The number of light photons produced in the scintillation fluid is proportional to the energy transferred by the incoming gamma rays. For example, gamma rays emitted by potassium-40 have 1.46 million electron volts (Mev) energy; those of cesium-137 have 0.660 Mev energy. When both these radionuclides are producing flashes of light in the scintillation fluid at once, the photomultiplier tubes produce two different strengths of electrical pulses. Electronic devices called multichannel pulse-height analyzers sort and record the number of each. POTASSIUM-40 IN HUMAN BODIES Data from whole body counters indicate that potassium-40 is the most abundant radionuclide in the human body. Our bodies also contain other naturally radioactive substances but the numbers of atoms usually present are so low (as with radium for instance) that they cannot be detected with whole body counters. Several man-made radionuclides also have found their way into body tissues and organs in quantities that sometimes are large enough to be detected and counted. Measurement of disintegrating potassium-40 atoms in the tissues of a human body can be used to determine the total amount of potassium (both radioactive and stable) in the body. It is known that potassium-40 makes up 0.0119% of all potassium and that 11% of all disintegrating potassium-40 atoms emit high-energy gamma rays that are measurable by the counter. The method of determining the amount of potassium in a human subject is to compare the number of gamma rays from a known amount of potassium placed in a phantom, or dummy body, with the number counted from the human subject (see photo on next page). Phantoms are artificial bodies, approximately the size, shape, and density of a human body, used for calibrating counters. They are designed so the radioisotopes they contain have similar distribution to the distribution of the isotopes expected in the real body. This is how a test might work: Counts per minute from 140 grams of potassium in the phantom 16,800 Counts per minute with nothing in counter (background) 12,000 Net counts per minute from 140 grams of potassium 4,800 Counts per minute with a 77-pound boy in counter 14,400 Background cunts per minute 12,000 Net count per minute from boy 2,400 Calculated amount of potassium in boy 70 grams [Illustration: Figure 7 _Phantom used for iodine-131 studies. The radiation spectrum from the thyroid area in the neck is being obtained with a sodium iodide crystal, left._] We can appreciate the sensitivity of whole body counters by comparing the number of gamma rays recorded by the instruments with the total number emitted from the body being counted. The following data, also simplified, illustrate this comparison: