The accident at the Fukushima No. 1 nuclear power plant has prompted a surge in sales of radiation detectors--and also an increase in inaccurate measurements.
The characteristics of such devices for ordinary citizens vary considerably in terms of accuracy, method of measuring and price.
The best-known radiation detector is the Geiger counter. But a Geiger counter cannot distinguish between different types of radiation nor can it accurately evaluate the toxic effects on human health. It can only count how many times it has detected radiation.
It would be like owning a device that counts the number of animals but does not differentiate the species, whether they are elephants, rabbits or mice.
The least penetrative type of radiation is the alpha ray, which can be stopped by even a sheet of paper. Alpha rays, emitted by plutonium and other substances, are clusters of two neutrons and two protons.
The beta ray can penetrate paper but can be blocked by thin aluminum or other metal plates. Beta rays, ejected by strontium, cesium and other radioactive substances, are in fact electrons.
The gamma ray can only be stopped by lead or thick iron plates. Emitted by cesium and other substances, gamma rays are not particles but are electromagnetic waves of extremely high energy.
Neutron beams are emitted during nuclear fission of uranium and other matter. Water or concrete can be used to block them.
Radiation crashing against a human body generates electrons that disrupt DNA, but the amount of electrons generated by the collision differs according to the type of radiation. The levels of toxicity, which are also different, are measured in units of sieverts across all radiation types.
The Geiger counter is a simple mechanism that is sensitive to low radiation levels. Its central component is a container called the Geiger-Mueller tube. When radiation hits gas particles contained within the tube, electrons are ejected, generating electric currents in a circuit that the device can detect.
Since only a very small number of electrons are ejected, a voltage is applied to the GM tube so that the electrons accelerate and hit other gas particles, creating a cascade of electron flows. This contraption makes it possible to detect very small doses of radiation.
Different numbers of electrons are generated by different types of radiation. The radiation type could be identified if the initial amount of electrons was known, but this is impossible after the electrons have been multiplied in the GM tube.
Therefore, the dose in sieverts indicated on the Geiger counter is a mere estimate. It's like estimating the total weight of all the animals counted under the assumption they were all rabbits.
"Geiger counters can be used to check if food or other products are contaminated or not, but they do not tell you how harmful they are," said Keiji Kusama, head of the radiation safety section of the Japan Radioisotope Association.
He said the public should not rely on products available for tens of thousands of yen (hundreds of dollars).
Using the animal-counting comparison, some products cannot even reliably count the number of creatures.
The performance of a Geiger counter depends on the size of the GM tube. A larger tube can hold more gas for electrons to be generated, resulting in a higher measurement capability.
Geiger counters sold at tens of thousands of yen, however, have GM tubes the size of a fingertip. Their measurements, spanning only several seconds, can produce estimates ranging from half to double the real values, Kusama said.
"Cheap products can detect radiation only several times a minute," said Kouichi Yamada, director of the measuring systems sales division at Hitachi Aloka Medical Ltd. "Errors are large because the calculations are based on scanty data."
Other types of detectors, including the ionization chamber, can distinguish between radiation types.
The ionization chamber takes measurements from electrons generated by radiation crashing into air particles. The ionization chamber resembles the Geiger counter in this sense, but the electrons are not multiplied to be counted.
It would be like summing up the weights of elephants, rabbits and mice measured individually and carefully. This enables a precise evaluation of the total weight of all animals, or the total level of toxicity of all types of radiation.
But to measure low radiation levels in a short time, the device must be so big that is no longer portable. And the prices for the larger ionization chambers can cost millions of yen. These devices are used at monitoring posts or fixed measurement stations.
Portable types of ionization chambers are available for hundreds of thousands of yen, but their sensitivity is so low that they cannot even measure radiation levels at a distance of 50 kilometers from the Fukushima No. 1 nuclear plant.
Municipal governments across Japan are introducing detectors of yet another type--scintillation survey meters--to locate radiation hot spots. These devices, which use crystals that emit light when they are irradiated, are not as accurate as ionization chambers, but they can still produce sievert estimates by distinguishing between different types of radiation. They have about a 15-percent margin of error and cost hundreds of thousands of yen.
Some detectors use semiconductors that generate electricity when they are irradiated. These devices also distinguish different radiation types, but products available for tens of thousands of yen have semiconductor chips only several millimeters in size.
A semiconductor detector can be used to measure total exposure to radiation if it is carried over an extended period of time. But it is not suited for measurements in short intervals.
Experts agree that there are no radiation detectors that are both cheap and accurate. The figures these products produce should be taken as rough estimates, they say.
- « Prev
- Next »