Periodic Table--Krypton

Krypton-81 is the product of atmospheric reactions with the other naturally occurring isotopes of krypton [⁷⁸Kr (0.35%), ⁸⁰Kr (2.25%), ⁸²Kr (11.6%), ⁸³Kr (11.5%), ⁸⁴Kr (57.0%), ⁸⁶Kr (17.3%)] and from decay of ²³⁸U. It is radioactive with a half-life of 250,000 years. Little use of this isotope has been made due to interference from ⁸⁵Kr; however, it has been used for age determination in old (50,000-800,000 year) groundwater (Oeschger, 1978; Lehman et al.,1985).

Krypton-85 is an inert radioactive noble gas (half-life = 10.76 years) that is produced by fission of uranium and plutonium. Sources have included nuclear-bomb testing, nuclear reactors, and the release of ⁸⁵Kr during the reprocessing of fuel rods from nuclear reactors (Sittkus and Stockburger, 1976). Concentrations of ⁸⁵Kr in the lower atmosphere show considerable spatial variability, primarily reflecting the locations of the major sources (Solomon et al, 1998). A strong gradient also exists between the northern and southern hemispheres where concentrations at the North Pole are approximately 30% higher than the South Pole due to convective mixing (Jacob et al., 1987; Zimmerman et al., 1989).

Although ⁸⁵Kr has a half-life similar to that of ³H. ⁸⁵Kr has the advantage of being an environmental tracer with steadily increasing atmospheric input, whereas the ³H atmospheric input is a more complex function of season and latitude and has declined since cessation of atmospheric nuclear-bomb testing in the mid-1960s. Because ⁸⁵Kr is a noble gas, it is not subject to microbial degradation and other chemical interactions that can alter the concentrations of organic environmental tracers. Krypton-85 enters ground water by equilibration of the infiltration water with air in the unsaturated zone, assumed to have a ⁸⁵Kr activity similar to that of the atmosphere. If the effects of hydrodynamic dispersion are small, the ⁸⁵Kr specific activity of groundwater defines the time since the infiltration water was isolated from the atmosphere. The very large amounts of water required for ⁸⁵Kr analysis (at least 100 L) have prevented much use of this potentially useful tracer.

Source of text: This review was assembled by Dan Snyder from the references below.