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foone @Foone
, 17 tweets, 3 min read Read on Twitter
I had a random thought: Someone mentioned "CT scan" in IRC, and for some reason I thought "Contra-terrene" which is an obsolete word for antimatter. The CT actually stands for "computed tomography".
But here's the thing: PET scans!
PET is "Positron emission tomography" and what's a positron?
It's antimatter! So you could call a PET scan a "CT scan" where CT means contra-terrene if you wanted to confuse everyone.
PET scans work by having a tracer injected/ingested that incorporates a radionuclide, which'll decay in your body and produce positrons. The positrons then annihilate with your body's electrons and emit gamma rays, which can be detected
this has two nice features:
first, the tracer goes exactly where you want it, and no other parts of the body clutter up your scan
secondly, gamma rays aren't blocked by anything short of thick lead or concrete, so they can be detected as if your body wasn't even there.
the whole "antimatter" and "gamma rays" stuff may sound scary but it's in small dosages so it's not terribly hazardous. (one scan is comparable to a year's worth of background radiation)
another fun part of it is that because it's short-lived isotopes (that's rather the point, you want them decaying in your body while you're in the scanner, not days or years later), they have to be produced on-site
or at least nearby.
So if you've ever wondered why a hospital might have a particle accelerator, this is why.
Another random fun fact: PET scanners are calibrated and tested using a Jasczak Phantom, an acrylic cylinder which can have tracers injected into several compartments.

I mainly share this cause of this fun image of one being tested on a very Disney-heavy PET scanner.
so a fun fact about the whole beta-decay -> positron+electron->gamma ray process: It's happening to you RIGHT NOW, even if you're not reading this inside a PET scanner.
Your body has about 140 grams of potassium in it, and a small portion of that (about a hundredth of a gram's worth) is going to be Potassium-40, a radioactive isotope.
It is undergoing beta decay all the time, which does the same thing: generates positrons, which collide with electrons and annihilate, producing gamma rays that fly out of your body until they eventually are absorbed by some other mass nearby.
This does mean that both you and bananas are weakly radioactive, yes.
based on how much potassium is in your body, and the half-life of Potassium 40, this is happening about 4,000 times a second.
The banana thing is also used in nuclear safety, it's called the "Banana Equivalent Dose". It's how much radioactive dose you get from eating 1 banana, given how much potassium is in it.
It's roughly 0.1 µSv
so a PET scan is equivalent to eating about 14,000 bananas.
If you lived near Three Mile Island during the partial melt-down, you got about 800 bananas worth of radioactivity.
The point with the banana equivalent dose is that nuclear reactors are a lot safer than people generally think. Living near one only increases your radiation dose by a couple of bananas a year.
There's plenty of other plants that have similarly "high" levels of radiation, like potatoes and most nuts (Brazil nuts in particular are far higher than bananas, because they contain traces of radium)

But bananas are far sillier.
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