# Exposed to radiation doesn't always equal being radioactive

**URL:** <https://boards.straightdope.com/t/exposed-to-radiation-doesnt-always-equal-being-radioactive/670465>\
**Category:** Factual Questions\
**Created:** [October 5, 2013, 9:41pm UTC](https://boards.straightdope.com/t/exposed-to-radiation-doesnt-always-equal-being-radioactive/670465 "2013-10-05T21:41:26Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![dstarfire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dstarfire/32/5762_2.png) [@dstarfire](https://boards.straightdope.com/u/dstarfire)\
**Post date:** [October 5, 2013, 9:41pm UTC](https://boards.straightdope.com/t/exposed-to-radiation-doesnt-always-equal-being-radioactive/670465/1 "2013-10-05T21:41:26Z")

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So, a quick review of relevant GQ threads shows that pure water can become radioactive (for a few days) after being exposed to radiation. A search of the web shows that some elements can remain radioactive for a significantly longer time (e.g. Cobalt 60 has a half-life of 5.27 years).

Yet on a recent mythbusters segment (will cockroaches survive a nuclear blast?) Grant said “… just because they were exposed to radiation, doesn’t mean they’re radioactive”. He say this while handling an array of ~1" plastic cages each holding a single test insect.

**So, why aren’t those radioactive?** If simple water can be radioactive for a couple of days why wouldn’t the larger molecules in plastic also become radioactive? (I’m not considering the bugs as even the simplest living organisms are incredibly complex at the molecular level, and also consist primarily of elements that are hard to activate)

Also, **why can water become radioactive when hydrogen and oxygen are both considered difficult to activate?** Is it simply that difficult does not mean impossible, or is there something about the combination of the two that makes them more susceptible to (neutron) activation?

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**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\
**Post date:** [October 5, 2013, 9:49pm UTC](https://boards.straightdope.com/t/exposed-to-radiation-doesnt-always-equal-being-radioactive/670465/2 "2013-10-05T21:49:35Z")

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There are different kinds of radiation, and not all of them induce radioactivity in exposed materials.

> **[Induced radioactivity](https://en.wikipedia.org/wiki/Induced_radioactivity)**
>
> Induced radioactivity, also called artificial radioactivity or man-made radioactivity, is the process of using radiation to make a previously stable material radioactive. The husband and wife team of Irène Joliot-Curie and Frédéric Joliot-Curie discovered induced radioactivity in 1934, and they shared the 1935 Nobel Prize in Chemistry for this discovery.
> Irène Curie began her research with her parents, Marie Curie and Pierre Curie, studying the natural radioactivity found in radioactive isotope...

The article specifically mentions Cobalt-60, which is what’s used to irradiate food, and presumably the roaches on Mythbusters.

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**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\
**Post date:** [October 5, 2013, 10:09pm UTC](https://boards.straightdope.com/t/exposed-to-radiation-doesnt-always-equal-being-radioactive/670465/3 "2013-10-05T22:09:08Z")

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> [@bump](#):
>
> The article specifically mentions Cobalt-60, which is what’s used to irradiate food …

It also supplies an answer to one of the OP’s questions:

> [@](#):
>
> The isotopes used in food irradiation (cobalt-60, caesium-137) both have energy peaks below this cutoff and thus cannot induce radioactivity in the food.

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**Author:** ![dstarfire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dstarfire/32/5762_2.png) [@dstarfire](https://boards.straightdope.com/u/dstarfire)\
**Post date:** [October 5, 2013, 11:26pm UTC](https://boards.straightdope.com/t/exposed-to-radiation-doesnt-always-equal-being-radioactive/670465/4 "2013-10-05T23:26:29Z")

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> [@bump](#):
>
> There are different kinds of radiation, and not all of them induce radioactivity in exposed materials.
> 
> [Induced radioactivity - Wikipedia](http://en.wikipedia.org/wiki/Induced_radioactivity)
> 
> The article specifically mentions Cobalt-60, which is what’s used to irradiate food, and presumably the roaches on Mythbusters.

Thanks. That link mentioned photodisintegration, a 2nd form of induced radioactivity that’s far more common the the neutron activation I’d read about.

Thus, the answers to all my questions seem to be related to the difference between photodisintegration and neutron activation.