 What you see here are the three types of radiation streaming towards you. On the right-hand side here are the gamma photons. In the middle is the helium nucleus, the alpha particle, two protons, two neutrons. And on the left-hand side is the beta particle, the very fast electron. You'll notice that the gamma rays and the beta particles are almost travelling at the same speed. That's because the beta particle is travelling at a speed quite close to the speed of light. If I bring a piece of paper in, what you'll notice is the alpha particles are now unable to penetrate through. The gamma rays and the beta particles can still get through. If I bring a piece of aluminium in, that's enough to absorb the beta particles and they are no longer getting through and there's no significant reduction in the number of gamma rays. If I bring in a large piece of lead, that is enough to absorb a significant number of the gamma rays and the count rate drops right down.