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Questions about radioactive material

HappyCat

Thinker
HappyCat
Joined
Oct 6, 2005
Messages
126
I have two questions with regards to radioactive material. First, the standard model of the universe as I understand it states that all the elements through iron can be formed during fusion in stars, and the rest of the elements are created in super novas. How is it, then, that we see elements with half-life times of only a few days? Radon is one example of an element whose half-life is only about a week, IIRC. It seems to me that if these elements were created in a super nova blast, they would have deteriorated into stable elements in the billions of years since the super nova blast.

My second question is with regards to radioactive dating. I know that they date really old rocks by measuring the Uranium content and comparing it to the Lead content in the rock, since Uranium decays into Lead. It seems to me that in order to use this as an accurate dating method, we would need to know both how much Uranium and Lead were in the rock when it was formed. Since the Uranium in the earth is decaying into Lead at the same rate as the Uranium in the rocks, how is this possible?
 
HappyCat,

I have two questions with regards to radioactive material. First, the standard model of the universe as I understand it states that all the elements through iron can be formed during fusion in stars, and the rest of the elements are created in super novas. How is it, then, that we see elements with half-life times of only a few days? Radon is one example of an element whose half-life is only about a week, IIRC. It seems to me that if these elements were created in a super nova blast, they would have deteriorated into stable elements in the billions of years since the super nova blast.
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The short half-life elements we see in nature are often the products of the decay of other longer half-life elements. In other cases, they are created through nuclear reactions from cosmic rays. For example, Carbon 14 is created in the upper atmosphere a Proton in a Nitrogen atom is turned into a Neutron.

My second question is with regards to radioactive dating. I know that they date really old rocks by measuring the Uranium content and comparing it to the Lead content in the rock, since Uranium decays into Lead. It seems to me that in order to use this as an accurate dating method, we would need to know both how much Uranium and Lead were in the rock when it was formed. Since the Uranium in the earth is decaying into Lead at the same rate as the Uranium in the rocks, how is this possible?
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By comparing with other dating methods. The reliability of the various dating methods is derived largely from the consistency between the results the different methods give.


Dr. Stupid
 
Stimpson J. Cat said:
The short half-life elements we see in nature are often the products of the decay of other longer half-life elements. In other cases, they are created through nuclear reactions from cosmic rays. For example, Carbon 14 is created in the upper atmosphere a Proton in a Nitrogen atom is turned into a Neutron.
Click to expand...

Ok. This makes sense. Thank you.

Stimpson J. Cat said:
By comparing with other dating methods. The reliability of the various dating methods is derived largely from the consistency between the results the different methods give.
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My question is not about the reliability of Uranium dating, but rather how we arrive at a number at all. Say Earth contains X concentration of Uranium and Y concentration of Lead when a rock is formed. This rock will have X and Y concentration of Uranium and Lead respectively. After one Uranium half life, the rock will have X/2 Uranium concentration and Y+X/2 Lead concentration. However, since the Uranium in the earth is decaying at the same rate as the Uranium in the rock, the earth now contains X/2 concentration of Uranium and Y+X/2 concentration of Lead as well. So a new rock formed will have X/2 and Y+X/2 concentration of Uranium and lead respectively. How do we distinguish the new rock from the old rock? It seems to me they will both have the same concentrations of Uranium and Lead. I do not doubt the reliability of the dating method, I just don't understand how they arrive at the conclusion.
 
You'll have to as a geologist about that. I don't really know enough about Uranium dating to answer.

Dr. Stupid
 
HappyCat said:
My question is not about the reliability of Uranium dating, but rather how we arrive at a number at all. Say Earth contains X concentration of Uranium and Y concentration of Lead when a rock is formed. This rock will have X and Y concentration of Uranium and Lead respectively. After one Uranium half life, the rock will have X/2 Uranium concentration and Y+X/2 Lead concentration. However, since the Uranium in the earth is decaying at the same rate as the Uranium in the rock, the earth now contains X/2 concentration of Uranium and Y+X/2 concentration of Lead as well. So a new rock formed will have X/2 and Y+X/2 concentration of Uranium and lead respectively. How do we distinguish the new rock from the old rock? It seems to me they will both have the same concentrations of Uranium and Lead. I do not doubt the reliability of the dating method, I just don't understand how they arrive at the conclusion.
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My off the cuff answer is that there are probably nuclear reactions going on beneath the Earth's crust.
I think I remember reading this a long time ago and I can't see what else could cause it.
 
