Primordial nuclide

Primordial nuclides are the ancient building blocks of our world. In the fields of geochemistry, geophysics, and nuclear physics, these terms describe specific bits of matter. These nuclides have existed in their current form since before the Earth was even formed. They were present in the interstellar medium before our Solar System began. They originated from many different cosmic events. Some were created during the Big Bang. Others were formed through nucleosynthesis inside stars. Some were created during supernovae, which are massive star explosions. Other processes, like cosmic ray spallation, also contributed to their creation.

To understand these nuclides, we must look at their stability. A nuclide is stable if it does not change over time. There are 251 known stable nuclides. However, many primordial nuclides are radioactive, meaning they are unstable. These radioactive nuclides must have very long half-lives to survive since the formation of the Earth. A half-life is the time required for half of a sample to decay. For a nuclide to be detectable on Earth today, its half-life must be greater than about 100 million years. This is because the Earth is approximately 4.58 billion years old. If a half-life is too short, the nuclide will vanish. For example, a nuclide with a 60-million-year half-life would see its atoms disappear rapidly. If you started with one mole of such atoms, only four atoms would remain today.

There are 286 known primordial nuclides in total. This group includes the 251 stable nuclides and 35 radioactive ones. These 35 radioactive nuclides are isotopes of 28 separate elements. Some elements carry more than one radioactive primordial isotope. For instance, cadmium, tellurium, xenon, neodymium, samarium, osmium, and uranium each have two. Because many primordial elements are made of multiple isotopes, there are only 83 distinct primordial chemical elements. Of these, 80 elements have at least one stable isotope. Three elements, bismuth, thorium, and uranium, consist only of radioactive primordial isotopes.

Scientists categorize these nuclides based on how long they last. Some are so long-lived that they are effectively stable. For example, 87Rb, 187Re, 176Lu, 232Th, and 238U have half-lives long enough that their decay is limited over geological time. Other nuclides have much shorter half-lives but are still long-lived enough to be present in significant amounts. These include 40K and 235U. On the other end of the spectrum are extinct radionuclides. These are nuclides that were present in the primordial solar nebula but have since decayed away completely. Niobium-92 is an example of an extinct radionuclide. Other examples include 146Sm and 244Pu, which should theoretically still exist in the Earth. However, they are very difficult to identify in the Earth's crust through human assays.

Not every naturally occurring nuclide is primordial. Some isotopes are created through constant regeneration. These are called cosmogenic nuclides if they are made by cosmic radiation. Examples include beryllium-10 and aluminum-26. Others are formed by geonuclear transmutation, such as when neutrons are captured by uranium. Some nuclides are radiogenic daughters. These are new bits created from the decay of older ones. For example, the stable argon isotope 40Ar is often radiogenic. It forms from the electron capture decay of the primordial isotope 40K. This process has been generating argon since the early days of Earth.

We also see geogenic nuclides in the Earth's crust. These are decay or fission products of uranium or other actinides in subsurface rocks. These nuclides have shorter half-lives than their parent primordial nuclides. Some geogenic nuclides come from the spontaneous fission of long-lived nuclides like 232Th, 235U, and 238U. An example is 126Sn, which makes up about 28 percent of all natural tin. Another example is 99Tc. These processes help scientists trace the history of rocks and the planet itself.

Understanding primordial nuclides connects us to the very beginning of the universe. By studying the ratio of stable to radioactive isotopes, researchers can learn about the age of the Earth. They can also understand the processes that occurred in the stars before our sun existed. Even though some isotopes, like those of tungsten, are theoretically radioactive, their half-lives are so long they appear stable. This deep connection between nuclear physics and geology allows us to map the timeline of our cosmic history.