Have you ever wondered how a Geiger counter and how do you measure radioactivity? In this article we are going to see how it works in a way that everyone can understand.
What is a Geiger counter?
A meter or Geiger counter-Müller is a device used to measure and detect ionizing radiation.
When the ions that produce these radiations pass through the active volume of the detector they are accelerated by an electric field, producing a current pulse that signals the passage of radiation.
Geiger counters are part of a family of radiation detectors called «gas detectors«. These detectors, as the name suggests, are full of gas (some kind of inert gas at low pressure, over 0.1 atmospheres).
They have an electrode in the center of the chamber and are configured so that there is a electrical voltage between the electrode and the metal wall of the chamber.
This is how a Geiger counter works
The moment the radiation hits the gas molecules in the tube, separates electrons from atoms, this process is called ionization. The electron is attracted by the effective charge of the anode and the rest of the atom (a positively charged ion) is thrown towards the wall of the tube. The electron then travels through the wires that make up the electrical circuit and recombines with the ion. One part of the Geiger counter is a device that measures that flow of electrons.
Of course, it is very difficult to measure a single electron, fortunately the instruments should not. At the moment when the electron and ion are accelerated towards the electrode and the walls of the chamber gain Lots of energy Due to the high voltage, they collide with other atoms and suppress electrons in a process called secondary ionization. Those electrons and also ions, meanwhile, cause even more ionizations and so on repeatedly, this amplifies the original signal to a colossal level, up to a point where it can be measured.
The creation of discrete avalanches on a proportional counter
In a Geiger counter, the voltage is so high that the entire gas chamber is ionized, this provides a very high sensitivity to incoming radiation. In other detectors (called ion chamber and proportional counters) the voltage is lower and the gas amplification ratio is also lower; only a few gas atoms are ionized in these detectors. The amplification ratio with increasing voltage is shown in the following graph.
To make sense of this graph, trying mainly to understand how a Geiger counter works and some of its limitations, it is useful to know that alpha particles have considerably more energy than beta particles.
Furthermore, it helps to know that the radiation dose is a measure of the proportion of energy that is deposited by radiation on an element, more energy means more dose.
Thus, when viewing this graph, we can see that high-energy radiation striking a detector leaves a larger signal than low-energy radiation.
Now look at the right part of the graph, the Geiger-Muller region; In this region, high-energy radiation generates exactly the same signal as low-energy radiation. So we don't have the ability to distinguish between high-energy and low-energy radiation, which means that we don't have the ability to always tell how much radiation dose an individual was exposed to if we only found ourselves measuring with a Geiger counter. This is one reason why not in all cases we have the possibility to use a Geiger counter to measure the radiation dose rate accurately.
If a Geiger counter, among other things, is calibrated to measure the radiation dose rate of radionuclide Cs-137, it will be really useful as long as you try to measure the radiation dose of this nuclide.
So far we have seen how a Geiger counter works in a simple way but it is important to know that these types of radiation meters have drawbacks.
Limitations of Geiger counters
But what if you are trying to measure cobalt 60 (Co-60) radiation?
Well then you are going to have a slight drawback: radiation from Co-60 has twice the energy of Cs-137So whatever the detector reads it will only be half the actual radiation dose. On the other hand, various radionuclides have lower energy. In such a case, your meter is going to read a higher dose rate than it is, anyway. The bottom line is that A Geiger counter is only going to give an accurate reading of the radiation dose rate if it is measuring the exact same radioactive material with which it was calibrated. Hence, Geiger counters are not in all cases the appropriate instruments for measuring radiation dose rates.
It turns out that there is a type of Geiger tube that is going to give very accurate readings of a wide range of radiation energies, they are known as Geiger energy compensated counters. These are designed to provide a constant reading over a wide range of radiation energies, so if you are using an energy compensated GM detector you can take accurate dose rate readings. If not, it may be better to carry out dose rate measurements with an ion chamber.
One more thing about Geiger counters: They are large general purpose radiation detectors because they have the possibility to measure the alpha, beta and gamma radiation. If you are measuring gamma radiation, you may be more interested in measuring the dose rate (measured in mR / hr or, for really low scenarios, in µR / hr - microR per hour). If you are measuring alpha or beta radiation, then you may be trying to find contamination and you should measure either the counts per minute (cpm) or the counts per second (cps), depending on the type of meter you are using.
Summary of the characteristics of a Geiger counter
- Gas-filled detectors (such as Geiger counters and ion chambers) are filled with a gas to which an electrical voltage is applied.
- The moment the radiation interacts with the gas it causes ionizations, and this small signal is amplified by the electrical voltage. The amplification ratio is voltage dependent.
- Ion chambers have the ability to measure the difference in radiation energy, for this reason they are special for measuring radiation dose rate.
- Geiger counters, on the other hand, have full signal amplification for whatever radiation energy reaches them. For that reason, not in all cases give accurate dose rate readings, mainly if the radiation energy is different from the one that was calibrated.
- With the above mentioned, GM energy compensated detectors are designed to help address this issue, now that they offer very accurate dose rates over a wide range of radiation energies.
- And in the end, Geiger counters also have the possibility to measure not only the radiation dose gamma (measured in mR / hr or µR / hr), but rather also contamination alpha and beta (measured in cps or cpm).
I hope you liked this basic explanation. If you have fallen short and want to expand your knowledge of how a Geiger counter works, I recommend that you see this wikipedia article.