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Articles by S Tokonami
Total Records ( 12 ) for S Tokonami
  L Tommasino , S Tokonami and P. M. Tommasino

In the present paper, the long noses of Pinocchio, Tengu and Trolls are used to measure, respectively, radon, thoron, and their decay products both by track-etch detectors and by Geiger–Müller (G–M) counters. Just recently, four new passive samplers (termed quatrefoil) have been developed which greatly simplify the detection of all airborne radionuclides by using either passive or real-time detectors. In particular, surface-deposited radon (thoron) decay products are sampled by films with large area and small surface density (0.1–1 mg cm–2). Once exposed, these films are stacked together for their detection by a pancake G–M counter. For the measurements of radon and thoron in soil, 25-cm-long tubes with sampling films along their internal surfaces can be successfully used. Once exposed, these films can be counted by a pancake G–M for the selective measurement of radon and thoron.

  Z. S Zunic , I Celikovic , S Tokonami , T Ishikawa , P Ujic , A Onischenko , M Zhukovsky , G Milic , B Jakupi , O Cuknic , N Veselinovic , K Fujimoto , S. K Sahoo and I. Yarmoshenko

This paper deals with the results of the first-field use in the Balkans, i.e. Serbia and Republic of Srpska (Bosnia and Hercegovina), of a passive polycarbonate Mark II type and poliallyldiglycol carbonate (Cr-39) alpha track detectors sensitive to thoron as well as to radon. Both types of solid state nuclear track detectors were designed and supplied by National Institute of Radiological Sciences (NIRS), Chiba, Japan. The commercial names for these detectors which all have been field tested in Balkan rural communities are known as: UFO and RADUET passive discriminative radon/thoron detectors. No database of thoron and thoron progeny concentrations in dwellings in Serbia or Balkans region exist, and as a result, the level of exposure of the Serbian population to thoron and its progeny is unknown so far.

  R. C Ramola , G Prasad , G. S Gusain , B. S Rautela , V. M Choubey , D. V Sagar , S Tokonami , A Sorimachi , S. K Sahoo , M Janik and T. Ishikawa

This paper presents the preliminary results of radon and thoron measurements in the houses of Chhatrapur area of southeastern coast of Orissa, India. This area is one of the high radiation background radiation areas in India, which consists of monazite sand as the source of thoron. Both active and passive methods were employed for the measurements. Radon and thoron concentrations were measured in the houses of Chhatrapur area using twin cup radon dosemeters, RAD7 and radon–thoron discriminative detector (Raduet). Thoron progeny concentration was also measured in the houses using deposition rate measurements. Radon and thoron concentrations in the houses of study area were found to vary from 8 to 47 Bq m–3 and the below detection level to 77 Bq m–3, respectively. While thoron progeny concentration in these houses ranges between 0.17 and 4.24 Bq m–3, preliminary investigation shows that the thoron concentration is higher than radon concentration in the houses of the study area. The thoron progeny concentration was found to be comparatively higher, which forms a base for further study in the area. The comparison between the results of various techniques is presented in this paper.

  V. W. Y Choi , C. K. M Ng , R. K. K Lam , M Janik , A Sorimachi , C Kranrod , D Nikezic , S Tokonami and K. N. Yu

The ‘proxy equilibrium factor’ (Fp) method has been developed for long-term determination of airborne radon progeny concentrations using LR 115 solid-state nuclear track detectors. In this paper, the effects of 220Rn on the Fp method have been studied. The correction to the track density was related to a parameter which was the ratio of the sum of activity concentrations of alpha-particle emitting radionuclides in the 220Rn decay chain to the activity concentration of 220Rn alone. Under commonly encountered circumstances, could not be smaller than 2. An attempt was made to verify this using the exposure chamber at the National Institute of Radiological Sciences (NIRS), Chiba, Japan. A most interesting observation of < 2 for very high 220Rn concentrations and very low equilibrium factors for 220Rn in the exposure chambers was made. A possible explanation was the substantial deposition of 216Po under the extreme conditions inside the exposure chambers.

  Y Yasuoka , A Sorimachi , T Ishikawa , M Hosoda , S Tokonami , N Fukuhori and M. Janik

It was shown that radon and thoron concentrations exhaled from soil were separately measured using the AlphaGUARD and liquid scintillation counter (LSC) methods. The thoron concentrations from the RAD 7 were used to create the conversion equation to calculate thoron levels with the AlphaGUARD. However, the conversion factor was found to depend on the air flow rate. When air containing thoron of ~60 kBq m–3 was fed to the scintillation cocktail, thoron and thoron progeny could not be measured with the LSC method. The radon concentration of about 10 kBq m–3 was measured with three methods, first with the LSC method and then with two AlphaGUARDs (one in the diffusion mode and the other in the flow mode (0.5 l min–1)). There were no significant differences between these results. Finally, it was shown that the radon and thoron concentrations in air could be measured with the AlphaGUARD and LSC methods.

  S. K Sahoo , T Ishikawa , S Tokonami , A Sorimachi , C Kranrod , M Janik , M Hosoda , N. M Hassan , S Chanyotha , V. K Parami , H Yonehara and R. C. Ramola

Several industrial processes are known to enrich naturally occurring radioactive materials (NORM). To assess such processes with respect to their radiological relevance, characteristic parameters describing this enrichment will lead to interesting information useful to UNSCEAR. In case of mineral treatment plants, the high temperatures used in smelting and refining processes lead to high concentrations of 238U and 232Th. Also due to thermal power combustion, concentration of U and Th in the fly ash increases manifold. NORM samples were collected from a Thailand mineral treatment plant and Philippine coal-fired thermal power plants for investigation. Some studies are initiated from a high background radiation area near Gopalpur of Orissa state in India. These NORM samples were analysed by gamma-ray spectrometry as well as inductively coupled plasma mass spectrometry. The radioactivity in case of Orissa soil samples is found to be mainly contributed from thorium. This study attempts to evaluate levels of thorium activity in NORM samples.

