Technetium and promethium have no isotopes that are long-lived on a geological timescale. When I were a lad they were considered purely synthetic, in an earthly context. (It has long been known that they can be produced in stars.)

In 1999, David Curtis of LANL used X-ray emissions from a uranium sample to determine that Tc-99 exists in association with uranium in amounts of around 1 in a trillion, as a product of natural fission. ( Some have taken this as confirmation of results reported by a German team of Tacke, Noddack and Berg in 1925, which were not widely accepted at the time. Technetium is usually said to have been discovered in 1936 by the Italian researchers Perrier and Segre.) This would mean there are several tonnes of technetium in the crust, and hundreds of tonnes in the Earth altogether.

Promethium is also now known to exist naturally in the crust, again as a product of natural fission of uranium. It is much rarer than technetium, and it is thought there is about half a kilogram in the crust, and a few dozen kilograms in the Earth as a whole.

Suppose this half kilo of Promethium is distributed evenly through all the solids and liquids of the Earth’s crust and oceans, lakes etc.

What is the probability of swallowing an atom of it in a cup of tea? (say a 0.5 litre cup, for the sake of argument).

Have I used promethium today? Almost certainly, if you have a fine vintage or chunky diver’s watch. Promethium-147 was widely used in luminous paints, but today’s regulators don’t like radioactive material. If you have a heart pacemaker, you’re carrying around your own piece of promethium. And check out the on-off switch on your grandma’s electric blanket.

http://businesslife.ba.com/Ideas/Features/Rare-earth-metal-of-the-month-Promethium-.html

Technetium:

dv said:

Technetium and promethium have no isotopes that are long-lived on a geological timescale. When I were a lad they were considered purely synthetic, in an earthly context. (It has long been known that they can be produced in stars.)

In 1999, David Curtis of LANL used X-ray emissions from a uranium sample to determine that Tc-99 exists in association with uranium in amounts of around 1 in a trillion, as a product of natural fission. (Some have taken this as confirmation of results reported by a German team of Tacke, Noddack and Berg in 1925, which were not widely accepted at the time. Technetium is usually said to have been discovered in 1936 by the Italian researchers Perrier and Segre.) This would mean there are several tonnes of technetium in the crust, and hundreds of tonnes in the Earth altogether.

Promethium is also now known to exist naturally in the crust, again as a product of natural fission of uranium. It is much rarer than technetium, and it is thought there is about half a kilogram in the crust, and a few dozen kilograms in the Earth as a whole.

Nice, I hadn’t heard that. I had heard that plutonium and neptunium, once thought to be purely synthetic, are now known to exist is significant quantities in the Earth’s crust.

From memory, technetium is a natural product of nuclear fission, yes, you’ve said that. It is even be a bit more than that, because with a mass of 98 it’s actually one of the most commonly produced elements from the spontaneous fission of 235U and 238U. On the other hand, it’s not the most commonly produced element by the fission of 232Th and 233U whose (lower mass) products peak at a mass of 91-93. According to wikipedia, the half-life of 238U with respect to spontaneous fission is 8.4×10^15 years.

I had thought that promethium was common on Earth. I hadn’t even known that all its isotopes were radioactive. Let me check: “The most stable isotope of the element is promethium-145, which has a half-life of 17.7 years via electron capture”. Compare that with “Technetium-99 has a radioactive half-life of 212,000 years”. isotopes with mass 145 are almost the most common (higher mass) fission product of 239Pu, which is itself rare. Promethium is not the most common isotope produced by the spontaneous fission of 238U, but still, isotopes with a mass of 145 are produced in about 4% of fissions of 238U.

Aha, now I know why I thought promethium was common on Earth. According to wikipedia it had been thought that promethium was a decay product of natural niobium, which is common. This was only disproved in the year 2003.

The Rev Dodgson said:

Suppose this half kilo of Promethium is distributed evenly through all the solids and liquids of the Earth’s crust and oceans, lakes etc.

What is the probability of swallowing an atom of it in a cup of tea? (say a 0.5 litre cup, for the sake of argument).

That’s a big cup, like two metric cups.

Under those circumstances, I’d expect your two-cup cup to contain about 51 promethium atoms on average, and the probability of there being at least one atom of promethium in the cup to be extremely close to 100%.

Of course, promethium is NOT distributed in that way. It is found either in association with the uranium from which it derives, or in laboratories, and the “natural” isotope doesn’t last very long.

I have some natural geological specimens here, that are about 30% U3O8. They weigh about 25g in total.

Are they likely to contain some promethium?

Michael V said:

I have some natural geological specimens here, that are about 30% U3O8. They weigh about 25g in total.

Are they likely to contain some promethium?

The mass ratio of uranium to promethium is supposedly about 2×10^17 to one.

From what you’ve said, this sample would contain 6.3 grams of U, so 3.1×10^-17 grams of promethium, which is about 120000 atoms.

Thanks for that.

:)

dv said:

The Rev Dodgson said:

Suppose this half kilo of Promethium is distributed evenly through all the solids and liquids of the Earth’s crust and oceans, lakes etc.

What is the probability of swallowing an atom of it in a cup of tea? (say a 0.5 litre cup, for the sake of argument).

That’s a big cup, like two metric cups.

Under those circumstances, I’d expect your two-cup cup to contain about 51 promethium atoms on average, and the probability of there being at least one atom of promethium in the cup to be extremely close to 100%.

Of course, promethium is NOT distributed in that way. It is found either in association with the uranium from which it derives, or in laboratories, and the “natural” isotope doesn’t last very long.

Uranium is found in seawater, in sufficiently large quantities that there have been proposals to mine seawater for uranium, 0.0033 ppm (that may not seem much, but it’s the same as the concentration of iron in seawater). But a cup of tea would have less uranium than a cup of seawater.