potassioum isotope perhaps not the same as banana

Submitted by Anonymous on Tue, 2011-05-31 16:19

From the owner of radiationnetwork.com--- "I have read and heard that bananas are radioactive?, so I scanned one, but that effort also proved "fruitless" - the same 21 CPM. Now, I can imagine what the radiation mythologists are protesting, "Ahhh, but the peel was on, so surely that explains why your banana did not turn up radioactive." So okay, I peeled the banana, cut it in half revealing its cross section, and scanned the nitty gritty center, seeds and all. Answer, 20.4 CPM - sorry, guys (or maybe I just had an uncooperative banana). My theory on this is that someone long ago confused benign potassium content in the banana with radioactive potassium isotopes, and everyone since then has repeated the myth, instead of doing original research like I just did".

To the BRAWN team: Is this correct and have you measured the "radioactivity" in a banana?
CME

‹ Michio Kaku on CNN 5/13 and 5/31 Fear doesn't take a holiday! ›

Signal-to-noise problem with a banana; should try KCl

Submitted by bandstra on Tue, 2011-05-31 18:32.
Hi CME, I once tried to measure a whole box of bananas with a Geiger counter, and it turns out to be almost impossible to detect their radiation! And there's a good reason why this doesn't work: Using the figure of 360 mg of potassium in one banana, there are only about 10 decays per second of radioactive Potassium-40. Only 11% of these decays produce the 1460 keV gamma-ray that a Geiger counter could detect, so that's about 1 per second. It gets worse. That decay could occur in a potassium nucleus anywhere inside the banana, and the gamma ray could go in any direction, so only a fraction — say 5% — would actually pass through the detector. And even worse — the gamma-ray energy is so high that fewer than about 1% of the gamma-rays that pass through the detector would actually cause the Geiger counter to click. So in total, that's fewer than about 0.0005 counts per second. This is why the banana cannot be seen over background. [Incidentally, the other 89% of the decays produce rather high-energy beta particles, but these would not be detected outside of the banana. The K-40 in our body also produces these betas, and since they are produced internally they contribute a good deal to our normal radiation dose. But many of the 1460 keV gamma rays pass harmlessly out of our bodies without interacting.] However, I once was able to measure the radioactive content of a 300 gram canister of salt substitute (KCl) from the grocery store. I did this with a germanium detector once, but it is probably measurable with a Geiger counter. A 300 gram canister of KCl will have an activity of about 4,900 decays per second. If the other efficiency numbers remain the same, then you might expect about 0.27 counts per second from the KCl in the Geiger counter. So for a background rate of 20.4/60 = 0.36 counts per second, there is a good chance of readily seeing the radioactivity of a small canister of KCl. Mark [BRAWM Team Member]

Thanks for your post,

Submitted by Anonymous (not verified) on Tue, 2011-05-31 20:10.

Thanks for your post, Mark!

So this quote from wikipedia about bananas being "sufficiently radioactive to be detected by radiation sensors used to detect possible smuggling of nuclear material at U.S. ports" is just an urban legend, or the equipment used in this case is much more sensitive?

Banana Equivalent Dose

It's plausible

Submitted by bandstra on Tue, 2011-05-31 21:38.

It's plausible since the equipment is more sensitive. I found a Newsweek article that quotes a Homeland Security official saying that bananas trip the sensors.

Say you have a ton of bananas -- that would be around say 4,000 bananas. That would be a total of 40,000 decays of K-40 per second, 11% of which produce 1460 keV gamma rays. That's about 4,000 gamma rays per second.

A portal monitor might surround say 20% of a truck. This means about 800 gamma rays go through the detectors. If the detectors are scintillators -- usually plastic or sodium iodide, you might see up to 5% of those. So you would have a count rate of about 40 per second. This could easily be above the background level, especially if the background level is stable and well-known.

Mark [BRAWM Team Member]