SENSITIVE DETECTORS Whole body counters are sensitive radiation detecting and measuring instruments that provide information not easily obtainable otherwise about that most important of all chemical systems, the human body. They can do this because, strange as it may seem, every person who ever lived is slightly radioactive. Quickly, accurately, and painlessly, whole body counters reveal the kinds and amounts of radioactive substances that have accumulated in the body from natural sources, from man-made fallout, or from tracer isotopes given for medical purposes. They count emissions from these radioactive materials, as do other kinds of instruments known as “counters”. In contrast to devices that disclose concentrations of radioactivity in a small area or a particular organ, whole body counters usually are used to total up the burden of radioactivity in all parts of a human body. They also are distinguished from many radiation detecting instruments with the same general purpose by their large size, their heavy shielding, and their sensitivity to low levels of radioactivity. Whole body counters are useful in many studies of physiological activity in living persons and animals. They have proved valuable in calculating the radiation absorbed by victims of overexposure to radioactive materials. They can show a doctor how much of his patient’s body is fat and how much lean. Whole body counters also gave medical scientists clues to the relation of potassium deficiency to muscular dystrophy and other diseases. And new medical and scientific uses are being found regularly. The need for an instrument that would measure whole body radioactivity was first felt in the 1920s when the hazardous nature of radium was recognized. Other sorts of instruments had to be used to estimate the amount of radium that some factory workers inadvertently had absorbed while painting luminous watch dials with a radium-containing coating. (See pages 15 and 16.) But instruments then available were without adequate shielding to eliminate background radiation, and so the measurement efforts were of limited value. It was not until the 1950s that new types of radiation-detecting instruments were designed, making use of the discovery that some crystals, liquids, and plastics give off light when struck by gamma rays (one form of radioactive emission). Two principal types of instruments have been developed to detect these emissions in human tissues. The most common whole body counter employs a sodium iodide crystal as the radiation detector. The person being examined usually sits in a tilted chair in a room that has thick steel walls to absorb background radiation. During the counting period, the crystal is centered a few inches above the subject. This type is useful for examination involving low levels of radiation or emissions from more than one kind of radioactive atom.