Skip to 0 minutes and 6 seconds[Radioactivity setup and safety] Using radioactive sources and Geiger counters can be intimidating for physics teachers, let alone non-specialists and early career teachers. Here we will be showing you the best ways to set up the equipment safely, to minimise the risk to both yourself and your students. This is a teacher-led demonstration to show the fundamentals, but we will show you how you can get your own students involved later on. Here we see the usual set up in the classroom. With this set up you have to be careful which direction the source is facing and where you sit your students.
Skip to 0 minutes and 50 secondsIt also means as a teacher you are often left holding the sources, or the barriers with tweezers which is not ideal. This is a similar set up to before, except instead of it being sideways on, we’ve flipped it vertically. This means the radioactive source is now pointing directly down as opposed to the walls or windows. The Geiger counter is also clamped into place to ensure you’re getting much more reliable readings. It is vital students understand background radiation. This can come from cosmic rays, rocks, soil, food and drink as well as artificial sources like nuclear power or medical uses. To take a measurement of this, turn on the Geiger counter with no source close by and then time for one minute.
Skip to 1 minute and 46 seconds[60 sec] You can repeat this 2-3 more times to take an average to show students it is not a constant number. This is something we will mention again later in this topic.
Skip to 2 minutes and 5 secondsTo make the holder for the source you use a piece of plastic piping and a piece of cork with a hole big enough to fit the source in.
Skip to 2 minutes and 34 secondsThis means you can clamp the source in place and be able to move it up or down to be able to see how this affects the count rate.
Skip to 2 minutes and 48 secondsIf you have a counter that makes noise this can be a lot more impactful on students.
Skip to 3 minutes and 10 secondsRecord the count for one minute but then remember to take away the background count from this total. One other advantage of this set up is when demonstrating barriers, you are able to place them over the clamp and leave them there, instead of holding them with tweezers or trying to clamp them vertically.
Skip to 3 minutes and 35 secondsThis means you can move away, and reduce the levels of radiation that you are exposed to. We will be looking at barriers and how this can be linked to a real world context in the next section of this topic. Finally, let’s see the set up in action. Here we have a beta source. Beta can travel a short distance through air before transferring most of its energy to the surroundings.
Skip to 4 minutes and 5 secondsBy having a metre rule next to the setup you can measure the distance from the source to the counter quite reliably. You can see how the count varies when you double the distance between the source and the counter.
Skip to 4 minutes and 21 secondsYou could get your students to make predictions on this.
Skip to 4 minutes and 30 secondsFinally, you can add different thicknesses of barrier to see how that affects the count as well.
Simple radioactivity demonstration
In the video above we look at the basic demonstration of using radioactive materials in the school laboratory. The commonly used setup is the one you see initially, which has the Geiger-Müller tube held horizontally and the radioactive source held in tweezers with the teacher’s hand. With this, you need to consider where the students are sat and how to arrange your room.
The vertical setup removes a lot of health and safety issues as the cork will house the radioactive source, and it is always pointing towards the floor instead of across the room. It also allows you to be free to look around the room and distance yourself from the source should you have any worries or concerns. The vertical set up is also very cheap and easy to reproduce and make which means it should be accessible for most science departments.
As you can see in the video we look at the background radiation. Ensure students understand that background radiation is always there and is random, as well as the value can be higher or lower depending on where you live. They will need to take the background radiation away from the number detected from the source.
- Background radiation = 15 counts per minute (cpm)
- Count with beta source = 356 cpm
- Actual count from beta source = 356 – 15 = 341 cpm
Counts per minute is a common unit used for measuring radiation levels in the classroom. They are the units used in the video.
This is a useful investigation to get students making predictions and to assess their understanding of uses. Recording data and plotting a graph of count rate vs. distance from the source is an excellent way of developing students’ skills in working scientifically. Can they interpret the shape of the graph and make predictions?
When it comes to context the usual ones that are given are testing paper or aluminium thicknesses, but students can’t really relate to this. Smoke alarms are also another context, and is something we look at a bit later in this week. A good context to use with this is radiotherapy where you can see what affect materials have on different types of radiation. One interactive online resource is Inside Story available on the IoP website, where you have the option to treat a tumour in the lung and adapt the levels of radiation whilst thinking of absorption and the effects of radiation on healthy tissues.
Can you think of any real world contexts that this would link too? Share them below.
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