Abstract

Over the last 80 years numerous determinations have been made of the 147Sm half-life. The determinations since 1960 have converged on close agreement between the two primary determination techniques used in direct physical counting experiments—ionization chambers and liquid scintillation counters, and with radioisotope age comparisons using two meteorites. Thus the 147Sm half-life value of 106 ± 0.8 Byr has now been adopted for standard use by the uniformitarian geological community. This value is based on the weighted average of four direct counting determinations in the period 1961–1970 and the recalibration in the 1970s of Sm-Nd model ages of two meteorites by forcing them (essentially by circular reasoning) to agree with their Pb-Pb isochron and model ages. However, direct counting experiments in 2003 determined the 147Sm half-life value was 10% or more longer at 117 ± 2 Byr. This was achieved by using four standard Sm solutions with internal α-radioactive standards in 19 alpha spectrometer and 24 ionization chamber determinations, making it the most thorough and comprehensive effort to determine the 147Sm half-life. The thinner counting sources used, while resulting in low α-activities being measured, greatly reduced the counting uncertainty due to self-absorption of the emitted α-particles. Although rejected or ignored, this 117 ± 2 Byr value for the 147Sm half-life, which agrees with some earlier determinations, may well be highly significant and more reliable than the adopted value. Yet, in spite of the many experiments directly measuring 147Sm decay, preference has been given to the half-life value of 106 ± 0.8 Byr determined by forcing the Sm-Nd data to agree with Pb-Pb dates. But many unprovable assumptions are also involved, not the least being that the radioisotope systems closed at the same time and subsequently remained closed. Furthermore, even this “gold standard” has unresolved uncertainties due to the U decay constants being imprecisely known, and to measured variations of the 238U/235U ratio in terrestrial rocks and minerals and in meteorites. Both of these factors are so critical to the U-Pb method, as well as the additional factor of knowing the initial concentrations of the daughter and index isotopes, so it should not be used as a standard to determine other decay constants. In any case, the determined half-life of 147Sm has been shown to be dependent on the thicknesses of the Sm counting source and the detector. There is also evidence decay rates of the radioisotopes used for rock dating have not been constant in the past. This only serves to emphasize that if the Sm-Nd dating method has been calibrated against the U-Pb “gold standard” with all its attendant uncertainties, then it cannot be absolute, and therefore it cannot be used to reject the young-earth creationist timescale. Indeed, current radioisotope dating methodologies are at best hypotheses based on extrapolating current measurements and observations back into an assumed deep time history for the cosmos.

Keywords: radioisotope dating, decay constants, half-lives, samarium-147, 147Sm, α decay, direct counting, emulsions, liquid scintillation counters, ionization chambers, surface-barrier detectors, Geiger counter, counting efficiencies, geological comparisons, meteorites, U-Pb “gold standard”, 238U/235U ratio

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