The Geiger-Müller (GM) counter is used to detect ionising radiation such as alpha and beta particles or gamma rays. The radiation enters through a very thin window at one end of the tube. This window is usually made of mica.
Mica is a mineral that forms in layers called sheets. These sheets can be split apart into very thin layers, so thin that even an alpha particle can pass through it (remember that alpha particles can be stopped by something as thin as your skin or a sheet of paper). The mica window prevents the argon inside the tube from escaping and also stops air from getting into the tube.
When radiation enters the tube and collides with an argon atom, an electron may be knocked off the atom – we call this process ionisation. When ionisation occurs, a positively-charged argon ion and a negatively-charged electron are produced. The argon ion is attracted to the outside wall of the tube, which is connected to the negative terminal of the power supply, while the electron is attracted to the central electrode, which is kept at a high positive voltage – typically 500V.
A small pulse of current is produced each time an electron reaches the central electrode. These pulses can be counted by an electronic circuit and a displayed on a 7-segment display. Sometimes a small speaker is added to the system to produce a click for each pulse. On its own, the GM tube cannot tell the difference between alpha, beta and gamma radiation. We need to place different materials (e.g. paper, aluminium, lead) in front of the mica window to discover which type of radiation is responsible for the reading.
Here is a short video demonstrating the use of a Geiger-Müller tube.
I’ve had an email from Yenka to say that they are now offering many of their products for home use – free of charge! I think Yenka is the new name for Crocodile Clips, the company who made Crocodile Physics and Crocodile Technology. Their software allows you to create your own experiments to learn about
For those of you preparing for S Grade prelims straight after the Christmas holidays, I have added a 6 page summary on radioactivity & half-life calculations with worked examples and extra questions (with answers) for you to try.
You’ll find the pdf file containing all this radioactive goodness in the Health Physics section of the Standard Grade revision page.
We don’t need to cover chemotherapy as part of the Standard Grade Phyics course but I thought you might be interested in how it works, now that we have looked at the use of Radiotherapy. The attached pdf file contains some notes on chemotherapy taken from the CancerHelp UK website.
We’ve been looking at the various uses for lasers in surgey, including the correction of eyesight. I found this site that shows how one company does this in their clinic. The animation does a pretty good job of explaining the procedure.