Understanding radioactivity levels of different isotopes

[Jimmy87]

Homework Statement:: Understanding radioactivity levels of different isotopes
Relevant Equations:: None

Hi,

This isn't a homework problem but is an introductory question. Could someone explains what makes one isotope more radioactive than another isotope assuming you have the same amount of each. The reason I ask is because I was reading that Radium-226 is quite radioactive and was quoted as a million times more radioactive than uranium. However, Radium-226 has a half life that is quite large (1600years) so if it takes that long for half the nuclei to decay then how can it be so radioactive? Is it more radioactive than cobalt-60 that only has a half life of 5 years?

Thanks for any help offered

 

[DaveC426913]

Radium-226 is quite radioactive and was quoted as a million times more radioactive than uranium

You've compared a single isotope of radium (its most stable isotope) with uranium the element.

I think you'll find that radium the element has many isotopes that are very radioactive:
https://en.wikipedia.org/wiki/Isotopes_of_radium
whereas, many isotopes of uranium are quite stable, on the order of tens of thousands to billions of years:
https://en.wikipedia.org/wiki/Isotopes_of_uranium

 

[Jimmy87]

You've compared a single isotope of radium (its most stable isotope) with uranium the element.

I think you'll find that radium the element has many isotopes that are very radioactive:
https://en.wikipedia.org/wiki/Isotopes_of_radium
whereas, many isotopes of uranium are quite stable, on the order of tens of thousands to billions of years:
https://en.wikipedia.org/wiki/Isotopes_of_uranium

Thanks. So is it as simple as looking at the half life? So for example if I had 3g of radium 226 and 3g of cobalt 60 then the cobalt 60 will be much more radioactive as it has a half life of 5 years compared to 1600years?

 

[gmax137]

Activity is measured in units becquerels which is just decays per second. The older unit, curie, is 3.7x10^10 decays per second (because that's the activity of one gram of radium-226).

You are correct, short half life means more radioactive.

The wiki page gives a good account of relating the amount of material, its half life, and the activity.
https://en.wikipedia.org/wiki/Becquerel

 

[jbriggs444]

So is it as simple as looking at the half life?

Half life will give you the rate of decay events for a given number of atoms. If decays per second is what you are interested in then yes, it is that simple.

If what you are interested in is radioactivity level then the decay pathway may be important. If one isotope decays into another which decays into another and so on, that can give rise to more radiation than if a different isotope simply decays once and is done. Even though the half life of the initial isotopes might be comparable.

The type of decay is also important. Is it an alpha emitter? Beta? Gamma? With what energies? That can tell you how you have to shield.

Is it chemically active? Solid, liquid, gas? That can affect how it needs to be contained.

It can get about as complicated as you want to make it.

 

[Jimmy87]

Activity is measured in units becquerels which is just decays per second. The older unit, curie, is 3.7x10^10 decays per second (because that's the activity of one gram of radium-226).

You are correct, short half life means more radioactive.

The wiki page gives a good account of relating the amount of material, its half life, and the activity.
https://en.wikipedia.org/wiki/Becquerel

Thanks. So I have been doing some reading online and saw that the isotope polonium 210 is the most toxic isotope to humans and you need 0.1 micro grams to die from it. I know it’s an alpha emitter and that is highly ionising so that plays a part in its toxicity but is the fact its half life is 128 days a reason as well then? So hypothetically if polonium 210 had a half life of 100 years then it wouldn’t be as dangerous?

 

[kuruman]

So hypothetically if polonium 210 had a half life of 100 years then it wouldn’t be as dangerous?

Think of it this way. What ma is the energy deposited by the alphas vs. the ability of the body to repair the damage done by that energy per unit lifetime. Is an ingested alpha source that kills in two years less "dangerous" than another source that kills in two weeks? I think not. It's always dangerous to ingest an alpha emitter so don't try it at home.

 

[Vanadium 50]

It's not obvious that there is any practical distinction between substances that are toxic in microgram quantities with ones that are toxic in milligram quantities.

One of the issues with Po-210 is that the half life (4 months) is comparable to the time it takes to excrete it (2 months). So it delivers most of its dose in the body. Sr-90 is similarly dangerous - what doesn't get excreted immediately gets taken up in bones, where stays forever, and it's half-life (60 years) is comparable to human lifetimes.

On one side is uranium, which tends to be excreted with weeks, but has a half life of billions of years. So it doesn't have time for it to do all the damage it can. On the other is maybe O-13, which would certainly do damage if taken up, but its half-life is 1/100 of a second. You don't have time to get it inside you before its gone.

 

[snorkack]

Thanks. So I have been doing some reading online and saw that the isotope polonium 210 is the most toxic isotope to humans and you need 0.1 micro grams to die from it. I know it’s an alpha emitter and that is highly ionising so that plays a part in its toxicity but is the fact its half life is 128 days a reason as well then? So hypothetically if polonium 210 had a half life of 100 years then it wouldn’t be as dangerous?

Not even so hypothetically.Polonium 209 has a half-life 125 years - long thought to be 100 years. Both are alpha emitters. Polonium 209 is much less dangerous than polonium 210.

While one possible measurement of radioactivity is count of decay events, another possible one is decay energy.

 

[Jimmy87]

Not even so hypothetically.Polonium 209 has a half-life 125 years - long thought to be 100 years. Both are alpha emitters. Polonium 209 is much less dangerous than polonium 210.

While one possible measurement of radioactivity is count of decay events, another possible one is decay energy.

Great. Thank you. This is a great comparison for Po-209 vs Po-210 that I had no idea about.

 

[snorkack]

On one side is uranium, which tends to be excreted with weeks, but has a half life of billions of years. So it doesn't have time for it to do all the damage it can.

It is actually seriously unclear whether uranium is dangerous because of radioactivity, or because of biochemical toxicity.

 

[dlgoff]

or because of biochemical toxicity.

I think it has more to do with the biological half life.

 

[snorkack]

I think it has more to do with the biological half life.

Depleted uranium, natural uranium and enriched uranium have the same biological half-life. Is their toxicity proportional to their radioactivity (because it is radioactivity) or equal (because it is biochemical toxicity irrespective of nuclear properties)?

 

[dlgoff]

because it is biochemical toxicity irrespective of nuclear properties

good point.

 

[mfb]

It is actually seriously unclear whether uranium is dangerous because of radioactivity, or because of biochemical toxicity.

Both. Wikipedia has an overview.