The history of the K/Ar-method for absolute dating of minerals and rocks is full of surprises and good guesses. The β-activity of potassium was discovered, together with that of rubidium, by J. J. Thomson as early as 1905. It was confirmed by a considerable number of authors (for the early literature of potassium β-activity cf. Meyer and Schweidler (1927)). The γ-activity of potassium was discovered by Kohlhörster in 1928 and studied by him in potassium-bearing salt mines. In 1935, Klemperer and, independently, Newman and Walke (1935), ascribed, from reasons of isotope systematics, the activity of potassium to a-then unknown — rare isotope K40. This was the first good guess. In 1935, A. O. Nier actually discovered this isotope and found its abundance to be 1.19 · 10-4 of the total K. Smythe and Hemmendinger (1937) found that the β--activity of K is actually due to K40.
KeywordsElectron Capture Impact Crater Good Guess Iron Meteorite Atomic Mass Unit
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