In this experiment, a very narrow beam of alpha particles was fired at a very thin piece of gold foil.
- The thick sheets of lead are to stop the beam of alpha spreading out like the beam on a car headlight. They keep the beam narrow. The arrangement is called a collimator.
- The detector is moved around the outside of vacuum chamber and readings taken for different angles.
- The beam has to be narrow because this gives a very definite spot on the foil from which to measure the angle, as shown on the diagram below:
The moment of genius was when it was decided to move the detector round onto the same side as the alpha source. No one in their right mind would expect to find alpha particles on this side, but that is excatly what they did discover. It's called backscattering. 
We can tell that the nucleus is positive because the positive alpha particles are repelled. - We can tell that the nucleus is heavy by thinking of what happens in snooker when the cue ball hits a coloured ball. The cue ball stops but the coloured ball moves on. Momentum is transferred here because both balls have the same mass. However, the alpha particle bounces back and the nucleus is unmoved. This means that the nucleus is much heavier.
