How to measure radioactive substances


here are many different radioactive substances, such as iodine, cesium, and plutonium, leaking out of the Fukushima Daiichi nuclear power plant. How are they measured, and how can they be distinguished
from one another? (30s Kanagawa)


There are essentially three kinds of radiations that scientists are concerned about when measuring the radioactive substances as the result of fission:
a.    alpha particles (helium nucleus),
b.    beta particles (electron), and
c.    gamma rays (electromagnetic radiation).
For instance, iodine-131 and cesium-137 each emit a beta particle and a gamma ray per nucleus when they decay. While strontium-90 emits only a beta particle, whereas plutonium emits only an alpha particle. From the type of radiations and their energies, scientists can deduce the type of the radioactive substances present in environments.

Scientists use a variety of radiation counters depending on the type of radioactive substances they want to measure. A radiation counter is a detector instrument that counts the number of radiations that enter the detector. We must be careful here that the counter cannot directly measure the number of radioactive substances: it only counts the radiations emitted by radioactive substances when they decay into more stable substances.

The most common technique to distinguish radioactive substances is by analyzing the energy spectrum of gamma rays, since many of the substances emit gamma rays when they decay. Let us look at an actual data taken by the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba. The AIST scientists used what is called a gamma ray spectrometer to measure radioactivity in dusts accumulated on their premises in Tsukuba after the Fukushima Daiichi accident. Below is the energy graph they produced.
A gamma ray energy spectrum produced by the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba

The horizontal axis is the radiation energy measured in kilo-electronvolt (keV), and the vertical axis is the radiation count measured per 1,000 seconds. The red curve shows the data from a sample taken on March 15, and the green curve shows that from a sample taken on March 18. The blue curve is the measurement without any sample, showing the background noise. For example, cesium-137 (Cs-137) is known to emit gamma rays with the energy of 662 kilo-electronvolt (keV). By counting how many of the gamma rays in the environment are at this energy, scientists can compute cesium-137’s contribution to the radioactivity.

So the radioactive substances that emit gamma rays are relatively simple to measure, but there are radioactive substances require many complicated steps of analysis. For example, strontium-90 emits only beta particles whose energy is in the range where it is easily buried in the background spectrum, and requires more sophisticated equipment to eliminate the background noise. Plutonium emits alpha particles, for which scientists use a type of survey meter for a quick count, but the instrument cannot tell the alpha particles of plutonium origin from those of radon origin. The accurate measurement of plutonium densities requires more rigorous measurement and analysis, which can take as long as a week to get the final result!

Thus, the difficulty with which scientists measure the radioactivity of a substance depends on the type of the radioactive substances, and it does not necessarily mean that just because some are not detected by one type of counter, they don’t exist. Regardless of the difficulty, the radioactive substances we ought to watch out for, at this point, are long-lived ones that include strontium-90 and plutonium.

Science Communicator: Misato Hayashida