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Nuclear radiation can be incredibly dangerous, but it can also be incredibly useful to us. The Shedding Light on Nuclear Radiation series teaches students what nuclear radiation is and how humans have harnessed its awesome power.
Where do all the naturally occurring radioisotopes that exist on Earth come from? How were they formed? And do they have any practical uses? Shedding Light on Nuclear Radiation Episode 7: Natural Radioisotope Production brilliantly answers these questions. It begins by describing how cosmic rays produce a wide variety of radioisotopes on Earth. It then explains the big bang theory, nuclear fusion, what supernovas are and how they create new atoms, how stars and planets form, decay chains, and rock dating. This video takes us on a journey from a single proton to the whole universe!
Contents:
Part A: Radioisotope Production by Cosmic Rays.
Part B: Radioisotope Production by Supernova Explosions (including the big bang theory, nuclear fusion, supernovas, star and planet formation, decay chains, and rock dating).
The Episode 8 Question Sheet for Students:
The PDF version.
Google The Google Doc version. Google
Get the answers.
Transcript (more or less)
PART A: Radioisotope Production by Cosmic Rays
Hi everyone. Welcome to another episode of the Shedding Light on Nuclear Radiation series. In this episode, we’re going to look at where all the naturally occurring radioisotopes that exist on Earth come from.
We already looked at one example in our last episode: carbon-14.
Carbon-14 is made thanks to the action of cosmic rays. The Sun and other stars are gigantic balls of extremely hot hydrogen and helium. Every second, millions of tons of hydrogen nuclei (that is, protons), helium nuclei, and electrons are flung out of stars at extremely high speeds. These are called cosmic rays.
When a high-speed cosmic-ray proton crashes into an oxygen-16 atom’s nucleus in the atmosphere (which is a fairly common occurrence since oxygen-16 atoms make up about 21% of the Earth’s atmosphere), the nucleus breaks apart into smaller fragments,
which nearly always include at least one neutron.
When a neutron then slams into the nucleus of a nitrogen-14 atom, it results in the formation of an unstable carbon-14 atom.
Carbon-14 is just one radionuclide produced by cosmic rays. There are plenty more and they’re actually called cosmogenic nuclides. One that’s of particular interest is beryllium-10.
We’ve seen that cosmic ray protons can crash into oxygen atoms and neutrons are typically released (among the other fragments).
If a neutron from this original collision then goes on to hit another oxygen-16 atom, beryllium-10 is a fairly common product. This nuclear reaction is just one example of how beryllium-10 is generated. Beryllium-10 is unstable. It has a half-life of about 1.4 million years (1.39 × 106 years) and decays via beta minus emission to boron-10.
Its fairly long half-life allows it to be used in the study of erosion and other geological processes, similar to the way radiocarbon dating is used to examine ancient artefacts.
Beryllium-10 produced in the atmosphere ends up coming down to the Earth’s surface in the rain or snow. If this happens in extremely cold climates, where the water freezes into ice, the beryllium-10 gets trapped in the ice. The ice stays where it is and then, over time, gets covered by more snow and ice.
Scientists often drill into the ice in places like Antarctica where they extract what are called ice cores. Generally speaking, the deeper the ice, the older it is. The ice contains trapped atmospheric gases which can be analyzed to see how the ratio of different gases has changed over time. They can also examine pollen counts, volcanic ash and other things. By also examining how much beryllium-10 there is in the ice core, scientists can estimate how long ago the ice formed.
If the section of the ice core has a similar amount of beryllium-10 as you would find on the Earth’s surface today, it means that the ice was formed fairly recently. If though there’s less beryllium-10 than the normal amount, it means that some of the original beryllium-10 has decayed. As I said, generally, the deeper down you go in the ice the older it is. Scientists can use data like that shown on this graph to estimate how long ago the ice formed.
The data can be used to work out Earth’s climate history, that is, when we had colder periods like ice ages and warmer periods where plants and animals flourished.
Carbon-14 and beryllium-10 are just two radioactive substances produced by cosmic rays. There are plenty more (this list shows just a few examples) and they all contribute to the natural background radiation that we all receive. Most are created in the atmosphere, but small amounts are also produced on the surface of the Earth since a small number of cosmic rays make it all the way through the atmosphere. Many of these cosmogenic radioisotopes can be used to date things like rocks or to study water flow in underground lakes and rivers.
Now collisions of cosmic rays with atoms on Earth are not the only way the Earth’s radioisotopes are produced. Many radioisotopes are produced by stars. So how are they produced, and how did they get here? Let’s take a look.
