Radioactivity

stereo_steve

I have a question! For those of you that know about radioactivity. I understand half life, how the nucleus decays and all. But here's the part I get confused about! Why hasn't every radioactive source decayed? I was pondering that question for a while untill my friend said that gamma rays from space create new radioactive isotopes?

But then i was wondering how carbon dating would work if that were true. The gamma rays would keep creating new radioactive isotopes and the decay of C14 wouldn't be noticed!

Could someone please explain exactly what happens?

seamus

Space rays create more? hmmmm.....

When a radioactive isotope decays, it usually gives off a type of radiation (duh) which I don't need to go into the specifics of. The half-life of a substance is the length of time it takes for one-half of a sample of a radioactiev substance to decay. Some elements have half lives of thousands or millions of years. Many, when they decay, decay into another radioactive substance, so the process goes on and on. Also the conservation of energy law means that all elements must become something, so they won't just disappear after a while. Sorry, I wrote this in dribs and drabs, it mightn't make sense

nesf

Short answer:

Some Radioactive Isotopes of Elements are naturally occuring. Heavy Water (H3O) for example. They arrise from natural chamical reactions daily.

That's why they haven't all decayed theres always new ones taking the places of of the old ones. Plus as Seamus said some have very long half-lives.

bonkey

Originally posted by stereo_steve
Why hasn't every radioactive source decayed?

<snip>
But then i was wondering how carbon dating would work if that were true. The gamma rays would keep creating new radioactive isotopes and the decay of C14 wouldn't be noticed!

Where do you think the radioactivity came from to begin with?
Matter is not created from nothing, nor does it remain unchanging over time.

Chemical reactions will cause certain radioactive substances to be created - such as heavy water (as nesf pointed out). C14 dating is based on the fact that *living* matter undergoes constant chemical change, which results in an almost fixed percentage of its/their makeup being Carbon 14. Now, once death occurs, growth ceases, and thus (over long periods of time) the chemical reactions cease as well, resulting in a "static" amount of carbon 4, which decays over time. Thus, we can use C14 dating to approximate age. Its is only approximate because we cannot say for certain that no external influences will have effected the carbon content over time.

If you want to look at naturally occurring non-reactive radioactive materials, I would suggest you find you answer by asking a simpler question - where did this matter come from? How did it become radioactive? If you understand that, then you'll understand why its still radioactive

jc

stereo_steve

If you want to look at naturally occurring non-reactive radioactive materials, I would suggest you find you answer by asking a simpler question - where did this matter come from? How did it become radioactive? If you understand that, then you'll understand why its still radioactive

I did ask that. A friend told me that cosic gamma rays create new radioactive atoms. Is that true? To me it sounds a bit odd. If someone could explain that, it would really help.

Victor

Originally posted by stereo_steve
I did ask that. A friend told me that cosic gamma rays create new radioactive atoms. Is that true? To me it sounds a bit odd. If someone could explain that, it would really help.

I can't comment on that but imagine there being (relatively) huge amounts of heavier radioactive material create by the big bang (and possibly in the cores of some stars), gradually that material is being degraded and we are left with smaller amounts today.

I can't remember much about carbon dating, but I was under the impression that a small amount of carbon in living (not dead) material (more importantly in plants) does convert to heavier isotopes.

kiffer

Some Radioactive Isotopes of Elements are naturally occuring. Heavy Water (H3O) for example. They arrise from natural chamical reactions daily.

Chemical reactions will cause certain radioactive substances to be created

tehehehehehe
you said chemical...
I do have a good post .. but it'll take me a while to put it together.

my friend said that gamma rays from space create new radioactive isotopes

um not sure about this ... I know that you can make Radioactive Isotopes by bombaring an Element with sub atomic particals. the basic idea is that if you make the atom unstable it will decay untill it becomes stable ... I dont know if Gama Rays do this ... but if they do I dont think that they would make that much of a differance to Carbon dating ...
through out your life you are constantly replacing the carbon in your body, and a certan percent of carbon is C14 , how ever when you die , you stop replacing the Carbon... so as you body lies in the cold cold ground your carbon-14 is slowly decaying away (into cabon-12 and Alpha particals.) and not geting replaced... so a few 1000 years later I dig you up and look at your bleached bones... and I find that you have lost some of your Carbon-14 , and as i know the its Halflife i can find how long this would take ...
now back to the gama rays ... could they make a differance to this (if they are able to make things radio active at all) ... well if you where deep under ground ... say in deep cave then there wont be any reaching you, so they cant effect that... but even if you where on the surface i dont think that it would make a noticable differance ... for two reasons ...
A) chances are that only a tiny tiny amount of radio atcive matirial will be created. Gama rays are poor ionisers and tend to pass through nondense mater... chances of it hitting the nucleus is minuet ,so even if it hits an atom on its way through your body the chance of it being a carbon atom followed by the chance of it hitting the nuclues (not just ionising the the atom) well um it's not realy very likely at all ... so if it did happen it would chnage the answer less than the marging for error is allready ...
If Gama rays do create radioactive isotopes , they would not create Carbon 14 from Carbon 12 ... although they might prompt C-14 to decay ... `again this would be even less likly than A.

