Dr. Tsubokura's Radiation Lecture Vol.37

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

73. Radiation is emitted to stabilize the nucleus

Radioactive uranium, potassium, and cesium transform themselves into stable substances by emitting a predetermined amount of radiation (alpha-, beta-, and gamma rays).

However, they do not emit radiation endlessly. In fact, radiation emission will eventually stop, and radioactive substances will no longer emit radiation once their nuclei are stabilized.

For example, when there are 100 molecules of a radioactive substance, they will all emit radiation to stabilize themselves. Some will emit radiation immediately, while others will take a while to begin emitting radiation.

The time taken for 50 out of 100 molecules of a radioactive substance to emit radiation and transform themselves into a stable substance is called a “half-life.”

After another half-life cycle passes, 25 radioactive molecules out of the remaining 50 will turn into the stable substance. The half-life for cesium-134 is about two years, while it is 30 years for cesium-137.

74. Our body’s excretion mechanism shortens the half-lives of radioactive substances ingested in our bodies.

Radioactive substances emit radiation to stabilize their nuclei. The time taken for half of the radioactive substance to transform itself into a stable substance is called its “half-life.”

The half-life of cesium-137 is about 30 years. Does this mean it takes almost 30 years to eliminate half of the cesium ingested into our bodies? The answer is no. Urination and defecation will excrete ingested cesium at a much faster rate than 30 years. The younger the age, the shorter the time taken to discharge: It takes approximately 100 days for adults, 30 days for 6-year-olds and 10 days for 1-year-olds to excrete half of an ingested radioactive substance.

Your body will be exposed to radiation while cesium remains in your body, but once it is out of the system, it will no longer have any effect on your body.

Your body will receive 1 millisievert of internal radiation by ingesting tens of thousands Becquerel of cesium. This indicates the fact that your body will receive 1 millisievert of radiation exposure in total while ingested radioactive cesium remains in your body (a few months), but it does not mean that you will receive 1 millisievert of radiation exposure at the moment of ingestion.


The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on May 29th and June 5th 2016 was reproduced for MRIC Global under the author's permission.

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