Donald J. Rokop

Determination of deuterated methanes for use as atmospheric tracers

The deuterated methanes, /sup 13/CD/sub 4/ and /sup 12/CD/sub 4/, are useful as tracers for long-range (>500-km) atmospheric transport. They are easily synthesized and released. Sampling is accomplished by collecting about 330 l of air and chromatographically separating the methane fraction, which is then analyzed by mass spectrometry for the /sup 13/CD/sub 4///sup 12/CH/sub 4/ and /sup 12/CD/sub 4///sup 12/CH/sub 4/ ratios. Detection limits in air are about 7 x 10/sup -16/ STP mol m/sup -3/ for /sup 13/CD/sub 4/ and 2 x 10/sup -15/ STP mol m/sup -3/ for /sup 12/CD/sub 4/. 11 refs., 5 figs.

The Geochemistry of Natural Technetium and Plutonium

It is generally assumed that element 43—technetium—and element 94— plutonium--are exclusively anthropogenic, found in nature only as contaminants. However, these nuclides also occur naturally as transient products of nuclear processes. In nature, 99Tc is produced by 238U spontaneous fission and 235U neutron induced fission and subsequently destroyed, with a half-life of 2.13 × 105 y, by radioactive decay. Plutonium-239 is produced naturally by 238 U neutron capture and decays with a half-life of 2.41 × 104 y. Rates of production of these nuclear products depend on the neutron fluence in the geologic environment, and the abundance of their mutual parent, uranium. Therefore, once the production and decay rates are equal (a condition we will refer to as nuclear equilibrium) samples of those environments should contain correlated abundances of the two elements. If the nuclear parent and daughters are undisturbed, such a state of equilibrium will be achieved in about 106 y. Chemical changes with respect to these three elements in this time frame can be identified and quantified by comparing measured abundances with those predicted by the systematics of nuclear equilibrium. Thus, determination of the abundances of 99Tc, 239Pu, and U should provide a useful tool for studying aspects of the chemistry of geologic environments during the last million years.