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During the last 84 years, determinations of the 238U and 235U decay constants and half-lives have been made using direct counting experiments and geological age comparisons, as well as by critical reviews and reevaluations of those determinations. By 1971 the direct counting experiments had provided 238U and 235U half-life values with small uncertainties which ever since have been the recommended values used in all U-Pb age calculations. All the geological age comparison studies have utilized those recommended values, in spite of the admitted philosophical circularity involved. And the critical reviews and reevaluations have all converged on these same recommended values because of the meticulous care taken in the 1971 direct counting experiments, which then gave those experimental results the dominant weight in the calculation of mean values. But there have still been repeated calls for more modern, more accurate direct counting experiments to more precisely determine the 238U and 235U half-lives. It is difficult to determine precise values for the 238U and 235U half-lives because the 238U and 235U peaks in the a-energy spectrum have to be accurately delineated where they overlap and from the background a-particles. Secular equilibrium is also assumed, yet 234U in-grows during the time periods of the experiments, which of necessity have to be long enough to collect statistically large sets of counting data. And the 235U half-life is ultimately determined from the determined 238U half-life by assuming the 238U/235U ratio is constant, which is also crucial in every U-Pb age calculation. Yet significant variations in this crucial 238U/235U ratio have now been measured in all the rocks, accessory U-bearing minerals, and meteorites that are routinely U-Pb dated. Also, clearly observable trends of deceasing 238U and 235U half-life values were obtained from the direct counting experiments between 1932 and 1974. Such experiments should be given the most weight in determining the 238U and 235U half-lives, because in them the numbers of parent 238U and 235U atoms that decay over given time periods are directly counted. Yet to admit that the 238U and 235U decay rates may not have been constant in recent decades is tantamount to admitting that the 238U and 235U half-lives might never be determined precisely. Furthermore, since the 87Rb, 176Lu, 187Re, 147Sm, and 40K half-lives have all been determined by cross-calibration with the 238U half-life by forced agreement of Rb-Sr, Lu-Hf, Re-Os, Sm-Nd, K-Ar, and Ar-Ar ages respectively with U-Pb ages obtained for the same rocks, minerals and meteorites, none of these decay half-lives are really known accurately. Therefore, without accurately known decay half-lives, all radioisotope ages cannot be accurately determined or be considered absolute ages. Thus, all these radioisotope dating methods cannot be used to reject the young-earth creationist timescale, especially as 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. Instead, the actual observable experimentally-determined radioisotope decay data suggest that radioisotope decay rates have been decreasing in recent decades. This is consistent with the several lines of impeccable evidence that radioisotope decay rates were grossly accelerated during the year-long biblical global Flood cataclysm, and then the decay rates decelerated. That we may still be detecting the radioisotope decay rates decelerating is likewise consistent with the Flood occurring only about 4300 years ago.
https://answersingenesis.org/geology/radiometric-dating/determination-decay-constants-half-lives-uranium/