Alpha recoil

There are two main sources of Rn to the ocean (1) the decay of sediment-bound "Ra and (2) decay of dissolved "Ra in a water column. Radon can enter the sediment porewater through alpha recoil during decay events. Since radon is chemically inert, it readily diffuses from bottom sediments into overlying waters. The diffusion of radon from sediments to the water column gives rise to the disequilibrium (excess Rn) observed in near-bottom waters. Radon is also continuously being produced in the water column through the decay of dissolved or particulate "Ra. 

Such alpha-recoil plays a fundamental role in fractionating the nuclides from one another in the low-temperature environment. During igneous processes, on the other hand, alpha recoil is probably not important in the generation of disequilibria ( °Th, Ra, and Pa). Beattie (1993) pointed out that the time scale of annealing of alpha decay damage at high temperatures was much shorter than the time scale of decay of these nuclides. 

Dran J-C, Langevin Y, Petit J-C (1988) Uraruum isotopic disequilibrium reappraisal of the alpha-recoil effect. Chem Geol 70 126... 

Fjeld RA, DeVol TA, Goff RW, Blevins MD, Brown DD, luce SM, Elzerman AW (2001) Characterization of the mobilities of selected actinides and fission/activation products in laboratory columns containing subsurface material from the Snake River Plain. Nucl Tech 135 92-108 Fleischer RL (1980) Isotopic disequilibrium of uranium alpha-recoil damage and preferential solution effects. Science 207 979-981... 

Fleischer RL (1982) Alpha-recoil damage and solution effects in minerals uranium isotopic disequilibrium and radon release. Geochim Cosmochim Acta 46 2191-2201 Fleischer RL (1988) Alpha-recoil damage relation to isotopic disequilibrium and leaching of radionuclides. Geochim Cosmochim Acta 52 1459-1466... 

Kigoshi K (1971) Alpha recoil " Th dissolution in water and the " U/ U disequilibrium in nature. Science 173 47-48... 

Ku 1965). However, the wide range in riverine values (0 to 2000 Cochran 1982) suggests that the elevation of the marine value above secular equilibrium results from a complex combination of weathering and alpha-recoil processes (see above). Thus, it is possible that the marine value has changed by small amounts with time. In practice, the main impediment to a chronometer is the sensitivity of to... 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Figure 20. Secular variation in 5 U(0) for Bahamas flowstone sequence. Changes in 5 U(0) are related to uranium-series disequilibrium conditions in host limestone, periodic addition of new material with elevated (marine) 5 U(0), alpha recoil effects and variation in recharge, and hence water-rock interaction times (see text for details).

The chemical behavior of U and its daughter nuclides in the ocean environment was extensively studied in the 1960s and 1970s and has been well summarized (Cochran 1992). The most important mechanism by which nuclides are separated from one another to create disequilibrium is their differing solubility. For U, this solubility is in turn influenced by the redox state. The process of alpha-recoil can also play an important role in producing disequilibrium. 

Uranium-237 Alpha recoil tracks and uranium series dating... 

Fleischer, R. L., Isotopic disequilibrium of uranium, alpha-recoil damage and preferential solution effects, Science, 207. 979-981 (1980). 

The ratio of to would be expected to be unity as long as the uranium stays locked inside undisturbed crustal rock in secular equilibrium with its progeny, but measurements show that the ratio is typically different than unity (EPA 1994). This disequilibrium occurs when the rock is disturbed by chemical or physical changes involving water. In the environment, a portion of the separates from the by what is theorized to be a physical process (alpha recoil ejection of the Th decay product from surfaces of soil particles) or a combination of physical and chemical processes (a transformation at the soil particle surface fractures the surface allowing access for water to dissolve the more soluble Th product) (NCRP 1984a). These processes can change the uranium isotope ratios in air, soil, and water. 

Fleischer R. L. (1982) Nature of alpha-recoil damage evidence from preferential solution effects. Nuclear Tracks 6, 35-42. 

Kigoshi K. (1971) Alpha-recoil Th dissolution into water and the disequilbrium in nature. Science 173,... 

Ewing RC, Headley TJ (1983) Alpha-recoil damage in natural zirconolite (CaZrTi207). J Nucl Mater 119 102-109... 

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