The biological hazard from radioactivity depends on all three of these characteristics. No matter whether the radioactive nuclei are produced in an atomic explosion or in an atomic reactor, some time will in general elapse before a human population can become exposed. If this time is long compared to the half-life of the radioactive species, most of the nuclei will have disintegrated, and the hazard will thereby be reduced. If, on the other hand, the half-life is long compared to this time, as well as to the life-span of a human being, the rate at which disintegrations occur will be low, and again the hazard will be reduced. In short the dangerous half-lives are the intermediate ones, not too long, not too short. Sr⁹⁰ is an example. CHAPTER V Breakup of the Nucleus The positive electric charges within an atomic nucleus repel one another. In the most heavily charged nuclei this repulsion becomes so great that the nucleus can break into two parts, simultaneously releasing a considerable amount of energy. In the case of _spontaneous nuclear fission_ the two parts are more or less equal in size. In the process of _alpha decay_ one of the parts (the alpha particle) is much smaller than the other. An alpha particle consists of two neutrons and two protons and is identical with the nucleus of the helium atom. (The symbol for this nucleus is He⁴.) Since two neutrons and two protons can simultaneously occupy the lowest energy state, the alpha particle is an especially stable nuclear unit. As a result, from time to time in heavy nuclei, two neutrons and two protons will coalesce into an alpha particle, which may then attempt to escape. In attempting to escape from the nucleus, however, an alpha particle encounters considerable resistance because of the short-range nuclear attraction of the other neutrons and protons. This resistance which an alpha particle experiences in trying to leave the nucleus is usually referred to as an “energy barrier.” If the alpha particle could acquire a little additional energy, it would be able to overcome the barrier and get away from the nuclear attraction. Once outside the nucleus, just beyond the reach of the nuclear attraction, the alpha particle would be accelerated violently outward by the large electrical repulsion between its two protons and the other protons in the residual nucleus. [Illustration: How an alpha particle escapes from the nucleus. From A to B it goes “uphill,” losing speed. At B its speed is zero and it almost always turns around. With a small probability it may sneak through the energy barrier B to C. Beyond C, it is repelled and emerges with increasing speed.]