Shedding Light on Nuclear Radiation Episode 8: Synthetic Radioisotope Production describes how particle accelerators (including linear particle accelerators and cyclotrons) and nuclear reactors are used to produce synthetic radioisotopes that have a wide variety of uses in medicine and in industry.
A 4-minute excerpt followed by a 1-minute trailer.
Contents:
Part A: Intro
Part B: Radioisotope Production by Particles Accelerators
Part C: Radioisotope Production by Nuclear Reactors
The Episode 8 Question Sheet for Students:
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Transcript (more or less)
Part A: Introduction
Hi everyone and welcome. In this video, we’re going to look at how humans produce synthetic radioisotopes.
Cobalt-60, for example, a beta-minus and gamma emitter used to sterilize medical equipment, does not exist in nature, but it can made using cobalt-59.
Fluorine-18, a beta-plus emitter, does exist in tiny tiny amounts in nature thanks to cosmic rays, but the fluorine-18 used to make chemicals to use in PET scans is made artificially using naturally occurring oxygen-18 and it then decays back into oxygen-18.
Synthetic radioisotopes are produced mostly either by what are called particle accelerators or by nuclear reactors. Let’s take a look.
Part B: Radioisotope Production by Particles Accelerators
Fluorine-18, the beta-plus-emitting radioactive tracer used to generate PET scans, is made artificially by a machine called a particle accelerator. The particle accelerator accelerates protons to really high speeds and they are then made to smash into oxygen-18 atoms.
The high-speed protons stick to the oxygen-18 nuclei and knock out a neutron. The nuclear equation is… an oxygen-18 nucleus plus a proton produces a fluorine-18 nucleus plus a neutron.
This type of nuclear reaction is often called a (p,n) reaction, and they’re actually fairly common. The equation is often written like this: (see image above)
An oxygen-18 is struck by a proton, which knocks out a neutron and a fluorine-18 atom is left behind. You can also get (n,p) reactions, (
,p) reactions and other combinations.
Particle accelerators use electric and magnetic fields to accelerate protons and other charged particles. One type of particle accelerator is called a linear particle accelerator or more simply a linear accelerator.
The basic principle of its operation is that if you connect some kind of voltage source to two metal plates, one will become positively charged and the other one will become negatively charged. If you then place a negatively charged particle like an electron in between, it will experience a force of attraction towards the positive plate and a force of repulsion away from the negative plate. As a result, it will accelerate towards the positive plate. If you place a positively charged particle, like a proton, in between the plates, the proton will be repulsed by the positive plate and attracted to the negative plate, and it will therefore accelerate towards the negative plate. When charged, the plates are called electrodes. This is the beginning of a very simple particle accelerator.
So, let’s look at the basics of how an actual linear accelerator works. There’s a chamber called the ion source at one end that contains the particles that you want to accelerate, let’s use protons as an example, and next to it is a series of electrodes in the shape of hollow tubes. Their job is to accelerate the particles to higher and higher speeds. At the end is the target.
The ion source and the hollow electrodes are attached to an alternating voltage source that continuously changes their charge: positive, negative, positive, negative and so on. So let me slow things right down.
When the voltage source is like this, these electrodes are positive (note that they’re connected to the positive side of the voltage source) and these ones are negative, because they’re all connected to the negative side of the voltage source. Note the little… let’s call it a bridge which means that the wires are not touching. When the voltage source flips its voltage, the charge on each electrode flips as well.
Initially the charge on the electrodes is like this. The negative charge on the first electrode attracts the proton towards it and the positive charge of the ion source repels the proton away from it. This accelerates the proton.
Then at just the right moment, the charges on the electrodes are reversed so the proton continues to experience a force towards the right. By alternating the charge on the electrodes, the proton gets faster and faster until it finally smashes into the target.
In reality large numbers of charged particles are all accelerated at the same time not just one a time like I’m showing here. Linear accelerators can range in length from half a metre right up to several kilometres. The tubes get longer and longer because of the fact that the charged particles get faster and faster.
