Measurement of environmental radioactivity with photo-diode (PD) and Imaging Plate (IP) was tried. Commercially available Si PIN type PD's generally have depletion layer thickness more than a few hundred micrometer, which is enough for alpha particle measurement. PD's have various features: being usable in normal temperature, high energy resolution and low cost. Radon daughter nuclides positively charged in atmosphere were collected on the PD surface with negative electric potential and measured the pulse height spectra of alpha-particles from the daughter nuclides of Radon in thorium oxide, uranium ore, granite, and concrete,.

  Counting of alpha-particles with IP was tried. Lead plates usually contain Pb-210 (RaD) and emit alpha-particles from Po-210. The alpha-particles from the plate were counted with PD and the plate was exposed to IP. By adjusting the gradation level on the reading out of the latent image, it was possible to count alpha-particle incident image one by one, and the number per 1 cm² was compared with the number of count with PD.

  The present status of a network of radiation monitors (NORM) working at KEK is described in detail. NORM consists of three parts; stand-alone radiation monitors (SARM), local-monitoring stations (STATION) and a central data-handling system (CENTER). NORM has developed to a large-scaled monitoring system in which more than 250 SARMs are under operation for monitoring the radiation fields and radioactivities around accelerators in KEK.

  The following problems on radon face us: establishment of the dose assessment, exposure due to thoron and technologically-enhanced exposure due to radon/thoron based on human activities. The radon problem has not yet been solved. When estimating dose, two different approaches are used for assessment. One is an epidemiological approach. In the epidemiological approach, the dose is derived from the relationship between the alpha exposure, i.e., the potential alpha energy concentration and the estimated excess relative risk for lung cancer among miners. The other is a dosimetric approach. The dose is derived from laboratory investigation in this case. There is a three-time difference between the two. It is obvious that information on the particle size of radon progeny is indispensable for accurate dose assessment. In order to combine the two estimates from the two approaches, both mine and home aerosols should be sufficiently characterized. As the second problem, recent studies have shown that attention must be paid to thoron because high thoron concentrations were occasionally observed in some areas. Although most of passive radon detectors are generally designed to minimize the entry of thoron, some of them were affected by the presence of thoron. It is important to understand the performance of the detector with an adequate thoron exposure facility before practical use. As the third problem, radon/thoron concentrations might be enhanced in a closed environment. In this circumstance, their concentrations should be mitigated with proper countermeasures. Building materials can be recognized as the their sources. Exhalation rates of radon and thoron from walls were measured in a traditional Japanese wooden house where a high thoron concentration was observed. No radon exhalation was detected but thoron exhalation rate was as high as that from soil. The dose due to thoron progeny cannot be ignored in such indoor environment.

  Tritium originating from seneral different sources is present in the atmosphere in various chemical forms, such as tritiated water vapor (HTO), tritiated hydrogen (HT) and tritiated methane (CH₃T). It is important to know the present background levels in each chemical form of tritium in the atmosphere before large-scale tritium handling is performed. Therefore, an analytical technique for atmospheric tritium with a low background liquid scintillation counter was developed and applied to actual air samples collected in Fukuoka, Japan from 1984 to the present. HTO concentrations ex pressed in Bq/L-H₂O vary within a range of 1.01 to 2.45, giving an overall average value of 1.86 ± 0.077. HTO concentrations expressed in mBq/m³-air vary within a range of 6.1 to 46.1 and have a strong correlation with the atmospheric humidity, being high in summer and low in winter. In the case of HT and CH₃T, no seasonal variations were observed with average monthly values of 20.7 to 61.0 mBq/m³-air and 8.2 to 23.9 mBq/m³-air, respectively. The present HTO concentrations are already close to the tritium level before nuclear testings. However, the present HT and CH₃T concentrations are still higher by a factor of about 125 and 30, respectively, than those before the testings.

  By the use of extremely low-background counting system it became possible to detect as low as 1 count per day of radioactivity. Fundamental techniques needed to attain low-level counting and outline of Ogoya underground laboratory are introduced first and examples of the recent works conducted at our laboratory are explained mainly in the fields of environmental radioactivity and geochemistry.

  A simplified method for the monitoring of environmental ¹⁴C was developed and applied for the determination of ¹⁴C concentrations in tree rings collected in Fukuoka, Japan. As results obtained by our method reproduced the historical ¹⁴C levels in the environment, it was shown that our method was useful for the monitoring of environmental ¹⁴C. Recent ¹⁴C levels in Fukuoka were about 15 dpm/g-C. The difference between the years when the highest ¹⁴C level occurred in tree ring and rice was explained by their different growing season of the year. Therefore ¹⁴C concentrations in tree rings directly reflected the ¹⁴C levels in the atmosphere at the time of its growth. The deviation of ¹⁴C concentration due to the deposition of sap was not observed.