Potassium-41

Potassium (K) has 24 known isotopes from ³²K to ⁵⁶K. Three isotopes occur naturally: stable ³⁹K (93.3%) and ⁴¹K (6.7%), and the long-lived radioisotope ⁴⁰K (0.012%).

The relative atomic mass is 39.0983(1).

Naturally occurring radioactive ⁴⁰K decays to stable ⁴⁰Ar (10.72% of decays) by electron capture or positron emission (giving it the longest known positron-emitter nuclide half-life). Alternately, and most of the time (89.28%), it decays to stable ⁴⁰Ca by beta decay. ⁴⁰K has a half-life of 1.248×10⁹ years. The long half life of this primordial radioisotope is caused by a highly spin-forbidden transition: ⁴⁰K has a nuclear spin of 4, while both of its decay daughters are even-even isotopes with spins of 0.

⁴⁰K occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. In healthy animals and people, ⁴⁰K represents the largest source of radioactivity, greater even than ¹⁴C. In a human body of 70 kg mass, about 4,400 nuclei of ⁴⁰K decay per second.

The decay of ⁴⁰K to ⁴⁰Ar enables a commonly used 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 (i.e., ⁴⁰Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic ⁴⁰Ar that has accumulated.

All other potassium isotopes have half-lives under a day, most under a minute. The least stable are ³³K and ³⁴K, both with half-lives shorter than 25 nanoseconds. The half-life of ³²K is unknown.

Outside of its use in dating ⁴⁰K has been used extensively as tracers in studies of weathering. Various potassium isotopes also been used for nutrient cycling studies because potassium is a macronutrient required for life.

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