Isotopic Abundance: What is it and how does it affect nuclear processes?

[Makveger]

Hello

I'm new to Nuclear Physics and I'm stuck in understanding the Isotopic Abundance
the book I am reading says this : "The isotopic abundance of the i-th isotope in a given element is the fraction of the atoms in the element that are that isotope."

can you explain this please?or just explain the Isotopic Abundance in your words?
thanks,

 

[daveb]

Suppose an element has an isotopic abundance of 10% for one of its isotopes, 15% for a second, and the remainder a third. This would mean out of every 100 atoms found in nature, on average, 10 of them would be the first isotope, 15 the second, and 75 of them the last.

 

[splitringtail]

Well, elements can have different masses, which is due to the number of neutrons. However, they are chemically the same because that depends on the number of protons (atomic number). These care called isotopes i.e. U-233, U-235, U-238, etc. They are all uranium, but differ in neutron number.

Isotopic abundance is how much of a given element is a particular isotope i.e. how much of the Uranium is actually U-235 and not say U-236,U-237,U-238.

However, isotopic abundance is usually treated as the amount of an isotope of a element found in nature. Sometimes you will hear Natural Abundance instead.

If you picked a hunk for pure Iron and ran it through a mass spectrometer and did it right, you would not get one mass, you would get several corresponding to each isotope. The amount of that isotope relative to the whole sample is your isotopic abundance. On Earth, Iron occurs in the isotopes of Fe-54, Fe-56, Fe-54, Fe-57 and Fe-58.

Why does this happen?

Most of the time these isotopes are stable and do not undergo radioactive decay. In addition, they are some radioisotopes like U-235, U-238, K-40 that are radioactive, but their half-lives are so long you can still find it in nature.

 

[Astronuc]

Isotopic abundance is how much of a given element is a particular isotope i.e. how much of the Uranium is actually U-235 and not say U-236,U-237,U-238.

Um - one may wish to rethink this statement.

Isotopic abundance normally refers to the fraction of a given isotope for a given element in its natural state.

The isotopic abundance of U-235 would be the enrichment of the uranium. Natural uranium is mostly U-238 (~0.99274), some U-235 (~0.0072), and traces of U-234 (0.000054). U-236 (half-life = 2.342E7 y) and U-237 (half-life = 6.75 d) are not normally found in natural U.

See - http://www.nndc.bnl.gov/chart/reCenter.jsp?z=92&n=146 (select Zoom 1 if the details are not visible)

Commercial (enriched) un-irradiated and irradiated uranium fuel has an entirely different isotopic vector than natural U.

 

[splitringtail]

I apologize for the poor wording, but I would think that isotopic abundance is a more general concept than natural abundance, which is the amount of isotopes of a particular element found in nature.

I would think, but my astronomy background is superficial, that some elements could possibly have different distribution of isotopes depending what exactly makes the materials that form the planet.

Speaking of Uranium, there were natural reactors in Gabon Africa that changed the concentrations of U-235. So, they are other processes that can alter isotopic distributions.

Also, I would think you could still use the term isotopic abundance when describing the amount of isotope of say fresh reactor fuel, spent reactor fuel, or nuclear transmuted material.

 

[Astronuc]

I apologize for the poor wording, but I would think that isotopic abundance is a more general concept than natural abundance, which is the amount of isotopes of a particular element found in nature.

I would think, but my astronomy background is superficial, that some elements could possibly have different distribution of isotopes depending what exactly makes the materials that form the planet.

Speaking of Uranium, there were natural reactors in Gabon Africa that changed the concentrations of U-235. So, they are other processes that can alter isotopic distributions.

Also, I would think you could still use the term isotopic abundance when describing the amount of isotope of say fresh reactor fuel, spent reactor fuel, or nuclear transmuted material.

Usually, we are concerned with the enrichment, which is the mass fraction of U-235. It's either U-235 or U-238, with a tiny amount of U-234. However, if the uranium is recycled, then it could have U-236.

I use the term isotopic vector.

The natural reactors in Gabon were of course natural, but also rather unique or atypical. They would not necessarily reflect the tendencies of other deposits in N. America, Australia or India for example.