[en] Researchers have used various methods to obtain the exhalation rates of radon and thoron from soil and building materials. One of the typical methods for radon exhalation rate is the circulation method using an accumulation container, an external or internal sampling pump and a continuous radon monitor. However, it is necessary to consider sampling flow rate if this method is applied to exhalation rate measurement for thoron due to its short half-life. Based on a calibration experiment, the measured thoron concentrations obtained by an electrostatic collection type radon and thoron monitor (RAD7) were found to be influenced strongly by the sampling flow rate. It was also found that the thoron exhalation rate from a soil sample depended on the pressure difference which was proportional to the increasing sampling flow rate. The thoron exhalation rate measured at the generally used sampling flow rate of the internal sampling pump of the RAD7 was overestimated compared with the value at 0 L min^-1. (authors)

[en] This study aimed to establish a walking survey technique for rapidly determining the distribution of the ambient gamma dose rate. The measurement system consists of a scintillation spectrometer and a Global Positioning System receiver, which are controlled using a laptop. In addition to locality information, the ambient gamma dose rates were measured continuously and corrected based on shielding by the measurer body. The measurement interval was optimized at 12 s based on the locality resolution and fluctuations of the measured dose rates. A comparison was also made between the dose rate distribution maps depicted by the walking survey technique and spot measurements at fixed points. Based on the measurement data selected for the comparison, it was found that the values of 80% of the data obtained using the walking survey technique deviated within ±20% relative to those obtained by spot measurements. Compared to the spot measurements, the time taken to depict the map was reduced to one-fifth, and the localized contamination points could be depicted. (author)

[en] Using a PICO-RAD detector, a determination of a radon concentration in air was examined with a simplified liquid scintillation counter. After activated carbon of PICO-RAD detector adsorbed radon for 24 hours, the scintillator was poured into the PICO-RAD detector. After standing for a certain period of time (elution time), the radon concentration of PICO-RAD detector was measured with conventional LSCs and a simplified LSC using the integral counting method. When the PICO-RAD detectors were exposed in the radon chamber in NIRS, the conversion factor was determined to be 0.016 cps m³ Bq^-1 and at least three days elution time was found to be needed. According to the minimum detectable concentration calculated from the background value by the measurement of the simplified LSC, it is difficult to determine the indoor radon concentration. However, exhalation of radon from the soil can be measured with the simplified LSC. (author)

[en] The radiation doses from natural radiation sources in Japan are reviewed using the latest knowledge. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and the Nuclear Safety Research Association report the annual effective doses from cosmic rays, terrestrial radiation, inhalation, and ingestion as natural sources. In this paper, the total annual effective dose from cosmic-ray exposure is evaluated as 0.29 mSv. The arithmetic mean of the annual effective dose from external exposure to terrestrial radiation is 0.33 mSv for the Japanese population using the data of nationwide surveys by the National Institute of Radiological Sciences. Previously in Japan, although three different groups have conducted nationwide indoor radon surveys using passive-type radon monitors, to date only the Japan Chemical Analysis Center (JCAC) has performed a nationwide radon survey using a unified method for radon measurements conducted indoor, outdoor, and in the workplace. Consequently, the JCAC results are used for the annual effective dose from radon and that for radon inhalation is estimated as 0.50 mSv using a current dose conversion factor. In this paper, UNSCEAR values are used for the mean indoor and outdoor thoron-progeny concentrations, and the annual effective dose from thoron is reported as 0.09 mSv. Thus, the annual effective dose from radon and thoron inhalation is 0.59 mSv. From a JCAC large-scale survey of foodstuffs, the committed effective dose from the main radionuclides in dietary intake is 0.99 mSv. Finally, the Japanese population dose from natural radiation is given as 2.2 mSv, which is similar to the reported global average of 2.4 mSv. (review)

[en] Anhydrous calcium sulfate has been identified for use as a desiccant in radon monitoring. However, the influence of a desiccant on radon measurements has not been investigated. Therefore, AlphaGUARD monitors working in flow and mode with and without a desiccant, and in diffusion mode as reference were used for radon measurements in the radon chamber at the National Institute of Radiological Sciences, Japan. The radon concentration in the air in the radon chamber was maintained at a constant level and measured at three levels of relative humidity: low, normal and high. Comparison of the results with the reference value showed that the differences were below 4%, which is within the acceptable range (below 10%). Consequently, when radon concentration was measured with the desiccant, we considered that the influence of the desiccant can be ignored. (author)

[en] The spatial distribution of ambient gamma dose rates in a high-rise steel-reinforced concrete building in Fukushima, Japan, was examined relative to the gamma-ray emissions from building materials and radionuclides derived from the 2011 nuclear accident. The results revealed the minor role of accident-derived radionuclides in ambient gamma dose rates ∼7 y after the accident. The ambient gamma dose rates were higher in the upper floors because of gamma-ray emissions from natural radionuclides in the floor slabs. The fractional contribution of natural radionuclides to the ambient gamma dose rates indicated compositional differences in concrete between the upper- and lower-floor slabs. (authors)

