So…in the year and a half since I resumed writing this blog, every now and again I’ll get a request to explain various aspects of how radiation interacts with matter – taken one at a time, none is really involved enough to warrant an entire post but, combined, I think they will. So for all the various requests (and requesters), here we go!
Photons are the particles/waves that make up light; photons come in a huge variety of wavelengths that range from radio waves with wavelengths as long as tens to thousands of kilometers to gamma rays that are a millionth of a micron from peak to peak. The shorter the wavelength, the more energy the photon has; the longest wavelength that can cause ionizations (ultraviolet) is about a hundredth of a micron.
Photons can be absorbed by the body’s molecules, adding enough energy to break molecular bonds, or they can undergo the interactions described below.
Ultraviolet radiation is much like visible light; when UV photons reach our bodies, they can be absorbed by an electron circling an atom, stripping the electron from the atom and creating an ion pair. This ion pair might recombine or it can go on to catalyze the formation of reactive molecules called free radicals that can then go on to attack and damage the DNA, causing damage that can lead to mutations or that can damage the cells. This can lead to inflammation, which (among other things) can manifest as sunburn.
X-ray and gamma radiation have more complicated interactions, depending on the energy of the photons. The three primary interactions are called the photo-electric effect, Compton scattering, and pair production.
The photo-electric effect is what earned Einstein his Nobel Prize in 1921. This is what I just described with UV light – when a high-energy photon is absorbed by an electron it can add enough energy to the electron to be ejected from the atom. And, while the photoelectric effect seems sort of mundane – it’s how solar cells produce electricity, for example, and we see them every day – its explanation was actually profoundly important to the nascent field of quantum mechanics and it helped to explain the quantum nature of light as well as how electrons are arrayed around the atom. The photo-electric effect is the most important type of interaction with lower-energy photons that are interacting with denser elements.
Intermediate-energy photons are more likely to interact through Compton scattering, named for Arthur Holly Compton, who was awarded the 1927 Nobel Prize for his discovery. In Compton scattering, the incoming photon is scattered by an electron in a random direction at a slightly lower energy; most of them are “forward-scattered,” meaning that they end up continuing on in roughly the same direction, although they can “bounce back” as well in a process called “backscatter” as well. My mental image here is of directing a stream of water against a window screen – some of the droplets pass through without touching any of the strands that make up the screen, and even the droplets that do interact with the strands are likely to end up on the opposite side of the screen. The scattered photons will be scattered again…and again and again and again, each time in a random direction and losing a part of their energy until he photons are finally absorbed by an atom or a molecule. If you’re interested in digging into the math of Compton scattering – how to calculate the loss of energy with various scattering angles (for example), you can find a pretty good explanation in Wikipedia.
At the risk of sounding too much like a card-carrying geek, my favorite photon interaction happens only with photons that have an energy of 1.022 million electron volts (MeV) or higher – it’s called pair production. The reason I love this interaction is that it is – quite literally – a demonstration of Einstein’s equation, E=mc2; in pair production, we see energy becoming mass and the mass turning back into energy. Not only that, but the minimum amount of photon energy to undergo pair production is exactly the amount of energy that’s released when the mass of two electrons is turned into pure energy (511 keV per electron, 1.022 MeV for the pair).
What happens is that when a high-energy photon passes close to an atomic nucleus it can spontaneously turn into an electron and positron pair (a positron is an antimatter electron, having a positive electrical charge to balance the negative charge on the electron). Having opposite electrical charges, the particles are attracted to one another and they recombine, annihilating each other (as matter and antimatter are wont to do) and converting their mass back into energy in the form of twin photons, each carrying the mass-energy of one electron – 511 keV.
