Is there evidence that is the actual reasoning, because I find it ridiculous. People who don’t care about the primary source are not going to follow a link to, e.g., a court ruling, government report, fed minutes, a leaked document, a research paper, etc; but those who are interested are going to immediately leave the site and likely not come back for a long while.
According to Wikipedia thorium is probably created from neutron stars or supernovae making it one of the rarest elements in the galaxy. But, it’s quite abundant on Earth, maybe 3x as abundant as Uranium on earths crust. Because of this abundance it is, along with Radon, the primary constituent of earths natural background radiation.
Iron is the last element that can form through a sustaining fusion process. All elements above iron are formed in super nova or neutron star collisions. Specifically gold and platinum are thought to be formed primarily in neutron star collisions
While it's correct that fusion stops producing energy once the fusion product is iron, it is not true that all elements beyond iron are only produced in cataclysmic events (supernovae / neutron star collisions).
Stars can function as "breeder reactors", where the natural
flow of neutrons produced as intermediate / side effect of the various fusion reactions going on in a later-stage-life star get absorbed by nuclei and then (by beta decay) produce beyond-iron elements. This is called the "s-Process" (slow), and responsible for a large range of elements into the Lanthanides or so. And stellar winds, or the planetary nebula stage at the end, will return some of this to the interstellar medium.
It's correct that the heaviest "naturally present" elements require the so-called "r-Process", heavy overabundance of neutrons / extremely high neutron flow as in supernovae, or direct fusion of beyond-iron nuclei as in neutron star collisions.
There is rather active research happening here, both astronomical (trying to detect various nuclei from x ray spectroscopy of cataclysmic events) and theoretical (because r-Process cannot be simulated in labs since the neutron fluxes needed are not within our reach).
That said, it remains
true that many "transition groups" elements are bred via s-Process in relatively-ordinary stars.
Iron 56 is the isotope with the highest nuclear binding energy per nucleon, so it can form from the fusion of any lighter nuclei.
After iron 56, the binding energy decreases slowly, so the next heavier nuclei can still form from the fusion of certain lighter nuclei, but not from any of them, but only from pairs with a lower average binding energy.
So the following heavier nuclei after iron 56 can still form through fusion, but with increasing atomic mass the probability of their formation decreases quickly, until it becomes negligible.
Relatively large amounts of cobalt, nickel, copper, zinc, gallium and germanium still form through the fusion of lighter elements, but after germanium the amount of chemical elements formed through fusion becomes extremely low. Already the amount of germanium formed through fusion is almost ten thousand times less than the amount of iron.
The binding energy per nucleon decreases very slowly, so even uranium has a higher binding energy per nucleon than helium, so energetically it could form through the fusion of hydrogen or helium, but such an event has a completely negligible probability (because there is a negligible chance for so many hydrogen or helium nuclei to collide simultaneously and if they fuse into nuclei of intermediate mass those block the propagation of the fusion reaction by having higher binding energies than the heavier nuclei).
The elements heavier than germanium form almost only through neutron capture, with the exception of some proton-rich isotopes, which form through collisions with protons. There are several kinds of environments with abundant neutrons where heavy elements can form, where the concentrations of neutrons and their energy distributions are different, so in any of these environments there are different classes of isotopes that form preferentially there.
In a relatively young stellar system like ours, the matter from which the star and the planets have condensed is a mixture of chemical elements coming from different sources.
In environments with extremely high neutron abundances (which include the nuclear explosions on Earth, not only supernova explosions, neutron star collisions and the like), all chemical elements up to fermium (Z = 100) are formed. Nonetheless, while the matter composed of these elements travels through space until the formation of a new stellar system, most of the trans-uranium elements, except the plutonium, decay. When the Solar System was formed, it still contained relatively large amounts of plutonium, not only thorium and uranium, as the heaviest elements, but since then until now the plutonium has decayed, like also most of the uranium 235 that is used now in nuclear reactors.
Potassium is everywhere, including in every living cell.
So the exposure to potassium radiation is constant for any living being on Earth.
Exposure to thorium and radon varies greatly depending on the location, i.e. it can be significant in places with granitic rocks or with sediments whose origin is in the erosion of granitic rocks.
So some people, animals etc. may be exposed to more thorium/radon radiation than from other sources, especially if inhaling dust or radon gas, but for most the greatest exposure is from the content of potassium 40 and carbon 14 that is inside their bodies (which produce only weak beta radiation).
Potassium is an element absolutely essential for any life form on Earth.
Any cell of any living being, including humans, contains potassium ions, which are required to neutralize the excess negative charge of proteins, otherwise the interior of the cells would become acidic and it would self-destroy.
