furnace walls causes problems in postwar steel, too, so old warship armor is used when possible.

Some whole body counters have additional shielding. In the counter at the Brookhaven National Laboratory, Upton, New York, the steel room is lined with ¼-inch lead sheets, covered by thin layers of cadmium and copper. The lead is intended to absorb the secondary X rays produced in the iron by the interaction of high-energy gamma and cosmic rays. The cadmium and copper absorb the secondary radiation that is similarly produced in the lead.

The doors of these rooms often weigh 6 tons or more. A special escape hatch was built into the counter room at the University of California at Los Angeles, to be used if the main door should be jammed by an earthquake. In newer whole body counting laboratories, such as the one at the National Institutes of Health, Bethesda, Md., the steel rooms are concealed in the interior design and are so pleasantly furnished that the patient scarcely is aware of the thick walls around him.

Figure 9 A sodium iodide crystal, right, and a cluster of 7 photomultiplier tubes that fit under it to record its scintillations.

Figure 9 shows a sodium iodide crystal used to react with the gamma rays that traverse it. To the left of the crystal is a cluster of seven photomultiplier tubes that “watch” for the scintillations, convert them to electrical pulses, and amplify them so they can be sorted, counted, and recorded. A trace of thallium added to the sodium iodide improves its scintillation properties.

In addition to being of convenient size and easy to maintain, crystal detectors have another advantage over liquid systems. The energy of the incident gamma rays from crystals is more accurately indicated by the quality of the flashes of light impinging on the photomultiplier tubes. If two or more radionuclides are emitting gamma rays, a crystal detector distinguishes between their energy levels with much more precision and sensitivity than does a liquid system. Crystal instruments separate gamma rays differing by no more than 0.05 Mev.

The energies of the gamma rays emitted by nuclides have all been determined and are listed in handbooks. A scientist can thus identify the data delivered by a multichannel pulse-height analyzer as coming from potassium-40, zinc-65, or any other nuclide.

Counters using sodium iodide crystals intercept, and therefore count, a much smaller fraction of the gammas emitted by the subject’s body than liquid systems, but they also pick up a smaller amount of background. When speed is important, the liquid counter is more effective, but the crystal counter is preferred when radionuclides emitting gammas of nearly the same energies are to be separated and counted.

THE RADIUM STORY

Radium-226 in the human body poses unique problems for whole body counters. People who have accumulated this nuclide only because of the minute amounts occurring naturally in food and water have counts of only two or three disintegrating atoms per second, and this amount cannot be distinguished from background radiation. Whole body counters are useful, however, in diagnosing effects in persons who have been overexposed to radium. These include persons who formerly were employed to paint watch dials with a luminous paint containing radium. (See table below.)

Figure 10 A group of radium dial painters at work in a watch factory in 1922. Almost all these employees have been identified and the living ones recently have participated in a study at Argonne National Laboratory to determine the extent of radium accumulation in their bodies. Whole body counters aided in their examinations.

Excerpts from case records of one research center show the high counts found in several patients and the source of the radium or thorium (a closely related element) that their bodies had taken up:

Scientists at the Argonne National Laboratory, Argonne, Illinois, have attempted to improve crystal whole body counters so that they will be more useful in determining the amount of radium-226 in humans. Rolf Sievert at the Swedish Atomic Energy Commission also has studied the radium-226 detection. He devised a highly accurate whole body counter with 10 ion chambers arranged around a curved aluminum bed on which the subject rested. The instrument was installed below ground to reduce the interference of background radiation.

Figure 11 The spectrum of gamma radiation emitted by the body of a man three years after he accidentally inhaled radium-226. The solid line shows the normal radiation due to potassium-40, the dotted line the total from potassium-40 and radium-226.

A NEW BODY CONTAMINANT

In 1955, Charles E. Miller and L. D. Marinelli were measuring human potassium levels with the whole body counter at the Argonne laboratory. They were puzzled by finding several people who emitted 0.660-Mev gamma rays. Gamma rays of this energy, which are emitted by cesium-137, had not previously been detected in humans. To add to the perplexity, when the same persons were examined a few months later, the count of these gamma rays had increased. The Argonne findings indicated strongly that radiocesium, which is known to occur in fallout from nuclear explosions, was finding its way into people’s bodies. (See Fallout from Nuclear Tests, another booklet in this series.)

INVESTIGATION OF FALLOUT CESIUM IN LAPLANDERS’ DIET

Long-lived radionuclides, particularly cesium-137, accumulate on plants.

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