[Illustration: _Dr. Frederick Reines (left) and Dr. Clyde L. Cowan (right), co-discoverers of the neutrino, lower a fellow worker into the first “whole body counter”, the scintillation assembly used in their experiment. Below, Dr. Wright Lanham, inside the counter, peers from the opening._] [Illustration: Figure 5, below] According to theory, neutrinos are created whenever negative beta-emitting atoms are produced. On this basis, Drs. Reines and Cowan were convinced that the fission of the nuclear fuel in the reactors of the Hanford atomic plant at Richland, Washington, should create high densities of neutrinos. They went to Hanford and set up an elegant neutrino-catching experiment that hinged on detecting and counting gamma rays of definite energy. To accomplish this, they built a large liquid scintillation detector and shielded it from stray gamma radiation. As their work progressed, someone realized that the equipment was large enough to allow a person to crawl inside. After further research at the Atomic Energy Commission’s Savannah River Plant in South Carolina, they were successful in finding their long-sought neutrino. In doing so, they also developed the sort of instrument that can study the human body. Figure 6 shows a person about to enter one version of a Los Alamos counter, the instrument’s 140-gallon tank of scintillation fluid, and 45 of its 108 photomultiplier tubes. When the instrument is in use, the tank slides into, and is shielded by, a 20-ton barrier of 5-inch lead. [Illustration: Figure 6 _A liquid scintillation whole body counter at Los Alamos National Laboratory, showing (left) a subject in the chute before it is slid into the shielded detector chamber. Below, the same instrument’s detecting assembly, showing the photomultiplier tubes, removed from the shielding._] The loading chute will hold a person 6 feet 4 inches tall and weighing up to 260 pounds. The subject lies in the chute as it slides into the counter. A lead plug behind his head closes the end of the cylinder to add shielding. Counters of this type have “panic buttons” with which subjects may signal if they become uneasy on being confined. Most counts are completed in less than 5 minutes, however, so the buttons are rarely used. Since the detector fluid almost completely surrounds the subject when the chute is in place, this type of counter captures twice as large a fraction of the emitted gamma rays as does the Geneva type. Each radioactive substance emits gamma rays with an energy level characteristic of that substance. Whole body counters are able to measure this specific energy spectrum, or “fingerprint”, and so identify the kind of atom producing the radiation.