[Illustration: _A technician holds an inexpensive portable X-ray unit that was developed by the Argonne National Laboratory. Compare its size with the standard X-ray machine shown at left and above._] Still more recently, ¹²⁵I has been used very successfully in a portable device as a low-energy gamma source for radiography. The gamma rays from this source are sufficiently penetrating for photographing the arms and legs, and the necessary shielding is easily supplied to protect the operator. By contrast with larger devices, the gamma-ray source can be as small as one-tenth millimeter in diameter, virtually a point source; this makes possible maximum sharpness of image. The latest device, using up to one curie[12] of ¹²⁵I, weighs 2 pounds, yet has adequate shielding for the operator. It is truly portable. If this X-ray source is combined with a rapid developing photographic film, a physician can be completely freed from dependence upon the hospital laboratory for emergency X rays. A finished print can be ready for inspection in 10 seconds. The doctor thus can decide quickly whether it is safe to move an accident victim, for instance. In military operations, similarly, it becomes a simple matter to examine wounded soldiers in the field where conventional equipment is not available. Tritium More than 30 years ago, when deuterium (heavy hydrogen) was first discovered, heavy water (D₂O) was used for the determination of total body water. A small sample of heavy water was given either intravenously or orally, and time was allowed for it to mix uniformly with all the water in the body (about 4 to 6 hours). A sample was then obtained of the mixed water and analyzed for its heavy water content. This procedure was useful but it was hard to make an accurate analysis of low concentrations of heavy water. More recently, however, tritium (³H) (radioactive hydrogen) has been produced in abundance. Its oxide, tritiated water (³H₂O), is chemically almost the same as ordinary water, but physically it may be distinguished by the beta rays given off by the tritium. This very soft (low-energy) beta ray requires the use of special counting equipment, either a windowless flow-gas counter or a liquid scintillator, but with the proper techniques accurate measurement is possible. The total body water can then be computed by the general isotope dilution formula used for measuring blood plasma volume. [Illustration: _The total body water is determined by the dilution method using tritiated water. This technician is purifying a urine sample so that the tritium content can be determined and the total body water calculated._] Activation Analysis