Science (Double Award) | Radioactivity and particles

Welcome to Radioactivity and Particles!

Hello future Physicists! This chapter might sound intimidating, but don't worry—it deals with the tiny core of the atom, the nucleus, and the powerful energy it can release. We will break down complex concepts like radioactive decay and half-life into manageable steps.

Understanding radioactivity is crucial, not just for passing exams, but because it explains how nuclear power works, how doctors treat cancer, and why the Sun shines! Let's dive into the fascinating world of unstable atoms.

1. Atomic Structure Refresher & Isotopes

1.1 The Nucleus: The Heart of the Atom

Before we discuss radioactive decay, let’s quickly remind ourselves about the atom:

• The atom consists of a tiny, dense centre called the nucleus.
• The nucleus contains two types of particles: protons (positive charge, mass = 1) and neutrons (no charge, mass = 1).
• Electrons orbit the nucleus (negative charge, negligible mass).

When writing nuclear symbols, we use two key numbers:
\[ \begin{array}{c} \text{Mass Number (A: Protons + Neutrons)} \\ \text{Atomic Number (Z: Number of Protons)} \end{array} \]

1.2 Isotopes: Different Flavors of the Same Atom

Atoms of the same element always have the same number of protons (the same Atomic Number, Z). But they can have different numbers of neutrons.
Key Term: Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A).

Analogy: Imagine all hydrogen atoms are brothers. They all have one proton (same name/element). But one brother might weigh 1 kg, another 2 kg, and another 3 kg, depending on how many neutrons they have.

Why does this matter?
Many isotopes are unstable because they have too many or too few neutrons compared to protons. These unstable isotopes are called radioisotopes, and they decay by emitting radiation to become stable.

2. Radioactive Decay: Random and Spontaneous

2.1 The Nature of Decay

Radioactive decay is the process where an unstable nucleus breaks down. This process is:

• Random: We cannot predict *when* a specific nucleus will decay. It's totally down to chance.
• Spontaneous: The decay is not affected by outside conditions like temperature, pressure, or chemical bonding. We can't speed it up or slow it down.

Quick Review: An unstable nucleus is looking for stability, and it achieves this by throwing out energy or particles—this is what we call radiation.

2.2 The Three Types of Radiation (\(\alpha\), \(\beta\), \(\gamma\))

The three main types of radiation emitted are Alpha, Beta, and Gamma.

Alpha Radiation (\(\alpha\))

Alpha particles are the largest and heaviest type of radiation.

• Nature: A helium nucleus (2 protons and 2 neutrons).
• Symbol: \(^4_2\text{He}\)
• Charge: +2 (positive)
• Speed: Slowest (travels only a few cm in air).

Memory Tip: Think of Alpha as the big, slow, but very damaging cannonball.

Beta Radiation (\(\beta\))

Beta particles are essentially high-speed electrons. They are released when a neutron inside the nucleus converts into a proton.

• Nature: A fast-moving electron.
• Symbol: \(^0_{-1}\text{e}\) or \(^0_{-1}\beta\)
• Charge: -1 (negative)
• Speed: Very fast (close to the speed of light).

Did you know? The electron does not come from the orbiting electrons; it is created when a neutron breaks apart!

Gamma Radiation (\(\gamma\))

Gamma radiation is pure energy, not a particle. It usually happens immediately after alpha or beta decay when the nucleus rearranges itself into a more stable state.

• Nature: Electromagnetic wave (a high-energy photon).
• Symbol: \(\gamma\)
• Charge: 0 (neutral)
• Speed: The speed of light.

Memory Tip: Gamma is like light, but with immense energy.

2.3 Comparing the Radiations (Penetration & Ionisation)

The ability of radiation to pass through materials is called penetrating power. The ability to knock electrons off atoms (and thus cause damage to living tissue) is called ionising power.

Shielding and Penetrating Power

Radiation	Penetration	What Stops It?
Alpha (\(\alpha\))	Very Low	A sheet of paper, or a few centimeters of air.
Beta (\(\beta\))	Medium	A thin sheet of aluminum (or other metal).
Gamma (\(\gamma\))	Very High	Thick lead or meters of concrete.

Ionising Power (Danger Level)

• Alpha (\(\alpha\)): Very High ionising power. Because it is large and slow, it crashes into atoms and causes massive disruption. (It is very dangerous if ingested or inhaled, as it damages cells locally.)
• Beta (\(\beta\)): Medium ionising power.
• Gamma (\(\gamma\)): Very Low ionising power. It usually passes straight through materials without hitting atoms. (It is very dangerous outside the body because it penetrates deeply.)

3. Nuclear Equations (Balancing the Books)

When a nucleus decays, the laws of physics demand that both the Mass Number (A) and the Atomic Number (Z) must balance on both sides of the equation.

3.1 Alpha Decay Equations

When an atom emits an alpha particle (\(^4_2\text{He}\)), the daughter nucleus (the product) must change:

• Mass Number (A) decreases by 4.
• Atomic Number (Z) decreases by 2.

Step-by-Step Example (Radium-226 decaying into Radon):
1. Start: \({}^{226}_{88}\text{Ra}\)
2. Subtract the alpha particle (\(^4_2\text{He}\)):
\[{}^{226}_{88}\text{Ra} \longrightarrow {}^{A}_{Z}\text{X} + {}^4_2\text{He}\]
3. Calculate A: \(226 - 4 = 222\)
4. Calculate Z: \(88 - 2 = 86\)
5. Find element with Z=86 (which is Radon, Rn):
\[{}^{226}_{88}\text{Ra} \longrightarrow {}^{222}_{86}\text{Rn} + {}^4_2\text{He}\]

3.2 Beta Decay Equations

When an atom emits a beta particle (\(^0_{-1}\text{e}\)), the daughter nucleus changes because a neutron turns into a proton.

