The world’s first man-made atomic explosion took place only 28 months after the arrival of the first scientific contingent at Los Alamos. Few greater tributes to human ingenuity have ever been written.
The theoretical basis for nuclear weapons was already understood, in its outlines, when the Lab¬oratory was established. Many of the engineering problems were foreseen in a general way, but much remained to be done. The following summary of weapon theory (all of it known in early 1943) will serve to suggest the enormous difficulty of the task that lay ahead.
The nucleus of an atom of uranium-235 contains 92 protons and 143 neutrons. When this nucleus absorbs an additional neutron, it becomes unstable and usually divides approximately in half. The two fragments become nuclei of two lighter elements, having a total mass somewhat less than the mass of the original uranium nucleus plus the additional neutron. Most of the mass difference between the original material and the products is converted into kinetic energy-rapid flight of the fragments.
