Use count rate measured in counts/s or counts/minute

What is a Count Rate?

When a radioactive source emits particles, a detector records how many particles hit it over a given time. The count rate tells us how many counts are recorded per unit time.

Common units:

• \$\text{counts per second (cps)}\$
• \$\text{counts per minute (cpm)}\$

📈 Analogy: Think of a rain gauge. The number of raindrops that fall in one second is like cps, while the number that fall in a minute is like cpm.

Calculating Count Rate

If a detector records C counts in a time interval t, the count rate R is:

\$R = \dfrac{C}{t}\$

Example:

• 20 counts in 10 seconds → \$R = 20/10 = 2\$ cps
• 120 counts in 2 minutes → \$R = 120/(2\times60) = 1\$ cps

💡 Tip: Always check the units of time before dividing.

Common Detectors

1. Geiger–Müller tube – counts individual particles.
2. Scintillation counter – flashes light when a particle hits a crystal.
3. Semiconductor detector – measures charge produced by particles.

Each detector gives a count rate that can be converted to activity (decays per second) if the detection efficiency is known.

Example Table: Counts vs. Time

📊 Notice the count rate stays constant – this indicates a stable source.

Exam Tips

• Always state the unit of your count rate (cps or cpm).
• If a problem gives counts in minutes, convert to seconds before calculating cps.
• Check that your answer makes sense: a higher number of counts in a shorter time → higher cps.
• Remember that the count rate is an average over the time interval; short bursts can still give a high average.

??

Quick Check: If you get 500 counts in 5 minutes, what is the cps?

Solution: \$500/(5\times60)=1.67\$ cps.