Can you make an X-ray image with bananas?
June 22, 2021 10:54 AM   Subscribe

The potassium in a banana includes potassium-40, and 11% of the time potassium-40 decays, it emits gamma radiation. Does that mean I could create an x-ray image using unexposed film and a bunch of bananas? How many would I need?
posted by paul_smatatoes to Technology (10 answers total) 16 users marked this as a favorite
Scientists, or people who pretend to be them, talk/joke about the BED, the Banana Equivalent Dose, which is 0.1 microsieverts of radiation.

> X-Rays and Computed Tomography (CT) use high-energy electromagnetic radiation to generate images of structures inside of the body. Similar to bananas, there is exposure to radiation whenever someone gets an x-ray. A dental x-ray provides 50 BED and a chest x-ray can provide between 200 and 1000 BED. A single mammogram is closer to 20,000 BED and a Chest CT is around 70,000 bananas, whereas a targeted radiation procedure, which might be done during a Radiation Oncology procedure, is closer to 20,000,000 BED.

100BED is basically a standard daily radiation dosage.

I'm not sure about the energy of the banana's gamma rays; x-rays are usually considered lower energy, as a category, but the real difference is that, though both types are high-energy photons, gammas are made by nuclear decay, and X-rays are made outside the nucleus. (I forget how-- extreme electron transition?)
posted by Sunburnt at 11:23 AM on June 22, 2021 [1 favorite]

Hmm. Let's see. Bananas subject you to about 1/100 as much radiation as a dental radiograph. So assume that 1/9 of the radiation is gamma radiation, then you're at about 900 bananas to image your teeth, if you could direct all of the banana radiation through your body. And to get a coherent image, you have to have it not just go through your body but be focused somehow. So if you had a big parabolic gamma ray mirror and maybe a gamma ray lens, assuming those have a little loss along the way, you might be able to do it with a few couple thousand bananas. I assume a banana dose of radiation is measured by how long the potassium is in your system, so you would probably want to do this in a lead-lined bunker* over the course of 12 hours or something.

* wait it doesn't have to be so grim. A lead-lined palace would work just as well!
posted by aubilenon at 11:27 AM on June 22, 2021 [8 favorites]

I'm kinda tempted to buy a geiger counter, or get some x-ray film from a doctor.

We deal with many pallets of bananas, each pallet must have about 5000 bananas (30lb cases, 48 cases on a pallet). And we might receive 10 pallets at a time, so 50,000 bananas. I feel if the danger was as high as you mention aubilenon, even with focusing it-- working in a banana processing facility surrounded by millions of bananas, or shipping hundreds of containers of bananas across the ocean would be a death-sentence?
posted by Static Vagabond at 12:06 PM on June 22, 2021 [8 favorites]

a big parabolic gamma ray mirror

Does such a thing exist? Does anyone know? I've realized this is a huge gap in my understanding of light, and you'll save me a lot of googling if you help me out. :)

Gamma rays are such high energy that they hit the nuclei of atoms only. I've always pictured it as either getting absorbed and re-emitting in a random direction, or passing right on by.

I *think* that metal can reflect coherent light efficiently because of the way the electrons are somewhat delocalized, and I don't see how you could have any normal matter that has nuclei like that. But this understanding would also say you wouldn't really get good reflection on microwaves and radio waves, and of course they have parabolic antennae.
posted by mark k at 12:14 PM on June 22, 2021

I think part of the problem is going to be that medical x-rays use a directional source, whereas your banana is probably going to be spraying it in all directions, meaning that you're going to need many more (6x maybe?) bananas to get the equivalent amount of radiation in the direction you're interested in (towards your exposure plate).
posted by parm at 12:16 PM on June 22, 2021 [3 favorites]

Gamma rays aren't x-rays, so the photons would interact with material differently than x-rays would (it's the equivalent of asking how many x-rays you'd need to see a rainbow; different photons do different things.) Gamma radiation is used in PET scans, though, and based on a quick calculation and this quickly-snagged reference, the dosage a body receives from the radio tracer alone in a single PET scan is the equivalent of the concentrated dosing power of about 60,000 bananas. That's a more reasonable comparison not only because it's the same type of radiation, but because the radio labeling also emits radiation in all directions, is what your bananas would be doing.

