Unstable nucleus.
Random decay!

A model is an explanation, not a photograph.

Protons and neutrons are drawn as colored balls packed together; real nucleons are not hard spheres, and colors and sizes are chosen to help you see the ideas. A nucleus is about 1/100 000 of an atom’s width; the electrons around the atom are not shown. Every motion is slowed down enormously: real alpha particles leave at about 5% of the speed of light, beta particles at up to 99%, and gamma rays travel at the speed of light.

“Decay this nucleus” makes one nucleus decay on demand so that you can watch it. Real decay is random: nothing can make a particular nucleus decay, or predict when it will. Americium-241 has a half-life of 432 years, carbon-14 5730 years and technetium-99m 6.0 hours. In the half-life view, one half-life always takes a few seconds on screen, whatever the isotope. Beta-minus decay also releases an antineutrino, which is drawn faintly; knowledge of antineutrinos is not required for GCSE. Technetium-99m emits almost only gamma rays, a reason it is chosen as a medical tracer.

Penetration ranges are typical classroom values: alpha is stopped by a sheet of paper or a few centimeters of air; beta by a few millimeters of aluminum; gamma is reduced, but never stopped completely, by several centimeters of lead. In the test, a quarter of the gamma rays get through the lead to show this; the real fraction depends on the thickness.