In the other type the subject is surrounded by a tank of a liquid that detects gamma rays. This type is faster, but less sensitive, than the crystal type. This booklet is intended to enable you to make imaginary visits to several whole body counters, to understand the scientific principles that are applied in their design, to learn the interesting ways they are used, and to appreciate the promise they hold for increasing our knowledge of ourselves and the world we live in. THE GENEVA COUNTER In general, all whole body counters must have (1) a mechanism that reacts to the energy emitted by some kinds of disintegrating, or radioactive, atoms; (2) a device that displays or records these reactions; and (3) adequate shielding to exclude unwanted rays from other sources. [Illustration: Figure 1 _Types of whole body counters._] [Illustration: A _The subject may be seated in a chair in an iron-shielded room and under a scintillation detecting crystal._] [Illustration: B _The subject may lie in a bed that slides into the end of a hollow cylindrical tank filled with scintillation fluid._] [Illustration: C _The subject may stand in a semicylindrical double-walled tank filled with scintillation fluid._ (_See Figure 2._)] [Illustration: D _The subject may lie on a wheeled cart and be wheeled beneath a shielded detecting crystal._] One of the first whole body counters was shown at an atomic science conference in Geneva, Switzerland, in 1955 (Figures 1D and 2). While it was on display, 4258 visitors to the meeting climbed a set of stairs to enter a 10-ton lead-walled chamber. Here they stood still for 40 seconds while the radioactive atoms in their bodies were being “counted”, or recorded. This device, because it was the first one persons could walk into, aroused great interest. [Illustration: Figure 2 _How a “walk-in” whole body counter, such as the one demonstrated at Geneva, works._] Showing: Signal lights Lead glass windows Lead shield Scintillation shield Photomultiplier tube Solution storage tank under platform Shielding for the Geneva counter consisted of 3 inches of lead. Only the most energetic background gamma rays and cosmic rays can penetrate this amount of shielding, and the number that do so remain almost constant during successive counting periods. This constant remaining “background” radiation level, once determined, could be subtracted from the recorded number of emissions to provide the correct radiation total from the body of each person examined. [Illustration: Figure 3 _Typical crystals and liquid materials used to produce scintillations for whole body counters and other radiation-detecting instruments. Scintillation counters provide much faster recording of radiation than Geiger counters, and are widely used in experiments with high-energy particle accelerators, as well as in whole body counters._]