Part 4

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