Physics (0625) | Radioactivity

Radioactivity: Unlocking the Secrets of Unstable Atoms (IGCSE Physics 0625)

Welcome to the fascinating world of Radioactivity! This chapter explains how certain unstable atoms spontaneously release energy and particles. It might sound complex, but radioactivity is everywhere—from natural rocks to smoke alarms and medical scanners.
We will learn what causes this radiation, the different types, how we measure it, and most importantly, how to use it safely.

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1. The Nature of Radioactive Emission (5.2.2 Core)

Radioactivity is all about unstable atomic nuclei trying to become stable by spitting out excess energy or particles. This process is called Radioactive Decay.

Key Properties of Decay

There are two critical characteristics of radioactive decay that you must remember:

1. Spontaneous: This means the decay happens without being caused by external factors (like heating, pressure, or chemical reactions). You cannot predict when a specific nucleus will decay.
2. Random: This means we cannot predict which specific nucleus in a sample will decay next. It is purely down to chance.

Analogy: Imagine a huge bowl of popcorn. It is spontaneous because you don't know exactly when any individual kernel will pop. It is random because you cannot point at one kernel and say, "That one will pop next!"

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2. Background Radiation (5.2.1 Core & Supplement)

We are constantly exposed to small amounts of radiation from our surroundings. This is called Background Radiation.

Sources of Background Radiation (5.2.1 Core)

It is essential to know the main sources of this natural and artificial background radiation:

• Radon Gas (in the air): This is usually the largest natural contributor. It seeps out of rocks and soil, especially granite.
• Rocks and Buildings: Small amounts of radioactive isotopes (like uranium and thorium) are found naturally in rocks, bricks, and concrete.
• Food and Drink: We consume tiny amounts of radioactive isotopes (like Potassium-40) in our diet.
• Cosmic Rays: High-energy particles entering the Earth's atmosphere from space.

Measuring Radiation (5.2.1 Core & Supplement)

Ionising nuclear radiation is typically measured using a Geiger-Müller (G-M) tube connected to a counter (a scaler).

• The detector measures the number of decays that occur per second or per minute. This measure is called the Count Rate (measured in counts/s or counts/min).

Calculating the Corrected Count Rate (5.2.1 Supplement):
Since the detector always picks up background radiation, even when no source is present, we must subtract this background reading to find the radiation coming only from the source.

Step 1: Measure the background count rate (count rate with no source nearby).
Step 2: Measure the total count rate from the source.
Step 3: Calculate the corrected count rate:

Corrected Count Rate = Total Count Rate – Background Count Rate

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3. The Three Emissions: Alpha, Beta, and Gamma (5.2.2 Core & Supplement)

There are three main types of radiation emitted during radioactive decay: Alpha ($\alpha$), Beta ($\beta$), and Gamma ($\gamma$).

3.1 Alpha Particles ($\alpha$)

• Nature: Consists of 2 protons and 2 neutrons. This is the same as a Helium nucleus (\( ^4_2 He \)).
• Charge: +2 (Positive, due to the two protons).
• Mass: Relatively heavy.

Analogy: Think of an Alpha particle like a slow, heavy cannonball.

3.2 Beta Particles ($\beta$)

• Nature: A fast-moving electron (\( ^0_{-1} e \)) emitted when a neutron turns into a proton and an electron inside the nucleus. (We only study $\beta^-$ particles).
• Charge: -1 (Negative).
• Mass: Very small (negligible mass).

Analogy: Think of a Beta particle like a light, fast-moving bullet.

3.3 Gamma Radiation ($\gamma$)

• Nature: High-frequency electromagnetic wave (like X-rays or light, but much more energetic).
• Charge: 0 (Neutral).
• Mass: 0 (A pure energy photon).

Analogy: Think of Gamma radiation like an invisible, high-energy light wave.

Comparison Table (5.2.2 Core)

The syllabus requires you to know their relative ionising effects and penetrating abilities.

Property	Alpha ($\alpha$)	Beta ($\beta$)	Gamma ($\gamma$)
Penetration (Stopping material)	Low (Stopped by paper or a few cm of air)	Medium (Stopped by thin aluminium (~5 mm))	High (Only reduced by thick lead or concrete)
Ionising Effect	Strongest (High kinetic energy, high charge)	Medium	Weakest (No charge, pure energy)

Explanation of Ionisation (5.2.2 Supplement):
Ionisation is the ability of the radiation to knock electrons out of atoms, creating ions.

