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The Beginning of Modern Physics
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Radiological Safety Training For Accelerator Facilities - index
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Van de Graaff Generator - hdbk11080098


DOE-HDBK-1108-97
Radiological Training for Accelerator Facilities
Student's Guide
The reason accelerators are of use in basic research can be described in terms of "black box"
experiments. Early studies in the structure of atoms approached the problem much like you may
approach the question of what is in a box that you cannot look into. You can fire "small bullets"
at the box and observe the effects. Did the bullet pass through? Did the bullets form a "shadow"
pattern? Were some of the bullets deflected? Did the bullets knock out pieces (particles). The
Geiger-Marsden experiment (the discovery of the nucleus) used alpha particles as the "tiny
bullets."
In the early 1900s, radioactive particles could be obtained only from materials found in nature.
The studies that physicists wanted to perform required both higher intensities and higher energies
than were obtainable from the natural sources. The ability to vary energy and intensity to suit a
particular experiment was also desirable. In addition, there was a need to know precisely the
composition of the beam of particles, where the beam was hitting the target, and the spread in
energy at the target. In other words, what was needed was control, which is the essence of the
experimental method.
In the 1930s, scientists began to build machines that produced the needed degree of control.
These machines were called accelerators.
C.
The Development of the Accelerator
The source of particles for the first accelerators was the simplest atomic element, hydrogen.
Hydrogen atoms are composed of a proton with a positive charge and a much lighter electron
with a negative charge. Since opposite electrical charges attract, there is a force holding these
two particles together in a normally stable configuration, somewhat analogous to the moon in its
stable orbit around the earth. If hydrogen gas and a few extra electrons are introduced into a
chamber where there is a positive electrical charge on one side of the chamber and a negative
electrical charge on the other side, the result will be collisions between electrons, protons, and
atoms that knock apart many of the atoms. When this happens, we say that the hydrogen gas has
been ionized. The free protons produced by such ionization are used as the particles to be
accelerated.
5


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