Radioactivity Detection
Section: Nuclear Physics | Syllabus: Cambridge IGCSE Physics (0625)
Background Radiation
We are constantly exposed to low levels of ionising radiation from natural and artificial sources - this is called background radiation.
Background radiation is the radiation that exists around us all the time
The main sources of background radiation
Sources that Make a Significant Contribution to Background Radiation
| Source | Description |
|---|---|
| (a) Radon gas | A radioactive gas that seeps into the air from rocks and soil |
| (b) Rocks and buildings | Contain small amounts of naturally radioactive elements |
| (c) Food and drink | Contain naturally occurring radioactive isotopes, e.g. carbon-14 |
| (d) Cosmic rays | High-energy radiation reaching the Earth from space |
- Background radiation levels vary from place to place
- Areas with granite rocks have more radon gas
- Background radiation increases with altitude because there is less atmosphere to absorb cosmic rays
- Background radiation must always be accounted for when measuring the radiation from a particular radioactive source
Detecting Ionising Radiation
Ionising radiation cannot be detected by our senses, so instruments are needed to detect and measure it.
- Ionising nuclear radiation can be measured using a detector connected to a counter
- Ionising radiation can be measured using a detector connected to a counter the detector responds to radiation, and each event is registered and displayed by the counter
The Geiger-Müller (GM) Tube
The most common instrument for detecting and measuring ionising radiation is the Geiger-Müller tube, connected to a counter.
A Geiger-Müller tube connected to a counter
- Radiation enters the tube through a thin window and ionises the argon gas inside
- The electrons produced are attracted to the positive central wire, and the positive ions move towards the negative outer tube, producing a pulse of electric current
- Each pulse is registered as one count by the counter
- The thin window allows alpha particles to enter - a thicker window would block them
- A GM tube detects the presence of radiation but cannot distinguish between alpha, beta and gamma radiation
Count Rate
The count rate is the number of radiation counts detected per unit time.
- Count rate is measured in counts/s (counts per second) or counts/minute
Worked Example: Converting Count Rate Units
Muchindu records 600 counts in 1 minute using a GM tube and counter.
- Count rate = 600 counts/min
- In counts/s: 600 ÷ 60 = 10 counts/s
Corrected Count Rate
Extended Only
Because background radiation is always present, measurements of background radiation are used to determine a corrected count rate - the count rate due to a radioactive source alone.
- Corrected count rate = measured count rate − background count rate
Experimental Procedure
- Measure the background count rate with no source present, ideally repeating the reading and averaging to reduce the effect of the random nature of radioactive decay
- Measure the count rate with the radioactive source present
- Subtract the background count rate from the reading taken with the source present
Worked Example: Calculating Corrected Count Rate
Muumbe measures background radiation over 5 minutes and records 150 counts. With a radioactive source present, 450 counts are recorded over the same 5 minutes.
- Background count rate = 150 ÷ 5 = 30 counts/min
- Total count rate (with source) = 450 ÷ 5 = 90 counts/min
- Corrected count rate = 90 − 30 = 60 counts/min
Interactive revision notes, videos and practice questions load below.