Radioactive decay and half-life

Watch 400 nuclei decay at random, and a perfect curve emerge.

Interactive

time0s

half-lives passed0

left400

curve predicts400

Half-life4 s

The math, with your numbers

N = N_0 \left(\tfrac12\right)^{t/T_{1/2}} = 400 \times \left(\tfrac12\right)^{0/4} \approx 400

The real count wobbles around the curve, more so near the end when few nuclei are left: that's the randomness showing through.

Try this

→Before you press Play, predict how many will be left after two half-lives.
→Watch a single nucleus. Can you predict when it will decay?
→Reset and run it three times. Does the count after one half-life come out exactly 200 each time?

Want to know why it behaves like this?

The lesson “Radioactivity & Nuclear Energy” explains it step by step: How can random decays add up to a clock you can set your watch by?

Radioactive decay and half-life

Watch 400 nuclei decay at random, and a perfect curve emerge.

Radioactive decay and half-life

Interactive

time0s

half-lives passed0

left400

curve predicts400

Half-life4 s

The math, with your numbers

N = N_0 \left(\tfrac12\right)^{t/T_{1/2}} = 400 \times \left(\tfrac12\right)^{0/4} \approx 400

The real count wobbles around the curve, more so near the end when few nuclei are left: that's the randomness showing through.

Try this

→Before you press Play, predict how many will be left after two half-lives.
→Watch a single nucleus. Can you predict when it will decay?
→Reset and run it three times. Does the count after one half-life come out exactly 200 each time?

Want to know why it behaves like this?

The lesson “Radioactivity & Nuclear Energy” explains it step by step: How can random decays add up to a clock you can set your watch by?