Activity Report
NASHIM Trainee Physicians’ Internal Radioactivity Measurement
Naoko MORITA
Research Assistant in charge of whole-body counter management
Radiation Effect Research Unit, Atomic Bomb Disease Institute
GraduateSchool of Biomedical Sciences, NagasakiUniversity
NagasakiUniversity’s whole-body counter was installed in 1968 to measure low-dose internal radioactivity remaining in human bodies, mainly of A-bomb victims. The measurement of residual radioactivity in the bodies of Hibakusha, as well as in soil and agricultural produce in Nagasaki, continued until the early 1980s, when it reached undetectable levels, considered equivalent to non-existent. Since the 1990s, we have been measuring people irradiated in the April 26, 1986 explosions at the Chernobyl nuclear power plant in the former Soviet Union (present Ukraine). This measurement has been maintained as a joint program with NASHIM.
The whole-body counter is effective as one of the precious few internal radioactivity measurement systems in Japan. It is used to measure internal radioactivity in Hibakusha of Hiroshima, Nagasaki, Chernobyl and other sites in the world, as well as in Thorotrast-treated patients, and to measure internal potassium. It is also ready for use in checking for internal irradiation following the emergency admission of irradiated patients to NagasakiUniversityHospital.
The whole-body counter is mainly composed of a large iron chamber (20 mm thick; internal lead lining 3 mm thick; synthetic resin board 3 mm thick; approximately 50 tons total weight), which completely shields against naturally occurring external radioactivity, a gamma-ray-detecting NaI (TI) scintillator, and a 511-channel wave-height analyzer, which together enable the measurement of trace radionuclides in the human body.
The whole-body counter can measure gamma-ray-emitting radionuclides such as potassium-40 (K-40) and caesium-137 (Cs-137). K-40, naturally occurring radioactive potassium, accounts for about 0.012% of natural potassium and is indispensable in the human body. Cs-137, on the other hand, is an artificially generated substance deriving from nuclear fission of uranium and is found in radioactive fallout from atomic bombs or past nuclear tests. Cs-137 is contained in soil on and around the site of the Chernobyl disaster, likely due to fallout from explosions during the disaster.
From 2002 to 2007, we have measured 25 NASHIM trainee physicians. Internal cesium was detected in 13 of them. Table 1 indicates their places of residence, numbers and annual radiation doses, estimated on the basis of the measurement results. No internal cesium has been detected in NASHIM trainees from areas near the Semipalatinsk nuclear test site.
Table 1Places of residence, numbers and estimated annual radiation doses of NASHIM trainee physicians in whom internal cesium was detected
Place of residence |
No. |
Annual radiation (from ICRP Pub.67) |
|---|---|---|
Gomel, Belarus |
8 |
0.002〜0.070 mSv/year |
Minsk, Belarus |
1 |
0.006 mSv/year |
Kiev, Ukraine |
1 |
0.020 mSv/year |
Korosten, Ukraine |
1 |
0.008 mSv/year |
Klincy, Russia |
1 |
0.015 mSv/year |
Obninsk, Russia |
1 |
0.001 mSv/year |
Many NASHIM trainee physicians come from areas near the Chernobyl disaster site (in Belarus, Russia and Ukraine). They are believed to regularly eat agricultural produce cultivated in areas contaminated with radioactive fallout. The possibility has already been suggested that Cs-137 in fallout from the nuclear power plant disaster was absorbed in the nearby farmland soil and in agricultural produce grown there, and was consumed by local residents, accumulating in their bodies. Past measurements of Nagasaki Hibakusha pointed to a similar phenomenon.
The physical half-life of Cs-137 is about 30 years, but its biological half-life (period of time required for a substance taken into the body to be reduced by half through excretion and other biological functions) is known to be relatively short, about 110 days. Therefore, the presence of a detectable level of Cs-137 in the human body 20 years after the Chernobyl disaster suggests continuous intake of radioactivity via soil and agricultural produce.
At the same time, however, since 2002 the levels of Cs-137 detected in our measurements using the whole-body counter and converted to annual figures have been below 1% of the world average annual radiation dose (2.4 mSv/year) in everyday life for 12 of the 13 persons and about 3% for the remaining person. Therefore, adverse effects from internal contamination of Cs-137 due to the Chernobyl disaster are highly unlikely. We usually inform trainees of the measurement results by the time of their departure from Japan, and they, including those in whom cesium has been detected, go home assured.
On behalf of NagasakiUniversity, I would like to thank NASHIM and its trainee physicians, who come to Nagasaki each year, for their kind cooperation in our radioactivity measurement using the whole-body counter. We hope to maintain mutually cooperative ties with NASHIM, which we hope will also be beneficial for developing NagasakiUniversity’s international Hibakusha medical care.
