[Easy Science] Radiation Exposure
[Easy Science] Radiation Exposure
  • Reporter Lee Sang-min
  • 승인 2011.04.13 00:50
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The Fukushima nuclear accident, which occurred after the Tohoku earthquake and tsunami on Mar. 11, is still going on and seems to be far from a complete resolution. With an enormous amount of public interest, the influence of radiation the human body has been explained in many kinds of media.

In spite of the high interest of readers and media in the issue, scientific details of contents seem to be insufficient. For members of POSTECH, The Postech Times prepared scientific explanations about how radiation affects the human body and how to measure radiation. Professor Moo-hyun Cho of the Department of Physics explained them easily.

▲ Ionizing radiation and its effect on matter

The fundamental effect of radiation on the human body

The influence of ionizing radiation, strong enough to ionize atoms, can be easily categorized into external exposure, which comes from a direct hit of external radiation, and internal exposure, occurred by the inflow of the radioactive source into the body.

In both cases what affects the body is radiation from radioactive decay of source materials and conversion of unstable atoms to stable atoms. Depending on its components, radiation can be divided into four kinds as they are represented in the picture. Its influence varies with the scale of the subject.

The nucleus can be influenced by radiation. A neutron can be combined with a nucleus, form unstable nucleus, make it decay to other products, and prevent it to function. Gamma rays excite a nucleus and makes it release protons or neutrons, like the light emission of an excited hydrogen atom. The nucleus remains unchanged, but its influence can spread.

On the atomic scale, ionizing radiation can ionize an atom, which should go back to its ground state, and then it will radiate X-rays while returning. X-rays are harmful for covalent bonds in Carbon, Nitrogen, Hydrogen, and Oxygen. Also they can be affected by ionizing radiation directly, meaning an influence of radiation on the molecular scale.

The influence of radiation acts mechanically at the crystal scale. If strong ionizing radiation hits a lattice structure of atoms, there will be an interstitial dislocation of affected atoms. A local change of lattice structure leads to a change of properties of matter. For example, the elasticity of rubber band changes when it is exposed to radiation.

When those effects from different scales add up enough, commonly known symptoms like radiation poisoning and DNA mutation occur.

The Measurement of Radiation

Measuring radiation seems not simple, because it propagates in any direction. Its influence varies on what part of the body it hits. Normally three SI derived units, which can be represented by the seven basic SI units like the kilogram and second, are used to count the amount of radiation.

When the media report detections of radioactivity from various sources, the unit ‘Becquerel’ is commonly seen. It refers to the amount of radioactive decay of the measuring source per second.

▲ Measurement of radiation

The previous expression of radiation is related to its source, but exposed objects do not absorb all the radiation occurred from sources. To describe the absorbed radiation dose of exposure, the unit ‘Gray’, equivalent to joule per kilogram, is applied.

However, measurement or estimation of radiation per area should not be simply integrated in the case of the human body. Cells of each region have different sensibility and effects of different rays are different. Thus, considering a weighing factor is needed, and finally the unit ‘Sievert’ is gained.

Being aware of where measurements come from, the concept of radiation will be developed and it is really needed to be an active reader.