Part B: Radioisotope Production by Supernova Explosions
So, we’ve seen that some of Earth’s radioisotopes are produced by cosmic rays. But where do the rest come from? Well, they’re made by stars! In fact, most of the atoms on Earth, both stable and unstable, are made by stars. So how do stars make atoms and how did some of those atoms end up here? Well, to provide explanations, we need to merge the study of nuclear physics with astrophysics, which is the physics of stars. The two often go together.
Back in the 1920s and 1930s, it was discovered that most galaxies are getting further and further away from each other.
So, if everything’s getting further apart, then if you go back everything was closer together. After studying the light from the farthest reaches of the universe and by making calculations based on our knowledge of atoms, scientists suggested that about 13 billion years ago, everything was contained in a very small space, which suddenly started expanding. They called this event the Big Bang.
Though there’s still a lot we don’t know, the best mathematical models of the universe that we have suggest that only hydrogen and helium atoms were produced in the Big Bang, and maybe tiny amounts of lithium and beryllium.
These atoms that were formed eventually coalesced together under gravity to form stars. So, where did the rest of the elements come from? Well, stars made them.
Stars produce new nuclei in a process called nuclear fusion, where two smaller nuclei under intense heat and pressure join together (that is, they fuse together) to form larger nuclei.
In a typical star, including our sun, most of the energy being produced comes from the fusion of hydrogen nuclei into helium nuclei.
In one process, 2 hydrogen nuclei (that is, protons) collide and fuse together. The protons have to be moving at a high enough speed of course to overcome the force of repulsion by the positive charge that they both carry. Immediately after the collision, one of the protons turns into a neutron resulting in the production of a positron and a neutrino. The two protons have fused together into a single hydrogen-2 nucleus.
A hydrogen-2 nucleus created like this can then collide with another proton to form helium-3. A gamma ray is emitted in this step.
Finally, 2 helium-3 nuclei created in step 2 can collide and a helium-4 nucleus and 2 protons are produced which then fly apart at huge speeds. So, in stars, hydrogen nuclei end up fusing together to form helium nuclei and a huge amount of energy is also released. As I said nuclear fusion is the process by which smaller nuclei fuse together to form larger nuclei.
In similar kinds of collisions, small amounts of other, larger nuclei like carbon and oxygen nuclei are also formed.
The outwards pressure of all the fusion reactions counteracts the inwards pressure of gravity and so stars typically remain the same size for billions of years.
However, the largest nuclei that normally get produced in stars are iron nuclei. There’s usually just not enough heat and pressure inside stars to force more protons onto an iron nucleus. So how are larger nuclei formed?
Well, when really large stars (much larger than our sun), start running out of smaller nuclei to fuse together the outwards pressure of fusion reduces and they collapse in on themselves. This triggers a gigantic explosion called a supernova explosion.
Incredible amounts of light and heat are produced, but importantly to our topic today, larger atoms that are heavier than iron atoms are produced.
Vast quantities of all these new atoms plus the ones that were already there are flung out into space and form huge clouds of gas and dust.
Eventually, these huge clouds of gas and dust, rich in all the different elements, collapse in on themselves under the influence of gravity and new stars form. In this process, planets, like our own, also form.
So that’s basically a 4-minute summary of billions of years of earth’s history. I obviously haven’t gone into much detail. However, it’s fascinating to think that we are made in large part of atoms like for example carbon and oxygen atoms that were made by large stars billions of years ago.
You and I are made in large part of star dust!
Now amongst all the atoms that are created in and by stars, there are unstable atoms. Most of them have short half-lives and so they don’t last long out in space before decaying into other atoms. However, some radionuclides created by stars have very long half-lives and these are the ones that have ended up on our planet.
For example, three of them with long half-lives that are still with us from way back then are uranium-238 (that has a half-life of 4.5 billion years), uranium-235 (that has a half-life of 700 million years), and thorium-232 (that has a half-life of 14 billion years). The Earth started forming about 4.5 billion years ago so there are still plenty of these particular radionuclides around. They’re of particular interest because they only turn into stable atoms after they’ve gone through a series of quite a few decays.
So, let’s look at what’s called the decay chain of thorium-232. Thorium makes up about 0.001% of the earth’s crust, obviously a very small number, but it’s about 3 times as abundant in the Earth’s crust as uranium is.
Thorium-232 has 90 protons and a total of 232 nucleons (that is, protons and neutrons) and it has a half-life of 14 billion years. It decays by emitting an alpha particle and so it loses 2 protons and 2 neutrons (which is 4 nucleons). This leaves behind radium-228, which is also radioactive. With a relatively short half-life of 5.7 years, radium-228 decays into unstable actinium-228 via beta-minus emission, which then decays into thorium-228, followed by radium-224, radon-220, polonium-216, lead-212, bismuth-212, polonium-212, and finally into lead-208, which is stable. Just to complicate things, bismuth-212 actually only decays via beta-minus emission into polonium-212 64% of the time. 36% of the time it decays via alpha emission into thallium-208, which then decays into stable lead-208.