So, I know that this sounds kind of abstract…except that in 2016 I was in a helicopter flying over an operating nuclear reactor with a fantastic gamma spectroscopy device and I saw a clear gamma peak at precisely 511 keV – proof that, directly beneath me, high-energy gamma ray photons from nuclear fission, while passing through the materials that made up the reactor vessel, were forming electron/positron pairs that were annihilating each other, shooting the annihilation photons into my detectors. For me it was quite a thrill – the pilot and copilot seemed less impressed. The spectrum I collected is in the figure here with the annihilation peak marked with the vertical line. And, yeah, I know it doesn’t look like much – but it made my day and gave me a chance to thoroughly bore a few police department pilots.
As photon energies increase even more we can produce other phenomena – a photon can eject a neutron from an atomic nucleus if the energy is above about 1.7 MeV, and even higher-energy photons can cause an atom to fission. But these interactions are rare, usually requiring high-energy astrophysical processes or the production of extremely high-energy gammas in particle accelerators.
That’s pretty much it for photon interactions – now let’s see what happens when neutrons, alpha, and beta particles come to rest in matter. Here, what matters the most is whether or not the particles have an electrical charge I’ll start with the charged particles.
Alpha and beta particles are both electrically charged – alpha particles have two protons and a charge of +2; betas consist of a single electron with a charge of -1. One way they interact with atoms is electrostatically – like charges (e.g. two things, each with a positive charge) will repel each other while opposite charges attract each other. So, for example, when a positively charged alpha particle approaches an atom it will first be attracted to the negative charge of that atom’s electron cloud and will curve towards the atom, picking up energy and pulling the atom ever so slightly towards it. As it passes by and starts moving away from the atom again, it will start to slow down – just as a space probe will speed up as it approaches a planet, slowing down as it flies past and away again. If the alpha particle passes closely enough, it might strip an electron from the atom, but it might also penetrate the electron cloud to interact directly with the atom’s nucleus. Every such interaction transfers some of the particle’s energy to the atoms with which it interacts; since energy is related to the particle’s speed, this means that the particle is slowing down the whole time, knocking off the occasional electron and creating the odd ion pair as it does so.
By the way, did I mention bremsstrahlung? It’s German for “braking radiation” and it’s yet another way for charged particles to interact; electrons in particular. As beta particles are slaloming through material the various tugs and pushes from positive and negative charges cause them to change direction and every time they do so there’s a chance that they’ll emit an x-ray photon. In fact, that’s how x-ray machines create x-rays – by slamming a beam of electrons into a metal target, letting the electrons bounce around a bit, and beaming out the ensuing x-rays. Heavier atoms give a greater likelihood of creating bremsstrahlung x-rays and they tend to have higher energies; something to keep in mind not only when generating x-rays in a machine as well as when trying to shield beta radiation sources.
And finally, there’s collisions! Alpha and beta particles – neutrons too – can simply plow into an atom’s nucleus. As with shooting pool, the particle might strike head-on, bouncing back and imparting energy to the atom, but it’s more likely to strike a glancing blow, losing energy, albeit somewhat less than striking a direct blow. Neutrons will do this too, also exchanging energy each time; they, too, will sometimes break chemical bonds within the molecule, or they can even be absorbed by the nucleus, causing it to become radioactive.
The last bit is how readily these interactions occur and how far into material radiation will penetrate. With a relatively high electrical charge, alpha particles interact readily; as a result, they yield their energy readily and can’t penetrate more than a few microns into most substances and no more than a few millimeters in air. The more-nimble beta particles exchange energy somewhat more grudgingly and are able to penetrate up to about a centimeter into water, plastic, and flesh. And neutrons, lacking an electrical charge, can only exchange energy via collision, and can penetrate fairly large distances (several inches to a foot or more) into most materials
Having gone through all of this, there are still a number of radiations and interactions that I haven’t even mentioned, let alone described. But, to be honest, neutrinos (for example) just aren’t something that anyone except a physicist is going to take much of an interest in, while high-energy protons are usually only found in space or in specialized medical devices. If anyone is interested, I can cover some of the more exotic radiations in a future piece.
Image Reference:
- Alpha, Beta, Gamma image by Kailogical:
https://commons.wikimedia.org/wiki/File:Alpha-beta-gamma_decay.png