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Potassium-40 undergoes four different types of radioactive decay, including all three main types of beta decay: electron emission (β −) to 40 Ca with a decay energy of 1.31 MeV at 89.6% probability, positron emission (β + to 40 Ar at 0.001% probability, [1] electron capture (EC) to 40 Ar * followed by a gamma decay emitting a photon [Note 1 ...
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The decay of 40 K to 40 Ar is used in potassium-argon dating of rocks. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40 Ar that has accumulated. Typically, the method assumes that the rocks contained no argon at the time of formation and all subsequent radiogenic argon (i.e., 40 Ar) was retained.
The decay scheme of a radioactive substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships. Examples are shown below. It is useful to think of the decay scheme as placed in a coordinate system, where the vertical axis is energy, increasing from bottom to top, and the horizontal axis is the proton number, increasing from left to right.
This involves electron capture or positron decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the closure temperature is fairly low in these materials, about 350 °C (mica) to 500 ...
Known instances include cosmic ray interactions and the decay of certain isotopes, such as potassium-40. This rare form of potassium makes up only 0.012% of the element on Earth and has a 1 in 100,000 chance of decaying via positron emission. Positron emission should not be confused with electron emission or beta minus decay (β − decay ...
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Ar by electron capture or positron emission (11.2%) or to stable 40 Ca by beta decay (88.8%). [30] The decay of 40 K to 40 Ar is the basis of a common method for dating rocks. The conventional K-Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (40