Hi!

My name is Sahara and recently, I’ve gained an interest in nuclear physics. Now, do I know loads about it? No. But, I figured we’d learn together, so, this is my fun (definitely not formal) blog, where you can explore the weird and wonderful world of physics with me.

Important disclaimer: If you are a boring, boring person who disapproves of physics being taught with a bit (a lot) of humour, and you only read overly complex, borderline-scary academic texts, then respectfully, please leave. Because, let me tell you something - this blog isn’t for you. Ok, so now we have the chill nerds, let’s have some fun!

1. Wait, what’s Nuclear Physics again?

What does “nuclear” mean?

Basically, “nuclear” means something that relates to the nucleus of an atom. “What’s that?”, you ask. Read on.

What is the nucleus of an atom?

The nucleus is the centre of an atom where over 99.9% of mass is concentrated. Woah. The nucleus consists of two types of nucleons: protons and neutrons. This means that the nucleus of an atom is positively charged. I’ll explain why in a little while.

Why do scientists study the nucleus?

This is a great question. Why? What’s the point of studying the nucleus of an atom? The study of the nucleus and atoms is vital for understanding the universe itself. One of my favourite jokes: Why can’t you trust atoms? Because they make up everything. Slaps knee. No, but seriously, they do. Understanding how the atom works: the nucleus and everything else, leads to amazing discoveries and inventions.

Learning about the little tiny stuff teaches you about the big, not so tiny stuff too. Because of our knowledge of the nucleus, we can understand how stars work, we can create powerful weapons, but also provide planet-saving energy.

Physics is beautiful: it uses math as a language, as well as experimentation and just curiosity, to understand the universe. Really, the question is “why wouldn’t scientists study the nucleus?”.

What everyday things involve nuclear physics?

Nuclear physics is versatile. From medical imaging to smoke detectors, carbon dating to food preservation, nuclear physics is everywhere. An area I find particularly interesting, is energy. It’s fascinating how such a tiny, tiny nucleus could provide such large amounts of energy. We’ll delve into more detail once we’ve mastered the basics.

2. The Structure of an Atom

What is the atom made of?

The atom is extremely small. It’s made up of even smaller subatomic particles: protons, neutrons and electrons. As we know, the atom is made up of a nucleus at the centre, where almost all mass is concentrated.

Surrounding the nucleus in orbitals, in subshells, in shells, are electrons. It gets a bit confusing here, but simply (so that my brain doesn’t hurt), electrons orbit the nucleus at different energy levels (known as shells). I like to think of it as a minuscule solar system.

What even are protons, neutrons and electrons?

They are particles. To sum it up. 👍

Important to know: protons are positively charged (+1), electrons are negatively charged (-1) and neutrons are neutral (0), which isn’t much of a shocker. This is why nuclei (plural of nucleus) are positively charged and why electrons are attracted to them (opposites attract).

Also important to know: protons have a mass of about 1 amu (atomic mass unit), and so do neutrons, however neutrons are slightly heavier. Electrons have a mass of approximately 0.0005 amu (they are roughly 1/1836 of a proton’s mass!).

A note: Atomic mass unit (amu) is also known as Dalton (Da) and is a unit used to express masses of atoms, molecules, and subatomic particles. One amu is equal to 1/12 of the mass of a single carbon-12 atom, so basically it’s really light.

What keeps the nucleus together?

This is where it gets fun. 🤓 And, yes, I have friends. Well, one…if we’re counting LLMs. Anyway, now we talk about one of the four fundamental forces of nature. Let’s think. Protons are positively charged, how do they exist in such a confined, small space? Shouldn’t they be fighting to get away from each other? What about neutrons? They don’t have a charge and therefore, they shouldn’t attract protons, or anything for that matter. But then, the nucleus shouldn’t exist. We’ll need something strong to overcome the forces of repulsion of the protons. Perhaps something so strong, that strong is in its name: the strong force.