No other positive ion can replace potassium, because any other abundant positive ion has a much greater tendency of forming solid precipitates, which would also destroy the cell. That is why any living cell expels the abundant ions of sodium and calcium outside it, while pumping inside any potassium ions from the environment. The moment when a living cell stops pumping potassium and magnesium inside and sodium and calcium outside, is when the cell dies.
Among the primordial chemical elements, which already existed at the formation of the Solar System, there are many which are weakly radioactive, i.e. they have some isotopes with half lives that are of at least many hundred million years, but of many billion years for most of them.
Among these weakly radioactive elements, the human body contains not only potassium 40, but also calcium 48, but the latter is much more weakly radioactive than potassium. Besides primordial radioactive isotopes, there are also radioactive isotopes that are formed continuously by the cosmic radiation, like carbon 14, which is also present in the body of any living being.
Among the primordial radioactive isotopes, the most radioactive are uranium 235 and potassium 40, followed by uranium 238, thorium 232 and platinum 190. The existence of weakly radioactive isotopes is not random, but it is determined by a set of rules of nuclear stability. For instance, potassium 40 is radioactive because any isotope with an even atomic mass of a chemical element with an odd atomic number and heavier than nitrogen is radioactive. Potassium 40 just happens to have an unusually long half life, so it has not decayed yet (the long half life is because potassium 40, like Buridan's ass, cannot decide whether it should decay into argon or into calcium, so it stays in limbo).
All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.
How do they know the accuracy of the clock? Eg: 2s of drift in 65M years.
Naively, measuring that seems more difficult than the clock. Wouldn’t you need a perfect clock as a baseline? There must be something really clever here.
They measured the stability by comparing the nuclear clocks with more stable conventional atomic clocks. In Vienna, they used a commercial single-ion optical clock [1], and in Beijing they used a hydrogen maser (a type of microwave atomic clocks) that was calibrated to the International Atomic Time scale via GNSS satellites.
By adding up the uncertainties of all intermediate measurements they made.
As a parallel, for a regular clock, you'd measure and compute the uncertainties of:
- The distance between the axis of rotation and the center of mass of the pendulum,
- The local acceleration of gravity.
And deduce the pendulum period and its uncertainty by calculus.
In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).
That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.
Standard atomic clocks probe outer-shell electrons sensitive to stray EM noise; locking directly to the nucleus is how you actually kill environment drift.
The hope is that with nuclear clocks there will be no need for the very complex single-ion traps or optical lattices required for clocks that use ionic or atomic energy levels.
Even with the atoms of thorium contained in a small crystal, the frequency of their clock transitions should not be influenced by the environment.
Unfortunately, for now no other suitable nucleus has been found, because the frequency corresponding to nuclear energy levels is normally too high for the current or near future technology.
With thorium 229, the potential simplicity of the clock is counterbalanced by the great expense required for producing this artificial isotope.
Am I missing something?
Reality is amazing!!
Stars can function as "breeder reactors", where the natural flow of neutrons produced as intermediate / side effect of the various fusion reactions going on in a later-stage-life star get absorbed by nuclei and then (by beta decay) produce beyond-iron elements. This is called the "s-Process" (slow), and responsible for a large range of elements into the Lanthanides or so. And stellar winds, or the planetary nebula stage at the end, will return some of this to the interstellar medium.
It's correct that the heaviest "naturally present" elements require the so-called "r-Process", heavy overabundance of neutrons / extremely high neutron flow as in supernovae, or direct fusion of beyond-iron nuclei as in neutron star collisions.
There is rather active research happening here, both astronomical (trying to detect various nuclei from x ray spectroscopy of cataclysmic events) and theoretical (because r-Process cannot be simulated in labs since the neutron fluxes needed are not within our reach).
That said, it remains true that many "transition groups" elements are bred via s-Process in relatively-ordinary stars.
Iron 56 is the isotope with the highest nuclear binding energy per nucleon, so it can form from the fusion of any lighter nuclei.
After iron 56, the binding energy decreases slowly, so the next heavier nuclei can still form from the fusion of certain lighter nuclei, but not from any of them, but only from pairs with a lower average binding energy.
So the following heavier nuclei after iron 56 can still form through fusion, but with increasing atomic mass the probability of their formation decreases quickly, until it becomes negligible.
Relatively large amounts of cobalt, nickel, copper, zinc, gallium and germanium still form through the fusion of lighter elements, but after germanium the amount of chemical elements formed through fusion becomes extremely low. Already the amount of germanium formed through fusion is almost ten thousand times less than the amount of iron.
