Part 3

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.

Pre-World War II surplus armor plate came to be the preferred shielding material. Thick slabs of battleship steel were available after the war at low cost. Furthermore, steel produced since the war may contain unwanted radioactivity originating in fallout from nuclear tests and make it undesirable for shielding. Sometimes cobalt-60 used as a tracer to measure deterioration of blast 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 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.)

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:

Case Body burden in disintegrating atoms per second Born 1900, drank 210 bottles of “Radithor” 63,640 in 1927 Born 1897, drank approximately 78 ounces of 32,780 “Radium Water” in 1932 Born 1925, worked as radium chemist since 14,800 1946 Born 1922, radium chemist for 7 years 7,000 Born 1898, two injections of “Thorotrast” 3,300 for X-ray diagnosis Born 1898, radium dial painter in watch 72,500 factory, 1918 to 1921 Born 1902, radium dial painter for 4½ months 1,924 in 1924

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.

Showing: COUNTS/MINUTE PER 95-KEV CHANNEL Subject G (total) Subject G (potassium-40)

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

The idea that this nuclide had entered the body with food later was tested by placing various foods in the whole body counter. All foods tested were found to contain some cesium-137, but beef and dairy products had highest levels. The radiation spectra of persons of the same age but different diet habits were compared, and correlation was found between their cesium-137 content and the amount of dairy products they ate.

In October 1960, Kurt Liden at the University of Lund in Sweden encountered evidence of the source of cesium-137 in humans. While he was using the whole body counter at the University’s Radiation Physics Department, Liden found several Norwegians whose bodies contained quantities of cesium-137 several times higher than previously recorded. He substantiated these data by counting 15 additional Norwegians from Oslo and 6 from Bergen. The Oslo residents averaged 21 nanocuries (abbreviated nc) of cesium-137 and the Bergen group 60 nc. Swedes averaged only 8 nc. (A nanocurie is one billionth of a curie, the standard unit of radioactivity.)

Curiosity regarding these high values in the Norwegians led him to investigate goat cheese, which Norwegians consume in larger quantities than Swedes. Goat cheese at that time showed a high cesium-137 content of 41 nc per kilogram. In northern Norway, near Bergen, another main food is reindeer meat, which was found to contain 28 nc of cesium-137 per kilogram, compared to 0.1 nc per kilogram in beef. These factors indicated that goat cheese and reindeer meat were responsible for the high cesium-137 count in the Norwegians.

Investigation of the soil in northern Norway revealed that it averaged 20 nc of cesium-137 per square meter. Only thin covers of lichens grow in this region, and reindeer must graze over large areas to obtain sufficient food. It was obvious that cesium-137 from fallout was collecting on lichens and then was being concentrated in the bodies of the reindeer before they were killed for meat.

Similar studies have been conducted by scientists of the Pacific Northwest Laboratory at Richland, Washington, who since 1959 have been measuring radioactivity of plants and animals in Alaska. When they found high levels of cesium-137 in lichens and caribou, they became interested in the body burdens of fallout isotopes in the Eskimos.

A portable crystal type of whole body counter was used to measure the cesium-137 content of Eskimos at five villages. The Eskimos cooperated willingly; in some communities nearly everyone accepted the invitation to be counted. This table presents the results: