Radioactive Bananas?

Are bananas radioactive? We've all heard they are rich in potassium, good to eat to restore electrolyte balance in the human body, but since they have potassium they're bound to have some potassium-40 in them, like our own bodies do.

How much ⁴⁰K? Bananas have 400-450mg of potassium; interestingly the state of Colorado says potatoes are a better source (750mg per medium potato). Potassium-40 is isotopically 0.01177% of natural potassium, so that's 0.05mg or 50ug of ⁴⁰K.

***This sentence was the error in the original post:
The specific activity of potassium-40 is about 30Bq/g (A Becquerel is one radioactive event per second), so there is only 1.5 Bq in a single banana. ***

In fact, the specific activity of potassium-40 is 258,000 Bq/g. 30Bq/g is the specific activity of generic potassium; that is, potassium that has 0.01177% K-40.
So, the banana actually has 13.5Bq.

It's not detectable, by me at least. My body has 4000Bq of K-40, but the geiger counter doesn't change from background when I'm near it. The geiger counter didn't see the bunch of bananas either.

Sorry, Kristin, the counter didn't move.

It does move for pure potassium chloride though.

potassium part 2

Sure enough, as I suspected in my previous post, taking the potassium chloride out of the container brought the count rate up 50% to 120 microrads/hr. I celebrated by tasting it, confident in my body's metabolic control over potassium. It was salty.

Radioactive potassium

A store-bought container of potassium chloride, aka Salt Substitute, contains enough of the naturally occurring potassium-40 isotope ( 0.0117% abundance per the CRC, page 4-24) to produce 5.5x the amount of background radiation when placed on a geiger counter, with the container intact. The rate would have been higher if I had taken the salt out of the container, as the metal base shields some percentage of the 1.31MeV(max) beta electrons.



Paul Frame at Oak Ridge has a great site about radioactive consumer products at http://www.orau.org/ptp/collection/consumer%20products/consumer.htm
and see the potassium info page at http://www.orau.org/ptp/collection/consumer%20products/potassiumgeneralinfo.htm