Whole Body Counters

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.

Part 2

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.)

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