Author Topic: A physics question (radioactivity-related)  (Read 1150 times)

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Offline rumborak

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A physics question (radioactivity-related)
« on: April 12, 2011, 09:22:34 AM »
So, with all the reporting about Fukushima, they're talking a lot about the different isotopes, most notable Iodine 131 and Cesium 137.
The usual commentary is "oh, Iodine has a half-life of 8 days so it's not so bad, but Cesium has a half-life of 30 years!"

My question is: Say you have 10 grams of Iodine 131 and 10 grams of Cesium 137. Doesn't the higher half-life of Cesium in turn imply that its radiation is subsequently lower? I mean, an isotope emits radiation when it splits off its extra neutron an then becomes its stable equivalent of the element, correct? So, if it takes Cesium 30 years for half of its atoms to emit radiation, this means much less radiation over a time span than Iodine, which emits the same amount of radiation, but within 8 days, right?

So, I can see the argument that Cesium is worse because you can't send people back into the area because they would be accumulating the same amount of radiation over time. Is that the argument then? The commentaries always make it sound as if Caesium is magnitudes worse because it radiates as much as Iodine, but for 30 years and not 8 days.

rumborak
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Offline orcus116

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Re: A physics question (radioactivity-related)
« Reply #1 on: April 12, 2011, 09:24:53 AM »
In terms of mass that logically makes sense. My only question would be how or if the different isotopes react with the surrounding environment.

Offline j

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Re: A physics question (radioactivity-related)
« Reply #2 on: April 12, 2011, 10:14:09 AM »
In terms of mass that logically makes sense. My only question would be how or if the different isotopes react with the surrounding environment.

Yeah, that's what I was going to ask.  Don't environmental conditions affect the rate of radioactive decay of certain isotopes in some cases?

-J

Offline XJDenton

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Re: A physics question (radioactivity-related)
« Reply #3 on: April 12, 2011, 10:59:49 AM »
Well comparing the two directly the main difference bwteen the two isotopes is the amount of energy that they release. They both decay via beta channels (ie emission of electrons) but the maximum energy for caesium is about twice that for iodine 131.

The main issue with the two isotopes is how they propagate in the enviroment. Iodine will be pretty much safe in a few weeks and only really gathers in the thyroid, with a biological half life of around 20 days, and you can easily stop the uptake of the dangerous stuff by taking iodine supplements. Caesium on the other hand will propagate into most of the enviroment and will also stick around in humans longer (bhl of 70 days) and accumulates in more places in the body, particularly in muscle tissue. Therefore with the two iostopes you will most likely get a larger accumulated dose with caesium.

Offline rumborak

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Re: A physics question (radioactivity-related)
« Reply #4 on: April 12, 2011, 12:19:04 PM »
Well comparing the two directly the main difference bwteen the two isotopes is the amount of energy that they release. They both decay via beta channels (ie emission of electrons) but the maximum energy for caesium is about twice that for iodine 131.

I just looked up the decay of Iodine, and I was surprised that it decays into Xenon. Xenon has 54 protons, whereas Iodine has 53. Does one of the neutrons of Iodine 131 transform into a proton?
EDIT: Just answered my own question. Indeed it does :lol

Quote
The main issue with the two isotopes is how they propagate in the enviroment. Iodine will be pretty much safe in a few weeks and only really gathers in the thyroid, with a biological half life of around 20 days, and you can easily stop the uptake of the dangerous stuff by taking iodine supplements. Caesium on the other hand will propagate into most of the enviroment and will also stick around in humans longer (bhl of 70 days) and accumulates in more places in the body, particularly in muscle tissue. Therefore with the two iostopes you will most likely get a larger accumulated dose with caesium.

That's what I kinda figured, that it's more about the mode of transmission than of the immediate radiation the elements put out.

rumborak
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Offline Silver Tears

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Re: A physics question (radioactivity-related)
« Reply #5 on: April 12, 2011, 12:29:40 PM »
Well comparing the two directly the main difference bwteen the two isotopes is the amount of energy that they release. They both decay via beta channels (ie emission of electrons) but the maximum energy for caesium is about twice that for iodine 131.

The main issue with the two isotopes is how they propagate in the enviroment. Iodine will be pretty much safe in a few weeks and only really gathers in the thyroid, with a biological half life of around 20 days, and you can easily stop the uptake of the dangerous stuff by taking iodine supplements. Caesium on the other hand will propagate into most of the enviroment and will also stick around in humans longer (bhl of 70 days) and accumulates in more places in the body, particularly in muscle tissue. Therefore with the two iostopes you will most likely get a larger accumulated dose with caesium.


