Learning outcomes
- Describe radioactive emission as spontaneous and random.
- Use count rate in counts per second or counts per minute.
- Define background radiation.
- Identify the main background sources required by the syllabus.
- Calculate a corrected count rate.
3.1 What radioactivity means
Radioactivity is the emission of ionising radiation from unstable nuclei. The emission occurs because a nucleus changes to a more stable arrangement. The process is spontaneous: it happens without being started by heating, pressure, chemical reaction or an external electric field.
The direction in which radiation is emitted is random. For a single unstable nucleus, the exact time of decay cannot be predicted. Radioactive decay is therefore random at the microscopic level, although the behaviour of a very large population can be described statistically.
3.2 Random does not mean patternless
If many identical radioactive samples are observed, individual counts fluctuate from one equal time interval to the next. One minute may give 94 counts and the next 103, even when the apparatus is unchanged. This variation is expected because decays occur randomly.
However, the average count rate from a large sample decreases in a regular way over time. Half-life is a statistical property of a large population, not a countdown clock for one nucleus. A nucleus does not become “due” to decay after one half-life.
3.3 Count and count rate
A detector records counts, each corresponding to an ionising event that produces a pulse. Count rate is the number of counts divided by the measuring time. It may be expressed as counts/s or counts/min. A longer measurement usually reduces percentage random uncertainty.
If 720 counts are recorded in 3.0 minutes, the mean measured count rate is 240 counts/min. When comparing data, all rates must use the same time unit.
3.4 Background radiation
Background radiation is ionising radiation present in the environment even when the intended laboratory source is absent. Required sources include radon gas in air, radioactive materials in rocks and building materials, natural radioactive substances in food and drink, and cosmic rays from space.
Background varies with location, altitude, geology, building material and time. It should be measured using the same detector, position, time interval and settings used for the source measurement.
3.5 Correcting a count rate
The detector records both the source and background. Therefore, corrected source count rate = measured count rate − background count rate. The background rate should be an average from a reasonably long measurement because its counts also fluctuate randomly.
If a measured rate is close to background, the corrected value has a large percentage uncertainty. Repeating readings and measuring for longer can improve reliability, but cannot remove the fundamental random nature of decay.
Worked examples
Corrected rate
A source plus background gives 286 counts/min. Background is 34 counts/min. Corrected source rate = 286 − 34 = 252 counts/min.
Converting to count rate
A detector records 450 counts in 150 s. The measured rate is 450/150 = 3.0 counts/s.
Practical focus
Investigation
With no source present, record background counts for several equal intervals and calculate a mean. Plot the readings to show random variation. Then analyse supplied source data and subtract the mean background from every measured count rate.
Examination guidance
- Use “spontaneous and random” precisely.
- Random means the exact decay time of one nucleus cannot be predicted.
- Name specific background sources: radon, rocks/buildings, food/drink and cosmic rays.
- Subtract background before using a decay curve or half-life calculation.
- Keep counts and count rates distinct.
Check your understanding
- What is meant by spontaneous radioactive decay?
- A measurement gives 180 counts in 60 s and background is 0.5 counts/s. Find corrected count rate.
- Why do repeated one-minute counts differ slightly?
Answers
- The nucleus decays without needing an external trigger and the exact time cannot be predicted.
- Measured rate = 3.0 counts/s; corrected rate = 2.5 counts/s.
- Radioactive decay and background detection are random processes.
