There’s radiation, and then there’s radiation. When a scientist hears the word “radiation”, they think of the electromagnetic spectrum – radio waves, microwaves, infra-red, visible light, ultraviolet light, x-rays and gamma rays. These are all types of electromagnetic radiation. However, thanks to popular culture, when the average person hears the word “radiation” they think of the stuff that causes both giant carnivorous ants and cancer/radiation sickness. This post is going to be about the difference between the two, and why that is.
Harmful radiation is what’s known as ionizing radiation. Ionisation is a process in which a particle/wave of radiation careens into an atom/molecule and knocks loose one of the atoms from its outer shell. Losing an electron gives the atom/molecule a net electric charge (since most atoms are neutral and electrons are negatively charged) and turns it into an ion. Thanks to their charge ions are very, very reactive since they will be drawn towards objects carrying the opposite charge and swap electrons with them in an attempt to become electrically neutral. If the molecule which gets ionized happens to be part of a living organism this can be very bad news indeed since ionization damages and mutates DNA structure. At its most basic level this is harmless since DNA can undergo a process of self-repair or else a programmed die-off, removing the mutated cells from the body. If the mutation cannot be repaired or removed, however, it is passed on during DNA replication and becomes worse with each cell division. This is how radiation causes cancer. And if you’re unlucky enough to receive a particularly heavy dose of radiation you won’t have to wait for cancer to kill you; the cells in your body receive so much damaging radiation that your organs break down and you die of radiation poisoning.
Pleasant, no? Fortunately for us it takes quite a lot of energy to liberate an electron from the outer shell of a molecule. This means that electromagnetic radiation with larger wavelengths (radio, micro, infra-red, visible and the lower portion of ultraviolet) simply doesn’t have the energy required to do it, and so you can take very high doses of these kinds of radiation without any long-term ill-effects. Short-term ill-effects may include rapid heating and bursting into flame, but you can’t win them all; I’ve certainly never heard of anyone dying as a result of taking a massive dose of radio waves. It’s the shorter wavelengths – X-rays and gamma rays – which are the truly dangerous types of electromagnetic radiation.
They are, however, not the only dangerous type of radiation; X-rays and gamma rays are streams of high-energy photons, but errant atomic matter such as protons and neutrons can also muster up enough energy to play snooker with electrons. In the case of neutrons this is quite literal: while they carry no electrical charge and thus cannot electromagnetically interact with electrons, they can bulldoze straight into an atomic nucleus and knock loose a proton which does carry a charge. The two main variants of this type of radiation are alpha particles (two protons and two neutrons stuck together into a Helium-4 nucleus) and beta particles (an electron or positron).
So, that’s what radiation is. How do we get it? On Earth, most radiation is produced by decaying radioactive isotopes. An isotope is a variant of one of the basic elements you might find on the periodic table which has more neutrons than it should do; while the proton numbers defines what element that particular atom is, the element can in theory have any number of neutrons. However, the additional neutrons make the isotope unstable and it seeks to regain atomic equilibrium by emitting mass or energy in the form of radioactive particles. This is radioactive decay, and it can go on for a long, long time.
The above graph is the decay chain for uranium-238. The Z scale on the X-axis is the proton number of the decaying atomic nucleus and the N scale on the Y-axis is the neutron number. Reading from right to left, the u-238 nucleus first emits an alpha particle, losing two protons and two neutrons. Z and N both decrease by two, and the u-238 becomes thorium-234. Then it undergoes beta decay; what happens here is that one of the neutrons in the nucleus emits an electron, losing its negative charge and turning it into a proton. The thorium thus loses a neutron but gains a proton, and becomes protactinium-234. Gamma rays (and X-rays, which are similar) differ from alpha particles and beta particles in that they are electromagnetic waves. After undergoing alpha or beta decay and becoming a new element an atomic nucleus can often be left in an excited, high-energy state, and it bleeds off this energy by emitting a gamma ray and moving to a lower energy state. The nucleus will continue to emit radiation particles/waves in this way until it reaches a stable form, and while it does so it will shift naturally between a number of different element states in a radioactive decay chain.
That’s one way of doing it, anyway. You can also get radiation as a byproduct of nuclear fission (splitting an atom into two parts plus some radiation), nuclear fusion (smooshing together two atoms into one bigger atom plus some radiation), and smashing streams of charged particles into each other really really fast (emitting radiation as the universe attempts to purge itself of the horrible mutant particles you just created). This is all stuff that tends to happen naturally inside massive clumps of matter like stars which is why we often get concerned about cosmic radiation. It’s exactly the same as the radiation we get out of nuclear bombs and reactors, though; there’s nothing particularly exotic about it.
The lethality of each type of radiation is dependent on your method of exposure. Alpha particles are very slow and fat and so they are absorbed very easily, only making it a few centimetres through air and being stopped by a piece of paper or a layer of human skin1. Beta particles are faster and slimmer and can penetrate further, but are still stopped by something as simple as aluminium foil. Gamma rays are the most energetic kind of radiation and will penetrate just about anything; the only way to stop them is to use shielding made of a very dense material like lead.
Since alpha particles can be blocked by skin and beta particles can only penetrate a centimetre or so into the body, it would be logical to say that gamma rays are the most dangerous kind of radiation as they slice right through your body and ionize everything in their path – while this is useful for getting a nice X-ray picture of your skeleton, exposure to more than mild amounts of gamma rays would make you feel very poorly indeed. Unless you’re very unlucky the worst you have to fear from alpha and even beta radiation is moderate radiation burns on your skin, whereas gamma rays can potentially kill you. However, if you were to somehow ingest a piece of matter that was emitting alpha or beta particles you would have a very serious problem, as you’ve now bypassed the layer of skin that protects your vital organs from radiation and alpha particles in particular are very strongly ionizing due to their high mass-energy. This is why radioactive sources getting into the food chain or the water table could potentially be catastrophic, since it often take decades or even centuries for a radioactive isotope to decay into a “safe” state2.
Still, we have many, many safety measures in place to stop some of the dumber things people did with radioactive materials from ever happening again (more on this next week) and the Earth’s magnetosphere deflects most of the cosmic radiation the sun spits out, so we’re mostly safe from radiation these days. If we went outside the magnetosphere it would begin to be a big problem, though, and solar radiation is going to be one of the many hazards that must be accounted for if long-range manned spaceflight is ever to become a serious possibility.
1. Funny story: I was doing a demonstration for a GCSE class where they had to place a bunch of materials in between a Geiger counter and a variety of radioactive sources and try to figure out what proportion of each type of radiation the sources were emitted based on how much radiation each material stopped. The sources were all inside plastic bags to stop the students from handling them directly, and after a few minutes one group came to me and said they couldn’t get any reading from one of their sources. I went over and checked and double checked the experiment to make sure they were doing it right; they were, indeed, getting no reading. Radioactive sources can’t break, so the only explanation was that the Geiger counter was broken, right? But we tried the source on a second Geiger counter, and that picked up no radiation either. Turned out the source was emitting only alpha particles which were all being stopped by the plastic bag before they ever got to the counter, and I looked like a right pillock.
2. This would have been the worst case scenario for the Fukushima nuclear accident; that the radioactive material inside the reactor would go into meltdown and that the meltdown would carry on out of control to the point where it burned through the bottom of the reactor and the ground and seeped into the water table. As it was the reactor went into meltdown but the meltdown was contained within the reactor. Mostly, anyway.