulent conditions, thoron concentrations in the middle of the room increased and the concentration gradient of thoron gas became lower. An obvious vertical distribution of thoron was also observed. Prominent diurnal variation of radon progeny concentrations was observed whereas that of thoron progeny concentrations was not. Concentration of thoron progeny changed little at different air flow levels, although the thoron gas level at the middle of the room varied significantly. The influence of air flows on detection efficiencies of the two types of thoron monitors were also checked. The mechanism of behavioral change of thoron and its progeny in turbulent atmosphere is discussed.

Vertical distribution of outdoor radon and thoron in Japan using a new discriminative dosimeter.

Passive measurements of outdoor radon and thoron concentrations were conducted from June 1992 to June 1993 at a monitoring station over a soil area (10 m x 6 m) in Chiba city, Japan. The measurement period was divided into 4 parts to investigate seasonal variations of radon and thoron concentrations. Ten passive radon-thoron discriminative dosimeters (R-T dosimeters) were placed in duplicate at 5 different altitudes to show the vertical distributions of outdoor radon and thoron concentrations. Outdoor radon concentrations showed no significant difference within 1.0 m above the ground, and the annual average of outdoor radon concentration was 3.85 +/- 0.19 (SE) Bq m-3. Annual averages of outdoor thoron concentrations at 0.04, 0.15, 0.25, 0.70, and 1.0 m above the ground were 40.5 +/- 4.4, 22.5 +/- 3.7, 13.9 +/- 3.1, 9.5 +/- 2.9 (SE) Bq m-3, and < 9.0 Bq m-3; the lower detection limit of the dosimeter, respectively, and their vertical profiles, n(z) (Bq m-3), were expressed well by the formula n(z) = alpha z beta. Vertical profiles of the atmospheric turbulent diffusion coefficient were also estimated from the observed thoron profiles, as expressed by the power function K(z) = AzB, of which B values were estimated to vary from 1.034 to 1.609 if averaged thoron exhalation rates during the measurement periods were within 0.3 to 2.8 (Bq m-2 s-1).

Protective Effect of 5-hydroxytryptophan against Lethal Doses of X-radiation in Mice

Many investigators have shown that 5-hydroxytryptamine (5-HT), one of the biogenic amines of pharmacodynamic action, has an excellent radiation protective ability (Gray, Tew and Jensen 1952, Bacq 1954, Langendorff and Koch 1957, Langendorff, Melching and Ladner 1959). Its metabolic precursor, 5-hydroxytryptophan (5-HTP), on the other hand, has been reported to have only a weak or insignificant protective effect and so far has not received attention as a radioprotective agent (Langendorff and Melching 1959, Thomson 1962, Van den Brenk and Haas 1961). In the course of study on the role of biogenic 5-HT in the development of radiation injury, however, we found that 5-HTP could be as good as, or even better than, 5-HT in protecting mice against lethal dose of ionizing radiation. In this paper we examine the optimum time of 5-HTP administration prior to irradiation and the relationship of 5-HTP dose and protective effect at this optimum time. Animals used were male mice of ddN strain, 8 weeks of age at irradiation time, weighing 23 to 30 g. They were housed individually, given laboratory chow and tap water ad libitum. The radiation factors were: 200 kvp x-ray; 20 mA ; 05 mm Cu plus 05 mm Al filters; h.v.l., 115 mm Cu; dose-rate, 96 R/min as measured in air with Victoreen dosimeter (Type 575). Ten mice were irradiated simultaneously at a single whole-body exposure in individually separated lucite chambers. 5-HTP was obtained from Sigma Chemical Co. (Lot No. H102B-205) or Nutritional Biochemicals Co. (Control No. 5754), U.S.A., was dissolved by twice glass distilled water into solution of 10 mg per ml. concentration immediately before use. No contamination by 5-HT was found in the 5-HTP samples from both sources, as is shown by thin-layer chromatography (figure 1).

Analysis of body-burden measurements of 137Cs and 40K in a Japanese group over a period of 5 years following the Chernobyl accident

A selected group of about 20 male researchers at the NIRS that reside in Chiba, Japan, was measured for total body content of radiocesium and 40K every 3 mo from February 1986 to May 1991. A whole-body counter at the NIRS was used to measure their radioactivity in a scanning mode of 5 cm min-1 in a shielded iron room with walls 20 cm in thickness. A maximum radiocesium level of 59 Bq was observed in May 1987. The annual change in the body burden decreased with an apparent half-time of 1.8 y after May 1987. The period of five years was sufficient to eliminate the effects of the accident in this group. Even in the most contaminated period, the dose from radiocesium was below 2 microSv y-1. The cumulative dose for 5 y was estimated to be 5.6 microSv, which is nearly equal to the total dose to the Japanese people caused by the artificial radionuclide fallout for the first year following the accident. It is much smaller than the committed dose of 82 microSv for internally deposited 137Cs resulting from nuclear explosions in 1961 and 1962 and the annual dose of 170 microSv from internal 40K. No detectable health risk was expected for the present group.

Consequences of the Chernobyl reactor accident on the 137Cs internal dose to the Japanese population

Abstract The consequences of the Chernobyl reactor accident on the Japanese population are assessed here, for the one-year period from May 1986, in terms of the internal dose due to 137Cs. The calculations are made via an approach which combines whole body counting with analysis of food intake data. First, the dose from 137Cs is assessed, for a group of healthy adult males, on the basis of their observed body burdens of 137Cs determined by whole body counting. The annual individual dose estimate thus obtained is 1·5 μSv, which is 6–15% and 3·7% respectively of the doses determined by whole body counting in UK and the Federal Republic of Germany. The temporal change in the average body burden is successfully explained here by a single-compartment model. Secondly, this latter model is used, along with the daily 137Cs intake data for each district in Japan, to calculate the dose for the whole of Japan. Appropriate values were chosen for the relevant biological parameters for each age and sex group. The estimates of the population dose and the average individual dose thus obtained are 148 man Sv, for the population of 120 million, and 1·24 μSv, respectively. Although comparatively small, these values nonetheless also include the residual contribution from past nuclear weapon tests. The average annual individual dose of 1·24 μSv corresponds to 0·7% of the dose from natural 40K in the body. Although whole body counting indicates that 137Cs burdens were still increasing as of May 1987, it is concluded that, in terms of radiocaesium, the effect of the Chernobyl reactor accident on Japan was negligible.