But how are the protons produced in the first place?
Well, the ion source, let me enlarge it, contains an electrical device that produces a shower of electrons, not unlike a simple spark generator. Hydrogen atoms are injected into the ion source chamber through a small tube. Of course, hydrogen atoms are always bonded to other atoms, but let’s just keep the animation simple. When the electron shower is switched on, the electrons knock the hydrogen atom’s electrons off the hydrogen atoms, leaving just the protons behind. Compared to everything else, creating ions is relatively simple. The protons are then accelerated as we saw before.
Another type of particle accelerator like the one behind me, is called a cyclotron. Cyclotrons use electric and magnetic fields to accelerate charged particles. To understand the basics of how they work, we need to know two things, one of which you already know.
Firstly, we already know that charged electrodes can create an electric field which can accelerate charged particles, as we’ve just seen.
Secondly, we also need to know that magnetic fields can deflect charged particles.
This vacuum tube attached to a high voltage power source is producing a beam of electrons from this metal plate called the cathode. When the electrons hit the fluorescent screen, light is produced so we can see their path. Now If I hold a magnet near the beam of electrons, it’s obvious that the electrons are deflected. Magnetic fields deflect moving charged particles.
So, if we get a flat magnet with its NORTH facing upwards and then place another magnet above it with its SOUTH facing downwards (I’ve drawn it slightly see through, so that we can see the S for South on the underside of the magnet), we create a magnetic field between the magnets.
Magnetic fields are invisible but the iron filings sprinkled in the magnetic field that surrounds these two magnets make it possible to see where the magnetic field is and its shape. By the way, there’s a clear sheet of plastic above the magnets to stop the iron filings actually touching the magnets. Notice the straight lines that are directly between the north of one magnet and the south of the other. Cyclotrons make use of the even magnetic field that exists directly between two magnets.
If a charged particle moves into this magnetic field, it will be deflected into a circular path. It won’t get any faster or slower, it will just be deflected. As I said magnetic fields deflect moving charged particles. The amount of deflection depends on the mass of the particle and on how strong the magnetic field is.
So let’s look at how cyclotrons combine electric and magnetic fields to accelerate charged particles.
A cyclotron accelerates charged particles using two hollow D-shaped electrodes that are placed between the two powerful magnets, which, by the way, are electromagnets, not permanent magnets. The dees, as they’re called, are kind of like a hamburger bun cut in half and with the middle section of bread scooped out. Cyclotrons vary in size but a diameter of about 2 or 3 metres is fairly typical.
Now let me show you the dees from the top without the electromagnets in the way. I’ve chopped a little bit off this dee just to help with the animation that you’re about to see.
They’re attached to an alternating voltage source that makes them either positive or negative. Near the centre of the dees, there’s an ion source which I won’t show from now on. When a proton comes out of the ion source, it will accelerate towards the negative dee on the right and speed up a little. Then, as a result of the magnetic field (which I’m not showing), it will follow a curved path inside the dee and move back towards the opening of the dee.
Now while the proton is inside the dee on the right, that dee (on the right) becomes positive and the dee on the left becomes negative. So, when the proton reaches the gap, it accelerates towards the left. It then enters the left-side dee and again follows a curved path. However, while inside the left-side dee, that dee becomes positive and the right-side dee becomes negative, so the proton accelerates across the gap again and gets even faster.
So if I start from the start, the proton gets faster, faster, faster, faster, and faster, before finally exiting the dees at a really high speed and then colliding with the target. The changing electric field accelerates the proton when it’s in the gap between the dees, while the magnetic field changes the proton’s direction so that it can swing around to get a boost in speed every time it crosses the gap. Obviously the timing of the reversals in charge AND the strength of the magnetic field have to be very carefully controlled.
It is in fact cyclotrons that are used to produce fluorine-18 atoms for use in PET scans. As I said earlier, a proton smashes into an oxygen-18 atom and a neutron is knocked out, leaving behind an unstable fluorine-18 atom. In this case, a single neutron is knocked out, but in other nuclear reactions, various combinations and numbers of neutrons and/or protons can be knocked out.