• Mass Number (A) stays the same (0 mass change).
• Atomic Number (Z) increases by 1 (because you gain a proton).

Step-by-Step Example (Carbon-14 decaying into Nitrogen):
1. Start: \({}^{14}_{6}\text{C}\)
2. Subtract the beta particle (\(^0_{-1}\text{e}\)):
\[{}^{14}_{6}\text{C} \longrightarrow {}^{A}_{Z}\text{X} + {}^0_{-1}\text{e}\]
3. Calculate A: \(14 - 0 = 14\)
4. Calculate Z: \(6 - (-1) = 6 + 1 = 7\)
5. Find element with Z=7 (which is Nitrogen, N):
\[{}^{14}_{6}\text{C} \longrightarrow {}^{14}_{7}\text{N} + {}^0_{-1}\text{e}\]

4. Half-Life

4.1 Definition and Concept

Since radioactive decay is random, we cannot predict when a single atom will decay. However, for a very large sample of atoms, we can measure how long it takes for half of them to decay.
Key Term: The half-life is the time taken for the number of unstable nuclei in a sample to halve, or for the count rate (activity) from the sample to halve.

Analogy: Imagine you have a pizza (representing the radioactive material). If the half-life is 1 hour, after 1 hour, half the pizza is gone. After another hour, half of the *remaining* pizza is gone (leaving 1/4 of the original).

The half-life can range from fractions of a second (highly unstable) to billions of years (like Uranium).

4.2 Calculation Steps

The calculation usually involves working out the number of half-lives that have passed.

Example: A source has an initial activity of 800 Bq (Becquerels) and a half-life of 2 days. What is the activity after 6 days?
Step 1: Calculate the number of half-lives passed.
\[\text{Number of half-lives} = \frac{\text{Total Time}}{\text{Half-life}} = \frac{6 \text{ days}}{2 \text{ days}} = 3\]
Step 2: Halve the activity the required number of times.
Start: 800 Bq
After 1 half-life (2 days): \(800 \div 2 = 400\) Bq
After 2 half-lives (4 days): \(400 \div 2 = 200\) Bq
After 3 half-lives (6 days): \(200 \div 2 = 100\) Bq
The final activity is 100 Bq.

5. Sources, Hazards, and Uses of Radiation

5.1 Background Radiation

We are constantly exposed to small amounts of radiation from our surroundings. This is called background radiation.
Sources include:

• Natural Sources:
  • Radon gas (from rocks and soil). *The largest natural source.*
  • Cosmic rays (from space).
  • Radioactive elements in food and water (e.g., potassium-40).
• Artificial (Man-made) Sources:
  • Medical procedures (X-rays, CT scans).
  • Nuclear weapons testing fallout.
  • Nuclear power generation/accidents.

5.2 Hazards and Safety

Radiation can cause cell damage, mutations, and cancer because of its ionising power.

Safety Precautions:

• Limiting Dose: The three main rules (Time, Distance, Shielding). Spend less time near the source, maximize distance from the source, and use appropriate shielding (lead, concrete).
• Storage: Radioactive materials must be stored in lead-lined containers and monitored frequently.
• Waste Disposal: High-level waste (e.g., spent fuel rods) requires remote handling and deep burial because of its very long half-life.

5.3 Uses of Radioactivity

Despite the dangers, radiation has many essential applications:

• Medical Tracers: A radioactive isotope with a short half-life is injected into a patient (e.g., Technetium-99m). Its path can be tracked by a detector to diagnose blockages or organ function.
• Sterilisation: Strong gamma sources are used to kill microbes on medical equipment (like syringes) or in food, as gamma radiation is highly penetrating and non-polluting.
• Thickness Gauging: Beta sources are used in industry (e.g., manufacturing paper or aluminum foil). If the sheet gets too thick, less radiation gets through, triggering a mechanism to adjust the rollers.
• Carbon Dating: Using the long half-life of Carbon-14 to estimate the age of archaeological finds.

6. Nuclear Energy: Fission and Fusion

Don't worry, we only need to understand the basic difference between these two massive energy release processes.

6.1 Fission (Splitting)

Nuclear Fission is the splitting of a large, unstable nucleus (like Uranium-235 or Plutonium-239) into two smaller nuclei.
The Process: A slow-moving neutron hits the unstable nucleus, causing it to split. This split releases a huge amount of energy, plus two or three more neutrons. These new neutrons can go on to hit other nuclei, causing a chain reaction.
Application: This process is used in nuclear reactors to generate electricity (the chain reaction is carefully controlled) and in atomic bombs (uncontrolled chain reaction).

6.2 Fusion (Joining)

Nuclear Fusion is the joining of two small, light nuclei (usually isotopes of Hydrogen) to form one larger nucleus.
The Process: This process releases far more energy than fission, but it requires extremely high temperatures (millions of degrees Celsius) and high pressures to overcome the electrostatic repulsion between the positive nuclei.
Application: Nuclear fusion is the source of energy for the Sun and other stars. Scientists are working hard to try and harness controlled fusion on Earth, as it produces less radioactive waste and uses common fuel sources, but it is currently not commercially viable.