\(\Large \star\) Study Notes: Radioactivity and Particles \(\Large \star\)
Welcome to the World of Invisible Energy!
Hello everyone! This chapter might sound complex, but radioactivity is actually a fascinating topic about unstable atoms trying to find balance. Don't worry if this seems tricky at first; we will break down every concept into small, easy-to-understand pieces. By the end of these notes, you’ll know exactly what radiation is, where it comes from, and how scientists use (and safely manage) this powerful energy. Let's get started!
1. Atoms, Isotopes, and Instability
1.1 Reviewing the Atom
Before we talk about radiation, let’s quickly remember the basic structure of an atom:
- The tiny, central part is the nucleus, which contains protons (positive charge) and neutrons (no charge).
- Electrons (negative charge) orbit the nucleus.
- The number of protons determines the element (e.g., all Carbon atoms have 6 protons).
1.2 What Makes an Atom Radioactive?
Sometimes, atoms of the same element have different numbers of neutrons. These are called isotopes.
Example: Carbon-12 is stable (6 protons, 6 neutrons). Carbon-14 is unstable (6 protons, 8 neutrons).
An isotope is unstable if its nucleus has too many or too few neutrons compared to protons. To become stable, the nucleus must release energy and/or particles. This process is called radioactive decay.
Key Term: Radioactivity is the process by which an unstable atomic nucleus spontaneously emits radiation to become more stable.
\(\Large \star\) Quick Review: The Source of Radioactivity \(\Large \star\)
Radiation comes only from the nucleus of an unstable atom. It has nothing to do with the orbiting electrons!
2. The Three Types of Radiation
When an unstable nucleus decays, it can release three main types of radiation: Alpha, Beta, and Gamma. You must know their nature (what they are), their penetrating power, and their ionising ability.
2.1 Alpha Particles (\(\alpha\))
Nature: An alpha particle is the same as a helium nucleus (2 protons and 2 neutrons stuck together). They are relatively large and heavy.
Penetrating Power: Very Low. They are easily stopped.
Analogy: Imagine a large, slow football. It stops immediately when it hits a thin sheet.
Stopping Material: A simple sheet of paper, or a few centimetres of air. They cannot penetrate skin.
Ionising Power: Very High. Because they are large and charged (+2), they smash into other atoms and knock electrons off, causing a lot of damage over a short distance.
Safety Note: Alpha sources are safe outside the body but extremely dangerous if ingested (eaten or breathed in) because the high ionisation occurs directly inside delicate body tissue.
2.2 Beta Particles (\(\beta\))
Nature: A beta particle is a high-speed electron ejected from the nucleus when a neutron changes into a proton (and releases the electron).
Penetrating Power: Medium. They are much smaller and faster than alpha particles.
Stopping Material: A few millimetres of aluminium or other thin metals.
Ionising Power: Medium. They cause less ionisation damage than alpha particles because they are smaller and faster, meaning they are less likely to interact with surrounding atoms.
2.3 Gamma Rays (\(\gamma\))
Nature: Gamma rays are part of the electromagnetic spectrum (like light or radio waves). They have no mass and no charge; they are pure energy. They are often emitted alongside alpha or beta decay.
Penetrating Power: Very High. They are extremely difficult to stop.
Analogy: Imagine a flash of pure light energy – it passes right through most materials.
Stopping Material: Several centimetres of dense material, like thick lead or concrete.
Ionising Power: Very Low. Because they are pure energy and interact only rarely with atoms, they cause the least ionisation damage per meter traveled.
Summary Table: Properties of Radiation
| Radiation Type | Nature | Charge | Penetrating Power | Ionising Power |
| Alpha (\(\alpha\)) | Helium nucleus (2p, 2n) | +2 | Low (Stopped by paper) | Highest |
| Beta (\(\beta\)) | Fast electron | -1 | Medium (Stopped by Aluminium) | Medium |
| Gamma (\(\gamma\)) | Electromagnetic Wave (Photon) | 0 | Highest (Reduced by Lead/Concrete) | Lowest |
Memory Tip: Think of the alphabet: A, B, C (Gamma comes after Beta!). A is the weakest penetrator, G (Gamma) is the strongest penetrator.
3. Detection, Background, and Safety
3.1 Detecting Radiation
Since we cannot see, smell, or feel radiation, we need special equipment to detect it. The most common device is the Geiger-Müller (GM) tube (often connected to a counter).
How it Works: The radiation enters the GM tube and causes ionisation of the gas inside. This produces a small pulse of electrical current, which the counter registers as a "click" or a "count."
The count rate is measured in counts per second (cps) or counts per minute (cpm).