>I'm kinda tempted to buy a geiger counter

Bananas do make them click a little faster than hanging out in air, but only a little. Most of the time, the photon is going to be absorbed by something else (most likely in the situation you describe? Another banana.) It's far below OSHA workplace dosage limit risks to handle your thousands of bananas.

>Does such a thing exist? Does anyone know?
There are ways to mess around with focusing x-rays, but for a source the size of hundreds of thousands of bananas? I don't think that's a thing.
posted by tchemgrrl at 12:26 PM on June 22, 2021 [4 favorites]

Best answer: No, you can't. X-rays and gamma rays are different.

Ok, lets assume you meant x-ray in the general sense of nuclear imaging and carry on from there.

First thing to note, you can't use BED to calculate necessary bananas, because the BED is based adsorption from eating the banana, not being near it. Some of the gamma isn't even going to escape the banana. You certainly need to peel it for maximum efficiency. In terms of Static Vagabond's question, by the time you have enough millions of bananas to be dangerous you've already created a mile thick banana radiation shield.

Also those BED numbers for x-rays are based on how much of the x-ray is adsorbed by the body, and for a photo on film you need a lot of the energy to go through/around to give you contrast. That won't be the problem though, gamma decay is plenty energetic, as it turns out.

The comparison you're looking for is the electronvolt measurement.

In x-rays (easier to find sources):

Very low-energy superficial X-rays – 35 to 60 keV (mammography, which prioritizes soft-tissue contrast, uses very low-energy kV X-rays)
Superficial radiotherapy X-rays – 60 to 150 keV
Diagnostic X-rays – 20 to 150 keV (mammography to CT); this is the range of photon energies at which the photoelectric effect, which gives maximal soft-tissue contrast, predominates.
Orthovoltage X-rays – 200 to 500 keV
Supervoltage X-rays – 500 to 1000 keV
Megavoltage X-rays – 1 to 25 MeV (in practice, nominal energies above 15 MV are unusual in clinical practice).
Cite: External Beam Radiotherapy

Now the very bad news (for the patient that is), is that Potassium decay is releasing a 1.460 MeV gamma ray, which is higher then what is used in a gamma ray knife (1.17 to 1.33).

So assuming for the sake of argument you could focus the energy of some N million bananas to get the right volume of Potassium to get some constant rate of gamma decay you could make a gamma knife, but I doubt there's enough bananas in the world.

Which bring us to the real question, can you focus/reflect gamma radiation in the first place? and the answer is that it's basically impossible to focus/reflect gamma rays, and was thought to be literally impossible until relatively recently. Medical imaging use particle accelerators or much more radioactive substances that decay at a much higher rate such that focus isn't an issue (you just shield everywhere except your aperture).

Certainly you could win the Nobel prize if you find a substance capable of reflecting gamma rays effectively. Adsorbing them we have covered (although there's plenty of room for improvement here if we ever want to move into space), but reflecting them is still a very cutting edge area of research as far as I can tell.

TLDR: Due to the inverse square law you can't get close enough to a large enough volume of bananas to get any useable amount of radiation out of them, and due to the physics of gamma-rays you can't refocus them no matter how many you stack up.
posted by tiamat at 12:30 PM on June 22, 2021 [46 favorites]

What if you put al the bananas* on the other side of a black hole and used it as a lens? Surely that affects gamma radiation

* that exist, and probably a whole lot more
posted by aubilenon at 1:15 PM on June 22, 2021 [4 favorites]

a big parabolic gamma ray mirror

Does such a thing exist? Does anyone know? I've realized this is a huge gap in my understanding of light, and you'll save me a lot of googling if you help me out. :)
Sadly, I don't think so. That's why even X-ray mirrors are so weird looking.

Making a shadow image with the object pressed directly against fast film in an entirely dark space doesn't seem impossible. Figuring out the exposure you need is not trivial. But it might be worth a couple of weeks and a bundle of bananas to try it out.
posted by eotvos at 1:29 PM on June 22, 2021 [1 favorite]

This is all pretty great, esp. since it's an opportunity to post the Light Sharpener, a parabolic mirrored array from MeFi's Own™ Rob Cockerham at
posted by theora55 at 2:16 PM on June 22, 2021 [1 favorite]

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