  M Hosoda , S Tokonami , A Sorimachi , T Ishikawa , S. K Sahoo , M Furukawa , Y Shiroma , Y Yasuoka , M Janik , N Kavasi , S Uchida and M. Shimo

Field measurements of thoron exhalation rates have been carried out using a ZnS(Ag) scintillation detector with an accumulation chamber. The influence of soil surface temperature and moisture saturation on the thoron exhalation rate was observed. When the variation of moisture saturation was small, the soil surface temperature appeared to induce a strong effect on the thoron exhalation rate. On the other hand, when the variation of moisture saturation was large, the influence of moisture saturation appeared to be larger than the soil surface temperature. The number of data ranged over 405, and the median was estimated to be 0.79 Bq m–2 s–1. Dependence of geology on the thoron exhalation rate from the soil surface was obviously found, and a nationwide distribution map of the thoron exhalation rate from the soil surface was drawn by using these data. It was generally high in the southwest region than in the northeast region.

  C Kranrod , T Ishikawa , S Tokonami , A Sorimachi , S Chanyotha and N. Chankow

There is a well-known discrepancy between dosimetrically derived dose conversion factor (DCF) and epidemiologically derived DCF for radon. As the latter DCFs, International Commission on Radiological Protection (ICRP) recommends a value of ~6.4 nSv (Bq h m–3)–1 and 7.9 nSv (Bq h m–3)–1 for radon decay products (RnDP) in dwellings and workplaces, respectively. On the other hand, the dosimetric calculations based on the ICRP-66 respiratory tract model derived a DCF of 13 nSv (Bq h m–3)–1 and 17 nSv (Bq h m–3)–1 for RnDP in dwellings and workplaces, respectively, and 83 nSv (Bq h m–3)–1 for thoron decay products (TnDP) in dwellings. In addition, the DCFs derived from both approaches and UNSCEAR were applied to comparative dosimetry for two thoron-enhanced areas (cave dwellings in China and dwellings at a spa town in Japan), where the equilibrium equivalent concentration of radon and equilibrium equivalent concentration of thoron have been measured. In the case of the spa town dwellings, the dose from TnDP was larger than the dose from RnDP.

  N Kavasi , T Vigh , A Sorimachi , T Ishikawa , S Tokonami and M. Hosoda

In this study, short-term radon (RnDP) and thoron (TnDP) progeny measurements and dose estimation were carried out in winter and summer in a manganese mine, Hungary. Gamma-ray dose rate originating from external sources and 222Rn and 226Ra contents of spring-water from a mine was also measured. During working hours RnDP and TnDP concentration values changed between 12.1–175 and 0.14–0.42 Bq m–3, respectively. The 222Rn and 226Ra concentration values in the karst spring-water were ~6 Bq dm–3 and 16 mBq dm–3, respectively. The radiation dose resulting from the consumption of karst spring-water was negligible. The doses from the inhalation of TnDP and external gamma radiation were of the same magnitude, ~0.1 mSv y–1, which was rather negligible related to the estimated radiation dose of 5 mSv y–1 from RnDP.

  M Janik , S Tokonami , C Kranrod , A Sorimachi , T Ishikawa and N. M. Hassan

Intercomparisons of radon/thoron detectors play an important role not only for domestic radon/thoron survey but also for international or interregional discussion about radon/thoron mapping in dwellings as well as that in the soil. For these purposes, it is necessary to improve and standardise technical methods of measurement and to verify quality assurance by intercomparisons between laboratories. Therefore, the first thoron international intercomparison was provided at the NIRS (National Institute of Radiological Sciences, Japan) thoron chamber with a 150 dm3 inner volume. In addition, a second intercomparison of radon detectors was conducted at NIRS with a 24.4 m3 inner volume walk-in radon chamber. Only etched-track detectors were used during thoron intercomparison as well as three types for the radon intercomparison: etched-track, charcoal and electret. In general, 45 % results for thoron experiment do not differ more than 20 % from the reference value of thoron concentration and 69 % for radon.

  Y Shiroma , N Isa , M Hosoda , A Sorimachi , T Ishikawa , S Tokonami and M. Furukawa

Thoron exhalation rates from the ground surface were measured at 57 sites on Okinawa Island, Japan, using a ZnS(Ag) scintillation detector equipped with photomultiplier. The arithmetic means ± SD, median ± SD, minimum and maximum of the rates (unit: Bq m–2 s–1) were estimated to be 1.9 ± 1.4, 1.6 ± 0.3, 0.04 and 6.2, respectively. The soils distributed on the island are generally classified into dark red soils, residual regosols, as well as red and yellow soils. While it was assumed that the soils were originated from the bedrock, recent studies suggested that the main material of dark red soils is the East Asian eolian dust. In the dark red soils area, the exhalation rate is relatively higher than that in the other areas. This suggested that the eolian dust was an enhancer for the environmental thoron concentration on Okinawa Island.

  S Akiba , S Tokonami , F Bochicchio , J McLaughlin , L Tommasino and N. Harley

A roundtable discussion was made at the end of the workshop. All the presentations were summarised in this discussion. It involved measurement techniques, quality assurance and dose assessment and health effects of thoron and its progeny. In particular, major epidemiological studies may be affected by thoron interference in radon measurements. Since their data are not sufficient when compared with that of radon, further efforts in thoron studies will be needed.

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