HappyCat said:
Ok. This makes sense. Thank you.


My question is not about the reliability of Uranium dating, but rather how we arrive at a number at all. Say Earth contains X concentration of Uranium and Y concentration of Lead when a rock is formed. This rock will have X and Y concentration of Uranium and Lead respectively. After one Uranium half life, the rock will have X/2 Uranium concentration and Y+X/2 Lead concentration. However, since the Uranium in the earth is decaying at the same rate as the Uranium in the rock, the earth now contains X/2 concentration of Uranium and Y+X/2 concentration of Lead as well. So a new rock formed will have X/2 and Y+X/2 concentration of Uranium and lead respectively. How do we distinguish the new rock from the old rock? It seems to me they will both have the same concentrations of Uranium and Lead. I do not doubt the reliability of the dating method, I just don't understand how they arrive at the conclusion.
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A very smart question.

The key is the concept of "resetting the clock." This is why some radiating elements are used to date geological formations, but others are not. Specifically, a very popular one is K/Ar (Potassium/Argon) dating.

The advantage of K/Ar dating is that when magma erupts to the surface, the molten mix of Potassium and argon flows and the argon dissipates quickly into the atmosphere because it is a gas. We know, therefore, that when the rock hardened, its [Ar] was zero. Any argon found in the sample at a later time must have been formed since the lava was liquid, and we can date the formation.

Another technique is to look for gamma radiation tunnels in a crystal structure. You can assume that the crystal would not have any tunnels when it was liquid, so all the tunnels that can be observed have been formed since it last hardened. Counting these tunnels will show how many radioactive decays have taken place in the meantime, and you just throw it into a forumula to get an age.

Another one is thermoluminescence, which 'resets' when a stone is heated. The amount of luminescence the stone gives out today can be measured against its last heating, when it was reset to zero. This is useful for stones that contain quartz and have been heated by campfires - we use this technique to date ancient settlements, or layers of cave floors that have had human occupation.
 
HappyCat said:
My question is not about the reliability of Uranium dating, but rather how we arrive at a number at all.
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We use Zircon. When Zircon crystals are formed uranium can be incorperated in place of Zirconium. However lead can't be. Thus any lead we find must have been formed after the crystal was formed.

Zircon is useful in that it it is very hard so lasts a long time meaning that there are 4.4 billion year old bits of Zircon around.
 
Not hollow but quite solid

blutoski said:
But... but... the earth is hollow...
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Nope. Last time I heard scientists talking about that, they were discussing about the core being formed of molten iron, nickel, and some other heavy metals. Solid stuff. Others were arguing about the possibility that the Earth's core was indeed a nuclear reactor. Even if this last hypothesis is true, it is based on uranium, plutonimum, thorium and other heavy radioactiva materials that makes a quite solid core.

Only Jules Verne said it was hollow. Hollywood took the novel and made a funny picture.

Don't trust Hollywood on science. They made "The Day After tomorrow," and now Al Gore's "An Inconvenient Thruth". I would call it "An Inconvenient Blunder".
 
Edufer said:
Nope. Last time I heard scientists talking about that, they were discussing about the core being formed of molten iron, nickel, and some other heavy metals. Solid stuff. Others were arguing about the possibility that the Earth's core was indeed a nuclear reactor. Even if this last hypothesis is true, it is based on uranium, plutonimum, thorium and other heavy radioactiva materials that makes a quite solid core.

Only Jules Verne said it was hollow. Hollywood took the novel and made a funny picture.

Don't trust Hollywood on science. They made "The Day After tomorrow," and now Al Gore's "An Inconvenient Thruth". I would call it "An Inconvenient Blunder".
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i think blutoski was joking :)
 
blutoski said:
A very smart question.

The key is the concept of "resetting the clock." This is why some radiating elements are used to date geological formations, but others are not. Specifically, a very popular one is K/Ar (Potassium/Argon) dating.

