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.
Figure 17 Results of experiment studying transfer of nutrients from an expectant mother to her unborn child.