The Geiger-Mueller Tube
The Geiger-Mueller Tube Is A Type Of Tube That Is Used To Detect Radiation
The basic principles of functioning of Geiger-Mueller tubes are described and explained in this comprehensive introduction.
Materials & Apparatus
- For GM tubes, a scaler holder is required
- Slender window GM (General Motors) tube
- Co-60 with a stop filter or Ra-226 with a thick filter are examples of gamma sources that are as pure as feasible
- Pure beta source (Strontium 90)
- A container for radioactive materials
- If a gamma GM tube is available, use it
- Matches' box
Technical Notes & Health & Safety
Geiger-Mueller tubes are designed to work at a voltage that is inside their 'plateau.' This is set automatically in self-contained systems.
When an energetic particle enters a Geiger-Mueller tube, the voltage across it is usually kept low enough to avoid a roaring spark.
Geiger-Mueller tubes are extremely fragile, especially when used to analyze alpha particles. The alpha particles might enter the chamber through the tiny mica glass. It requires a protective cover to prevent it from being harmed by being accidently handled. Photons may be counted with a competent alpha detecting Geiger-Mueller tube. A few ultra violet photons will be detected if you light a match in front of it.
The sort of Geiger-Mueller tube you're using will determine this. If you have a self-contained system, all you have to do now is get it ready and turn it on. You'll need to set the voltage on the scaler if you're using an older model Geiger-Mueller tube that connects into a separate ratemeter or scaler. Follow these procedures to do this.
In a holder, place a radioactive source. Fix this to a retort stand in a clamp.
In a stand, place the Geiger-Müller tube. Adjust it so that it is 5 cm away from the source and pointed at it.
Switch on the scaler (counter) with the Geiger-Müller tube plugged in.
Begin with a voltage of around 200 volts. Make a note of how many counts there are in a 15-second interval, for example.
Increase the voltage in 25-volt increments.
The numbers will fluctuate with voltage until reaching a plateau. This is how a graph would appear (you don't have to plot it):
The count will approach a plateau after reaching the threshold voltage. Over a wide variety of voltages, it will remain constant. Set the voltage between 50 and 100 V higher than the threshold.
If the clicking gets louder as the voltage gets higher, you've broken through the plateau. Reduce the voltage once more.
Return the source to a secure location until the demonstration is completed.
Executing The Demonstration
On the Geiger-Mueller tube counting system, turn it on.
The fact that there is a background count should be highlighted.
Draw attention to the increase in counts by bringing a radioactive source up to the Geiger-Mueller tube.
The background count and the count with the source close might both be measured. Do this for 30 seconds at a time. Make a point of pointing out the difference.
Notes For Teachers
Talk about what's going on in the Geiger-Mueller tube. It is important to emphasize that it is more sensitive and stable than the spark counter.
Draw attention to the differences between the Geiger-Mueller tube and the spark counter in terms of count. The Geiger-Mueller tube monitors all ionisation events that occur within it. Everyone has signed up.
As a development of the spark counter, discuss the Geiger-Mueller tube. You may put it this way: "The spark counter wire is encased in a metal shield that serves as the second electrode, and the high voltage source is built within the scaler. The scaler is in charge of counting the charge pulses given to the center wire by the electron avalanches. To ensure that each spark does not linger too long, the tube is filled with a proper gas combination. The tube and scaler are ready to count a 'bullet' from another radioactive atom's 'explosion' once the spark has been quenched. As you can see and hear, the tube can react to each avalanche extremely fast."
The Geiger-Mueller tube operates on the same principle as the spark counter: ionization between two high-voltage electrodes causes a pulse of current (an avalanche of charge) between them. The Geiger-Müller tube differs in that it is sealed, contains a low-pressure gas (often argon with a small amount of bromine), and is normally part of a circuit with a scaler counter.
Each charge pulse is recorded and counted by the scaler counter.
Inside a tube, the real occurrences are far more involved than the basic scenario of ionisation causing an avalanche of electrons. Ultra violet photons, as well as colliding electrons and ions, are likely to play a role within the tube, and the detailed picture is exceedingly complicated.
An ionising particle will create a charge pulse that is nearly constant in size. The energy or quantity of ionisation produced by the ionising particle has no effect on the pulse size.
The number of pulses indicates how many ionizing particles are entering the tube.
If a particle enters the tube and does not pass directly through, Geiger-Mueller tubes do not differentiate between one type of particle and another, or between a more energetic particle and a less energy particle (as most gamma rays do).