class="caption small">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 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.

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.

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.

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 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