• $\alpha$ particles are the best ionisers because they are heavy, slow, and have a high electric charge (+2). This means they spend more time interacting with the atoms they pass, causing maximum damage.
• $\gamma$ radiation is the weakest ioniser because it has no charge and often passes right through atoms without interacting.

Deflection in Fields (5.2.2 Supplement)

Because $\alpha$ and $\beta$ particles are charged, they are affected by electric fields (E) and magnetic fields (M), while neutral $\gamma$ rays are not.

1. Electric Fields:

• $\alpha$ (Positive) is attracted towards the negative plate.
• $\beta$ (Negative) is attracted towards the positive plate.
• $\gamma$ (Neutral) passes straight through, undeflected.

Since $\alpha$ is much heavier than $\beta$, the lighter $\beta$ particles are deflected much more sharply and in the opposite direction.

2. Magnetic Fields:

• $\alpha$ and $\beta$ particles are deflected in opposite directions (due to their opposite charges).
• You can find the direction of deflection using Fleming's Left-Hand Rule (for $\beta$ particles) or Right-Hand Rule (for positive $\alpha$ particles) if treated as a current flow.
• $\gamma$ radiation is undeflected.

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4. Radioactive Decay Equations (5.2.3 Supplement)

When an unstable nucleus decays, it changes its composition. We use Nuclide Notation to track these changes: \( ^A_Z X \).

• A is the Nucleon Number (Mass Number) = Protons + Neutrons.
• Z is the Proton Number (Atomic Number) = Number of Protons (which determines the element, X).

Alpha Decay (5.2.3 Supplement)

The nucleus emits an $\alpha$ particle (\( ^4_2 He \)).

• The Nucleon Number (A) decreases by 4.
• The Proton Number (Z) decreases by 2 (changing the element).

General Equation:

\( ^A_Z X \rightarrow ^{A-4}_{Z-2} Y + ^4_2 He \)

Example: Uranium-238 decaying into Thorium-234:

\( ^{238}_{92} U \rightarrow ^{234}_{90} Th + ^4_2 He \)

Beta Decay (5.2.3 Supplement)

A neutron turns into a proton and an electron (the $\beta$ particle, \( ^0_{-1} e \)). This happens in isotopes with an excess of neutrons (5.2.3 Supplement).

• The Nucleon Number (A) stays the same (we lost a neutron but gained a proton).
• The Proton Number (Z) increases by 1 (changing the element).

The change in the nucleus during $\beta$-emission is:
neutron $\rightarrow$ proton + electron

General Equation:

\( ^A_Z X \rightarrow ^A_{Z+1} Y + ^0_{-1} e \)

Example: Carbon-14 decaying into Nitrogen-14:

\( ^{14}_6 C \rightarrow ^{14}_7 N + ^0_{-1} e \)

Gamma Emission (5.2.3 Supplement)

Gamma emission often happens straight after $\alpha$ or $\beta$ decay when the nucleus is still in an excited state and needs to shed energy.

• The Nucleon Number (A) does not change.
• The Proton Number (Z) does not change (the element remains the same).

General Equation:

\( ^A_Z X^* \rightarrow ^A_Z X + \gamma \) (where * indicates an excited state)

Key Takeaway on Decay: Only $\alpha$ and $\beta$ decay change the nucleus into a nucleus of a different element (5.2.3 Core).

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5. Half-Life (5.2.4 Core & Supplement)

Since radioactive decay is random and spontaneous, we cannot predict the life of a single nucleus. Instead, we use the concept of Half-Life.

Definition and Calculation (5.2.4 Core)

The half-life is the time taken for half the nuclei of that isotope in any sample to decay.

It is also defined as the time taken for the activity (count rate) of a sample to fall to half its initial value.

Example: If a radioactive sample has 100 g of material and a half-life of 5 days:

• After 5 days (1 half-life): 50 g remains.
• After 10 days (2 half-lives): 25 g remains.
• After 15 days (3 half-lives): 12.5 g remains.

Core Calculation Example:
A source has an initial count rate of 800 counts/min and a half-life of 2 hours. What is the count rate after 6 hours?

