Episode 511: Absorption experiments
Lesson for 16-19
- Activity time 40 minutes
- Level Advanced
This gives students the opportunity to work with radioactive sources.
- Student experiments: Absorption of radiation and report back (40 minutes)
- Demonstration: Absorption of radiation by living matter
- Student experiment (optional)
Groups could work in parallel and report back to a plenary session.
Remind them to correct for the background count (taken at least twice – at the start and end of the main experiments and the two results averaged).
Range of alpha radiation
Range of beta radiation
Range of gamma radiation
An optical analogue for the absorption of γ particles by lead is the absorption of light by successive microscope slides.
Absorption of γ particles is an example of exponential decrease – check the data for a constant
half thickness, thus suggesting the type of physics involved. (Each mm of absorber is reducing the intensity by the same fraction.)
Demonstration: Absorption of radiation by living matter
To simulate the absorption of radiation by living matter use slices of different vegetables as absorbers, or a slice of bacon to represent human flesh.
Student experiments: Optional
The first requires a sealed radium-226 source. Because Ra-226 is the parent to a chain of radioactive daughters, granddaughters and so on, you get a mixture of αs, βs and γs emitted. Challenge students to use absorbers to establish that all three radiation types are being emitted. (The maximum energies are: E α = 7.7 MeV E β = 3.3 MeV E γ = 2.4 MeV)
The second is an extension of the β absorption experiment. You could speculate that some β particles might be
back-scattered (like Rutherford’s α particle scattering that first demonstrated the existence of the nucleus). A quick try shows that some β particles are indeed back-scattered.
Follow the local rules for using radioactive sources, in particular do not handle radioactive sources without a tool or place them in close proximity to your body. Deliberately placing a radioactive source in contact with the skin would increase your dose of ionising radiation unnecessarily and increase the risks to your health. This is a criminal offence.