Science Fun: Radioactivity

[cthulu2016]

On Edit: As noted, this is a very simplified description, and is not aimed at people highly knowledgeable about physics. It is intended to counter a few broad misperceptions about radiation "making things" radioactive.

It has been commonplace, since the 1950s, for the public to think that radiation makes things radioactive.

Shoot a bunch of radiation at a chair and the chair will then give off radiation... and perhaps make other chairs it comes in contact with radioactive.

But that is not how it works.

Radiation, itself, is like light. All the radiation from an H-bomb explosion behaves like the light from an H-bomb explosion. It flies off at about the speed of light. It doesn't stay around.

Some of it flies through your body and happens to hit and break chemical bonds in your cells for things you need. Get a big enough blast of radiation and you will die... too much damage in too many cells.

But all that damage is done in an instant, as the radiation goes through you at or near light-speed. And then the H-bomb explosion is done spitting out radiation. Blink of an eye.


Now, radioactive material is something else. Radioactive material is isotopes of elements where a lot of the atoms are going to spit out a particle of radiation... which then acts like one particle/wave of radiation from an H-bomb blast.

Radioactive material will spit out these little bits of radiation over time. (see: half-life... the average time it takes for half the atoms in a material to spit out a quantum of radiation when transforming into a lower energy atom.)



When an H-bomb goes off (or other radiation event) the area where it happened is not safe because it is radioactive.

But this is not because the bomb made material in that area radioactive.

There is no such thing as radioactive dirt, wood, cloth or flesh. Nothing can make those things radioactive.

The radioactivity in the blast area is all particles of radioactive material from the bomb itself. (Or close enough the be a useful model.)

The post-blast radiation is all tiny (like.... tiny!!!) particles of radioactive material from the bomb itself that have been spread over the area.

Again... nothing in the area has been made into radioactive material. It has gotten radioactive dust on it.

Shoot all the radiation you want at a person through a sheet of glass into a sealed room and that person will not be radioactive... though they will soon die.

And yes, it is amazing that material in a bomb that fits in a pick-up can be spread so finely over an entire city as to make the city uninhabitable. And it is amazing that these atoms are so numerous as to be dangerous. One little quantum of radiation is not likely to cause any harm. It takes a lot of those radioactive blips to mess you up.

(This is why you can evacuate away from fall-out, which is radioactive dust/particles. The radiation from the explosion went right through you when you saw the flash of light, but the fall-out is matter, not radiation. It travels at the speed of dust. Driving away from the area will leave most of the dust behind.)

But there are a LOT of atoms in a bomb. Maybe everyone in the city gets a million or ten million radioactive atoms on their clothes... there are that many atoms and a lot more in play.


Long story short: Radiation does not make things radioactive. (In a sense that is meaningful to our health or safety) In a bomb blast it appears to because it covers everything with radioactive matter. If a pound of bacon is x-rayed to kill the germs in it, however, that bacon has not become radiocative. It does not then give off x-rays of its own. It did not have radioactive matter added to it.

[Xipe Totec]

1. Interaction of Radiation With Matter

When neutrons collide with nuclei they may bounce off (an elastic collision) or they may stick (an inelastic collision) producing a new nucleus with the same charge (+Ze) but with a mass increased by one "amu" (atomic mass unit) and atomic mass number, A = Z + N also increased by one. If the original isotope was the most massive stable form of that chemical element, then the new isotope will itself be radioactive. If a macroscopic sample is irradiated with neutrons, then eventually some portion of the sample will be transmuted into a radioactive isotope of that element. If the naturally occurring element consists only of isotopes with even numbers of neutrons, then each naturally occurring nucleus may be rendered radioactive upon absorption of a single neutron.

http://www.ohio.edu/people/piccard/radnotes/penetrate.html

[cthulu2016]

2. Very true, but the OP is not meant to be high-level and is

useable as a practical understanding of what really happens from a bomb or reactor melt-down, etc.

Some atoms or some things will be energized into a radioactive state, but if the aftermath of an H-bomb was limited to that effect I doubt it would pose much risk unless one had a lot of cobalt around or something.

And many of those products will probably be very short half-life themselves, with the effect being that some of the initial radiation is delayed for a few seconds... works out much the same, in those cases, I guess.

The OP is aimed at the popular misconception that radiation behaves like infectuous disease. Winess people's fear of irradiated foods, for instance... as if bacon exposed to x-rays to kill germs is now radioactive bacon.

[broiles]

3. Tell me about the water pouring into the ocean.

How does that work? Why is the water radio active and are the fish safe to eat?

[cthulu2016]

5. The water has radioactive particles in it (or doesn't)

A fish in water with radioactive particles in it will ingest some, and thus have radioactive particles in the fish.

So it depends on how much.

Dilution matters. Eating one (or a thousand, probably) radioactive atom will tend to not harm you. You get a lot worse from the sun or TV or god knows what. Radon in the soil.

So it gets down to how much material is spread over how wide an ocean or area.

I would think the risk from, for instance, fish in the Pacific ocean from an event like Fukishima would be very small.

From fish living right where that local water table drains into the sea, the risk would be much higher. But the dilution of the whole ocean is incredible. Hard to see how it could have any health effect for fish who aren't where it is the most concentrated.

I don't know if those fish (fish right off-shore from the plant) are dangerous, but I don't know.

[broiles]

6. Thanks.

[intaglio]

4. Gross over simplification and confussion of the terms "Radiation" and "Radioctive"

There is no such thing as radioactive dirt, wood, cloth or flesh. Nothing can make those things radioactive.

Dirt is a mixture of substances if radioactive material gets mixed with dirt the dirt is radioactive
Wood, try running a Geiger counter over a sample of wood from Chernobyl or from near Selafield - it will be radioactive and the same applies to cloth and flesh.

What you are trying to say is that shooting nuclear radiation at an object will not make it radioactive and that is generally true. Gamma no effect, beta a tiny effect, neutron a larger effect. But firing neutron radiation at stuff is how you make materials radioactive and that effect depends upon the neutron cross section of the target, the intensity of the source, the length of time exposed and the stability of resultant fragments.

[cthulu2016]

7. You probably got that the OP is aimed at people who think that

irradiated food is radioactive, and that people who work at nuke plants glow in the dark. So the oversimplification is not harmful to the usability of the message. If this were aimed at physicists it would be a very silly OP indeed!

Yes, there is radioactive dirt. And I guess carbon-dating wouldn't work if there was no radioactive wood and flesh.

In that sense, everything that is an aggregate of atoms is radioactive... it all contains some radioactive atoms. There is probably not a cubic meter of the Earth that doesn't contain a radioactive atom. (No cubic meter of seawater is plutonium free--that's for sure)

But it is not a generally transferable property where water exposed to radiation will then later give off radiation in a way that matters to people... even though a handful of atoms in that water will probably have been kicked up to some radioactive something or another.

In a nuclear blast zone, if someone's arm sets off a geiger counter it typically needs to be washed off with water, not amputated, because it has not become intrinsically radioactive.

That's all.