1. Calculate number of half-lives: 6 hours / 2 hours = 3 half-lives.
2. Initial Rate = 800
3. After 1st HL: 800 / 2 = 400
4. After 2nd HL: 400 / 2 = 200
5. After 3rd HL: 200 / 2 = 100 counts/min.

Note: Core calculations usually ignore background radiation.

Calculating Half-Life from Decay Curves (5.2.4 Supplement)

If you are given a graph of count rate vs. time, you must determine the half-life even if background radiation has *not* been subtracted.

Step-by-step method:

1. Determine the constant background count rate (usually the count rate when the curve flattens out, or if specified in the question).
2. Calculate the starting corrected count rate (Total initial rate - Background rate).
3. Calculate half of this corrected rate.
4. Add the background rate back to this value. This gives the 'target' total count rate you need to find on the Y-axis.
5. Read the time from the X-axis corresponding to this target rate. This time is the half-life.

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6. Applications of Radioactivity (5.2.4 Supplement)

The type of radiation and the length of the half-life are key factors in choosing an isotope for a specific job.

A. Short Half-Life, Penetrating Radiation (Medical Diagnosis)

Isotopes used in medicine (diagnosis and treatment) must have short half-lives so they decay quickly and don't remain in the body long enough to cause excessive damage.

• Diagnosis and Treatment of Cancer: Gamma sources are used because gamma radiation is highly penetrating and can pass out of the body (for scanning) or deeply into tissue (for radiotherapy).

B. Long Half-Life, Weak Penetration (Smoke Alarms)

Smoke alarms use a small source of Alpha radiation.

• The alpha source ionises the air between two electrodes, allowing a current to flow.
• If smoke enters, it absorbs the alpha particles, the ionisation stops, the current drops, and the alarm sounds.
• Alpha is chosen because it is easily stopped by smoke, and the source needs a very long half-life so it doesn't need replacing frequently (5.2.4 Supplement).

C. Thickness Control (Industrial)

Radiation is used to monitor and control the thickness of materials like paper or metal foil.

• A radioactive source is placed on one side of the material and a detector on the other.
• If the material gets too thick, the count rate drops (more absorption), and rollers are adjusted.
• Beta radiation is used for paper or thin foil because it is partially absorbed, allowing small thickness changes to be measured accurately.
• Gamma radiation might be used for thick steel plates due to its high penetration.

D. Sterilisation and Food Irradiation

Gamma sources (often Cobalt-60, which has a moderately long half-life) are used.

• Irradiating food to kill bacteria: Gamma rays pass completely through the food, killing microorganisms without making the food itself radioactive.
• Sterilisation of equipment: Gamma rays sterilise medical equipment (like syringes) efficiently and safely, as they destroy bacteria.

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7. Safety Precautions (5.2.5 Core & Supplement)

Exposure to ionising nuclear radiations can be harmful to living things, causing:

• Cell death: High doses kill large numbers of cells immediately (e.g., radiation burns).
• Mutations and Cancer: Lower doses can damage DNA, leading to uncontrolled cell division (cancer) or genetic changes (mutations).

Safe Handling of Radioactive Materials (5.2.5 Core & Supplement)

Radioactive materials must be handled, moved, and stored carefully to reduce exposure. The three main safety principles are summarised by the T-D-S rule:

T - Time: Reduce the exposure time.

• Use sources for the shortest possible time during experiments.

D - Distance: Increase the distance between the source and living tissue.

• Use long handling tongs when moving sources.
• Keep sources stored in locked boxes away from people.
• Radiation intensity decreases rapidly with distance (inverse square law, although you don't need to calculate it, the concept is key).

S - Shielding: Use shielding to absorb the radiation.

• Sources should be stored in thick lead-lined containers (a lead pot).
• The specific type of shielding depends on the radiation: paper for alpha, aluminum for beta, thick lead/concrete for gamma.

All waste radioactive materials must be disposed of safely, often involving long-term storage deep underground or monitored disposal, depending on the half-life.

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Key Takeaways Summary

You've covered the core concepts of radioactivity! Remember that decay is spontaneous and random. Alpha, Beta, and Gamma have dramatically different properties regarding their penetrating power and ability to ionise. Finally, safety relies on minimizing Time, maximizing Distance, and using appropriate Shielding. Keep practising those half-life calculations!