How is a radionuclide formed?

Answer 1

Radionuclides (or sometimes, radioisotopes) are unstable atomic nuclei, and they spontaneously "fall apart" by various means because of their unstable nature. These unstable atoms can be found as isotopes of any element we care to name. All the elements have unstable "versions" of themselves, that is, they have one or more unstable isotopes, which are radionuclides. But where do they come from?

Certainly we can make them in the nuclear physics lab, and we apply the nuclear reactor, the cyclotron, or other means to create whatever it is we wish to use for a specific purpose or to investigate and do research on. Nuclear medicine and radiation biophysics use a number of different radionuclides in radiation therapy, biological tracing and other applications. And physicists have different needs for radionuclides to investigate their different properties. Industry has needs for radionuclides, too. Just one example is the creation of 60cobalt for use as an X-ray source for X-ray analysis of materials, or for sterilization (by radiation) of goods or products. To supply the needs of these groups, the nuclear industry creates a smorgasbord of radionuclides by various means. Background radiation is higher today owing to nuclear bomb tests, reactor accidents and other releases of radioactive materials that were created by man. Nature, too, creates radionuclides.

Unstable atomic nuclei are formed by stars. Stars operate as big fusion reactors with the huge force of their fusion trying to "blast" everything outward and their giant gravity trying to pull everything in all operating in an equilibrium. Stars are fusing smaller nuclei into larger ones all the time. But stars can't make nuclei heavier than iron during the course of "regular" stellar nucleosynthesis. The trans-iron elements are created in a supernova, when a star of sufficient magnitude has exhausted its nuclear fuel and then collapses to set off the spectular blast that we wonder at. All the naturally occurring trans-iron elements through uranium are created by this mighty crushing event, and they're distributed out into the galaxy by the following blast. Among the elements created are a variety of isotopes that are unstable. In fact, bismuth and all the elements heavier than it have no stable isotopes. The radioisotopes decay according to their nature (there are several mechanisms), and often we see a radioactive product (called a daughter) created by the decay process.

We have seen radioactive isotopes formed by stars, and now we look at the fact that they often beget radioactive daughters when they break down. That's another way radionuclides are created. Radioactive decay continues until a stable isotope is created. The times that the decay event takes to occur vary as the isotope, and are called half-lives. No single unstable nucleus can be said to take a given amount of time to decay; there is no way to predict when a specific atom of a radionuclide will decay. So we talk about an "average" time it takes for the decay of a radionuclide to occur, and it's a function derived statistically. We look at a lot of atoms and figure out how long it takes for half of them to decay, and run with that time for a half-life. To repeat, the radioactive decay continues until a stable isotope appears as a daughter. Radioactive decay, in addition to creating a daughter (which may or may not be radioactive) produces ionizing radiation. If it hasn't been obvious, we live in a radioactive universe where there is radiation all over the place. And that's another source of radionuclides.

When ionizing radiation like cosmic rays slams into the upper atmosphere of the earth, some nitrogen atoms there end up getting transformed into 14carbon atoms. You may recognize this isotope as the one that is used in radio-carbon dating. Other radiation, both aloft and on earth, can create other unstable isotopes of an element, and now we've discovered a third way that radionuclides can be naturally created.

Wikipedia has a post on radionuclides, and a link to that post can be found below.