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.
In Shedding Light on Nuclear Radiation Episode 4: Beta-Plus Decay, we introduce students to beta-plus particles, also known as positrons. Beta-plus particles are a form of anti-matter. Though beta-plus particles don’t really exist in huge quantities naturally on Earth, scientists have learned how to actually make atoms that emit them! Beta-plus particles have now become a major tool for doctors to diagnose a wide variety of cancers.
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Contents:
Part A: Introduction
Part B: Positrons
Part C: Positron Emission Tomography (PET Scans)
Transcript (more or less)
Part A: Introduction
Hi everyone. In this video, we’re going to look at what’s called beta-plus decay which is also called positron emission. We’ll look at what it is, write some nuclear equations, and learn how beta-plus particles are used in medical imaging. Let’s begin.
Part B: Positrons
We have already looked at alpha, beta-minus, and gamma radiation.
In beta-minus decay a neutron spontaneously converts into a proton and an electron, and the electron is emitted at a really high speed.
However, there is another type of beta decay called beta-plus decay where a proton spontaneously converts into a neutron and an electron that has a positive charge (yes, you heard that correctly; an electron that has a positive charge).
So, let’s look at an example. Fluorine-18 is a beta-plus emitter. In its nucleus it has 9 protons and 9 neutrons. Most fluorine atoms on Earth are stable fluorine-19 atoms, with 9 protons and 10 neutrons. The fact that fluorine-18 is 1 neutron short makes it unstable. Let me double it up. In beta-plus decay, one of the protons converts into a neutron and a positively charged electron which is emitted at a really high speed. This positively charged electron is called a positron or a beta-plus particle, which is often written as 
+ or e+, e for electron. It is a form of what scientists call antimatter; it is identical to an electron (a normal electron) in every respect except that it has a positive charge instead of a negative charge. So, a positron can also be called an antielectron. Beta-plus decay is also called positron emission.
Let’s write the nuclear equation. We start with a fluorine-18 atom. In atomic notation, this is written as 18
9F
. One of the protons turns into a neutron and a beta-plus particle. The beta-plus particle can be written in atomic notation as 0
1 or as 0
1e. The “1” (positive 1) refers to the positive charge that the beta-plus particle has. The “0” means that the combined number of protons and neutrons is zero. So, it’s a positively charged particle that is not made of protons or neutrons; it’s a positron. Let’s add it to the equation. Finally, let’s work out what the daughter nucleus is. The daughter nucleus has one less proton than the parent nucleus, since one of the protons turned into a neutron, and so it has an atomic number of 8. This makes it the nucleus of an oxygen atom. Let’s fill it in.
18
9F
—> 18
8O
+ 0
1
The mass number doesn’t change since the combined total number of protons and neutrons is the same, so it remains 18. And there’s the whole equation. Fluorine-18 undergoes beta-plus decay and turns into oxygen-18. Notice that 9 = 8 + 1 and that 18 = 18 + 0.
Beta-plus decay typically occurs in nuclei that have too many protons or I suppose not enough neutrons, depending on which way you want to look at it.
Beta-plus decay occurs only very rarely in naturally-occurring radionuclides on Earth. One example is potassium-40. There are three naturally occurring isotopes of potassium on earth which together make up about 2.7% of the weight of the Earth’s crust, so potassium atoms are very common in the most rocks.
Potassium is never found naturally in its elemental state. It’s only found in various compounds that make up the various minerals that form rocks. I can list the three potassium isotopes that make up natural potassium. Potassium-39 and -41, which make up the vast majority of potassium atoms on Earth are stable, but potassium-40, which makes up only a tiny percentage of potassium, is unstable. It usually decays by beta-minus emission into calcium-40, but one in every 100,000 decays is a beta-plus decay which results in the formation of argon-40.
However, beta-plus emitters can be made by smashing the nuclei of various atoms together resulting in the fusion of two nucleuses into one. This is how fluorine-18 is made.
The first beta-plus emitter that was ever made in a lab, phosphorus-30, was made in 1934 by Frédéric and Irène Joliot-Curie by directing a stream of alpha particles towards an aluminium target. The alpha particles came from polonium which had been discovered by Irène’s parents Marie and Pierre Curie in 1898. What a family! So where did the phosphorus-30 come from? Well, whenever an alpha particle smashed directly into an aluminium-27 atom’s nucleus, a neutron was knocked out of it, but the rest of the nucleons fused together, resulting in the creation of a nucleus with 15 protons and 15 neutrons; this was phosphorus-30. They then extracted some of the phosphorus-30 and, using their equipment, the younger Curies then detected beta-plus particles being emitted from it. We can write a nuclear equation for what happened. An alpha particle crashes into an Al-27 nucleus, which sends a neutron on its merry way and a P-30 nucleus is formed.