The oxygen-18 atoms that make up the target when fluorine-18 is being made are not present in the form of oxygen gas, but rather in the form of water. Water is much more dense than oxygen gas, so there’s much more chance of collisions occurring. However, to distinguish it from normal water, the formula can be written like this: H218O. H subscript 2, two H atoms, superscript 18 O, indicating the oxygen isotope oxygen-18. This type of water is called oxygen-18-enriched water. Chemically, it’s the same as normal water, but it’s heavier per gram than normal water.
Now oxygen-18 atoms make up only about 0.2% of all the oxygen atoms on Earth, so oxygen-18-enriched water is very difficult to produce. Per gram, it actually costs more than twice what gold costs.
This list shows just a small number of examples of radioisotopes that are produced by cyclotrons. There are many more and research into new radionuclides is ongoing. There are actually more than a thousand cyclotrons all around the world and the production of radioisotopes using cyclotrons is a huge industry that employs tens of thousands of people.
So, synthetic radioisotopes can be produced by particle accelerators. They can also be produced using nuclear reactors. Let’s take a look.
Part C: Radioisotope Production by Nuclear Reactors
So, protons accelerated in particle accelerators can be used to produce artificial radioisotopes. But neutrons can also be used to produce artificial radioisotopes. Let’s look at an example.
We saw in Episode 3 of this series that cobalt-60 is a beta-minus and gamma-ray emitter which is used to sterilize medical equipment. Cobalt-60 doesn’t exist in nature but is produced artificially by irradiating cobalt-59 with neutrons.
If a neutron crashes into a cobalt-59 nucleus, it sticks to it and cobalt-60 is produced. The nuclear equation is pretty simple! The stable cobalt-59 nucleus becomes an unstable cobalt-60 nucleus. That one extra neutron changes everything. The cobalt-60 then decays with a half life of 5.3 years into stable nickel-60 by releasing a beta-minus particle and not one but two gamma rays. It’s the gamma rays that kill the germs in the sterilization plant. I haven’t actually mentioned that cobalt-60 releases two gamma rays until now, because I wanted to keep it simple. I think we’re now well past simple.
But, how are neutrons produced? Well, neutrons are produced in nuclear reactions.
The cheapest and easiest way to produce the large numbers of neutrons required to make a decent amount of cobalt-60 (and many other radionuclides) is to use nuclear reactors inside nuclear power stations. So, we need to look at the basics of how nuclear power stations work.
Let’s start by looking at a coal-fired power station, just because it’s a little easier. Coal is burned in a boiler and the water in the boiler turns to steam. Because of the large pressure involved, the steam passes through pipes and spins turbines which then spin the generator, which produces electricity. Nuclear power stations are more complicated but they basically work the same way. However, instead of burning coal to produce heat to produce steam, they use uranium-235 or plutonium-239 to produce heat to produce steam. The heat is generated inside the nuclear reactor.
So, how do nuclear fuels produce heat and what’s this got to do with producing cobalt-60?
Well, using uranium-235 as an example, if a neutron crashes into a U-235 nucleus, it undergoes nuclear fission, that is, the nucleus splits into two separate nuclei (typically in about a 60:40 % ratio). The word “fission” means splitting apart. The fission process releases a huge amount of heat and this heat is used to generate the steam that turns the turbines. Here are a few examples of nuclear equations for the fission reactions. A very wide variety of fission fragments is produced.
Now very importantly, the fission process typically releases 2 or 3 neutrons.
These neutrons can crash into other U-235 atoms and cause them to fission as well. A self-sustaining chain reaction can occur if the reactor has the right configuration and concentration of U-235 atoms.
Nuclear reactors are often built with irradiation chambers inside them to allow for the production of synthetic radioisotopes using the free neutrons that are released when U-235 atoms undergo fission.
The irradiation chambers are designed to be irradiated by large numbers of neutrons coming from the reactor core where the U-235 is.
