In the context of the composition of the earth, we can basically divide the nuclides into four sets:

A) stable nuclides

B) nuclides with a sufficiently long half-life that a significant amount of primordial content (ie content that existed at the time of the creation of the earth) still exists on earth

C) nuclides that are short lived but are continually replenished by the decay of long lived nuclides or bombardment by high energy particles from spaaaaaaace

D) nuclides that only exist in any significant quantity on earth synthetically

Although there is no clear boundary for B), we can get an idea by considering what fraction of the primordial content remains. Taking the age of earth to be 4.5 billion, a nuclide with a half-life of 150 million years, about one part in a billion would remain. A half-life of 100 million years would mean one part in 30 trillion would still be here.

If the half-life was 57 million years, the remaining portion would be one part in 6 × 10^23: ie, one atom per primordial mole.

Plutonium-244 has a half-life of 80 million years and there appears to be a few grams in the earth’s crust.

The nuclide that has the next lowest half-life is niobium-92. That has a half-life of 35 million years, less than half of that of plutonium-244. What this means is that the “survival ratio” is more than the square that of the Pu-244.

No natural primordial niobium-92 has been found on earth. Making order-of-magnitude estimates based on cosmic abundance, we might anticipate that between 1 and 10 atoms could exist in the entirety of the earth: not likely any in the crust.

So what’s it worth if I find one?

sends sprogs out

dv said:

In the context of the composition of the earth, we can basically divide the nuclides into four sets:

A) stable nuclides

B) nuclides with a sufficiently long half-life that a significant amount of primordial content (ie content that existed at the time of the creation of the earth) still exists on earth

C) nuclides that are short lived but are continually replenished by the decay of long lived nuclides or bombardment by high energy particles from space

D) nuclides that only exist in any significant quantity on earth synthetically

Although there is no clear boundary for B), we can get an idea by considering what fraction of the primordial content remains. Taking the age of earth to be 4.5 billion, a nuclide with a half-life of 150 million years, about one part in a billion would remain. A half-life of 100 million years would mean one part in 30 trillion would still be here.

If the half-life was 57 million years, the remaining portion would be one part in 6 × 10^23: ie, one atom per primordial mole.

Plutonium-244 has a half-life of 80 million years and there appears to be a few grams in the earth’s crust.

The nuclide that has the next lowest half-life is niobium-92. That has a half-life of 35 million years, less than half of that of plutonium-244. What this means is that the “survival ratio” is more than the square that of the Pu-244.

No natural primordial niobium-92 has been found on earth. Making order-of-magnitude estimates based on cosmic abundance, we might anticipate that between 1 and 10 atoms could exist in the entirety of the earth: not likely any in the crust.

I’m going to modify your A) B) C) D) slightly. A better arrangement perhaps is the following.

A) nuclides known to be stable. There are 90 of these.

B) nuclides that have never been observed to decay but theoretically ought to decay with half lives longer than 10^22 years. According to Wikipedia there are 163 such nuclides.

C) nuclides that have been observed to decay, but do so on such a long timescale that they are for all practical purposes stable. Te-128, Xe-136, Ge-76, Ba-130, Te-130, Se-82, Ca-48, Cd-116, Zr-96, Bi-209 etc.

D) primordial radioactive nuclides on Earth. There are 34 of these. eg. Th-232, U-238, K-40, U-235, Sm-146.

E) radioactive nuclides that are continually being produced on Earth by the decay of other radioactives or by cosmic rays. eg. Isotopes of Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa.

F) radioactive nuclides produced synthetically in significant quantity. eg. Isotopes of Neptunium, Plutonium, Americium, Californium together with lesser actinides and nuclear fission products from Ge-76 through to Sm-153, and nuclides produced in cyclotrons and similar (one document lists 41 nuclides produced in cyclotrons)

G) nuclides produced synthetically so rarely that none has ever been stored. eg. Hydrogen-4,5,6,7 up to for example Ununoctium-294.

I’m also going to add another hypothetical category

H) relatively stable nuclides that have never been observed. >= 113 protons and >= 52 more neutrons than protons.

> There are 34 of these.

Oops, that figure from Wikipedia would have included both class B and class C.

> Plutonium-244 has a half-life of 80 million years and there appears to be a few grams in the earth’s crust. The nuclide that has the next lowest half-life is niobium-92. That has a half-life of 35 million years, less than half of that of plutonium-244. What this means is that the “survival ratio” is more than the square that of the Pu-244. No natural primordial niobium-92 has been found on earth. Making order-of-magnitude estimates based on cosmic abundance, we might anticipate that between 1 and 10 atoms could exist in the entirety of the earth: not likely any in the crust.

In total agreement with you there.

>> There are 34 of these.

> Oops, that figure from Wikipedia would have included both class B and class C.

Oops, class C and class D.