National Institute for Fusion Science

Due to the nuclear power plant accident at Fukushima, the necessity of spreading proper understanding and education of radiation is increasing. Properly understanding the radiation in personal belongings in the natural world and the radiation being used in our daily lives makes it possible to safely deal with radiation rather than feel uneasy toward it. For that reason, education regarding radiation is now being conducted even in compulsory education. At the National Institute for Fusion Science (NIFS), we are conducting research on radiation in the environment. Here, we report on the production of sources of natural radioactivity and its practical use in education.

That radioactivity and radiation are found everywhere in nature is not readily noticed in our daily lives. Taking this as a starting point, in order to properly understand the qualities of radiation, first we will try to actually measure radiation. However, to do so, even though the necessary radiation sources are extremely weak and pose absolutely no safety problems, when used as educational material in an educational setting, it is true that junior high school students, high school students, and members of the public feel opposition. Thus, using various objects, including radioactive substances found around us in nature, we considered producing a radiation source.

As such a material we may mention dried seaweed and sinter deposits at hot springs. Because these objects include the radioactive isotopes potassium 40 and thorium 232 which are found in nature, experiential learning in radiation has been used frequently. However, there was no example of a radiation source having been produced. Thus, we began to produce a radioactive source from such materials and to develop radiation education methods that utilize that material.

First, in order to make seaweed and other objects into a solid source of radiation, we pulverized the material and set it in a mold. Then, by compressing the material and making it solid, we developed a method compression mold that formed the material into a solid that is strong and easy to use. We call the radiation produced by this method a natural source of radioactivity. To this point, we have succeeded in producing natural sources of radioactivity using potassium chloride (a kind of salt), dried seaweed, chemical fertilizers and sinter deposits. A special characteristic of natural sources of radioactivity is that they are made from common materials that may be purchased at shopping malls and home centers, or by mail-order. The strength of the radioactivity depends upon the material, but the radioactivity is merely from 2 to 10 times the atmospheric radiation level. And because we may easily measure using radiation measurement devices such as a common GM survey meter and other devices, these may be used in radiation education.

At NIFS, taking advantage of these special characteristics, together with NIFS researchers and high school students who visit our institute, we implemented various types of practical training for measuring radiation by utilizing natural sources of radioactivity. As a result, we confirmed that the radiation counting rate falls at a rate inversely proportional to the square of the distance, and that the radiation counting rate falls exponentially as a function of the thickness of the shielding. And through understanding the temporal variation of the radiation counting rate and through training in measuring radiation, we came to understand that natural sources of radiation that have been developed can be used. And we demonstrated that this knowledge can be useful as radiation education materials.

Natural sources of radioactivity that have been developed are made from materials used in everyday life that anyone may easily obtain. And because that which is emitted is radiation of natural origin, there are no laws restricting it. These may be used freely not only at junior high school and high school, but also at community center, children’s house and family residences. For that reason, we can say that natural sources of radioactivity do not select people or places and are sources of radiation that are easy to use. In the future, we hope to expand the sites of radiation education regarding natural sources of radioactivity, and we wish to contribute to radiation education that aims for a proper understanding of radiation.

A variety of natural radioactive sources. Top left : chemical fertilizer (potassium sulfate); top right : seaweed; middle : sinter deposit; bottom left : salt; bottom right : potassium chloride （diameter：3.5 cm、thickness：about 1 cm）

Image: Training in measuring radiation.