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Before the summer of 1945 it could be fairly stated that the problem of radioactive contamination of water did not exist. True, some radioactive springs were known, but these aroused little interest, except for possible use of the supposed healing properties of the water. The successful testing of an atomic bomb in the New Mexico desert on July 16, 1945, changed this situation. Fogging of some packages of photographic film was traced to radioactive débris from this bomb. The radionuclides had got into the process water and then into the wrapping paper. This is the first known damage to a commercial product from radioactively contaminated water. Radioactivity may be defined as the spontaneous emission of penetrating energetic radiation from certain atomic nuclei. The early workers in the field soon classified the radiation into three types, called alpha (α), beta (β), and gamma (γ) rays, on the basis of their ability to penetrate matter. They were also able to establish their nature, finding that alpha and beta rays were high-speed particles, while gamma rays were electromagnetic waves similar to X-rays. The results of these early studies can be tabulated as shown in Table I. The emission of these radiations is associated with a nuclear disintegration, which is the rearrangement of the constituents of the nucleus into a more stable configuration with an associated release of energy. The rate of emission is characteristic of the radioactive material (also called radionuclide), and is described by the time required for half of the unstable nuclei to complete their rearrangements and emissions: the half-life. After the lapse of one half-life, since the number of unstable nuclei has been reduced to one-half the original number, the emission rate (or activity) is also reduced to one-half of its initial value. The quantity of radioactive material is measured by the rate of disintegration; that amount of a nuclide in which 3.7 × 1010 atoms disintegrate per second is defined as a curie. Ordinarily, subdivisions of this unit, milli-, micro-, or even micromicrocuries, are used. These are 10−3,10−6, and 10−12 curies respectively. The term picocurie is occasionally used as a synonym for micromicrocurie.