The binding energy per nucleon decreases very slowly, so even uranium has a higher binding energy per nucleon than helium, so energetically it could form through the fusion of hydrogen or helium, but such an event has a completely negligible probability (because there is a negligible chance for so many hydrogen or helium nuclei to collide simultaneously and if they fuse into nuclei of intermediate mass those block the propagation of the fusion reaction by having higher binding energies than the heavier nuclei).
The elements heavier than germanium form almost only through neutron capture, with the exception of some proton-rich isotopes, which form through collisions with protons. There are several kinds of environments with abundant neutrons where heavy elements can form, where the concentrations of neutrons and their energy distributions are different, so in any of these environments there are different classes of isotopes that form preferentially there.
In a relatively young stellar system like ours, the matter from which the star and the planets have condensed is a mixture of chemical elements coming from different sources.
In environments with extremely high neutron abundances (which include the nuclear explosions on Earth, not only supernova explosions, neutron star collisions and the like), all chemical elements up to fermium (Z = 100) are formed. Nonetheless, while the matter composed of these elements travels through space until the formation of a new stellar system, most of the trans-uranium elements, except the plutonium, decay. When the Solar System was formed, it still contained relatively large amounts of plutonium, not only thorium and uranium, as the heaviest elements, but since then until now the plutonium has decayed, like also most of the uranium 235 that is used now in nuclear reactors.
The amount used in a nuclear clock would be very small, but even so, its availability would be a problem.
I have no idea which of these articles is more correct.
So the exposure to potassium radiation is constant for any living being on Earth.
Exposure to thorium and radon varies greatly depending on the location, i.e. it can be significant in places with granitic rocks or with sediments whose origin is in the erosion of granitic rocks.
So some people, animals etc. may be exposed to more thorium/radon radiation than from other sources, especially if inhaling dust or radon gas, but for most the greatest exposure is from the content of potassium 40 and carbon 14 that is inside their bodies (which produce only weak beta radiation).
Any cell of any living being, including humans, contains potassium ions, which are required to neutralize the excess negative charge of proteins, otherwise the interior of the cells would become acidic and it would self-destroy.
No other positive ion can replace potassium, because any other abundant positive ion has a much greater tendency of forming solid precipitates, which would also destroy the cell. That is why any living cell expels the abundant ions of sodium and calcium outside it, while pumping inside any potassium ions from the environment. The moment when a living cell stops pumping potassium and magnesium inside and sodium and calcium outside, is when the cell dies.
Among the primordial chemical elements, which already existed at the formation of the Solar System, there are many which are weakly radioactive, i.e. they have some isotopes with half lives that are of at least many hundred million years, but of many billion years for most of them.
Among these weakly radioactive elements, the human body contains not only potassium 40, but also calcium 48, but the latter is much more weakly radioactive than potassium. Besides primordial radioactive isotopes, there are also radioactive isotopes that are formed continuously by the cosmic radiation, like carbon 14, which is also present in the body of any living being.
Among the primordial radioactive isotopes, the most radioactive are uranium 235 and potassium 40, followed by uranium 238, thorium 232 and platinum 190. The existence of weakly radioactive isotopes is not random, but it is determined by a set of rules of nuclear stability. For instance, potassium 40 is radioactive because any isotope with an even atomic mass of a chemical element with an odd atomic number and heavier than nitrogen is radioactive. Potassium 40 just happens to have an unusually long half life, so it has not decayed yet (the long half life is because potassium 40, like Buridan's ass, cannot decide whether it should decay into argon or into calcium, so it stays in limbo).
All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.
https://en.wikipedia.org/wiki/Doramad_Radioactive_Toothpaste
Naively, measuring that seems more difficult than the clock. Wouldn’t you need a perfect clock as a baseline? There must be something really clever here.
[1] https://www.toptica.com/products/optical-quantum-clocks/topt...
As a parallel, for a regular clock, you'd measure and compute the uncertainties of:
- The distance between the axis of rotation and the center of mass of the pendulum,
- The local acceleration of gravity.
And deduce the pendulum period and its uncertainty by calculus.
In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).
That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.
Even with the atoms of thorium contained in a small crystal, the frequency of their clock transitions should not be influenced by the environment.
Unfortunately, for now no other suitable nucleus has been found, because the frequency corresponding to nuclear energy levels is normally too high for the current or near future technology.
With thorium 229, the potential simplicity of the clock is counterbalanced by the great expense required for producing this artificial isotope.
the former brings (cesium) atoms, including their electrons, into resonance.
the latter brings (thorium) _nuclei_ into resonance.
both then "just like any clock" measure that oscillation. "simples" ;-)
and I learned "environment" creating drift is temperature, magnetism, anything that influences electrons.