Dr. Tsubokura's Radiation Lecture Vol.32

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

63. Air dose rates are not consistent

"External exposure" occurs when the body is exposed to radiation originating from an external source. Glass badges and monitoring posts are used to measure the radiation.

The level of external radiation, measured by a device called dosimeters, does not remain the same over time, as radiation is not emitted at a regular pace. The emission of radiation occurs similarly to how sparklers give off sparks. If radioactive substances were sparklers, the fireball would be the nucleus and sparks would be ionizing radiations. In the same way that sparks come off randomly, in different directions, ionizing radiations are emitted inconsistently. Consequently, the levels of detected external radiation fluctuate. Therefore, the external radiation level can be determined by measuring repeatedly or calculating the average value over a fixed period.

Moreover, the measurement for external exposure is also affected by natural phenomena, such as rainfall and snow. When it rains, radioactive substances in the atmosphere descend with the rain, which leads to a temporal increase in the detected level of external exposure.

On the other hand, rain and snow on the ground cause a reduction in the measurement, as they act as a barrier to radiation emitted from the ground.

64. The level of radiation exposure is lower inside the buildings

"External exposure" occurs when the body is exposed to penetrating radiation from an external source. Throughout our days, people are coming from and going to different places. The health effects of the radiation can be determined by calculating the sum of the exposure received in those places that people have visited over the course of the day.

The air radiation dose rates inside of a building are lower than they are outside, because walls act as a barrier to radiation and the radiation level also decreases as it travels away from the ground. The type of building material has a slight influence on the level of protection afforded from radiation. When the level of external exposure received outdoors is 100, the level of external exposure received inside a wooden house, on the ground floor, inside of a building, on the 3rd or 4th floor, and inside of a high-rise, on perhaps the tenth floor, would be 40, 5, and 1, respectively.

Previous research revealed that the actual measurements of radiation exposure for children were much lower than the estimated level of radiation exposure, calculated on the basis of air dose rates for their residences. (The actual measurement of exposure was approx. 1/3 for of Minamisoma city in 2012) It was implied that spending a long time inside of a school building protected children from radiation exposure, which thereby contributed to the lower level of radiation exposure that was actually measured.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on Mearch 20th and 27th 2016 was reproduced for MRIC Global under the author's permission.