3.2 Background Radiation
Radiation is all around us! Even before you turn on a radioactive source in the lab, the GM counter will register a few clicks. This is called background radiation.
Sources of Background Radiation:
- Cosmic Rays: High-energy particles from space.
- Natural Rocks and Soil: Rocks containing radioactive elements (like granite).
- Radon Gas: Gas seeping out of the ground (main source in many areas).
- Food and Drink: Trace amounts of radioactive isotopes in our diet.
Important Step: When conducting experiments, you must always measure the background count rate first and then subtract it from your final measurement. This gives you the count rate only from your source.
3.3 Hazards and Safety Precautions
The Hazard: Radiation is hazardous because ionisation damages the DNA inside cells, potentially leading to cell death, mutations, or cancer.
Safety Measures (The 3 Cs):
- Containment/Control: Use suitable shielding (Paper for Alpha, Aluminium for Beta, Lead for Gamma).
- Close Distance Reduction (Distance): Keep sources far away using tongs, and work behind screens. The intensity of radiation decreases rapidly with distance.
- Clock Management (Time): Minimise the time spent near the source.
Did you know? People who work regularly with radiation (like hospital staff) wear special badges that monitor their total radiation exposure over time.
Key Takeaway
Radiation must be measured by subtracting background levels. Safety relies on controlling Time, Distance, and Shielding.
4. The Concept of Half-Life
Radioactive decay is random; we cannot predict exactly when one specific nucleus will decay. However, we can measure the average time it takes for a large sample to decay.
4.1 Defining Half-Life (\(T_{1/2}\))
The half-life is the time it takes for:
- Half of the original unstable nuclei in a sample to decay.
- The activity (the count rate) of the sample to fall to half of its original value.
Analogy: The Popcorn Analogy
Imagine a bag of unpopped popcorn is the "radioactive sample." You heat it, and kernels pop (decay). You can't predict when any single kernel pops, but you can measure that it takes, say, 2 minutes for half the kernels to pop. That 2 minutes is the half-life. After another 2 minutes, half of the *remaining* kernels will pop.
4.2 Calculations (Simple Examples)
Half-life problems usually involve simple halving:
Example: A source has an initial activity of 800 counts per second (cps) and a half-life of 2 days. What is the activity after 6 days?
Step 1: Determine how many half-lives have passed.
\(6 \text{ days} / 2 \text{ days per half-life} = 3 \text{ half-lives}\)
Step 2: Halve the activity for each half-life.
Start: 800 cps
After 1st half-life (2 days): \(800 / 2 = 400\) cps
After 2nd half-life (4 days): \(400 / 2 = 200\) cps
After 3rd half-life (6 days): \(200 / 2 = 100\) cps
Answer: The activity after 6 days is 100 cps.
Crucial Point: Radioactive decay never stops completely. The activity will always halve, but it never reaches exactly zero.
5. Uses of Radioactivity
Despite the dangers, radiation is extremely useful in medicine and industry, depending on the type of radiation and the half-life of the source.
5.1 Medical Uses
Medical Tracers (Diagnosis)
A short-lived radioactive source is injected into the body (often Gamma-emitters, as Gamma rays pass easily out of the body to be detected by external cameras). The tracer moves through the body, allowing doctors to monitor the function of organs (like the kidneys or thyroid).
Requirement: Tracers must have a short half-life (hours) so they decay quickly and limit patient exposure once the diagnosis is complete.
Radiotherapy (Treatment)
Gamma rays or high-energy Beta particles are used to destroy cancerous tumours. The radiation is carefully directed at the tumour to minimise damage to surrounding healthy tissue.
5.2 Industrial Uses
Sterilisation
Gamma rays are highly penetrating and are used to sterilise medical equipment (like syringes) and sometimes food. Gamma rays can kill all microbes without generating heat, keeping the equipment sterile even after packaging.
Thickness Gauges
Beta sources are often used in factories (e.g., paper mills) to monitor the thickness of materials as they are being produced.
- The source is placed on one side of the material, and a detector is on the other.
- If the material gets too thick, less radiation reaches the detector, triggering a warning to adjust the rollers.
Why Beta? Alpha radiation would be stopped completely, and Gamma radiation would pass through the thin sheet largely unaffected. Beta radiation is the ‘just right’ medium penetration.
Common Mistake to Avoid: Do not confuse a short half-life source (good for tracers) with a long half-life source (bad for tracers, but often found in rocks or used for things like smoke alarms where consistency is key).
\(\Large \star\) Final Key Takeaway \(\Large \star\)
Radioactivity is simply unstable atoms breaking down. We classify the resulting radiation by its ability to penetrate matter and its ability to cause damage (ionisation). The rate of this breakdown is measured by the half-life.