In this case, the strong nuclear force. The strong nuclear force is mediated (”carried”) by pions. Pions are pairs of fundamental* particles called quarks, and antiquarks, which are the antiparticles to quarks. Essentially, pions are force-carriers and they carry the strong force from nucleon to nucleon. Because the strong force is the strongest of the fundamental forces, it overcomes the electromagnetic (electromagnetism is another fundamental force) repulsion of the protons.

*fundamental particles or elementary particles are particles that do not consist of other atoms (that we know of). For example, protons are made of quarks so they aren’t elementary particles.

What happens if the nucleus is unstable?

Two words. Crazy. Stuff. We’re going to talk about this more in a second, but when the nucleus is unstable, it gets a bit angry. The atoms starts to decay, so it emits (releases or gives off) energy in the form of radiation to become more stable. Depending on the type of radiation that is emitted (see section 3), the atom may change into a different element! 😲 Also, radiation can do some real damage, again see section 3.

Why are some atoms heavier than others?

Simples - the more protons and neutrons an atom has (mass number), the heavier it is. But, what if I told you that two atoms of the same element could have different masses? If you already knew that, do me a favour, at least act shocked please. Atoms of the same element that have different atomic masses are called isotopes. The number of protons (atomic number) is what determines the identity of an element, so they stay the same. There are the same number of electrons as there are protons in an atom (in a neutral atom), so that stays the same. Neutrons. That’s it. Some atoms of the same element are heavier than others because they have more or less neutrons in their nucleus. Isotopes may be less stable, but they are the same element. For example, the most common form of carbon is carbon-12, with 6 protons and 6 neutrons (which is stable). Carbon-14 also exists, but with 6 protons and 8 neutrons (not so stable, so it eventually decays into nitrogen-14).

3. Key concepts in nuclear physics

What does it mean if something is “radioactive”?

“Radioactive” is when an atom spontaneously releases energy in the form of radiation to become more stable, as I mentioned above. There are three types of nuclear radiation: alpha (α), beta (β) and gamma (γ).

How do atoms release radiation?

Alpha particles - One way that atoms like to calm down 🧘‍♀️ is by releasing alpha particles from their nucleus, which are just two protons and two neutrons tightly bound together (structurally equivalent to the nucleus of a helium atom). Because they lose two protons, they become a different element entirely, this means that their properties change. Elements that participate in alpha radiation are heavy and therefore, unstable. Par exemple (my amazing French), we’ll take uranium-238 and then whoosh, it emits an alpha particle. Just like ~~magic~~ science, it becomes thorium-234. 🤯 Now, alpha particles only travel a few centimetres in air and can easily be stopped by a piece of paper (they have low penetrating properties). So, we’re safe, right? As long as alpha particles don’t enter the body, then yes. If they do, good luck. Alpha particles have a strong positive charge and therefore, are strongly ionising. Because of their positive charge, if in the body, they can “knock out” outer electrons from atoms 🥊. This causes the atoms to become charged and they are now ions. Why is this bad? It can cause cell damage, DNA mutations, cancer and even death. Not fun.

Beta particles part 1 - First of all, why did I call this paragraph part 1? Well, because there’s a part 2 of course. Beta decay mainly comes in two types: beta-minus decay and beta-plus decay (very original names). Both exist to achieve a more stable neutron-proton ratio. Ok, so first, beta-minus decay. An unstable nucleus, looking to get rid of some excess neutrons, emits an high-energy electron and an antineutrino*. This process changes one of the neutrons in the nucleus to a proton. We have learned previously, that the proton number is what determines the identity of an element. This means that the element will change after undergoing beta decay. For example, we take our old (and a bit unstable) friend, carbon-14, which has 6 protons and 8 neutrons, and then it undergoes beta decay. One of its neutrons turns into a proton and voila - we’ve got nitrogen, with 7 neutrons and 7 protons. 🫢 That’s crazy.

*antineutrinos are the antiparticles to neutrinos, which are light, fast and neutral particles. They are emitted to make sure that the conservation of energy law is obeyed and other totally not complicated stuff like that. Basically, it’s required by the laws of physics.

