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 2: Alpha Radiation, we look at what alpha particles are, introduce students to nuclear equations, and show how alpha-emitting substances are used to power space probes and to treat certain types of cancer.
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Part A: Introduction
Part B: Alpha Particles
Part C: Putting Alpha Emission to Work
Transcript (more of less)
Part A: Introduction
In this lesson we’re going to look at a particular type of nuclear radiation called alpha radiation. There are other forms of nuclear radiation, including beta and gamma radiation, which we’ll be looking at in our next episode, but in this lesson, we’re just going to look at alpha radiation.
Nuclides that emit alpha radiation, what we call alpha emitters, are used in, for example, various space probes to produce electricity and in certain types of cancer treatments. Alpha radiation is made of alpha particles.
So, what are alpha particles? Let’s take a look.
Part B: Alpha Particles
These are two photos of uranium. Uranium makes up about 3 parts per million of the Earth’s crust and is more abundant than gold, silver and mercury, but slightly less abundant than lead.
Uranium is used to produce atomic bombs and it’s one of the fuels used in nuclear reactors. Nuclear reactors produce heat in nuclear power stations which produce electricity, and nuclear reactors also produce radioactive materials artificially.
There are three isotopes of uranium that exist naturally on Earth. Uranium has an atomic number of 92, which means that uranium atoms have 92 protons. Each isotope has a different number of neutrons of course. Now remember, the number of protons + the number of neutrons = the number of nucleons, which is what we call an atom’s mass number. For example, 92 + 146 = 238. U-238 has a mass number of 238. In terms of abundance, U-234 atoms make up only about 0.005% of all uranium atoms, basically zero, U-235 atoms make up about 0.7%, while about 99.3% of uranium atoms on earth are U-238 atoms All uranium atoms have unstable nuclei and they are all alpha emitters. Let’s see what that means.
This is the nucleus of a U-238 atom. It has 92 protons (that’s the atomic number) and 146 neutrons, so, in total, there are 238 nucleons all together (what we call the mass number), hence U-238. This many nucleons is unstable, and so the nucleus spits out (the technical word is it “emits”) what’s called an alpha particle which is a group of 2 protons and 2 neutrons. It’s never some other combination like 2 proton and 3 neutrons, it’s always 2 and 2. The uranium atom (that we’re only showing the nucleus of) is no longer a uranium atom. It has lost 2 protons and so it only has 90 protons left, which, if you consult a periodic table, makes it a thorium atom. It has also lost 2 neutrons leaving it with 144 neutrons. Since it has lost a total of 4 nucleons (2 protons and 2 neutrons), the number of nucleons it now has is only 234. So, when the alpha emission occurs, the uranium-238 atom turns into a thorium-234 atom. The whole process is called alpha emission or alpha decay. Decay means breaking down into smaller bits. Since alpha particles comes from the nucleus of alpha emitting atoms, alpha particles are a form of nuclear radiation. We’ll look at beta and gamma radiation in our next episode. The leftover nucleus is called the daughter nucleus.
We can’t see nuclear radiation with our eyes but we can detect it with Geiger counters and with other specialized electronic equipment.
Alpha particles are emitted at speeds of about 30,000 km/s, or about 10% of the speed of light. Since alpha particles have 2 protons and 2 neutrons, they are identical to the nuclei of helium atoms. Alpha particles are given the Greek symbol alpha, , which is obviously very similar to an English “a”, but it’s got curvy lines. In atomic notation, alpha particles can be written as 4
or as 4
2+. Both of these tell us that the alpha particle has two positively charged protons and 4 nucleons in total (which means that there are 2 protons and 2 neutrons). The 2+ here means that the alpha particle carries a charge of 2+. There are two protons, each with a charge of 1+ and since there are no electrons, the overall charge is 2+.
We can write a nuclear equation to express what happens when a U-238 atom, let me double it up, emits an alpha particle and changes into a thorium atom. What begins as a 238 92 U atom loses two protons and 2 neutrons, which, as we’ve seen can be written as 4 2 alpha. 4 nucleons, 2 of them protons, alpha particle. The uranium atom, having lost 2 protons changes into an atom with only 90 protons, that is, a thorium atom. Having lost 4 nucleons there are only 234 nucleons left, so its mass number reduces to 234. The new atom can be written in atomic notation as 234 90 Th. We can put all this information together in a single nuclear equation. 238 92 U turns into 234 90 Th + 4 2 alpha. It’s pretty easy to do the mathematics. Notice that 238 = 234 + 4 and that 92 = 90 + 2.
