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 When the burst is close enough to the bottom, as in the BAKER shot of CROSSROADS, an underwater crater may be formed, and the material excavated from it will be radioactive and contribute to the residual radiation inventory.

The primary source of personnel exposure from the BAKER shot was not the radioactive pool of water, however, but from contact with the target ships, which had been bathed in the radioactively contaminated water, sand, and coral that rained down upon them from the cloud and from the radioactive mist (base surge) that rolled out from the base of the underwater explosion column.

A nuclear explosion produces three types of radiation that posed a potential hazard to CROSSROADS participants: alpha, beta, and gamma radiation. When any of these encounters living tissue, it transfers some of its energy to the target atoms, tearing off some or all of their electrons. This leaves the atoms with a positive electrical charge. This process is called ionization. This tearing off of the electrons destroys the bonds holding together the complex molecules making up living tissue and leaves the tissue damaged to some extent. At low levels of radiation, the damage is minor and probably does not adversely affect the individual's health or longevity. At higher levels, the reverse is true.

Gamma rays are electromagnetic radiation, differing from the more familiar radio waves and x-rays in that they have higher frequency and shorter wavelength. They are produced in great quantities and with very high energy during a nuclear explosion. They are also given off during the decay of the radioactive isotopes produced by a nuclear explosion. They can travel long distances and can readily penetrate clothing and skin. Because the personnel conducting Operation CROSSROADS were miles from the two detonations, the gamma hazard to them came from radioactive isotopes left in the target area or carried from it by wind or tide or on the participating ships or planes or even on the bodies of the personnel themselves. The radiation detection instruments used during CROSSROADS readily detected gamma rays.

Beta particles are electrons. Like gamma rays, they are given off by a nuclear explosion or by the radioactive isotopes produced by the explosion. Unlike gamma rays, however, beta particles do not travel far and, except at high energies, are stopped by clothing or the outer layers of skin. They are a greater hazard if isotopes emitting them are taken into the body or are left in contact with skin for a long period. Beta radiation was measured fairly well by several types of safety instruments used during CROSSROADS.

Alpha particles are made up of two protons and two neutrons. With the addition of two electrons, each becomes a helium atom. Alpha particles are given off by some radioactive isotopes created in a nuclear explosion and by unfissioned uranium or plutonium. Because alpha particles are relatively massive, they do not travel far, about 1 or 2 inches in air. The skin, clothing, or even a piece of paper will stop them. However, if the material emitting them enters the body and lodges there, the alpha particles can do great harm because they cause a high rate of ionization. The decay time of many alpha emitters is long. Plutonium only loses half of its alpha particles in 24,000 years! As described in detail in Chapter 2, the safety instruments available at CROSSROADS for detecting alpha particles directly were few in number and would not operate

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