Radioactive Sources
Introduction
In this title we are going to explore different radioactive sources, their properties and how they are detected.
We are going to do this by looking at an e-scenario of a set of sources, some material blocks or shields, a Geiger counter, and a rate meter.
Finally, we will look at a GCSE exam question where we can apply what we have learned.
Switch on the counter. The meter will count all radioactive particles reaching it which includes any background radiation.
You can move the source by dragging its base.
You can change the source by clicking the label above the source.
If you have moved the source, return the source to its most righthand position.
Switch on the counter. Change the source to gamma and drag the source closer to the detector.
Do that now and notice how the count increases to much faster when the source is close to the detector. Note this well, the closer you are to a source, the more radiation will be received.
Return the source to its most righthand position.
Blocking the gamma radiation
With the source all the way to the right, as shown here, drag the lead block in front of the source.
This will prevent the gamma rays from reaching the detector and the only radiation counted will be from the background. This is the background radiation.
For many experiments it is useful to know the average rate of the background radiation, and this is one way of measuring it.
Try blocking the gamma radiation by using the paper and aluminium blockers. For the lead and aluminium blockers, you need to have the source as far right as it goes.
You will observe that these have little or no discernible effect.
This is a very important observation, a lead block several cms thick will block gamma radiation, but other substances are not effective as protection from gamma radiation.
Measuring the background radiation
Move the lead block in front of the source.
Zero the counter and wait for 60 seconds. This will give you the background radiation rate in counts per minute.
What is the background radiation in counts per second?
What blocks different sources
Move the lead block in front of the source.
Zero the counter and wait for 60 seconds. This will give you the background radiation rate in counts per minute.
What is the background radiation in counts per second?
Try blocking the gamma and beta radiation by using the paper, aluminium and lead blocks. Note that the alpha radiation has a very short range, so you will need to drag it all the way to the right, so that it is as close as possible to the detector.
Note your results in a table like the one below.
Radiation fall-off for gamma radiation
With the gamma source selected. Count the number of hits for a distances of 10, 15, 20, 25 and 30cm for one minute.
Subtract the background radiation you found in the previous exercise.
Plot the count rate on the y axis against the distances on the x-axis. Can you identify the nature of the fall-off?
Exam Questions
Radiation fall-off for beta radiation
With the beta source selected. Count the number of hits for a distances of 10, 15, 20, 25 and 30cm for one minute.
Subtract the background radiation you have already found.
Plot the count rate on the y axis against the distances on the x-axis. Can you identify the nature of the fall-off?
Radiation fall-off for alpha radiation
This won’t yield much of a plot as alpha only penetrates a very short distance. However, you can get a few readings over short distances.
Complete the table shown on the belowand create a plot as for gamma and beta.
You’ll obtain a rapid fall-off in the first few centimetres and then virtually no radiation at all at larger distances.
If you have difficulty answering this, try the e-scenario again taking note of the scale on the side of the gas container.
Exam Queastion AQA May 2024 Foundation Paper 1
AQA May 2024 Foundation Paper 1
You should recognise this from the exercise immediately before this question. If you don’t then repeat the exercise.
This should be obvious from the exercise on the different blocks. If it isn’t, go back to trying out the different material blocks on the different sources.
You need to think about all the factors that effect the amount of radiation that will ‘hit’ the teacher.
Does the amount of time of the exposure matters?
Does direction of the source matter?
What sort of protection might be worn?
How might the sources be handled?
This is perhaps a little bit more subtle as it uses the word contaminated which implies that the source has got onto or even into the teacher. So, here we must think about preventing contact, so think about the possible contacts the teacher might have.
Could the teacher use something to pick up the sources?
What body protection could be worn?
Would eating and drinking be a good idea?
As an aside, consider this...
Alexander Litvinenko, a Russian agent, was poisoned by ingesting an alpha radiation source in November 2006.
Alpha radiation may seem weak, but it is lethal if ingested. So, precautions are essential.