CIE IGCSE Physics (0625) Detection of radioactivity Study Notes - New Syllabus
CIE IGCSE Physics (0625) Detection of radioactivity Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Detection of radioactivity
Key Concepts:
- Background Radiation
- Measuring Ionising Nuclear Radiation
Background Radiation
Background Radiation
Background radiation is the low-level radiation that is always present in the environment, even when no obvious radioactive source is nearby.
- It is natural and constant but can vary depending on location, altitude, and local materials.
Main Sources of Background Radiation:
| Source | Details |
|---|---|
| Radon gas | Naturally occurring radioactive gas released from rocks and soil; largest contributor to background radiation. It can accumulate indoors. |
| Rocks and buildings | Some rocks like granite contain radioactive elements which emit radiation; used in buildings, contributing to indoor background levels. |
| Food and drink | All foods contain small amounts of radioactive isotopes like potassium-40 and carbon-14, naturally present in biological material. |
| Cosmic rays | High-energy radiation from outer space; intensity increases with altitude (more exposure in airplanes or mountains). |
Example:
A student places a Geiger–Müller counter in a science lab and measures a steady count rate even when no radioactive source is nearby.
(a) What is the most likely explanation for the observed radiation?
(b) Suggest two possible sources that could contribute to this reading.
▶️ Answer/Explanation
(a)The radiation is due to background radiation, which is always present in the environment.
(b)
Radon gas from the ground or building materials
Cosmic rays entering from space
Measuring Ionising Nuclear Radiation
Measuring Ionising Nuclear Radiation
Ionising radiation (alpha, beta, gamma) can be detected using a radiation detector connected to an electronic counter.
- The detector commonly used is a Geiger–Müller (GM) tube, which clicks or displays a number when radiation enters.
- The output is measured as a count rate, in units of:
- counts per second (counts/s)
- counts per minute (cpm)
Corrected Count Rate:
Since background radiation is always present, the total count includes:
Radiation from the source + background radiation
To find the true radiation from the source, subtract the background reading:
Corrected count rate = Total count rate − Background count rate
Example:
A GM tube is used to measure radiation from a radioactive sample.
The background radiation is measured first as 30 counts/minute.
With the radioactive source present, the counter reads 110 counts/minute.
Calculate the corrected count rate from the source.
▶️ Answer/Explanation
Step 1:
- Background = 30 counts/min
- Total with source = 110 counts/min
Step 2:
Corrected count rate = Total − Background
\( = 110 – 30 = \boxed{80 \, \text{counts/min}} \)