Radioactivity

1. Detection of Radioactivity

Sources of Background Radiation

The main sources that contribute to background radiation are:

Natural sources:

1. Radon gas (in the air) – This is the biggest contributor, making up about 50% of background radiation. Radon is:

   • A radioactive gas that comes from rocks in the ground (uranium decays into radon)
   • An alpha emitter
   • Colourless, tasteless, and odourless
   • Particularly dangerous if breathed into the lungs in large amounts

2. Rocks and buildings – About 15% of background radiation comes from:

   • Natural radioactivity in building materials like stone, brick, and decorative rocks
   • Heavy radioactive elements like uranium and thorium that occur naturally in rocks
   • These elements are present in very small amounts

3. Food and drink – About 11% comes from:

   • Naturally occurring radioactive elements in food and water
   • These elements get into food from contact with soil and rocks
   • For example, bananas contain small amounts of radioactive potassium-40
   • The amounts are tiny and not harmful

4. Cosmic rays – About 10% comes from:

   • High-energy particles from the sun and outer space
   • Protons from the sun that enter Earth's atmosphere at high speed
   • When these collide with air molecules, they produce gamma radiation
   • Other cosmic rays come from supernovae and other space events

Man-made sources:

• Medical sources (about 13%) – X-rays, CT scans, radioactive tracers, and radiation therapy
• Nuclear waste – Does not contribute much to general background radiation but can be dangerous for people handling it
• Nuclear fallout – Radioactive material from nuclear weapon tests (very low now, but higher in areas where tests occurred)
• Nuclear accidents – Events like Chernobyl released large amounts of radiation into the environment

Detecting and Measuring Radiation

Ionising nuclear radiation can be measured using a detector connected to a counter.

The most common detector is the Geiger-Müller tube (also called a GM tube, GM counter, or Geiger counter). Here's how it works:

1. The GM tube contains argon gas at low pressure
2. When radiation enters through a thin mica window, it ionises the argon atoms
3. This means electrons are knocked off the argon atoms, creating ions (charged particles)
4. A high voltage (about 450V) inside the tube accelerates these ions and electrons
5. As they move, they collide with other atoms, creating more ions
6. When the ions reach the electrode, they create a pulse of electric current
7. This pulse is amplified and sent to a counter, which makes a clicking sound
8. The counter displays the number of radiation particles detected

Other types of radiation detectors include:

• Photographic film (often used in badges worn by people working with radiation)
• Ionisation chambers
• Scintillation counters
• Spark counters

Count Rate

Count rate is the number of radioactive particles detected per unit time. It is measured in:

• counts per second (counts/s), or
• counts per minute (counts/minute)

The count rate tells us how much radiation is being detected. Important points:

• The count rate decreases the further the detector is from the radioactive source
• This is because radiation spreads out in all directions as it moves away from the source
• The greater the count rate, the more radiation is present
• Each "click" from the GM counter represents one detected particle

Example calculation: If a GM tube counts 16,000 decays in 1 hour, what is the count rate?

Step 1: Convert time to seconds 1 hour = 60 minutes × 60 seconds = 3,600 seconds

Step 2: Calculate count rate Count rate = Total counts ÷ Time Count rate = 16,000 ÷ 3,600 = 4.4 counts per second (counts/s)

Accounting for Background Radiation (Extended Tier Only)

When measuring radiation from a specific source, we need to remove the effect of background radiation to get the true count rate from the source alone.

To do this:

1. First, measure the count rate with no radioactive source present – this is the background count
2. Then, measure the count rate with the source present – this is the total measured count
3. Subtract the background count from the total measured count to get the corrected count rate

Formula: Corrected count rate = Measured count rate - Background count rate

Example: A student measures radiation at different distances from a source. Far away (beyond 1 metre), the count rate stays constant at 15 counts per minute. This is the background radiation. Closer to the source, the count rate is 180 counts per minute.

What is the corrected count rate from the source? Corrected count rate = 180 - 15 = 165 counts per minute