Dr. Tsubokura's Radiation Lecture Vol.73

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

145. Detecting the number of electrons emitted from collisions against radiation rays

The extent of radiation exposure determines its impact on health. There are various measuring devices to detect levels of radiation and radioactivity.

Radiation rays have a feature of colliding against substances in their way and forcing those materials to emit electrons. When radiation rays pass through a measuring device filled with stable gas, they collide against the substances composing the gas, and electrons are emitted from those substances. Measuring devices that detect the amounts of emitted electrons are called Geiger counters.

Another measuring device is called a "semiconductor detector" (the common type being a Germanium-based semiconductor detector), which utilizes materials called "semiconductors" instead of gas. While materials that allow the flow of electric current (e.g., iron and copper) are called "conductors," materials that do not transmit electricity (e.g., rubber and glass) are called "insulators." A semiconductor lies between a conductor and an insulator; in other words, it transmits electricity to some degrees. A cooled germanium semiconductor detector can conduct electricity only when stimuli are added. Therefore, it can conduct electricity when electrons are emitted after radiation rays collide against it.

Although such a detector is expensive and requires large equipment, as it uses liquid nitrogen for its cooling system, the device has excellent specificity for distinguishing types of radioactive substances. For this reason, it is used to detect levels of radiation contamination in food.

146. The radiation emitted from our body

The extent of radiation exposure determines its impact on health. There are various measuring devices to detect levels of radiation and radioactivity.

Radioactive substances naturally present in humans, namely radioactive potassium, leak very small levels of radiation from inside our bodies. A measuring device used to detect these small levels of radiation by placement in either the front or the back of a seat near a patient’s body is called a "whole-body counter."

Various types of whole-body counters are present, including some incorporating fluorescent substances or semiconductors. The basic principle of whole-body counters is the same as that of devices used to detect levels of radiation contamination in the air or food. For example, for two minutes of the examination period, a whole-body counter detects the type and level of radiation leaking from the body. Then, based on these results, the device speculates the amount and type of radioactive substances that are present in our bodies.

In the majority of the screening tests conducted in Fukushima Prefecture as of October 2017, radioactive potassium has been detected in residents' bodies, yet radioactive cesium has not been detectable. This result suggests that natural radioactive potassium is present in the residents' bodies while radioactive cesium is no longer ingested.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on 22nd and 29th October 2017 was reproduced for MRIC Global under the author's permission.