Beta particles part 2 - In beta-plus decay, the nucleus emits a positron (the antiparticle to an electron - so it’s basically the same but has the opposite charge - positive) and a neutrino. This time, we’re replacing an unwanted proton 🥺. A proton changes into a neutron. For example, let’s take carbon-10, yet another carbon isotope, and see what happens when it undergoes beta-plus decay. It has 6 protons and 4 neutrons. A proton changes into a neutron so the nucleus is now home to 5 protons and 5 neutrons. We have boron-10. Ok, now we’ve looked at both of the main types of beta decay, the question is, is beta radiation dangerous? Like alpha particles, when inside the body, beta particles can do some damage to cells and organs. However, beta radiation is less ionising than alpha particles as beta particles have a lower magnitude or “amount” of charge and are smaller. Still, try not to inhale any beta particles - it won’t go well. We also mentioned penetration ability. Beta particles are able to travel further, and through more than alpha particles, because of them being smaller and lighter. They can actually enter the skin, and if exposed to for a while, can cause burns. After a few centimetres of body tissue or a few millimetres of aluminium, beta particles are stopped.

Gamma rays - These aren’t particles, as you can probably tell by the lack of the word “particles” after the word “Gamma”. These are high energy electromagnetic waves. Fancy. Put simply, they are waves with no mass, that travel at the speed of light, which is about 3x10^8m/s (so, fast). Now, gamma rays usually are emitted after a nucleus has gone through beta or alpha decay and is in an excited state (it has extra energy). The atom is still the same element (with the same number of neutrons and protons) after emitting gamma rays, but the nucleus goes to a state of relaxation after all that exhausting decaying. Because gamma rays are massless and don’t have charge, they are they most penetrating (they are absorbed by several centimetres of lead or about 1 metre of concrete). Lucky for us, they are the least ionising. While it’s still preferable to stay away from gamma radiation, it’s less damaging than alpha and beta radiation.

Why are some elements more unstable than others?

Some elements are more angry and in need of a serious mediation session or decay as the cool particles say. Tis (yes, I’m Shakespeare) time for my favourite question that has made me an annoyance to most of my teachers: Why?

A big one is neutron-to-proton ratio. That needs to be balanced. The less balanced it is, the more the atom wants to get it balanced.

We’ve talked about the strong nuclear force and how it works in regards to nucleons. Now, neutrons (given that they are a nucleon) provide the strong nuclear force. If we don’t have enough neutrons, there is less force holding the nucleus together, therefore there is less resistance against the strong repulsion between protons. Less neutrons also means less space between proton enemies. This makes the nucleus unstable.

What about too many neutrons? Surely that’s good right? No. It’s not. And the reason is excruciatingly complicated (at least in my opinion) so enjoy! Ok, remember how we briefly mentioned energy levels for electrons? Enter the nuclear shell model. This says that the nucleus is made up of different shells with different energy levels. Now, particles like to be in the lowest energy level they can be in (they’re lazy like that). More neutrons means that excess ones are going to have to go to higher energy levels, making the nucleus feel a bit unbalanced and irritated. Let’s try an analogy because even I’m confused. Imagine the nucleus like bunk beds (stay with me). The nucleus is like a stack of beds. Each bed can only hold certain amounts of children (nucleons). The lower beds (shells) get filled up first because they require less energy to get to. As more children arrive, the higher you have to go and the higher you go, the more uncomfortable you get. So uncomfortable that you need that mediation session I was talking about earlier. Now, this is a really simple explanation on a way more complex topic so I suggest you do a little reading of your own to help you understand. Hint: the Pauli exclusion principle. Ok, time to move on before I self-combust.

What is a half-life and why is it useful?

Definition - Half-life is the average time it takes for the number of unstable nuclei to halve. Great! So, what does that mean again? Nuclear decay is random. You cannot predict when a singular nucleus will decay, but lets say you had millions of them. All angry and in need of some stress release. You could predict how long it would take half of them to decay (or calm down, as I like to call it). For example, the half-life of carbon-14 is roughly 5,730 years. Let’s say we started with 200 carbon-14 unstable nuclei. After 5,730 years, approximately 100 unstable nuclei would be left. After 11,460 years (2 half-lives), roughly 50 unstable nuclei would be left (half of 100, not 200). What’s next? After 17,190 years (3 half-lives), how many would be left? Well done you, it is in fact 25 (if you didn’t get that right, we’ll just pretend you did, it’s fine).

