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Articles by S. Tokonami
Total Records ( 5 ) for S. Tokonami
  J Chen , E Schroth , E MacKinlay , I Fife , A Sorimachi and S. Tokonami

Naturally occurring isotopes of radon in indoor air are identified as the second leading cause of lung cancer after tobacco smoking. Winnipeg had the highest radon (222Rn) concentration among 18 Canadian cities surveyed in the past. There is great interest to know the current radon as well as thoron (220Rn) concentrations in Winnipeg homes. Therefore, radon–thoron discrimination detectors were deployed in 117 houses for a period of 3 months. The results confirmed that thoron is present at detectable levels in about half of the Winnipeg homes and radon remains significantly higher than the national average. In this study, radon concentrations ranged from 20 to 483 Bq m–3 with a geometric mean of 112 Bq m–3 and a geometric standard deviation of 2.07. It is estimated that 20% of Winnipeg homes could have radon concentrations above the Canadian indoor radon guideline of 200 Bq m–3. This conclusion is similar to the previous estimation made 20 y ago. Thoron concentrations were below the detection limit in 60 homes. Among the homes with detectable thoron concentrations, the values varied from 5 to 297 Bq m–3, the geometric mean and standard deviation were 21 Bq m–3 and 2.53, respectively.

  J Chen , B Walker , A Sorimachi , H Takahashi and S. Tokonami

The alpha-track detector was well designed for long-term radon measurements in the 1992 Winnipeg case–control study. However, its diffusion characteristic for thoron in comparison to radon was yet unknown. An investigation on radon and thoron response of these detectors was undertaken. The results showed that the relative sensitivity between thoron and radon is 2 %, i.e. the detector sensitivity to radon is about 50 times higher than the sensitivity to thoron. It can be concluded that there was no significant influence of thoron on the radon measurements with these detectors.

  S. Tokonami

New scientific findings based on the latest epidemiological analyses for lung cancer risk due to radon have been demonstrated. The residential radon concentration is mainly measured by passive radon detectors. Although the passive radon detector is usually designed to detect radon efficiently and exclusively, several types of them can detect thoron together with radon. In this case, these detector readings may include both radon and thoron signals. If the readings are overestimated, the lung cancer risk will be given as a biased estimate when epidemiological studies are carried out. In our experience, there seem to be no correlation among radon, thoron and thoron progeny concentrations. Therefore, one parameter cannot be estimated by the other. This study presents the importance of thoron measurement throughout results we have obtained in field and in laboratory so far.

  A Sorimachi , T Ishikawa , M Janik and S. Tokonami

The National Institute of Radiological Sciences (NIRS) has developed passive radon (222Rn)–thoron (220Rn) discriminative detectors for a large-scale survey and has established a thoron chamber to calibrate such detectors. In order to establish quality assurance and quality control for the 220Rn measurement at NIRS, intercomparison studies have been carried out. The intercomparisons using a scintillation cell method, which has been used as a standard for 220Rn measurement at NIRS, were conducted at New York University (NYU, USA) and Physikalisch-Technische Bundesanstalt (PTB, Germany). As a result, it was found that the result from the NIRS was in good agreement with that from the NYU. On the other hand, it was observed that the relative discrepancy between the 220Rn concentrations from the NIRS and PTB monitors was, on average, >50 %. Using the NIRS 220Rn chamber, the international intercomparison experiment for passive 220Rn detectors started in 2008.

  M Shimo , Y Ishimori , M Hosoda and S. Tokonami

Thoron exhalation rates were measured with a newly made portable instrument at 33 areas in 7 prefectures of Japan. Thoron exhalation rates ranged from 49 to 4890 mBq m–2 s–1. Radon exhalation rates were also measured in many of the areas at the same time and ranged from 2.1 to 11 mBq m–2 s–1. Thoron exhalation rates showed a rough correlation with radon exhalation rates. Both exhalation rates also showed a rough correlation with geological features.

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