4
2He
+ 27
13Al
—> 1
0n
+ 30
15P
Notice that 2 + 13 = 0 + 15 so all our protons and positive charges are accounted for, and that 4 + 27 = 1 + 30 so all our nucleons are accounted for as well.
As I said, phosphorus-30 is a beta-plus emitter. One of its protons turns into a neutron and a beta-plus particle (AKA a positron), which is emitted. The equation is 30 15 P produces 30 14 Si + 0 1 beta.
30
15P
—> 30
14Si
+ 0
1
The Joliot-Curies noticed that within about 2½ minutes the beta-particles they were detecting reduced by half. So, the phosphorus-30 didn’t last very long before it all turned into silicon.
Since 1934, scientists have learned to make lots of other radionuclides. Rather than just relying on alpha emitters, scientists now commonly use what are called particle accelerators to smash speeded-up protons or other light nuclei into other nuclei. We’ll talk more about that in a few lessons’ time.
So, what happens to a beta-plus particle after it’s emitted? Well, it doesn’t actually last very long. Within a tiny fraction of a second and typically within about a millimetre of where it was emitted, it crashes into an electron of a nearby random atom.
As soon as the positron hits the electron, the two completely annihilate one another and two high-energy gamma rays are produced. These gamma rays always travel in the opposite direction to each other. The two particles literally cease to exist, which is what annihilation literally means, but two gamma rays are created! The word annihilation is related to the word nil. The mass of the particles is converted into the energy of the gamma rays. Basically, in equation form, an electron + a positron produces 2 gamma rays.
The world down there at the atomic level is so weird. Positively charged electrons, that is, positrons, can be created seemingly out of nowhere but really out of the energy of the nucleus, and then when annihilation occurs two gamma rays are created out of the energy of the positron and the electron. Amazing.
Now in the 1950s, scientists discovered two other particles that are created and then emitted in a beta decay. One is the electron neutrino and the other is the electron antineutrino. How’s that for a set of names!
In beta-minus decay, not only is a newly created electron emitted, a particle called an electron antineutrino is also emitted. In beta-plus decay, not only is a newly created positron emitted, a particle called an electron neutrino is also emitted. I’m going to call both particles neutrinos for short.
Some 70 years after they were first discovered, we still don’t know a whole lot about them, and they may in fact be the same particle.
We know that they have no charge (that is, they’re neutral) and we know that they have a tiny mass, something like a millionth of the mass of an electron, but that’s only an estimate.
Neutrinos barely interact with ordinary matter and they have an extraordinary penetrating ability. Right now, the sun is creating huge numbers of them. This piece of paper has a square printed on it with a side length of 1 cm, in other words, it has an area of 1 square cm. If I hold the paper up to the sun, there are 65 billion neutrinos passing through that square per second.
The vast majority pass straight through the Earth without interacting with any atoms and then continue out the other side, travelling at like 99.9999 something percent of the speed of light.
The universe is amazing and there’s still a lot about it that we don’t know. Since neutrinos rarely ever interact with atoms once they’re produced, I’m not going to mention them again, so let’s get back to beta-plus emitters.
Beta-plus emitters and the gamma rays that are produced can be used in a fantastic medical imaging technique called Positron Emission Tomography or PET for short. Let’s take a look at it.
Part C: Positron Emission Tomography (PET Scans)
Scans taken using Positron Emission Tomography, PET Scans for short, are mainly used to detect the location of the tumours that cancer cells form. Knowing the size and location of the tumours allows doctors to treat the cancer more effectively.
Now X-rays are good at imaging bones and so doctors can see exactly where a fracture has occurred for example, but X-rays go straight through cancer cells (and normal cells) and so imaging cancer cells and tumours can’t be done very well with X-rays.
So how do PET scans detect the location of tumours? Well, there’s a few things we need to know.
Firstly, cancer cells are cells that are rapidly reproducing out of control at a much faster rate than ordinary cells reproduce. The cancer cells often form tumours which affect the ability of the organ to function.
Now because cancer cells are reproducing and growing at a faster rate than other cells, they usually absorb more nutrients like, for example, glucose, than other cells do. This is the key to understanding how PET scans work. When we eat carbohydrate-rich foods like bread, rice, pasta, or fruit our digestive system breaks down the carbohydrates mostly into glucose molecules.
The glucose enters our blood and is then carried to all the cells of our body. Glucose is basically our main fuel source.
However, as I said, cancer cells absorb more glucose than other cells, thanks to their huge growth and reproduction rate.
By tracking where there’s an unusually high concentration of glucose in the body, they can locate the tumour. This scan shows a number of tumours that have formed which need to be treated. But how do they track the glucose? Well, what’s called a radioactive tracer is used.
