From the Banana equivalent dose article in Wikipedia:
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The major natural source of radioactivity in plant tissue is potassium: 0.0117% of the naturally occurring potassium is the unstable isotope potassium-40. This isotope decays with a half-life of about 1.25 billion years (4×1016 seconds), and therefore the radioactivity of natural potassium is about 31 becquerel/gram (Bq/g), meaning that, in one gram of the element, about 31 atoms will decay every second. … Since a typical banana contains about half a gram of potassium, it will have an activity of roughly 15 Bq. Although the amount in a single banana is small in environmental and medical terms, the radioactivity from a truckload of bananas is capable of causing a false alarm when passed through a Radiation Portal Monitor used to detect possible smuggling of nuclear material at U.S. ports
Of course, when I read this, I wanted to measure bananas with my meters.
When looking at instant measurements (CPS and µSv/h), I noticed no increase.
And the spectrum measured with my Radiacode 103 over 24 hours showed a minuscule little bump.
It’s almost not noticeable in this spectrum, but when switching from log to linear, it stands out better:
This bump is around 1460 keV, which corresponds to potassium 40:
But it turns out that this bump is most likely not caused by potassium in the bananas, because it also shows up in a spectrum I measured of the background radiation, away from the bananas:
Although this spectrum is measured at the same location as the banana measurement was done, it was measured 24 hours later, so there is no absolute guarantee that the background radiation was the same at that time.
But I do know that my meter can pick up radiation from potassium 40. Here is a spectrum over 5 hours that I took with my meter stuck in low-sodium salt. In this salt, the sodium in sodium chloride has been replaced by potassium. Thus the concentration of potassium 40 is much higher than in bananas.
Here you can see a little bump that doesn’t show up in the background radiation spectrum.
Article Link: Quickpost: Banana Equivalent Dose | Didier Stevens