Long-Lived Radionuclides as Tracers in Terrestrial and Extraterrestrial Matter

Abstract

Environmental radioactivity / Cosmogenic radionuclides / Long-lived radionuclides / Accelerator mass spectrometry / Natural and mass-made tracers SummaryRadionuclides in nature offer unique opportunities for radioactive dating and tracing of natural and man-made processes. Naturally occurring radionuclides with half-lives between a few days and IO 12 a have found widespread applications some of which are surveyed here. After a short glance on radionuclide production in stellar nucleosynthesis and on extinct radionuclides in the early solar system, applications are discussed of the long-lived (5 ka < T 1/2 < 16 Ma) radionuclides 10 Be, 14 C, 26 Al, ,6 C1, 41 Ca, "Mn, 59 Ni, and 129 I in cosmochemistry and -physics and in the earth and environmental sciences. These nuclides occur in nature as cosmogenic and partially as radiogenic nuclides and some of them were brought into the terrestrial environment by man. Methods to analyze them at their natural levels are reviewed. Applications are highlighted which cover the cosmic ray record in extraterrestrial matter and in various terrestrial compartments and archives. Emphasis is put on the distinction of mechanisms affecting constancy or variability of the observed natural radionuclide abundances. Finally, the longterm human impact on the radiation environment and the relevance of long-lived radionuclides for the human radiation exposure is discussed. Radionuclides in natureRadioactive nuclides with half-lives between 7.2 · IO 24 a ( I28 Te) and 17 ns ( 212m Po) occur naturally in the solar system and in our terrestrial environment as primordial, radiogenic, nucleogenic or cosmogenic nuclides. More than 20 primordial radionuclides, among them <°K, 87 Rb, 232 Th, 235 U, and 238 U, have sufficiently long half-lives to survive the time since the closing of the solar system. Radionuclides with shorter half-lives are continuously produced. Radiogenic nuclides originate from radioactive decay or spontaneous fission of primordial radionuclides. Nucleogenic ones are produced by nuclear reactions induced by α-particles and neutrons from radioactive decay and spontaneous fission, respectively. Finally, cosmogenic nuclides are the products of interactions of primary and secondary solar and galactic cosmic ray particles with matter. *

Cosmogenic 26Al in the atmosphere and the prospect of a 26Al/10Be chronometer to date old ice

“…For the assessment of the extraterrestrial 26 Al fraction we measured Al and 53 Mn in meteorites and interplanetary dust (Reedy, 1990;Nishiizumi et al, 1995;Michel, 1999) Be ratio.…”

Organoiodine formation during humification in peatlands

“…Field measurements and laboratory studies on iodine speciation using the long-lived radioactive 129 I isotope have contributed to our knowledge of the behaviour of iodine in the environment (Fabryka-Martin et al 1985;Behrens 1988;Michel 1999;Rädlinger andHeumann 2000, Moran et al 2002). In this context, it has been suggested that humic substances play a major role in converting and binding iodine in soils.…”

Cosmogenic radionuclides in the recently fallen Kobe (CK4) meteorite.

“…Cosmogenic radioactive and stable nuclides in chondrites have preserved important records of their exposure history during the last ten million years (e.g., Michel, 1999;Vogt et al, 1990;Caffee et al, 1988). For example, activity of typical neutron-capture product, 60 Co has been used as an indicator of their preatmospheric size (e.g., Eberhardt et al, 1963;Spergel et al, 1986), and those of spallation products, 22 Na, 54 Mn and 26 Al etc.…”