Part 1

Whole Body Counters

UNITED STATES ATOMIC ENERGY COMMISSION

_Dr. Glenn T. Seaborg, Chairman_ _James T. Ramey_ _Dr. Gerald F. Tape_ _Dr. Samuel M. Nabrit_ _Wilfrid E. Johnson_

_ONE OF A SERIES ON UNDERSTANDING THE ATOM_

Nuclear energy is playing a vital role in the life of every man, woman, and child in the United States today. In the years ahead it will affect increasingly all the peoples of the earth. It is essential that all Americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them.

The United States Atomic Energy Commission provides this booklet to help you achieve such understanding.

Edward J. Brunenkant Director Division of Technical Information

Whole Body Counters/_{CONTENTS}

1 SENSITIVE DETECTORS 2 THE GENEVA COUNTER 8 THE LIQUID SCINTILLATION COUNTER 10 POTASSIUM-40 IN HUMAN BODIES 13 CRYSTAL COUNTERS 15 THE RADIUM STORY 17 A NEW BODY CONTAMINANT 21 PROTECTION OF LABORATORY PERSONNEL 24 SPECIAL USES 31 CONCLUSION 34 SUGGESTED REFERENCES

THE COVER

This smiling youngster in the chute of a large whole body counter has just emerged from the opening (beyond her feet) of a hollow tank of scintillation liquid, where she lay while the radioactivity in her body was being “counted.” In a minute she will step into her slippers (on the ramp, right) and be ready for play. The sensitive, heavily shielded radiation-detecting equipment shown has many uses that are described in this booklet.

THE AUTHORS

John H. Woodburn teaches chemistry at Walter Johnson High School in Rockville, Md. In the past he taught at Michigan State University, Illinois State Normal University, and Johns Hopkins University. He received his A.B. from Marietta College, his M.A. from Ohio State University, and his Ph. D. from Michigan State University. He is the author of the book _Radioisotopes_ (J. P. Lippincott 1962), which is a student’s introduction to this subject.

Frederick W. Lengemann is associate professor of radiation biology at New York State Veterinary College, Cornell University. He received his B.S. and M.N.S. from Cornell and his Ph. D. from the University of Wisconsin. He has been research associate in radiation biology and assistant professor of chemistry at the University of Tennessee and formerly was a biochemist with the Atomic Energy Commission, Division of Biology and Medicine.

Whole Body Counters

By JOHN H. WOODBURN and FREDERICK W. LENGEMANN

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.

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.

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.

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.

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 a dark room, you can see these fascinating scintillations, just as Rutherford saw them long ago.

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.)

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 radium or some other radioactive substance at some time in the past.

THE LIQUID SCINTILLATION COUNTER

A 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.

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.

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 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

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:

Total atoms in 70 grams of potassium 1.08 × 10²⁴[1] Number of atoms of potassium-40 in 70 grams of 1.3 × 10²⁰ potassium Half-life of potassium-40 in minutes 6.4 × 10¹⁴ Number of potassium-40 atoms disintegrating per minute in 70 grams of potassium 141,000 Number of potassium-40 atoms disintegrating per 15,510 minute with emission of measurable gamma rays Number of counts recorded 2,400 Detection efficiency: 2,400 ÷ 15,510 = 15.5%

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 enough. (See _Radioisotopes in Medicine_, another booklet in this series, for a full discussion of medical treatment with radioactive materials.)

CRYSTAL COUNTERS

When we visit a crystal counter, shown under construction in Figure 8, walls of battleship steel 6 to 8 inches thick are the first things we see. Rather than using shielding only around the detecting instrument, as was done in the Geneva counter and early versions of the Los Alamos counter, crystal counters have shielding around the entire counting room. With this arrangement the instruments are available for adjustment and servicing.

This type of counter also uses a different detection device: a solid, rather than a liquid scintillator. A large crystal, usually of sodium iodide sensitized with thallium, is used to convert gamma rays to light photons.

Let us return to the shielding problem for a moment. Tanks of water, bricks, stone, and lead have been tried by scientists seeking effective, cheap, and convenient shielding. Some early counters were built deep underground in the hope of avoiding cosmic radiation. Radioactive elements are so widely distributed in the rocks, soil, water, and air, however, that there is no place where background radiation does not exist. Not even the crystals, glass, or metals used in the detection system are free of radioactivity.