HR: 0800h
AN: V41F-1528    [Abstracts]
TI: Pressure- and chemistry-dependent electron capture radioactive decay
AU: * Lee, K K
EM: kanani@gps.caltech.edu
AF: California Institute of Technology, Division of Geological & Planetary Sciences 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: * Lee, K K
EM: kanani@gps.caltech.edu
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, 95440 Germany
AU: Steinle-Neumann, G
EM: g.Steinle-Neumann@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, 95440 Germany
AB: The half-lives of radioactive isotopes are often considered constant and are determined to high precision. For this reason, many radioactive isotopes are used to date Earth processes at all time and length scales. Among important radioisotopes are those that decay by electron capture, namely ⁷Be (→ ⁷Li, t_1/2 ~53 days), ²⁶Al (→ ²⁶Mg, t_1/2 ~720 kyr) and ⁴⁰K (→ ⁴⁰Ar, t_1/2 ~1.25 Gyr). Heat production due to the decay of ²⁶Al and ⁴⁰K were also important during the accretion process and current heat budget respectively. For electron capture decay schemes, external forces affect the half-life. These include chemical state, ionization, temperature and pressure. Here we investigate the change in decay constant for these radioisotopes due to pressure and chemistry by means of accurate total energy band-structure computations to determine the charge density at the nucleus of the respective atom as pure metals (Be, Al, K) and in oxides (BeO, Al₂O₃ and K₂O). We use an all-electron density functional theory based method, implemented in the Linearized Augmented Plane Wave (LAPW) method, with both the local density (LDA) and generalized gradient (GGA) approximations to many electron interactions. We predict that pressure decreases the half-life of ⁷Be by ~0.2% (~0.1 days) and that of ⁴⁰K by ~0.03% (~375 kyr) at 50 GPa, while the half-life of ²⁶Al increases by ~0.01% (~72 years) at 50 GPa. The oxide forms of each element show a further decrease in the length of the half-life.
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1155 Extinct radionuclide geochronology
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005