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Dr. Tsubokura's Radiation Lecture Vol.94

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

187. Cosmic rays pour down on Earth from outer space

The impact of radiation on health is determined by the extent of radiation exposure, not by whether one received any radiation exposure. Although the Fukushima Daiichi nuclear power plant accident widely dispersed radioactive substances and caused nuclear contamination, we have always been exposed to natural radiation to some extent, even before the disaster.

It is well known that natural radiation is emitted from radioactive substances in soil and rocks. In addition to natural radiation from soil, about 100 years ago, it was discovered that natural radiation reaches the Earth from outer space as well.

Back then, scientists measured radiation dose rates at various altitudes; for example, by climbing the Eiffel Tower in Paris or using a hot-air balloon. They originally assumed that radiation exposure due to radioactive substances in the ground would lessen as they were measured further from the ground. However, the results of these experiments were contrary to this hypothesis, as the dosage increased as the measurement points moved away from the ground.

In fact, researchers discovered that the dosage at 5000 meters aboveground was about ten times that of the measurement at ground level. In 1912, Victor Franz Hess, an Austrian scientist, discovered that radiation was reaching the Earth from outer space. This radiation is called cosmic rays, and the average annual dosage from cosmic rays is approximately 0.39 millisieverts globally.

188. Cosmic rays originate from the sun

In our daily lives, we receive a certain level of radiation exposure from radioactive substances in our surroundings. The previous article explained that these types of radiation do not only come from radioactive substances in rocks and soil but also originate from outer space. This radiation reaching Earth from the universe is called “cosmic rays.”

Well, the question is, where do cosmic rays originate?

The answer is that cosmic rays originate from the sun. The level of cosmic rays arriving on the Earth is not constant. When a solar flare—a sudden explosion on the surface of the sun—occurs, more cosmic rays than usual are emitted and reach the Earth. Moreover, excessive levels of cosmic rays can cause interference with electromagnetic waves. It is also known that cosmic rays invade the Earth’s atmosphere when a star explodes outside the solar system.

When cosmic rays from space enter the Earth’s atmosphere, they collide with air molecules and produce various types of radiation. The collision of cosmic rays with air molecules occurs repeatedly and results in cosmic rays pouring onto the Earth like a shower. However, because the level of these cosmic rays is very low, they do not cause any adverse health effects on our bodies.


The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 12th and 19th August 2018 was reproduced for MRIC Global under the author's permission.

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