When does an isotope begin to decay?

In summary, the decay of 60Fe found in meteorites starts from the moment it is created, regardless of its physical state or temperature. However, dating methods for radioisotopes look at the ratio of parent to daughter isotopes, which can only be measured under specific conditions. This allows us to determine the age of the sample, not the isotope itself. Different dating methods exist for different isotopes and rely on the assumption that the starting concentration of the parent is known.
  • #1
MarkL
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Take 60Fe found in meteorites. Does it start to decay the moment it is created. Or does it start to decay when it cools to a solid (a meteorite)? Does heat have anything to do with it? Thanks
 
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MarkL said:
Take 60Fe found in meteorites. Does it start to decay the moment it is created. Or does it start to decay when it cools to a solid (a meteorite)? Does heat have anything to do with it? Thanks
The individual atoms start decaying probabilistically according to their half-life from the moment that they are created (perhaps as decay products of other unstable isotopes, perhaps by nucleosynthesis in stars). Whether these atoms are assembled into a lump of iron or floating around in the heart of a star, whether they're part of a mass of solidified iron or a puddle of molten iron is irrelevant to their individual decay probability.

So now you're probably wondering how we can do radioisotope dating if the solidification of the meteorite doesn't start a "decay clock" in the individual atoms?
 
  • #4
Yes. Thank you. So we measure the age of the isotope, not the meteorite.
 
  • #5
MarkL said:
So we measure the age of the isotope, not the meteorite.

No.

The isotope starts decaying from its inception. However, in radioisotope dating, we measure two things: the parent nucleus and the daughter nucleus. We pick pairs where the properties differ: e.g. in K-Ar dating, the argon is a gas, so if the rock is still in liquid form, the argon bubbles out. Only after the rock solidifies is the argon trapped, so K-Ar dating tells us when the sample was last liquid.
 
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  • #6
MarkL said:
Yes. Thank you. So we measure the age of the isotope, not the meteorite.
I don't think I would put it that way.
You cannot know the age of an isotope (which particular nucleus would you be looking at?) and you can only do dating under particular conditions and with particular isotopes.
There's a general principle for radio isotope dating but different methods to achieve it. There needs to have been some physical or chemical process that isolates a suitable isotope with none of the daughter product there. Then you have a starting point for the process, after which there will be a steady exponential decay. As the parent decays, the proportion of parent to daughter after a given time will tell you how long that particular sample has been isolated (assuming you know the half life). The meteorite is assumed to have been formed with a particular concentration of the parent and the ratio that's measured will tell you how long since the meteorite was formed.
Radio Carbon dating looks at the Carbon isotopes that have been locked into plant tissue, from CO2 in the air. The proportion of C12 and C14 in the atmosphere is assumed to have been what it is now and the proportion of the two isotopes will change in time for the fixed C in the dead plant. The time is how long since the plant photosynthesised the food.
 

1. When does an isotope begin to decay?

Isotopes begin to decay at a completely random and unpredictable time. The process of decay is governed by the laws of quantum mechanics, making it impossible to predict exactly when it will occur.

2. What causes an isotope to decay?

Isotopes decay due to the instability of their atomic nuclei. This instability is caused by an imbalance in the number of protons and neutrons in the nucleus, which leads to the emission of radiation in order to achieve a more stable state.

3. How long does it take for an isotope to decay?

The amount of time it takes for an isotope to decay is known as its half-life. Each isotope has a unique half-life, which can range from fractions of a second to billions of years. This means that some isotopes decay very quickly, while others are incredibly stable.

4. Can we speed up or slow down the decay of an isotope?

No, the rate of decay for an isotope is constant and cannot be altered by any external factors. This is because the process of decay is governed by the laws of physics and is not affected by temperature, pressure, or any other conditions.

5. What happens to an isotope after it decays?

After an isotope decays, it transforms into a different element with a different number of protons and neutrons. This new element may also be unstable and undergo further decay until it reaches a stable form. This process continues until a stable element is reached, known as the final decay product.

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