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How radioactivity changes with size

Radiation can be absorbed internally

The total amount of radioactivity of a given isotope is directly proportional to the number of undecayed nuclei you have.  If you ignore the mass of any radiation emitted then doubling the mass exactly doubles the activity.

But radiation can be emitted at any point in the material.

Alpha particles that are emitted in the middle of a sample will just be absorbed by the sample and can’t be detected.  Only those particles emitted near the surface of a sample make it to the outside world.

Try Why Do Astronauts Float by Julian Hamm

So for a large sample of an alpha emitter the total amount of detected radiation depends on surface area more than volume.  In we can ignore this effect because the size of a laboratory sample typically stays constant.

In our animations we tend to show particles being emitted anywhere on a single face.

back to Lesson 3: Half-life part 1