  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. Figure 1 Types of whole body counters. A The subject may be seated in a chair in an iron-shielded room and under a scintillation detecting crystal. B The subject may lie in a bed that slides into the end of a hollow cylindrical tank filled with scintillation fluid. C The subject may stand in a semicylindrical double-walled tank filled with scintillation fluid. (See Figure 2.) 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. Figure 2 How a “walk-in” whole body counter, such as the one demonstrated at Geneva, works. Shielding for the Geneva counter consisted of 3 inches of lead. Only the most energetic background gamma rays and 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. To detect the gamma rays emitted by radioactive atoms disintegrating within the body, whole body counters take advantage of a property of radiation that has been known since 1896. In that year the English physicist William Crookes discovered that X rays react with certain chemicals to produce fluorescence. A few years later a New Zealand-born physicist, Ernest Rutherford (later Lord Rutherford), found that this glow consisted of many tiny individual flashes or scintillations, each caused by the emission of a single alpha particle. He laboriously counted individual flashes by observing them through a magnifying glass. If you examine a luminous watch with a hand lens in Today, scientists have found several crystals, liquids, and plastics that are especially effective in showing scintillations caused by nuclear radiations. One of these substances, with the challenging name 2,2'-p-phenylene bis [5-phenyloxazole], often shortened to POPOP, was used in the scintillating liquid of the Geneva counter. How the flashes are detected can be appreciated by considering the infinitely small world of individual atoms and following a single atom as it disintegrates. (For a more complete explanation of radioactivity, see the companion booklet Our Atomic World in this series.) Let us assume that we are looking at a single potassium-40 atom in the body of the person to be examined and that it is about to disintegrate. (Potassium-40 is naturally radioactive. It is the most abundant radioisotope in our bodies.) In any sizable portion of potassium-40, we know that half of the atoms will disintegrate over a period of 1.3 billion years, but, since this process is random, there is no way for us to know when any particular atom will do so. However, when it does, one of two alternative events will occur: either a beta particle (that is, an electron) will be ejected from the nucleus, creating an atom of nonradioactive calcium-40, or the nucleus will capture one of its own orbital electrons, resulting in creation of an atom of stable argon-40 and the emission of a gamma ray. (The beta emission process occurs in 89 out of every 100 disintegrations. See Figure 4.) Figure 4 Comparison of potassium-40 disintegration methods. Assume that the particular gamma ray is traveling in the direction of the scintillating liquid in the counter. Remember that the gamma ray is tiny in comparison with an atom, which is mostly empty space. Therefore, any one gamma ray probably will miss all the material part of the atoms in the body of the person being studied. Nor will it collide with anything as it passes through his clothes and the stainless steel tank. It also may fail to collide with any of the atoms in the molecules of scintillation liquid, of course. But let us assume that the one we are watching does make a hit there. Its total energy will be converted instantaneously to a flash of many bits or photons of light. These photons radiate from the collision scene and strike a light-sensitive surface in one or more of the counter’s photomultiplier tubes, which have been placed where they can “see” the scintillation liquid. Energy transformations result, and a tiny pulse of electricity is originated. These photomultiplier devices are similar to the equipment in the familiar “electric eye” door openers. As their name suggests, photomultiplier tubes (see Figures 6 and 9) do more than merely respond to the light flashes produced in the scintillation liquid. They also amplify the weak electron disturbances into electrical pulses to operate meters that record each scintillation and count the total. The Geneva counter recorded about 25% of the total gamma rays emitted by each subject. Since this sample was a constant proportion of the total body radiation, it could be converted to whole body measurements with about 97% reliability. In addition to finding persons with actual body contamination among those counted at Geneva, the 1955 counter revealed some interesting sideline information. People who failed to remove radium-dial watches were soon spotted. And one small boy who had picked up a sample of uranium ore at a nearby exhibit “jammed” the instrument. Each of the 25 persons who were found to have above-normal levels of radiation could recall having worked with THE LIQUID SCINTILLATION COUNTERA visit to this type of counter recalls the first glimmers of scientific insight that the radiation in the human body could be counted. In the early 1950s, Frederick Reines and Clyde L. Cowan, two scientists at the Los Alamos Scientific Laboratory, Los Alamos, New Mexico, built a large liquid scintillation counter hoping to prove or disprove that neutrinos really existed. Neutrinos are elusive, uncharged particles with essentially no mass. They had been predicted in theory nearly 20 years earlier to explain how beta particles of different energy levels can be emitted from atoms with apparently identical nuclei. Figure 5 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. 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 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. 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. 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 BODIESData 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
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:
Fatty tissues are known to have a low potassium concentration and muscle tissue a higher level. It is therefore apparent that potassium-40 determinations provide a way to indicate the amount of lean muscle in any individual, and indirectly the amount of fat. Estimates of the amount of fat based on the measurement of the specific gravity of the subject, often used in the past, have never been satisfactory. Not only do variable and unmeasurable air spaces change body density, but the process of submerging a person in a tank—to determine his specific gravity by the amount of water he displaces—is a clumsy and uncomfortable one. Significant variations in potassium content have been found in persons suffering from muscle diseases or malfunctions. For example, a sharp drop in potassium content accompanies the profound muscle weakness that follows diabetic coma. Administration of potassium produces striking improvement in the condition known as familial periodic paralysis. Whole body counter data from a study of muscular dystrophy and myotonia atrophica patients showed there is a gradual and progressive decrease of body potassium during the unrelenting courses of these diseases. Otherwise healthy children of muscular dystrophy patients, or their brothers and sisters, also may be deficient in potassium. By assisting in muscle research, whole body counters help doctors learn more of how potassium relates to muscle function and muscle health. Whole body counting is an improvement over potassium determination based on chemical analysis of body fluids. If counters are not used, one way to measure body potassium is to inject a known quantity of potassium-42 (another radioactive form of potassium), wait until this has been uniformly mixed with the potassium already in the body, and then record the radioactivity of a volume of blood serum. From the degree of dilution of the injected potassium-42, the total body potassium can be calculated. This widely used method is uncomfortable for the patient since it involves use of syringes to inject and withdraw fluids. Because about 95% of the body potassium is inside the cells, rather than in fluids between the cells, this method may also be inexact if the mixing process does not continue long Figure 8 A crystal whole body counter “iron room” under construction above, and in use. Figure 8, below
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