“Ours is a world of nuclear giants and ethical infants. We know more about war that we know about peace, more about killing that we know about living.” –Omar N. Bradley
Nuclear physics is one of the most daunting, emotionally charged phrases in all of science. You can hardly say the words without the image of a mushroom cloud popping into most people’s heads, followed by the devastations of radiation sickness and lingering radioactivity.
But — as a physicist — that’s not what I think of at all.
Think down to all the basic constituents of matter, down beneath your cells, your organelles, the molecules that make them up all the way down to the individual atoms that make up the elements of everything on Earth.
At the heart of every atom is an atomic nucleus made up of some combination of protons and neutrons, which in turn are made up of even more fundamental particles known as quarks and gluons. When I think of nuclear physics, I think of tremendous numbers of these little guys — Avogadro’s Number‘s worth of them — and how they combine together on the smallest scales.
At the most fundamental level (that we know of), the quarks and gluons bind together, with three quarks making up each and every nucleon, where a nucleon is a general term for either a proton or a neutron.
Neutrons and protons aren’t just made up of three quarks apiece, though, and you might have guessed that if you looked up the masses of the quarks and the masses of protons and neutrons.
We typically say that a proton is made up of two up and one down quark, and that a neutron is made up of one up and two downs. But an up quark has a mass of about 4-5 MeV (in natural units) and a down has a mass of 7-8 MeV; based on that, you might think a proton has a mass of around 17 MeV and a neutron at around 19 MeV.
Those guesses are reasonable, and yet they’re only about 2% of the actual proton and neutron masses, which are about 938 MeV and 940 MeV, respectively. Where does the rest of that mass come from?
From binding energy. Those numbers I gave you for quark masses are for theoretically free quarks, which aren’t bound at all to anything else. But — at least in the Universe’s current state — free quarks can’t exist, thanks to the rules of Quantum Chromodynamics, the laws that govern the strong force! Quarks can only stably exist in bound states, and the only bound states of quarks that are stable for longer than a microsecond are protons and neutrons.
So most of the energy that we observe as “mass” in a proton or neutron doesn’t come from the masses of the three quarks that define the nucleon themselves, but rather from the field of the strong force that keeps them bound together. Another way of looking at it is to consider the gluons and the sea of virtual particles (quarks and antiquarks of all types) that make up each nucleon as what really makes the mass of a proton or neutron as heavy as it is.
Which is to say, to have something just as simply as one proton or one neutron, there’s a lot more involved than just three quarks. In fact those three quarks that you hear of as making up a proton or neutron are more specifically known as valence quarks, and all the other quarks-and-antiquarks inside are known as sea quarks, which carry some 98% of the masses of these particles.
But with the sole exception of a single proton (which serves as a common hydrogen nucleus), these nucleons don’t exist in isolation in nature.
They exist in states where they’re bound to one another. That’s what makes atoms interesting: the fact that they have different numbers of protons (which makes for different elements) and different numbers of neutrons (which makes for different isotopes). And unsurprisingly, each unique combination of protons and neutrons has a unique binding energy, and only a very select few combinations are stable.
The simplest combination — one proton and one neutron — is known as a deuteron, and is about 2.2 MeV lighter than a free proton and neutron alone. Start adding more, like two protons and two neutrons (to make Helium-4), and you’re suddenly 28 MeV lighter than those four free particles. The most stable of all the elements is Iron-56, with 26 protons and 30 neutrons, and which has a mass that’s a full 492 MeV lighter than 26 free protons and 30 free neutrons.
Heavier elements and isotopes may have a larger total binding energy, but no element has a higher binding-energy-per-nucleon than this isotope of iron.
Low-mass particles are easy to fuse into higher-mass ones; they emit a lot of energy when they do so. So long as you can achieve sufficient temperatures and densities, this is something that happens spontaneously, and is both the great hope of commercial nuclear fusion and also how all the elements in the Universe heavier than lithium were produced: in the nuclear fusion furnaces of stars!
On the other hand, (mostly heavier) particles that have too little binding-energy-per-nucleon can spontaneously undergo one of three radioactive decays to reach a more stable state:
- Gamma decay, where a nucleus emits a gamma-ray (high-energy photon) to form a slightly lower-mass nucleus with the same number of protons and neutrons;
- Beta decay, where a nucleus emits an electron (and an antineutrino) to form a nucleus with a slightly lower mass, with one more proton and one fewer neutron than its parent nucleus;
- and Alpha decay, where a nucleus emits a Helium-4 nucleus (two protons and two neutrons, known as an alpha particle), and results in a nucleus with a lower mass, two fewer neutrons and two fewer protons than its parent.
These processes may be a one-off, as in the daughter nuclei they give rise to may be stable, or they may be part of a radioactive decay chain, which can require many steps until something stable is reached.
What’s particularly interesting is that:
- Each combination of protons and neutrons has a specific binding energy and therefore a specific rest mass,
- but there are only three types of radioactive decay, each which gives rise to a photon (massless), electron (of a small, given mass) or an alpha particle (of a larger, given mass), and therefore
- each radioactive element produces radioactive decays with specific, characteristic energies (and timescales) associated with them!
Each type of particle — alpha, beta and gamma — takes different types of material to stop them, and to shield sensitive things (like you and me) from them.
Alpha particles are mostly harmless; they can be stopped by a sheet of paper, and even if they actually reach your body, are stopped by the outer one-or-two layers of skin cells on your epidermal layers.
Beta particles can do some damage; they can penetrate your skin and, in large doses, can give you radiation sickness and kill you.
But gamma particles are the most deadly: it takes a full foot (30 cm) of lead to effectively shield you from gamma radiation, and most cases of radiation sickness related to, say, the Hiroshima bomb came from gamma radiation.
But that’s not all cases of radiation sickness.
Alexander Litvinenko was famously poisoned by being forced to ingest a radioactive isotope of Polonium (Po-210), which is an alpha-emitting radioactive particle. Although alpha decay is harmless if it takes place outside of your body, inside of you, all of that radiation is absorbed internally, and death is inevitable within a matter of days. (This is true for ingesting sufficient quantities of any radioactive material with a short half-life!)
But there are some specific characteristics that allow us to determine just what it was that poisoned him.
When an alpha particle is emitted from a nucleus, it will have a specific amount of kinetic energy that’s determined by the decay parent (Po-210 in this case) and the large, daughter nucleus (Pb-206, lead, which is stable). So when you want to determine whether there’s a particular radioactive substance, you look for alpha particles with a certain energy, and that’s a smoking gun for the radioactive parent.
There’s a question burning up around the world right now: is this how Yasser Arafat died?
Although at this point I don’t think anyone can say for sure, this is the type of detective work that will uncover the answer: nuclear physics! It’s an incredibly powerful thing; used irresponsibly, it can kill hundreds of thousands in an instant or poison a targeted individual slowly and painfully, but used responsibly, it can be a practically infinite source of power for mankind.
It’s to be respected and valued, and only feared in the wrong hands. No matter what, it’s a great opportunity to understand our natural world a little better, and how matter on the smallest scales can impact the largest things we know!