I had to look up. Most of the radioctive potassium 40 in bananas decay by electron emmision. A tiny part by positron emision 0.001%. So you will get a lot of noise in the radioacivity detector.

A few sources like https://alpha.web.cern.ch/science/positron-source and https://ifj.edu.pl/private/jdryzek/page_r12.html recomend sodium 22.