 With alpha particles, Rutherford has something to fire at atoms to see if they were indeed like a positive pudding with embedded electrons. Here's a graphic of the apparatus used to run the experiment. An alpha particle emitting substance is placed behind a lead screen with a single small hole in it to enable a narrow beam of particles to flow through. This beam is directed at a very thin gold foil. A movable zinc sulfide screen is placed on the other side of the foil. Zinc sulfide flashes when hit by an alpha particle. A microscope swivels to view all scattering angles. If the Thompson model was correct, the positively charged alpha particles would pass through the distributed and therefore diluted positive charge in the gold atoms with little or no deflections. But after days of observation, here's what they found. While most of the alpha particles do go right through with only minor deflections, some were scattered through very large angles. A few were even scattered in the backward direction. Rutherford described this as almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back at you. To explain these results, Rutherford was forced to picture an atom as being composed of a tiny nucleus with a positive charge and nearly all of its mass are concentrated, with electrons some distance away. Note that the closer the alpha particle is to the nucleus, the greater the angle of the deflection. We can use this angle to measure the maximum possible size of the nucleus. Here we have an alpha particle trajectory with an impact parameter B that scatters at the angle theta and reaches a closest distance labeled D. The number of protons in the alpha particle is 2 and the number of protons in gold is 79. The energy of the naturally occurring alpha particles used by Rutherford were 7.7 million electron volts. An electron volt is the energy it takes to move one electron across one volt. If we considered the direct hit trajectory, the initial kinetic energy of the alpha particle will drop to zero when it reaches its closest possible distance to the nucleus. All its kinetic energy would have been converted to electric field potential energy. Conservation of energy tells us that these two numbers must be equal. Rutherford's calculations show that the radius of a gold atom nucleus cannot be any larger than .0003 nanometers. That was 10,000 times smaller than the size of a gold atom. Here's a picture of the test apparatus Hans Geiger of Geiger counter fame and Ernst Marsden built to look for scattered alpha particles from every angle. This was the first experiment that fired a beam of particles at a target to detect the scattering effects and deduce what is going on. That was around 100 years ago. This is exactly what we are doing today at the European Center for Nuclear Research, CERN, analyzing the Higgs boson. We'll return for a deeper look at Rutherford's scattering when we get to particle accelerators.