Alpha particles can cause damage to living cells basically because as they fly past atoms in the cells, their strong positive charge attracts electrons away from the atoms and the chemical reactions that would have occurred don’t occur since the electrons have gone missing. This stops the cells from functioning properly. Depending on how much radiation there is, the cell can die or turn cancerous.
Alpha particles have a very low penetrating ability. They get stopped in only a few centimetres of air and they can’t penetrate the outer layer of our skin, which is made of dead skin cells. However, if you breathe in or swallow an alpha emitter, the alpha particles that are emitted can kill healthy cells in your stomach or lungs, for example.
In 1986, one of the nuclear reactors at the Chernobyl Nuclear Power Plant in Ukraine exploded. It released huge amounts of alpha-emitting uranium and other radioactive materials into the surrounding region. It wasn’t a nuclear bomb type of explosion; the uranium overheated when the operators of the plant were conducting some very poorly-thought-out maintenance tests, and the pressure of the water built up to the point where the pipes all exploded. The biggest danger to the people and animals was breathing in or swallowing dust that was contaminated with radioactive material. A lot of it was dangerous even from a distance, but it was way more dangerous if it was inside a person’s body because that’s where alpha particles (and beta and gamma radiation) can do the most damage.
The nearby town of Pripyat was evacuated and it’s still a ghost town today thanks to the way-above-average radiation levels. The forest trees are slowly taking over the land.
As alpha particles slow down—within a tiny fraction of a second—they pick up two electrons from atoms that they’ve crashed into and turn into ordinary, and harmless, helium atoms. About 99% of all the helium on Earth comes from alpha-particle emission by uranium and thorium atoms.
Now though alpha radiation can be harmful, we can also put it to good use. Let’s take a look.
Part C: Putting Alpha Emission to Work
The radioactive decay process generates heat.
At the atomic level, the atoms of a cold object move or vibrate relatively slowly, whereas the atoms of a hot object move or vibrate relatively quickly. We covered this concept, called the Kinetic Theory, in our Heat series.
When atoms undergo alpha decay, the daughter nuclei and the alpha particles that are produced fly apart with more speed than what the original nuclei had, a bit like firing a bullet out of a gun. All this extra movement at the atomic level results in the sample of the alpha emitter getting hot. Basically, whenever any atom emits an alpha particle, some of the nuclear energy of the nucleus is converted into heat energy.
In fact, the radioactive decay of uranium, thorium and potassium-40 deep below the surface of the earth is the main source of heat that keeps the interior of the earth really hot, hot enough to melt rock, which we call magma when it’s underground and lava when it flows up onto the surface.
Now earlier, I mentioned that uranium, an alpha emitter, is used in nuclear bombs and nuclear reactors. The huge amounts of heat generated in nuclear bombs and nuclear reactors is not generated by alpha emission but rather by a process called nuclear fission which involves the splitting of uranium atoms into two new atoms that are more or less equal in mass. Because it’s possible to set up a chain reaction, fission releases far more energy than alpha emission does, but we’ll talk more about that in another lesson, so let’s get back to alpha emission. (For the senior Physics students, the energy released is 210 MeV per fission vs about 5 MeV per alpha emission. Also fission can be set up so that lots of atoms fission at the same time, whereas alpha emission in uranium atoms is a slow process, so not much heat is generated in a sample of uranium).
Heat energy produced by alpha emission can be used to generate electricity using a simple device called a thermocouple.
A thermocouple consists of two different wires, like copper and steel, twisted together. When the junction is heated a voltage is generated. So even in the middle of nowhere (not that I was really in the middle of nowhere), if you have a source of heat and two different wires, you can, with no moving parts, generate a small amount of electricity.
NASA uses this principle to build what are called Radioisotope Thermoelectric Generators, or RTG’s. RTG’s use the heat generated by certain radioactive materials to produce electricity.
That’s an RTG right there powering the Voyager 2 space probe which flew past Jupiter, Saturn, Uranus and Neptune in the 1970s and 1980s.
