Part 2

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[2] Born 1897, drank approximately 78 ounces of 32,780 “Radium Water” in 1932[2] 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:

Cesium-137 (in nanocuries) Village Number of Minimum Maximum Average subjects Diomede 12 8 35 22 Barrow 259 8 166 51 Point Hope 107 3 119 17 Kotzebue 132 17 518 138 Anaktuvuk 52 83 719 421

(For comparison, the average body burden of cesium-137 of residents of Richland, Washington, during this period was between 5 and 7 nanocuries.)

These data show that the Eskimos who lived inland at Anaktuvuk and ate heavily of caribou meat carried cesium-137 burdens up to 20 times greater than Eskimos who lived in the four villages along the coast and had more variety in their food. (See pages 18 and 19 for photo story of a similar project in Finland.)

PROTECTION OF LABORATORY PERSONNEL

For chemists or others working with arsenic, cyanide, or other chemical poisons, safety depends on recognizing the materials and keeping them where they belong. When accidents do happen and poisons are swallowed or breathed in, successful treatment requires that someone find out exactly what and how much of the poisonous material was involved.

In the event of accidents in nuclear laboratories or reactors, it would be equally essential to identify accurately and quickly the quantity and kind of unstable nuclides the victim has absorbed. A reactor accident conceivably could add a unique hazard since neutron radiation might change the normally stable, nonradioactive atoms in the bodies of nearby workers into radioactive ones. Even gold or silver fillings in their teeth might become radioactive.

When human tissue (for example, hair) is bombarded by fast neutrons, sulfur atoms in molecules of proteins are converted to the radionuclide phosphorus-32. In atomic shorthand, this reaction is:

³²₁₆S + ¹₀n → ³²₁₅P + ¹₁p

or, in still more abbreviated form, ³²S_{n,p}³²P. Phosphorus-32 atoms do not emit gamma rays upon disintegration and so are not detected in standard whole body counters. Attempts are being made, however, to adapt some whole body counters to pick up the secondary radiations, called bremsstrahlung, that occur when the high-energy beta particles that are emitted by phosphorus-32 collide with other atoms.

_Figures 14 through 16 illustrate the kind of data that whole body counters provide to help physicians care for people involved in accidents._

Similarly, neutron bombardment of natural sodium (²³Na) atoms in the body produces sodium-24. This reaction is written: ²³Na(n, gamma ray)²⁴Na. The 1.38-Mev gamma rays emitted by sodium-24 are detected effectively by whole body counters. Since a given neutron dose converts a known proportion of ²³Na atoms to ²⁴Na, it is possible to determine how much neutron exposure a worker has received by obtaining his body count of radioactive sodium.

Crystal type counters also were used in an interesting special case of excessive radiation exposure. Seven natives of the Marshall Islands were examined by the whole body counter at the Argonne National Laboratory in 1957. Another whole body counter, mounted in a Navy amphibious landing ship, was taken to Rongelap Atoll in the Marshalls several times to check on the health of all the residents of the atoll. These people had accumulated zinc-65 in their bodies as a result of contamination of crabs and other food items by fallout from the March 1954 atomic bomb tests at the Pacific Proving Ground. Although normal radioactive decay progressively reduced the total amount of radioactivity in the area, the Marshallese still were carrying this nuclide in their bodies after several years. (See _Atoms, Nature, and Man_, another booklet in this series, for a more complete report of this study.)

SPECIAL USES

Studies of New Babies

Study of the transfer of nutrients and other substances from an expectant mother’s body to that of her unborn child is one of the most challenging areas of biological research. A team of scientists headed by N. S. MacDonald has used a whole body counter at the University of California at Los Angeles to study one aspect of this problem by comparing the concentrations of radioactive materials in newborn infants, in babies who are born dead, and in tissues of the mothers-to-be.

In these studies the scintillation crystal was placed directly beneath a plastic bassinet holding the babies. Twenty-eight infants, 6 to 24 hours old, were counted for 45 minutes each. The only radionuclide found was the ever-present potassium-40.

The bodies of seven stillborn babies were counted for at least 10 hours each. More kinds of radionuclides were found than in the living babies, although the large counting time may have affected the results.

The same counting techniques were used with placental tissues from mothers of three of the stillborns. The placenta is the organ that nourishes an unborn child and through which substances from the mother’s bloodstream are exchanged with those in the baby’s blood. The graphs in Figure 17 show data from this experiment and illustrate the method of interpreting whole body counter data. When the counts per minute at each band of gamma-ray energy recorded from the placental tissues (b) were subtracted from corresponding values from the stillborns (a), it was found (c) that the placentas contained more of the isotopes ruthenium-103, ruthenium-106, and zirconium-95 than did the babies that had been nurtured by these placentas. The babies’ bodies contained more niobium-95 and potassium-40 than the placentas. Niobium-95 is produced by the radioactive disintegration of zirconium-95. This suggested that zirconium-95 atoms do not pass readily through the placenta, but, after they have decayed to niobium-95, they pass into the baby’s bloodstream easily.

