Koji Tsuboi - Proton Beam Radiotherapy: Physics and Biology

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People have long been interested in the vastness of the universe as well as in the minute world that exists within materials and human bodies. In ancient times, people drew the numerous constellations seen in the night-sky and imagined romantic stories about them. The invention of the telescope in the early seventeenth century opened up a field of scientific research regarding the universe. New technology has led to the development of more powerful telescopes that are able to see long distances into space using optical lenses and mirrors along with a parabolic antenna for radio waves. The first optical microscope was developed in the late sixteenth century to observe the minute world that exists within materials and living things, and enabled research into small structures including biological cells and bacteria. In the late nineteenth century, a rush of scientific discovery arrived in the field of radiation physics. First, Wilhelm Conrad Roentgen discovered X-rays using a cathode ray tube (CRT) in 1895. Then, Antoine Henri Becquerel detected radioactivity in 1896 using a photographic plate and uranium salts of phosphorescent materials. This was followed by the identification of electrons for the first time using a CRT in 1897 by Sir J. J. Thomson. Two years later, in 1899, Ernest Rutherford discovered -particles in the radiation emitted from uranium salts, following which, in 1909, Geiger and Marsden observed that an -particle deflects at an angle of more than 90 when it strikes a gold foil. The following year, Geiger showed that the largest possible deflection of a particle passing through a thin gold foil was less than 1. It was very difficult to understand that the large deflection angle of the -particles after they struck thin gold foil was a result of the sum of multiple small deflections. In the same year, Sir J. J. Thomson proposed a theory to explain this strange behavior of -particles. His atomic structure model suggested that an atom consists of a number of negative charges accompanied by an equal number of positive charges uniformly distributed in a sphere. Using this model, he hypothesized that the large deflections would not take place without the positively- and uniformly-charged sphere being very much smaller than the size of the whole atom. Building on the knowledge gained from experimental work and theories, Ernest Rutherford suggested a simple atomic structure model that was able to explain the large deflection behavior. He described that, according to his model, an atom contains negative charges uniformly distributed in a sphere surrounding positive charges at its central point. He determined that the large deflection must have been caused by a single collision. This idea successfully explained how -particles exhibited large deflections while passing through thin gold foils. This came to be known as the Rutherford Scattering Experiment, which led to the discovery of the atomic nucleus in 1911. During that year, Rutherford published a new atomic model showing that an atom consists of electrons orbiting around a very small nucleus in which most of the atomic mass and charge is concentrated. He needed eight more years to uncover the detailed structure of the atomic nucleus.