New Horizons, a space probe that flew past Jupiter in 2007 and past Pluto in 2015 also has an RTG. As you can see from the photo, taken before New Horizons was launched of course, the RTG is about a metre long.
Solar panels would be no good for space probes that explore the outer solar system because the sun’s not bright enough out there.
The Perseverance Rover that landed on Mars in 2021 also uses an RTG, that’s it right there, to power all its electrical systems. Some earlier rovers like the Opportunity used solar panels. The sun is bright enough on Mars to make solar panels effective and the Opportunity mission was very successful. However, Mars has a very thin atmosphere which, even though it’s only about 1% of what the Earth has, nevertheless kicks up a bit of dust from the ground which, over time, covered the solar panels which ended the mission.
The RTG used on Perseverance doesn’t have this problem.
NASA RTGs use plutonium-238 as their heat source. Plutonium-238 is an alpha emitter. This photo shows a small amount of plutonium-238 dioxide glowing red hot after it was covered by a special insulating blanket for a few minutes. The heat generated is quite significant.
As the plutonium-238 atoms in an RTG decay, the power output of the RTG slowly decreases of course, because, as time passes, there are fewer and fewer plutonium-238 atoms. The power decreases by about 0.8% per year, but’s it’s still enough to power a rover or a space probe for decades.
The emission of alpha particles is also used to treat certain types of cancers. Let me explain. Our bodies are continuously making new cells as older cells die. For example, the outer layer of our skin is made of dead skin cells which just fall off, but new skin cells are continuously being made underneath. The process is carefully controlled.
Cancer is basically a disease where some cells in a part of your body reproduce at a much faster rate than normal. The cancerous cells typically form what’s called a tumour. A tumour interferes with the operation of the organ that it’s growing in or surrounding organs. Since cancer cells are more active than normal cells, they usually absorb more nutrients than normal cells.
For example, bone cells require calcium to be strong and healthy. Cancerous bone cells absorb more calcium than other bone cells because they’re growing and reproducing at a faster rate. This difference between normal cells and cancer cells can be used by doctors to kill cancer cells.
For example, patients with certain types of bone cancer can be treated with radium-223, which is an alpha emitter. Chemically speaking, radium is a little similar to calcium. All Group 2 elements on the periodic table have similar chemical properties.
When the alpha-emitting radium-223 is injected into the body the body treats it a little like calcium. The bone cells absorb the radium-223 in a similar kind of way to the way that they absorb calcium. But, the cancerous cells absorb more of the radium-223 than the normal cells do (because of the fact that the cancer cells are so active) and the alpha particles being emitted by the radium-223 kill the cancerous cells, though I’m not showing the alpha particles individually. The fact that alpha particles don’t have a high penetrating ability means that the healthy cells surrounding the cancer cells don’t get affected much.
However, healthy cells do still get killed, so radium-223 therapy is only used when doctors believe that there is no better alternative, and that the risk of dying from the bone cancer is greater than the risk of dying from exposure to the low levels of nuclear radiation that the patient will be exposed to. It’s a balancing act.
The radium-223 atoms and the decay products are eliminated from the body over the course of a few days or so.
As we’ve seen, the reason that alpha particles kill cells is that they are charged and they attract electrons away from the atoms that they go whizzing past, and so those atoms can’t chemically react properly.
This can, for example, rip a DNA molecule apart which will lead to the cell dying.
The process by which an atom has electrons ripped away from it is called ionization. Here, the carbon atoms have been ionized. An ion is an atom that has lost or gained electrons. Here, the carbon atoms have lost electrons.
Now because alpha radiation produces ions, it is said to be a type of ionizing radiation. Any type of radiation that can knock electrons off atoms is called ionizing radiation, because it ionizes the atom. Beta and gamma radiation, which also come from the nuclei of certain radioactive atoms are also forms of ionizing radiation, and it’s these two forms of radiation that we’ll be looking at in our next episode. See you then.
“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.
“Chernobyl Part 1 The Mother of all Nuclear Reactor Disasters 1986 | A Brief History of Documentary” by Plainly Difficult. https://youtu.be/IkBJU8BbrUs Creative Commons License.
Animations of the Perseverance Rover and the New Horizons space probe by NASA
Atomic Bomb footage produced by various US Government Departments.