[en] Before the 1995 Hyogoken-Nanbu earthquake, various geochemical precursors were observed in the aftershock area: chloride ion concentration, groundwater discharge rate, groundwater radon concentration and so on. Kobe Pharmaceutical University (KPU) is located about 25 km northeast from the epicenter and within the aftershock area. Atmospheric radon concentration had been continuously measured from 1984 at KPU, using a flow-type ionization chamber. The radon concentration data were analyzed using the smoothed residual values which represent the daily minimum of radon concentration with the exclusion of normalized seasonal variation. The radon concentration (smoothed residual values) demonstrated an upward trend about two months before the Hyogoken-Nanbu earthquake. The trend can be well fitted to a log-periodic model related to earthquake fault dynamics. As a result of model fitting, a critical point was calculated to be between 13 and 27 January 1995, which was in good agreement with the occurrence date of earthquake (17 January 1995). The mechanism of radon anomaly before earthquakes is not fully understood. However, it might be possible to detect atmospheric radon anomaly as a precursor before a large earthquake, if (1) the measurement is conducted near the earthquake fault, (2) the monitoring station is located on granite (radon-rich) areas, and (3) the measurement is conducted for more than several years before the earthquake to obtain background data. (author)

[en] The relationship between radon anomalies and earthquakes has been studied for more than 30 years. However, most of the studies dealt with radon in soil gas or in groundwater. Before the 1995 Hyogoken-Nanbu earthquake, an anomalous increase of atmospheric radon was observed at Kobe Pharmaceutical University. The increase was well fitted with a mathematical model related to earthquake fault dynamics. This paper reports the significance of this observation, reviewing previous studies on radon anomaly before earthquakes. Groundwater/soil radon measurements for earthquake prediction began in 1970's in Japan as well as foreign countries. One of the most famous studies in Japan is groundwater radon anomaly before the 1978 Izu-Oshima-kinkai earthquake. We have recognized the significance of radon in earthquake prediction research, but recently its limitation was also pointed out. Some researchers are looking for a better indicator for precursors; simultaneous measurements of radon and other gases are new trials in recent studies. Contrary to soil/groundwater radon, we have not paid much attention to atmospheric radon before earthquakes. However, it might be possible to detect precursors in atmospheric radon before a large earthquake. In the next issues, we will discuss the details of the anomalous atmospheric radon data observed before the Hyogoken-Nanbu earthquake. (author)

[en] This paper describes how humidity, wind and ambient aerosols in air influence the detection responses of passive detectors. Two types of alpha track detectors based on a passive radon (²²²Rn)–thoron (²²⁰Rn) discriminative measurement technique were used: the Raduet and Radopot detectors that were developed and calibrated by the National Institute of Radiological Sciences, Japan. The initial experiment showed that the infiltration rate of ²²⁰Rn onto sponges with a high air exchange rate for the Raduet detectors was one third lower than that onto filters for the Radopot detectors. Little distinct dependence on humidity was observed for the ²²²Rn detection responses of both detectors. For ²²⁰Rn, the detection responses of both detectors for the high air exchange rate seemed to decrease slightly at high humidity conditions. The ²²⁰Rn detection responses of the Radopot detectors had little influence from wind speed. The ²²⁰Rn detection responses of the Raduet detectors for the high air exchange rate seemed to decrease at low wind speeds. Furthermore, there was little difference between the detection responses in the presence and absence of ambient aerosol particles because the ambient aerosols were filtered out during their passive diffusion through the sponges and filters for the Raduet and Radopot detectors, respectively.

[en] During a one-year long measurement period, radon and thoron data obtained by two different passive radon-thoron discriminative monitors were compared at subsurface workplaces in Hungary, such as mines (bauxite and manganese ore) and caves (medical and touristic). These workplaces have special environmental conditions, such as, stable and high relative humidity (100%), relatively stable temperature (12°C–21°C), low or high wind speed (max. 2.4 m s⁻¹) and low or elevated aerosol concentration (130–60 000 particles m⁻³). The measured radon and thoron concentrations fluctuated in a wide range among the different workplaces. The respective annual average radon concentrations and their standard deviations (in brackets) measured by the passive radon-thoron discriminative monitor with cellulose filter (CF) and the passive radon-thoron discriminative monitor with sponge filter (SF) were: 350(321) Bq m⁻³ and 550(497) Bq m⁻³ in the bauxite mine; 887(604) Bq m⁻³ and 1258(788) Bq m⁻³ in the manganese ore mine; 2510(2341) Bq m⁻³ and 3403(3075) Bq m⁻³ in the medical cave (Hospital Cave of Tapolca); and 6239(2057) Bq m⁻³ and 8512(1955) Bq m⁻³ in the touristic cave (Lake Cave of Tapolca). The respective average thoron concentrations and their standard deviation (in brackets) measured by CF and SF monitors were: 154(210) Bq m⁻³ and 161(148) Bq m⁻³ in the bauxite mine; 187(191) Bq m⁻³ and 117(147) Bq m⁻³ in the manganese-ore mine; 360(524) Bq m⁻³ and 371(789) Bq m⁻³ in the medical cave (Hospital Cave of Tapolca); and 1420(1184) Bq m⁻³ and 1462(3655) Bq m⁻³ in the touristic cave (Lake Cave of Tapolca). Under these circumstances, comparison of the radon data for the SF and CF monitors showed the former were consistently 51% higher in the bauxite mine, 38% higher in the manganese ore mine, and 34% higher in the caves. Consequently, correction is required on previously obtained radon data acquired by CF monitors at subsurface workplaces to gain comparable data for SF monitors. In the case of thoron, the data were unreliable and no significant tendency was seen during the comparison therefore comparison of previously obtained thoron data acquired by either CF or SF is doubtful. There was probable influence by relative humidity on the detection response; however, the effects of the high wind speed and elevated aerosol concentration could not be excluded. The results of this study call attention to the importance of calibration under extreme environmental conditions and the need for using reliable radon-thoron monitors for subsurface workplaces