Without a little further explanation the title of this article may not convey a definite meaning to all readers, so I would like to start by pointing out that ''high energy'' refers to the energy of individual bombarding particles used to produce nuclear disintegrations, and not to situations where a large aggregate amount of energy is involved, as in an atomic power plant. The next question might be, when is energy considered ''high?'' To discuss this, we need a scale for measuring particle energies, and some feeling for the meaning of magnitudes on this scale. The basic scale unit is the ''electron volt'', which is the amount of energy acquired by a particle bearing an electric charge equal to that of an electron, when it falls through an electrical potential difference of one volt. This unit is rather small for use in nuclear physics, so a million electron volts, abbreviated ''MeV'', is the commonly used unit.