Potassium-#40# is an interesting isotope of potassium, that can undergo both beta-plus and beta-minus decay.
It has an #89%# chance of undergoing beta-minus decay, turning into calcium-#40#, and the equation for that is:
#""_19^40K->""_20^40Ca+e^(-)+barv#, where #barv# is an antineutrino, and #e^-# is an electron.
Meanwhile, the remaining #11%# chance makes it undergo beta-plus decay, which turns it into argon-#40#, and that equation is:
#""_19^40K->""_18^40Ar+e^++v#, where #v# is a neutrino, and #e^+# is a positron.
Source:
http://www.radioactivity.eu.com/site/pages/Potassium_40.htm
