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.

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 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.

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 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.)