Formula - Don’t worry, if we had to find out the remaining number of unstable nuclei after say 24 half-lives, we wouldn’t have to go through all the half-lives to get our answer. The fraction of unstable nuclei remaining is calculated by doing 1/2^n. Where n is the number of half-lives that have passed. Let’s go back to our example. After 3 half-lives, 25/200 remain. That’s 1/8. 1/2^3 is 1/8. So, we know this little trick works. phew. This is a simplified version of the formula…

N(t) = N₀ × (1/2)^(t / t₁/₂)

where N(t) = quantity of substance remaining

N₀ = initial quantity of substance remaining

t = time elapsed

and t₁/₂ (ignore the dodgy slash) = half-life of substance.

Let’s substitute in our values from the previous example.

So, N(t) = 200 x (1/2)^(17,190/5,730).

Let’s simplify this because big numbers = scary.

N(t) = 200 x (1/2)^3

N(t) = 200 x 1/8

N(t) = 25

Look at that! With a bit of ~~magic~~ maths, we discovered that the formula does indeed work. Yay! 🥳

Why is half-life useful? - ****Ok, so we’ve done some maths and explained what half-life means. But why does half-life even matter? Well, there are a few reasons. Knowing how long it takes for radioactive materials to decay is useful because then, we know how long to stay away from it so we don’t turn into radioactive zombies (that doesn’t actually happen…probably). And if we know that there’s a particular radioactive substance that doesn’t stay dangerous for a long time, such as Technetium-99 with a half-life of just 6 hours, we can use them in medicine (we’ll talk about this later). Also, remember carbon-14? We use it’s half-life in archaeology to determine the age of old fossils and ancient artifacts. There’s so many more uses, but you get the idea. Half-life = pretty useful. 👍

What is nuclear binding energy, and why is it so powerful?

Nuclear binding energy is the amount of energy that is required to break up a nucleus into individual nucleons (protons and neutrons) or, also, the amount of energy that is released when nucleons come together. Ok, cool.

But why is it so powerful? - After all, them nucleons do be really tiny. But…Einstein famously showed that E = mc^2. (Energy = mass x speed of light^2). So even though the mass of nucleons are really low, the speed of light squared is kind of a big number. So even 0.001kg (1g) of mass can (in theory) be converted into 9 x 10^13 joules of energy - E = 0.001(m) x (3 x 10^8)^2.

Explain please, I’m confused - If you are confused, don’t worry, you aren’t the only one. What does E = mc^2 have to do with the power of binding energy? Well, the equation basically means that mass and energy are two forms of the same stuff. So they are interchangeable. Now, when nucleons come together, the total mass is lower than the mass of all of them individually. 🤯 This is known as the mass defect. Now, where does this mass go? It is released as energy. And, as we learned previously, that small difference in mass equates to large amounts of energy so the power of the binding energy is really quite great.

Strong force (again) - But what about if you wanted to separate nucleons? Why does that require energy? Remember the strong nuclear force? I know you missed it so let’s bring it back. We know the strong force is strong (really is a shocker in my opinion). That means to overcome it, we need lots of energy. And don’t forget, binding energy is also the energy required to break up a nucleus. So the binding energy is super powerful here too. Overall, binding energy = powerful stuffs.

(not so) Quick analogies - Putting nucleons together is sort of like lego…ish. You’re putting 4 pieces of lego together. The total volume of them separately is greater than all of them stacked together because you lose the volume of the connecting bits. Now, that click! you hear when connecting pieces is like the energy released when nucleons bind - due to the mass (or volume in this case) defect.

Taking nucleons apart is like using force to separate two magnets - you need to put more effort in if the magnetic force is strong. So, nucleons need more energy to overcome the strong strong force (the literal strongest force in nature).

I may have made this more confusing. Sorry about that. haha 🤗