- Sep 5, 2012
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Please note that this thread is NOT about polonium halos, nor is it about uranium dating. Any reference to those topics will be considered off topic and ignored. We can discuss those in another thread, if you like, but I want to focus on this topic, please.
As Uranium 238 decays through alpha-decay, the emitted alpha particles create damage to the surrounding rock, forming a sphere around the inclusion whose radius is dependent on the alpha particle decay energy.
They are called halos because we study them by looking at cross-sections of the sphere.
Now, it takes many decay events to form this visible sphere, since each atom that decays only produces one dot on the sphere. Something like 10^9 atoms of uranium need to decay in order to produce this sphere.
Since the half life of Uranium is about 4.5 billion years, in order for enough atoms to decay to form a halo, it would take hundreds of millions of years.
This is evidence of an old earth.
But there is more, it's also evidence that the decay constant of Uranium 238 has been constant for that time. Why?
Because the decay rate is inversely related to the decay energy. So, if the decay rate was faster in the past, the halo radius would be a different size. So, since the Uranium halos have a radius consistent with the decay energy that we observe today, we know that decay rates have not changed for hundreds of millions of years.
As Uranium 238 decays through alpha-decay, the emitted alpha particles create damage to the surrounding rock, forming a sphere around the inclusion whose radius is dependent on the alpha particle decay energy.
They are called halos because we study them by looking at cross-sections of the sphere.
Now, it takes many decay events to form this visible sphere, since each atom that decays only produces one dot on the sphere. Something like 10^9 atoms of uranium need to decay in order to produce this sphere.
Since the half life of Uranium is about 4.5 billion years, in order for enough atoms to decay to form a halo, it would take hundreds of millions of years.
This is evidence of an old earth.
But there is more, it's also evidence that the decay constant of Uranium 238 has been constant for that time. Why?
Because the decay rate is inversely related to the decay energy. So, if the decay rate was faster in the past, the halo radius would be a different size. So, since the Uranium halos have a radius consistent with the decay energy that we observe today, we know that decay rates have not changed for hundreds of millions of years.