Welcome to the World of Nuclear Physics!
In this chapter, we are going to explore Radioactive Decay. While the word "radioactive" might sound scary (thanks to superhero movies!), it is actually a natural process that happens all around us. Think of it as unstable atoms trying to "relax" and find a more stable state. By the end of these notes, you will understand how atoms change, what they emit, and how we measure this process.
1. The Nature of Radioactive Decay
Radioactive decay is the process where an unstable nucleus loses energy by emitting radiation. There are two very important words you must remember about how this happens: Spontaneous and Random.
Spontaneous Nature
This means the decay is not affected by any external factors. Whether you heat the substance, freeze it, or put it under high pressure, the rate of decay stays exactly the same. It is an internal process of the nucleus.
Random Nature
We cannot predict exactly which nucleus will decay next or when a specific nucleus will decay. However, for a large number of atoms, we can predict a constant average rate of decay.
Analogy: The Popcorn Example
Think of making popcorn. You know that eventually, almost all the kernels will pop. However, you can’t point to one specific kernel and say, "That one will pop in exactly 3 seconds!" Also, shaking the pan harder won't make a specific kernel pop if it's not ready. That is exactly like radioactive decay!
Quick Review: Evidence for Randomness
We know decay is random because if you use a radiation detector (like a Geiger-Muller tube), the count rate fluctuates (it goes up and down slightly) over time, rather than staying perfectly steady.
Key Takeaway: Decay is spontaneous (ignores the environment) and random (unpredictable for individual atoms).
2. Activity and Background Radiation
Activity is the rate at which a source of radioactive nuclei decays. It is measured in Becquerels (Bq), where \(1 \text{ Bq} = 1 \text{ decay per second}\).
Background Radiation
Did you know you are being hit by radiation right now? This is Background Radiation. It comes from natural sources like rocks (radon gas), cosmic rays from space, and even the potassium in bananas! When doing physics experiments, we must always measure the background radiation first and subtract it from our readings to get the "true" activity of a sample.
Key Takeaway: Total Count Rate - Background Count Rate = Corrected Count Rate.
3. The Three Types of Radiation
Unstable nuclei usually spit out one of three things to become stable. Let’s compare them:
1. Alpha (\(\alpha\)) Particles:
- What is it? A Helium nucleus (\(^4_2\text{He}\)). It has 2 protons and 2 neutrons.
- Charge: \(+2\)
- Ionising Power: Very High (it’s big and crashes into things easily).
- Penetrating Power: Low (stopped by a sheet of paper or a few cm of air).
2. Beta (\(\beta\)) Particles:
- What is it? A high-speed electron (\(^0_{-1}\text{e}\)).
- Charge: \(-1\)
- Ionising Power: Moderate.
- Penetrating Power: Moderate (stopped by a few mm of Aluminum).
3. Gamma (\(\gamma\)) Rays:
- What is it? High-energy electromagnetic waves (\(^0_0\gamma\)).
- Charge: \(0\) (Neutral).
- Ionising Power: Low.
- Penetrating Power: Very High (stopped only by thick lead or several meters of concrete).
Memory Aid:
Alpha is like a bowling ball (heavy, causes damage, stops quickly).
Beta is like a ping-pong ball (lighter, travels further).
Gamma is like a ghost (no mass, goes through almost everything!).
Key Takeaway: As ionising power increases, penetrating power usually decreases.
4. Understanding Half-Life (\(t_{1/2}\))
Half-life is the time taken for a quantity (like the number of undecayed nuclei or the activity) to reduce to half of its initial value.
Don't worry if this seems tricky! You don't need fancy calculus for H1 Physics. You just need to be able to "jump" by halves.
Step-by-Step Example:
If a sample has an activity of \(800 \text{ Bq}\) and its half-life is \(2 \text{ hours}\), what will the activity be after \(6 \text{ hours}\)?
1. Find the number of half-lives: \(6 \text{ hours} / 2 \text{ hours} = 3 \text{ half-lives}\).
2. Start at \(800 \text{ Bq}\).
3. After 1st half-life: \(400 \text{ Bq}\).
4. After 2nd half-life: \(200 \text{ Bq}\).
5. After 3rd half-life: \(100 \text{ Bq}\).
Quick Review: Decay Curves
On a graph of Activity against Time, the curve always looks like a "slide" that gets flatter but never quite touches zero. You can find the half-life by picking a value on the y-axis, finding the time, then picking half of that value and seeing how much more time has passed.
Key Takeaway: Half-life is a constant for a specific isotope. It doesn't matter how much you start with; the time to reach 50% is always the same.
5. Nuclear Equations
When an atom decays, we write a nuclear equation. The golden rule is: The total mass (top) and total charge (bottom) must be the same on both sides.
Example of Alpha Decay:
\(^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^4_2\text{He}\)
Check: Top (\(238 = 234 + 4\)). Bottom (\(92 = 90 + 2\)). Perfect!
Example of Beta Decay:
\(^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^0_{-1}\text{e}\)
Check: Top (\(14 = 14 + 0\)). Bottom (\(6 = 7 - 1\)). Perfect!
Common Mistake to Avoid: In Beta decay, the bottom number (proton number) actually increases by 1 because a neutron turned into a proton!
6. Applications and Hazards
The properties of radiation determine how we use them and why they are dangerous.
Hazards
Radiation is hazardous because it is ionising. It can rip electrons off atoms in your DNA, leading to mutations or cancer.
- Alpha is very dangerous inside the body (high ionisation).
- Gamma is dangerous outside the body because it can pass through your skin to reach your organs.
Applications
1. Medical Tracers: We use isotopes with short half-lives so they don't stay in the patient's body for too long. We usually use Gamma emitters because they can penetrate out of the body to be detected by a camera.
2. Industrial Thickness Gauges: We use Beta radiation to monitor the thickness of paper or aluminum foil. If the foil gets too thick, fewer Beta particles reach the detector on the other side.
Did you know?
Smoke detectors use an Alpha source (Americium-241). The Alpha particles ionise the air to create a current. If smoke enters, it blocks the Alpha particles, the current drops, and the alarm goes off!
Key Takeaway: We choose isotopes based on their penetrating ability and half-life for the specific job.
Summary Checklist
- Can you define spontaneous and random?
- Do you know the charge and mass of Alpha, Beta, and Gamma?
- Can you calculate the remaining activity after several half-lives?
- Can you balance a nuclear equation?
- Do you understand why ionisation makes radiation hazardous?