Saturday, September 30, 2017

Notes on Radioactivity & Particle Physics

Cross-posted from Teaching Is . . .

BP Tech Applied & Advanced Physics

Some notes on how we could approach teaching radioactivity/nuclear structure


The State of Massachusetts has revised its high-school science curriculum finally. But there is an orphan unit: radioactivity. I think this must be a new unit in the science curriculum, and the State first tried to add it to the Chemistry curriculum. Then to Earth Science. And finally to Physics. Where it truly is simply added, like a wart, to the front of the Physics curriculum. No attempt is made to connect it to anything else in the curriculum.

Now of course radioactivity is a proper physics topic, and the study of radioactivity led to important developments in modern physics. At BP Tech, where I teach, I always took a bit of the school year to look at basic atomic structure, knowing that students would see it again in chemistry. The problem with just tacking on radioactivity is that explaining radioactivity (as opposed to just describing it) draws you into quantum and particle physics, which could easily eat up an entire semester, or more. I spent a whole year thinking about how to present the topic without getting completely derailed from the rest of the physics curriculum. These notes explain what I came up with, on behalf of the entire physics teaching team at my school.

Part I

Here is how I’ve tried to approach atomic structure in the past. After exploring the gravitational field and early into electrostatics (after introducing electric fields and electrons), I take a moment to look at a simple atomic model:

This model explains several things: the electric neutrality of atoms, the mobility of electrons, where our mass comes from. Later, when talking about electric current, I begin the discussion of how materials are constructed of atoms (or more usually molecules), and how electrons can basically hop from atom to atom. There is a net flow of electrons throughout a circuit but no single electron moves through the entire circuit (hence my distaste for the water model of electric circuits). I also take this moment to show various simulations that try to represent electrons moving through a circuit, and how they are incorrect and misleading.

Now it might be useful to discuss the residual charge (or residual electric field) of the electrons. Residual charge explains stickiness and friction and why chemical reactions happen and the unusual properties of water. Then when we get to the strong force, the idea of residual force will come into play, and the students will have already experienced the concept.

So this is as far as I have carried this in the past. We need to dig deeper in order to explain radioactivity.

Part 2

The nucleus, made up of protons and neutrons. What holds it together?

Wouldn’t the protons repel? Yes, of course, and it does happen in nature. Some atoms spit out a proton or neutron now and again. There’s your first taste of radioactivity. So there must be another force that’s really strong but has a tiny range. Call it a nuclear force, because it only operates in the nucleus, and more specifically call it the strong force.

Two issues: why the tiny range? And why does it apply to neutrons as well as protons? Let’s assume that protons and neutrons are made of something similar, and let’s call these constituent particles quarks. It turns out that protons and neutrons are made of 3 quarks each. And protons and neutrons differ by only one quark. The strong force is what holds the quarks together. Here’s a model of a proton:

The strong force that exists outside the “boundary” of the proton is the residual strong force. This is what holds protons and neutrons together.

At this point, I think there is no sense in complicating this picture. You could point out that there are different kinds of quarks, but I wouldn’t even take it that far. And I definitely wouldn’t mention specific force field particles, like gluons. This will just draw you into quantum physics, and really the point here is just to explain radioactivity.

Exploring the atomic nucleus is tricky (and abstract) enough – too much information will muddy the waters. We’re just building on the concept of force fields (gravity, electricity, magnetism, and now strong nuclear). If you have students who wish to pursue this on their own, here is an excellent website called The Particle Adventure:

Part 3

So now we’re ready to talk about radioactivity as the result of the instability of large atomic nuclei, like those of uranium, or nuclei with too many neutrons. Basically there isn’t enough residual strong force out on the margins to hold these nuclei together.

An unstable nucleus will:
  • spit out single neutrons (neutron emission)
  • spit out single protons (rare)
  • spit out a chunk of nucleus made of 2 protons and 2 neutrons (alpha emission)
  • during these processes, the nucleus might also emit very high energy EM radiation (gamma emission)

All these emission products (particles, if you will) have a LOT of energy. If absorbed by other atoms, this energy can damage molecules and make atoms radioactive.

At this point, we are welcome to explore further anything we wish about radioactivity, including health effects or nuclear fission/fusion, or mass/energy conversion, or commercial nuclear energy, or what fuels the Sun. We have to keep it short and simple, though, because we’re not quite done.

Part 4

An unstable nucleus will also spit out – an electron! This is beta emission, and it's really weird. Why is this weird and unexpected? Well, where did the electron come from? Protons and neutrons aren’t made out of electrons!

So there must be another force, another nuclear force. This one is called the weak force. The weak force is odd, though, in that it does not cause anything to happen, it allows something to happen. Here’s the something:

But there’s leftover negative charge and energy and mass. Where does it go? The weak force temporarily holds the charge, mass, and energy, and then releases it as an electron. So that’s where the beta emission comes from.

A neutrino is also emitted, but I don’t know how much you want to get into neutrinos, other than to say that they are especially tiny sub-atomic particles with no charge. They are often the result of energy converting into mass.

So here is what the full interaction looks like:


Ultimately all this is to say that there are only four fundamental forces in nature: gravity, electro-magnetism, and the two nuclear forces, strong and weak. And we might not ever have known about the nuclear forces if it hadn’t been for radioactivity.

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