The Beginning of Modern Physics

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Radiological Training for Accelerator Facilities

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today's synchrotrons, whose endless paths allow electrons, positions, and protons to gain huge

energies by passing millions of times through the electric fields that accelerate them.

The Beginning of Modern Physics

In 1911, Hans Geiger and Ernest Marsden performed an experiment that was the foundation of

modern particle physics. At the urging of the great physicist Ernest Rutherford, they used a

radioactive source to shoot alpha particles at a wafer-thin gold foil and detected the scattered

alphas by watching them hit a phosphorescent screen. At the time, alpha particles were known to

be related to helium atoms and to be much heavier than electrons, but the nature of the atom was

the subject of speculation. J. J. Thomson, the discoverer of the electron, believed that the

negatively charged electrons in an atom were embedded in an unknown way in a cloud of

counterbalancing positive charge, much like raisins in a plum pudding.

By studying how the alpha particles scattered off the gold foil, scientists hoped to learn

something of the nature of the gold atoms. According to Thomson's plum-pudding model, it was

expected that the alpha particles should pass through with only small changes of direction

because neither the electrons, which were too light, nor the positive charge, which was too

diffusely distributed, could exert enough force on the alphas to knock them noticeably off-course.

However, Geiger and Marsden found that some of the alpha particles were deflected at large

angles, and a few actually reversed direction altogether.

By analyzing the distribution of scattered alpha particles, Rutherford arrived at the modern

picture of the atom in which the electrons orbit a tiny central nucleus, as in a miniature solar

system. In the Geiger-Marsden experiment, most of the alphas go straight through the empty

spaces but occasionally one will get close enough to a dense, heavy nucleus that electrical

repulsion between the two will push the alpha off its path. This revelation of the atom as a tiny

mechanical system, consisting of electrons and a nucleus, was the beginning of modern atomic

physics.