The advantage of K/Ar dating is that when magma erupts to the surface, the molten mix of Potassium and argon flows and the argon dissipates quickly into the atmosphere because it is a gas. We know, therefore, that when the rock hardened, its [Ar] was zero. Any argon found in the sample at a later time must have been formed since the lava was liquid, and we can date the formation.

Another technique is to look for gamma radiation tunnels in a crystal structure. You can assume that the crystal would not have any tunnels when it was liquid, so all the tunnels that can be observed have been formed since it last hardened. Counting these tunnels will show how many radioactive decays have taken place in the meantime, and you just throw it into a forumula to get an age.

Another one is thermoluminescence, which 'resets' when a stone is heated. The amount of luminescence the stone gives out today can be measured against its last heating, when it was reset to zero. This is useful for stones that contain quartz and have been heated by campfires - we use this technique to date ancient settlements, or layers of cave floors that have had human occupation.
Click to expand...

geni said:
We use Zircon. When Zircon crystals are formed uranium can be incorperated in place of Zirconium. However lead can't be. Thus any lead we find must have been formed after the crystal was formed.

Zircon is useful in that it it is very hard so lasts a long time meaning that there are 4.4 billion year old bits of Zircon around.
Click to expand...
Thanks both of you. I feel more informed now :)
 
HappyCat said:
I have two questions with regards to radioactive material. First, the standard model of the universe as I understand it states that all the elements through iron can be formed during fusion in stars, and the rest of the elements are created in super novas. How is it, then, that we see elements with half-life times of only a few days? Radon is one example of an element whose half-life is only about a week, IIRC. It seems to me that if these elements were created in a super nova blast, they would have deteriorated into stable elements in the billions of years since the super nova blast.
Click to expand...

If I remember correctly Radon comes from the breakdown of uranium in granite. The problems people have with radon are they it seeps into their house and gets concentrated their.
 
russingram said:
the Earth's core is heated by nuclear reaction
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The Earth's core doesn't need heating. The Earth was formed from basically molten rock. So far only the top few kilometres have cooled enough to solidify, which traps the heat inside even more effectively, so the Earth is cooling even slower than in the past. It will take billions more years before the core cools noticeably, by which time it will probably have been fried by the Sun anway.

Also, since the core is thought to be almost entirely iron there are no nuclear reactions that could take place naturally. Iron is the most stable element known, so fusion reactions take place between lighter elements, and fission takes place in heavier elements, but without extreme conditions, such as supernovae, iron won't do anything.
 
Decay chains

Uranium doesn't decay directly to lead, it takes several steps with half-lives ranging from fractions of a second to many thousands of years. Check this out: http://en.wikipedia.org/wiki/Decay_chain .

As others have noted, knowledge of these chains allows different forms of dating.

// CyCrow
 
Cuddles said:
Also, since the core is thought to be almost entirely iron there are no nuclear reactions that could take place naturally.
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Not with iron, no. But even though it's mostly iron, it isn't ONLY iron. There's other stuff too, including uranium (which, being quite dense, is probably more abundant near the core than near the surface) as well as other radioactive elements. These won't undergo chain reactions, but they will undergo decay, and that will produce heat. As you said, the earth's core is VERY well insulated: even a little bit of radioactive decay could be a significant component to the thermal properties of the earth's core.
 
Ziggurat said:
Not with iron, no. But even though it's mostly iron, it isn't ONLY iron. There's other stuff too, including uranium (which, being quite dense, is probably more abundant near the core than near the surface) as well as other radioactive elements. These won't undergo chain reactions, but they will undergo decay, and that will produce heat. As you said, the earth's core is VERY well insulated: even a little bit of radioactive decay could be a significant component to the thermal properties of the earth's core.
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True. Do you know what proportion could be unstable elements? It seems to be mostly iron, with around 4% nickel and 10% oxygen or sulphur, but I can't find any mention of any other elements. How much would be needed to get significant heating, and would it even be detectable?

On the other hand, I read the post I replied to as implying some kind of chain reaction, as in Oklo, rather than just radioactive heating.
 
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