This is an ordinary glucose molecule. Its chemical formula is C6H12O6. To determine where it goes in the body, scientists chemically attach a radioactive tracer to it. A very commonly used radioactive tracer is fluorine-18 which, as we’ve seen, is a positron emitter. Let me duplicate the glucose molecule. The OH group here on the glucose molecule is chemically replaced by a fluorine-18 atom. The new molecule is called fluorodeoxyglucose or FDG for short. The amazing thing is that the body treats FDG very much like it’s an ordinary glucose molecule. As a result, it enters the cells of our body just like glucose does.
The FDG is injected into a patient’s bloodstream and it then enters the cells of the patient. However, just like ordinary glucose, cancer cells absorb more of it than the other cells do.
The fluorine-18 atoms randomly give off positrons which then, within less than about a millimetre of where they were emitted, smash into electrons, which results in the production of two gamma rays that travel directly in the opposite direction to each other.
The patient lies on a bed within a gamma-ray detector that surrounds the patient. The detector detects the pairs of gamma rays coming from the FDG inside the patient, and then a computer puts the information together to construct a false-colour image of where there’s a higher concentration of FDG, in other words, where the cancer cells are. This scan shows the spread of cancer cells in the liver and surrounding organs.
Now fluorine-18 is a very commonly used tracer, but other tracers can also be used with other molecules just depending on exactly what the doctor is looking for. Depending on the type, size, and location of the tumour, doctors may opt for surgery, radiotherapy, or chemotherapy, which is the use of certain drugs.
We can see here two PET Scans of a patient before and after treatment. The patient is now free of cancer. The doctors, the radiographers, the scientists, and the engineers who come up with all this stuff… it’s just awesome.
The amount of radioactive FDG that is injected into the patient is tiny, so the radiation dose is quite small and within about a day, all of the fluorine-18 that was injected is gone. So, what happens to it?
Firstly, even though FDG and glucose are similar and both can enter our cells, the kidneys can actually tell the difference between them. The job of the kidneys is to filter out unwanted chemicals from our blood as the blood passes through them.
The urinary system is the same for both men and women except for the very end bit of course, which is obviously different in men and women.
The kidneys collect the unwanted chemicals, which then pass into our bladder through the ureters. We then empty our bladders when we go to the toilet and urinate. FDG is one of the chemicals that gets filtered out by our wonderful kidneys!
As a result, FDG often shows up prominently in PET scans in a patient’s kidneys and bladder. It also often shows up, by the way, in the patient’s heart and brain, which both always use lots of glucose and which therefore also take up a fair amount of FDG, so doctors have to be a little bit careful when analysing PET scans.
Secondly, the amount of fluorine-18 decreases because the fluorine-18 atoms literally change into oxygen-18 atoms as they decay. Oxygen-18 is harmless. But how long does the whole decay process take?
Well, this graph shows the percentage of the original fluorine-18 that remains over time. Within about 6 hours, 90% of the original fluorine-18 is gone and then after another 6 hours, about 99% is gone. When radionuclides decay, the graph of their decay follows a curved path. Basically, within less than a day, pretty much all of the radioactive fluorine-18 that had been injected into the patient is gone, either due to decay or thanks to the kidneys. The patient is usually isolated from others for less than a day.
Now other radionuclides take a different amount of time to decay. Some decay very quickly while others take millions of years. It’s this really important concept of being able to state just how active a radioactive substance is that we’ll be looking at in our next episode. See you then.
CREDITS:
“Meet the experimental physicist who calls this particle accelerator home” by ANU TV. https://youtu.be/LXA-zv2xYrs. Creative Commons License.
“Cancer is not one disease” by Garvan Institute of Medical Research. https://youtu.be/BlajAw8exg4. Creative Commons License.
“The Evolution of Medical Imaging for Cancer Care” by IAEAvideo. https://youtu.be/EeZNhX0IqsY. Creative Commons License.
“How radiopharmaceuticals help diagnose cancer and cardiovascular disease” by IAEAvideo. https://youtu.be/mQjCTTKWOFU. Creative Commons License.
“How Does a PET Scan Work?” by NIBIB gov. https://youtu.be/GHLBcCv4rqk. Creative Commons License.
“PET_scan_image_02.gif” by Vislupus. https://commons.wikimedia.org/wiki/File:PET_scan_image_02.gif. Creative Commons License.
Urinary System Large Unlabeled.jpg by Andrewmeyerson. https://commons.wikimedia.org/wiki/File:Urinary_System_Large_Unlabeled.jpg. Creative Commons License.
Blausen 0592 KidneyAnatomy 01.png by BruceBlaus. https://commons.wikimedia.org/wiki/File:Blausen_0592_KidneyAnatomy_01.png. Creative Commons License.