now I'm off to write a post that makes sense.

stereo_steve

Thanks kiffer. That really makes sense. However ther is still the problem of how the radioactive isotopes are created. If they where all created during the big bang, then surely they would have all decayed at this stage. I agree that gamma rays seems unlikely. What else could create them?

kiffer

Originally posted by stereo_steve
Thanks kiffer. That really makes sense. However ther is still the problem of how the radioactive isotopes are created. If they where all created during the big bang, then surely they would have all decayed at this stage. I agree that gamma rays seems unlikely. What else could create them?

hum ok , first off the big bang made mostly hydrogen and a little Heilum , this formed stars ... the stars through nucular fussion created heavier elements ... the old we are stardust comment...
so to answer you question stars made all the heavy elements ...
but that really does'nt answer your question does it ... you want to know why we still have radioactive isotopes , why have'nt they all just decayed away ?
well alot of them have... well you see they start out as heavier radioactive substances... do a google search for "decay chains" and you should find alot on where they come from ... there are some wierd ones out there ...

Victor

Let us say that we started with 1% radioactive material over each half-life we then have:

Start			1.00000000000000000%
end of HL	1	0.50000000000000000%
end of HL	2	0.25000000000000000%
end of HL	3	0.12500000000000000%
end of HL	4	0.06250000000000000%
end of HL	5	0.03125000000000000%
end of HL	6	0.01562500000000000%
end of HL	7	0.00781250000000000%
end of HL	8	0.00390625000000000%
end of HL	9	0.00195312500000000%
end of HL	10	0.00097656250000000%
end of HL	11	0.00048828125000000%
end of HL	12	0.00024414062500000%
end of HL	13	0.00012207031250000%
end of HL	14	0.00006103515625000%
end of HL	15	0.00003051757812500%
end of HL	16	0.00001525878906250%
end of HL	17	0.00000762939453125%
end of HL	18	0.00000381469726563%
end of HL	19	0.00000190734863281%
end of HL	20	0.00000095367431641%

If we only start off with 1,000,000,000 tonnes of radioactive material on a moon (small in cosmic terms the earth is about 5,000,000,000,000,000,000,000 tonnes) and considering some half-lives are huge in length of time.

Start			1,000,000,000 tonnes
end of HL	1	500,000,000 tonnes
end of HL	2	250,000,000 tonnes
end of HL	3	125,000,000 tonnes
end of HL	4	62,500,000 tonnes
end of HL	5	31,250,000 tonnes
end of HL	6	15,625,000 tonnes
end of HL	7	7,812,500 tonnes
end of HL	8	3,906,250 tonnes
end of HL	9	1,953,125 tonnes
end of HL	10	976,563 tonnes
end of HL	11	488,281 tonnes
end of HL	12	244,141 tonnes
end of HL	13	122,070 tonnes
end of HL	14	61,035 tonnes
end of HL	15	30,518 tonnes
end of HL	16	15,259 tonnes
end of HL	17	7,629 tonnes
end of HL	18	3,815 tonnes
end of HL	19	1,907 tonnes
end of HL	20	954 tonnes

So even after millions of years, you have large amounts of material that are still radioactive.

stereo_steve

I was just looking round the net and came across this table:

Polonium-214 0.164 second
Oxygen-15 2 minutes
Bismuth-212 60.5 minutes
Sodium-24 15 hours
Iodine-131 8 days
Phosphorus-32 14.3 days
Cobalt-60 5.3 years
Carbon-14 5,730 years
Plutonium-239 24,110 years
Uranium-238 4.5 billion years

So I can see what you mean Victor about the isotopes taking a LARGE time to decay, but what about the shorter radioactive isotopes? Like polonium-214 above?

Surely radioactive isotopes are still being created. Not just from the big bang? Or am i completely wrong?

Victor

In stars and as a result of the decay of other elements.

I don't know anything about polonium. Perhaps it decays from a higher element over a long period of time and becomes yet another element very quickly or becomes a stable isotope.

---

Sorry, the 954 tonnes of radioactive material (after 20 HLs) in the above example would be scattered through the 1,000,000,000 tonnes of material and if it simply becomes another isotope, it would be chemically indistinguishable.

If it degenerates into other elements, those elements would be scattered through the material on an atomic level.

stereo_steve

Thanks for the help victor. I'm still a bit confused over a few little bits but the subject really irriates me so I'm gonna try and forget it!