This video is made by a very scientifically minded gentleman who walks you through his method for detecting the radioactive potassium 40 in bananas.
All bananas contain potassium (‘K’ in the periodic chart). All potassium contains 93+ % Potassium 39 which is stable potassium, and approximately 6% of the isotope, Potassium 41.
Potassium also contains a tiny fraction of the radioactive isotope of potassium, Potassium 40 (K40).
Potassium 40 undergoes three forms of decay, beta -, rarely beta+, and electron capture.
Watch the video to see more. The author also lists his calculations and sources for equipment.