Brian D. Marshall 1999 <p><span>Potassium has three isotopes (see&nbsp;</span><span class="InternalRef">Potassium</span><span>); potassium-40 (&nbsp;</span>⁴⁰<span>K) is radioactive and decays to both calcium-40 (&nbsp;</span>⁴⁰<span>Ca) and argon-40 (&nbsp;</span>⁴⁰<span>Ar). The combined half-life of&nbsp;</span>⁴⁰<span>K is 1.25 billion years. The branched decay scheme of&nbsp;</span>⁴⁰<span>K is shown in&nbsp;</span><span class="InternalRef">Figure P33</span><span>. It decays by β- decay to&nbsp;</span>⁴⁰<span>Ca and to&nbsp;</span>⁴⁰<span>Ar by both electron capture and positron decay (see&nbsp;</span><span class="InternalRef">Potassium-argon decay system</span><span>). Although 89.52% of the decays produce&nbsp;</span>⁴⁰<span>Ca, the system is of somewhat limited use due to the large natural abundance of&nbsp;</span>⁴⁰<span>Ca (96.9% of calcium, see&nbsp;</span><span class="ExternalRef">&nbsp;<span class="RefSource">Calcium</span></span><span>).</span></p> application/pdf 10.1007/1-4020-4496-8_260 en Springer Potassium-calcium decay system chapter