Fig 1.
Location of Tokyo municipalities consisting of 23 wards (A1) and 30 cities, towns and villages (A2).
The number for each administrative district (#1 - #53) is an ID number that is given in this paper by reference to the Japanese Industrial Standards. The color scale gives the altitudes within the districts. This map was drawn using the GMT [15] and GSI maps of the Geospatial Information Authority of Japan [16].
Fig 2.
The survey routes for measuring the count rates in metropolitan Tokyo.
Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer in November of the four years, 2015, 2016, 2017 and 2018. Total distances traveled were 725 km for each year. The circles represent the locations where fixed-point measurements were made outside the car (n = 61).
Table 1.
Calculated shielding factors SFs and dose conversion factors DCFs.
Fig 3.
Calculated absorbed dose rates in air from natural and artificial radionuclides measured in 2014 [5]– 2018 in metropolitan Tokyo based on the measurements by the car-borne survey technique.
The measurement was done on the same route (red line in Fig 2) using the same 3-in × 3-in NaI(Tl) scintillation spectrometer.
Fig 4.
Changes of absorbed dose rate in air from natural radionuclides in the eastern (A1) and western (A2) ends of Tokyo in 2014 [5]– 2018. The gamma-ray pulse height distributions were measured outside the car for 10 min, at 61 locations (Fig 2). The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and separated as natural radionuclides (K-40, U-238 series and Th-232 series).
Fig 5.
The distribution maps of absorbed dose rates in air in metropolitan Tokyo measured in 2015 (A), 2016 (B), 2017 (C) and 2018 (D). A minimum curvature algorithm was used for the data interpolation using the GMT [15]. Those maps were drawn using 4,018 data for 2015, 4,346 data for 2016, 4,717 data for 2017 and 5,138 data for 2018.
Fig 6.
The distribution maps of absorbed dose rates in air from artificial radionuclides in 2015 (A), 2016 (B), 2017 (C) and 2018 (D). The gamma-ray pulse height distributions were measured for 10 min, at 61 locations (Fig 2). The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and separated as artificial radionuclides (Cs-134 and Cs-137).
Fig 7.
Changes of absorbed dose rate in air from artificial radionuclides in the eastern (A1) and western (A2) ends of Tokyo in 2014 [5]– 2018.
Fig 8.
Changes of absorbed dose rate in air from artificial radionuclides measured at 1 m above the surface of porous and standard asphalt surfaces.
The gamma-ray pulse height distributions were measured 1 m above the surface of porous (n = 3) and standard asphalt (n = 5) materials for 10 min. The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and dose rates were calculated for the artificial radionuclides.
Fig 9.
The structures of porous and standard asphalt materials.