Thanks for explaining, that makes lots of sense to me now.  :)  I know about taking iodine supplements so the body doesn't absorb the radioactive isotope, but is there anything you can do for caesium?

Offline Chino

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Re: A physics question (radioactivity-related)
« Reply #6 on: April 12, 2011, 12:45:06 PM »
Just reading this thread makes me feel smart.

Offline jsem

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Re: A physics question (radioactivity-related)
« Reply #7 on: April 12, 2011, 01:33:15 PM »
Waiting for James.

Offline j

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Re: A physics question (radioactivity-related)
« Reply #8 on: April 12, 2011, 01:51:30 PM »
Well comparing the two directly the main difference bwteen the two isotopes is the amount of energy that they release. They both decay via beta channels (ie emission of electrons) but the maximum energy for caesium is about twice that for iodine 131.

The main issue with the two isotopes is how they propagate in the enviroment. Iodine will be pretty much safe in a few weeks and only really gathers in the thyroid, with a biological half life of around 20 days, and you can easily stop the uptake of the dangerous stuff by taking iodine supplements. Caesium on the other hand will propagate into most of the enviroment and will also stick around in humans longer (bhl of 70 days) and accumulates in more places in the body, particularly in muscle tissue. Therefore with the two iostopes you will most likely get a larger accumulated dose with caesium.

Thanks for explaining, that makes lots of sense to me now.  :)  I know about taking iodine supplements so the body doesn't absorb the radioactive isotope, but is there anything you can do for caesium?

As XJ pointed out, radioactive iodine contamination will have the most prominent effects on the thyroid gland, which has a lesser affinity for the isotope.  So, if you take potassium iodide, the thyroid will preferentially use that instead of the radioactive isotope.

Cesium tends to form water-soluble compounds and will thus become distributed to a relatively similar degree throughout body fluid and tissue, making it a little more complicated.  I know prussian blue is used to treat it, although I'm not sure of the mechanism by which it works.

-J

Offline Silver Tears

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Re: A physics question (radioactivity-related)
« Reply #9 on: April 12, 2011, 01:56:04 PM »
Hmm interesting. Any ideas as to why it affects the thyroid gland most?

Offline j

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Re: A physics question (radioactivity-related)
« Reply #10 on: April 12, 2011, 02:06:32 PM »
The thyroid uses iodine to synthesize thyroid hormone, and it stores it too.  I think close to a quarter of the body's iodine is used by the thyroid.

-J

Offline Silver Tears

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Re: A physics question (radioactivity-related)
« Reply #11 on: April 12, 2011, 02:08:49 PM »
Aaaah ok that makes sense. Thanks j  :)

Offline j

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Re: A physics question (radioactivity-related)
« Reply #12 on: April 12, 2011, 02:12:17 PM »
No prob. :tup

-J

Offline XJDenton

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Re: A physics question (radioactivity-related)
« Reply #13 on: April 12, 2011, 02:14:33 PM »
25% of iodine in the body can be found in the thyroid as its a key part of some hormones. And its not so much that the thyroid won't use the isotope as much as it is that the body will only store so much iodine so if you flood the body with safe iodine a greater proportion of the nuclear material will be ejected from the body.
« Last Edit: April 12, 2011, 02:21:53 PM by XJDenton »

Offline Silver Tears

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Re: A physics question (radioactivity-related)
« Reply #14 on: April 12, 2011, 02:21:31 PM »
Ah ok. Very interesting stuff.

Offline ricky

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Re: A physics question (radioactivity-related)
« Reply #15 on: April 12, 2011, 03:21:48 PM »
holy shit, you guys are smart.

reading this thread makes me feel dumb. :/

but technically, wasn't that a chemistry question, not a physics one?
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Offline Fiery Winds

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Re: A physics question (radioactivity-related)
« Reply #16 on: April 12, 2011, 03:25:28 PM »
holy shit, you guys are smart.

reading this thread makes me feel dumb. :/

but technically, wasn't that a chemistry question, not a physics one?


This thread has been burned.

Offline Silver Tears

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Re: A physics question (radioactivity-related)
« Reply #17 on: April 12, 2011, 03:27:13 PM »
I learnt most of that stuff regarding iodine and such in physics actually, but it was covered in chemistry too.

Offline XJDenton

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Re: A physics question (radioactivity-related)
« Reply #18 on: April 13, 2011, 07:42:27 AM »
As with most real world science, it doesnt easily split up into chemistry, physics and biology. Theres crossover on all disciplines.