5.2.1 Detection of radioactivity
- Syllabus
- 0625–2026–2027
- Topic
- 5.2.1
- Level
- —
Learning objectives
5.2.1.1What is meant by background radiation• Know what is meant by background radiation5.2.1.2Sources that make a significant• Know the sources that make a significant contribution to background radiation including: (a) radon gas (in the air) (b) rocks and buildings (c) food and drink (d) cosmic rays5.2.1.3Ionising nuclear radiation can be• Know: ionising nuclear radiation can be measured using a detector connected to a counter5.2.1.4Count rate measured in counts/s or• Use count rate measured in counts/s or counts/minute5.2.1.5Measurements of background radiation• Use measurements of background radiation to determine a corrected count rate
Define background radiation
Background radiation is ionising radiation that is always present in the environment, including when no deliberately placed radioactive source is nearby.
A detector can therefore record counts before an experimental source is introduced or after it is removed. This persistent reading is a real environmental contribution, not automatically a detector zero error.
The measured background level depends on the place, surroundings and measuring interval, so it should be measured for the experiment rather than assumed to be zero.
Background radiation names the radiation already present, not the reading produced by the source under investigation.
Identify sources of background radiation
| Significant source | Where it comes from |
|---|---|
| radon gas | radioactive gas in the air, often entering from the ground |
| rocks and buildings | radioactive materials in the ground and construction materials |
| food and drink | naturally occurring radioactive isotopes taken into the body |
| cosmic rays | high-energy radiation arriving from space |
A complete syllabus list includes all four: radon gas in air; rocks and buildings; food and drink; and cosmic rays.
Radon and rocks/buildings are terrestrial sources, food and drink contribute internally after intake, and cosmic rays come from beyond Earth.
Ordinary seismic waves are not ionising radiation and do not contribute to the background count. Do not replace the required source with a vague phrase such as 'the environment'.
Measure ionising radiation with a detector and counter
| Component | Function |
|---|---|
| radiation detector | responds when ionising radiation reaches its sensitive region |
| counter | records the detector pulses as a number of counts |
| timer | defines the measurement interval so a count rate can be calculated |
Place the detector in the required position, connect it to the counter, measure for a known time and record the counts. Repeating a measurement can provide a more stable average.
A count is one registered detection event. The detector-counter reading does not necessarily equal the number of particles emitted by the source because not every emission reaches or is registered by the detector.
A detector connected to a counter measures counts; it does not by itself identify the radiation type, energy or source activity without additional method and interpretation.
Express a count rate
Count rate is the number of registered counts divided by the measurement time: count rate = number of counts ÷ time.
| Time unit used | Count-rate unit |
|---|---|
| seconds | counts/s |
| minutes | counts/minute |
If 360 counts are recorded in 3 minutes, the count rate is 360 ÷ 3 = 120 counts/minute. In seconds, 3 minutes is 180 s, so the same result is 2 counts/s.
Keep the count and time interval together. To compare rates, first express both using the same time unit; 60 counts/minute equals 1 count/s.
Counts and count rate are different quantities. A total of 300 counts is not 300 counts/s unless the measurement lasted exactly 1 second.
Calculate a corrected count rate
The detector reading with a source contains both source radiation and background radiation. Correct it using: corrected source count rate = measured count rate with source − background count rate.
| Step | Action |
|---|---|
| 1 | measure background with the source absent, using the same detector position and a known time |
| 2 | repeat and average background measurements when data are provided |
| 3 | convert background counts to a rate using counts ÷ time |
| 4 | measure the total rate with the source present in the same unit |
| 5 | subtract the background rate from the total rate |
Background counts of 170, 164 and 185 in separate 10-minute intervals average to 173 counts in 10 minutes, or 17.3 counts/minute. If the total rate with the source is 949 counts/minute, the corrected rate is 949 − 17.3 ≈ 932 counts/minute.
A corrected decay curve lies below the uncorrected curve by the background rate. An uncorrected curve approaches the background level; a correctly background-subtracted curve approaches zero.
Subtract rates from rates, not a raw count from a rate. Background is removed once only; do not add it or halve the total before subtracting it.