So, if cobalt-59, for example, is placed into the chamber, the cobalt-59 nuclei absorb neutrons and turn into cobalt-60 nuclei. The process takes months. The cobalt-60 atoms are then removed and transported to facilities around the world that need them, like, for example, this plant that uses the gamma rays that cobalt-60 atoms produce to sterilize medical equipment.
Interestingly, many nuclear reactors around the world are made specifically to produce radionuclides, rather than electricity! These nuclear reactors are relatively small.
Other examples of synthetic radioisotopes produced by neutron irradiation are gold-198 and sodium-24.
Gold-198 is produced by irradiating stable gold-197 with neutrons. Gold-198 is a beta-minus and gamma emitter with a half-life of 2.7 days. Among other uses, it’s used as a radiotracer to study the movement of water in sewage systems, sewage treatment plants, and in the ocean once the treated water has been released into it. Remember, radioactive substances are fairly easy to track using either Geiger counters, gamma-ray cameras, or certain other detectors.
Sodium-24 is produced by irradiating sodium-23 with neutrons. It has a half-life of 14.9 hours. It’s used, among other things, to test for leaks in oil pipelines. If any oil leaks out of the pipes, the sodium-24 also leaks out of course and the radiation it produces as it builds up near the leak can be detected.
Now some useful radioisotopes are produced in the fission process itself.
A prime example of this is molybdenum-99. It’s extracted from the fuel rods in the nuclear reactor itself because it’s a fairly common product of the U-235 fission process.
Molybdenum-99 is used to generate what’s called technetium-99m, which is used as a radioactive tracer for diagnostic imaging. Technetium-99m is quite an unusual radioactive substance.
Most substances that are beta-minus and gamma emitters, like cobalt-60, emit the two forms of radiation pretty much at the same time.
However, when molybdenum-99 emits a beta-minus particle, the resulting technetium-99 nucleus has an excess of energy and remains in what’s called a metastable state and so it’s called technetium-99m; m for metastable. The emission of the gamma ray is delayed. With a half-life of 6 hours, a sample of technetium-99m decays to technetium-99 (no “m”) by emitting a gamma ray. As I said, this is a little unusual, because gamma rays are not usually emitted just on their own, but thanks to the delay, technetium-99m can be used as a radioactive tracer that doesn’t emit any beta-minus particles. This makes it a little safer because patients receive a smaller dose of
radiation. Technetium-99m is in fact the most commonly used diagnostic radioactive tracer in the world!
There are about 50 or so radioisotopes produced in nuclear reactors that have uses in medicine and in industry. This is just a small selection of examples.
So, over the past two episodes, we’ve seen that radionuclides can be produced naturally or artificially. All these radionuclides expose us to radiation like alpha, beta, and gamma radiation. But how much radiation do we receive from natural sources and how much do we receive if we have to, for example, have a PET scan. Well, the concept of radiation dose is what we’re going to look at in our next episode. See you then.
Credits:
Written and directed by Spiro Liacos
Additional footage and images:
https://commons.wikimedia.org/wiki/File:Cyclotron_ARRONAX.jpg by Popocatomar. Creative Commons 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
https://commons.wikimedia.org/wiki/File:KEK_Electron-Positron_Linac_P9024765.jpg by Kestrel. Creative Commons License.
https://commons.wikimedia.org/wiki/File:CERN_Linac.jpg by Florian Hirzinger. Creative Commons License.
https://commons.wikimedia.org/wiki/File:TRR1-M1-Reactor-TINT.jpg by Myesd. Creative Commons License.
Cherenkov Radiation in 60 seconds by IAEAvideo. https://www.youtube.com/watch?v=4hijBTgrvjY&ab_channel=IAEAvideo. Creative Commons License.
Nuclear Power in the 21st Century by IAEAvideo.
https://www.youtube.com/watch?v=kwn8rAYgZVw&t=232s&ab_channel=IAEAvideo. Creative Commons License.