So, all of these radionuclides exist on Earth naturally.
Thorium-232 was created by stars and the rest, which have relatively short half-lives, were generated and are continuously being generated thanks to radioactive decay.
Here we can see the decay chains of uranium-235 and uranium-238. The layout is obviously different to the graph-like layout that we saw just before. Both of these uranium isotopes, like thorium-232, produce a whole series of radionuclides. Eventually, lead-206 and lead-207 are produced which are stable, and so the decay chains stop.
There are about 35 radionuclides on Earth that were created by stars and which have such long half lives that they’re still with us today. They’re called primordial radionuclides. Their decay, and the decay of all the daughter radionuclides, is in fact what causes the Earth’s interior to be so hot. The three biggest contributors to the heat generated on Earth by radioactive decay are thorium-232, uranium-238, and potassium-40.
They’re also responsible for a lot of the natural background radiation that we are exposed to on Earth.
It turns out that neptunium-237 was also present when the Earth formed. However, there’s basically none left now. This is because it has a half-life of only about 2 million years. I say “only” because that’s only a short amount of time compared to the age of the 4.5-billion-year-old Earth. However, even though the neptunium-237 is all gone, some of the radionuclides in its decay chain are still going.
Now in our last episode, we looked at radiocarbon dating.
A similar concept to radiocarbon dating is used to work out how old rocks are.
Uranium-238 atoms are pretty widespread in nature. In molten rocks, whether they’re underground or flowing on the surface as lava, the uranium atoms and all the other atoms mix together pretty well. This is also true of what are called sedimentary rocks that are laid down layer by layer in, for example, oceans and lakes.
However, once the rock solidifies, no new atoms can enter into the rock and no atoms present inside the rock can leave. Unstable atoms present in the rock when it solidifies are stuck there.
So, uranium-238, which as I said is very widespread in rocks although only in tiny tiny amounts, decays into thorium-234, which then decays into protactinium-234, and so on until lead-206 is produced. Basically, cracking the rock open, not by smashing it onto another rock, but in a proper lab of course, and then analyzing how much of each radioisotope is inside, scientists can work out how old the rock is, that is they can work out how much time has passed since the rock first solidified. This technique can date rocks and fossils that are millions of years old.
So, in this video, we’ve seen where some naturally occurring radioisotopes come from: basically, from the action of cosmic rays which stars produce or from inside the stars themselves.
Many of these radioisotopes then also produce more radioisotopes in what are called decay chains.
But cobalt-60 which we saw in Episode 3 is used to sterilize medical equipment and fluorine-18 which we saw in Episode 4 is used to generate PET scans, are not produced naturally. They’re produced artificially.
In our next episode, we’re going to look at how scientists create radionuclides artificially. See you then.
CREDITS:
Thanks to
the National Archaeological Museum, Athens Greece, https://www.namuseum.gr/en/
Diros Caves, Vlixada, Greece, https://www.diros-caves.gr/
NASA
Scientists drill deep in Antarctic ice for clues to climate change by UW (University of Washington)
https://youtu.be/se-BRvZuu7k?si=uNn7tPMOp5auzcJ_ Creative Commons License
How ancient ice cores show ‘black swan’ events in history – even pandemics by The Conversation
https://youtu.be/ZHOdqb9ViLw?si=5zB9y23CQVWACY-Y Creative Commons License
Decay chain(4n+2, Uranium series).svg by User:Tosaka
https://commons.wikimedia.org/wiki/File:Decay_chain(4n%2B2,_Uranium_series).svg Creative Commons License
.svg){kind=link}
Decay Chain of Actinium.svg by User:Tosaka
https://commons.wikimedia.org/wiki/File:Decay_Chain_of_Actinium.svg Creative Commons License
File:McLaughlin Planetarium Star Theatre from Cove.jpg by Brucewaters
https://commons.wikimedia.org/wiki/File:McLaughlin_Planetarium_Star_Theatre_from_Cove.jpg Creative Common License.
Thorium-2.jpg by W. Oelen
https://commons.wikimedia.org/wiki/File:Thorium-2.jpg Creative Common License.
Footage of Gamma Irradiation © BGS Beta-Gamma-Service. Used with Permission. See “BGS Beta-Gamma-Service _ Using gamma rays to destroy germs_ How radiation sterilization works” by BGS Beta Gamma Service. https://youtu.be/hblMTH09KJQ