Actually, the gamma-ray energies of zirconium-95 and niobium-95 are so similar that the counter cannot distinguish between them. The two isotopes, however, were separated chemically, and whole body spectra were prepared from samples of the pure elements. The spectrum (d) of pure zirconium-95 subtracted from that of pure niobium-95 was strikingly similar to the spectrum of “stillborn baby minus placenta” on the graphs. Cesium-137 was added to the synthetic spectrum to provide a reference mark at the 0.660-Mev point. This revealed that the ratio of cesium-137 to potassium-40 is lower in babies than in adults.

Research on Body Processes

Radioactive tracer atoms, either natural or purposefully built into molecules of vital materials like proteins, are revealing how these substances function in the body to produce energy or to form new tissues. When we know accurately the normal totals and kinds of radioactive substances in the body, we can undertake new kinds of tracer studies without using large amounts of additional radiation. Small instruments called scanners (see Figure 18) usually are used to track tracer isotopes, but whole body counters are useful in special circumstances.

Two types of adaptations enable whole body counters to locate accumulations of radioactive materials in specific organs or small portions of the body. At the National Institutes of Health, Bethesda, Md., one counter is fitted with three rows of six 12-by-12-inch plastic blocks. Each block has four photomultiplier tubes to collect the scintillations from the crystal. The rows are curved so as to be equally distant from the patient’s body.

The current pulses from each of the 18 blocks can be fed individually into the pulse sorter, counter, and recorder. Thus the kinds and numbers of gamma rays from the sector of the patient adjacent to any block can be studied individually. Similarly, the path and speed of the administered materials can be followed by taking recordings from the blocks sequentially.

At the U. S. Naval Hospital in Bethesda, Md., a whole body counter is fitted with a crystal that can be moved at controlled speed past the body of the person being studied (Figure 19). To increase its scanning efficiency, the crystal is fitted with a slit and a focusing device. Instruments record the body radioactivity visibly at timed intervals as the crystal moves along the patient’s body. A television screen enables the operator to observe the patient during the counting.

The role of iron in preventing one form of anemia has been clarified by using iron-59 as a tracer. Persons suffering from chronic infections or such blood diseases as leukemia and polycythemia vera have been checked for the amount of iron carried by their red blood cells. Cobalt-60 atoms have been substituted for stable cobalt in molecules of vitamin B-12 so that the way the body makes use of this vitamin can be studied. Similarly, the body’s use of sodium can be studied by labeling sodium chloride with sodium-22 and then administering solutions of the tagged salt orally or by injection.

Whole body counters used in tracer studies cause a minimum of inconvenience for the patient. Their sensitivity permits use of smaller quantities of radioactive material than is required with small scanning instruments.

Those are unusual jobs for whole body counters, however. Scanners or other types of instruments are used more typically in following tracer isotopes.

Animal Research

How do dogs accumulate fallout isotopes in their bodies? This question was answered effectively by placing dogs in whole body counters and comparing the count from radioactive strontium-emitted gamma rays originating in their bodies with the count from a masonite phantom dog containing a known amount of radioactive strontium-85. It was found that female dogs increase their strontium retention while they are nursing newborn puppies. Strontium is much like calcium, which is a major component of milk. One dog measured had broken a leg in a fight. The counter showed above-average strontium accumulation for this dog, and it was conjectured that strontium, a “bone-seeking” element, had followed calcium to the point of bone repair and new bone growth.

Showing RETENTION, % versus TIME, DAYS for Man, Dog, Rat, Mouse

Figure 22 shows how animals differ in their retention of orally administered zinc-65, as revealed by a whole body counter. It is apparent that counters can be used to determine the differences in the metabolism of different animal species used for research. Standard data developed in this way can serve to reduce error that may occur if results from one species are used for interpretation of data for another species, such as man.

A University of Illinois project to breed meat animals with a high lean-to-fat ratio has been aided by whole body counters. The tendency to deposit fat seems to be inherited, and breeding stock with low fat content can be selected, using “muscle-seeking” potassium-40 to show the proportion of muscle in each potential parent. The Illinois counter is unique in being large enough to examine an adult steer (Figure 23). A similar counter at Cornell University has been used to study animals infested with internal parasites, comparing them with parasite-free animals. The counter revealed that a positive relationship exists between the level of parasite infestation and loss of iron-59-labeled blood from the digestive tract. The possibility of using this method to evaluate parasite-killing drugs is being considered. The Cornell counter is kept clean by covering the animals with plastic sheeting. The same counter also can serve human patients, who are positioned in a wheeled hospital stretcher. (See Figure 1D.)

CONCLUSION