Cambridge Syllabus Notes

5.2.1 Detection of Radioactivity

Objective: Know that ionising nuclear radiation can be measured using a detector connected to a counter.

Principles of Detection 📡

Think of a detector as a security guard who rings a bell every time a radioactive particle passes by. The guard (detector) is connected to a counter that keeps a tally of how many times the bell rings. The more particles that pass, the higher the count rate.

The activity of a radioactive source, measured in becquerels (Bq), is defined as the number of decays per second:
Activity: $$A = \frac{N}{t} \quad \text{with } 1\,\mathrm{Bq}=1\,\text{decay/s}$$

The counter records a count rate $R$, which is related to the activity by the detector efficiency $\epsilon$:
$$R = \epsilon A$$

Common Detectors 🧪

Detector	Principle	Typical Use	Example
Geiger–Müller Tube	Ionisation of gas → spark → pulse	General radiation checks	Geiger counter
Scintillation Detector	Particle excites crystal → light flash → photomultiplier	Medical imaging, high‑energy physics	NaI(Tl) detector
Semiconductor Detector	Charge carriers generated in a solid → current pulse	High‑resolution spectroscopy	Si(Li) detector

How a Geiger Counter Works 🔬

Radioactive particle enters the Geiger–Müller tube.
It ionises the gas inside, creating an electron‑ion pair.
The electric field accelerates the electrons, causing an avalanche of further ionisations.
A brief electrical pulse is produced and sent to the counter.
The counter increments the displayed count.

The count rate $R$ (counts per minute) is proportional to the source activity, but the proportionality constant (efficiency) depends on the detector geometry and the type of radiation.

Exam Tips 🎯

Remember that activity is a rate of decays, not a total number.
When given a count rate, you can estimate activity if the efficiency is known: $A = R/\epsilon$.
Use the analogy of a security guard to explain how detectors count particles.
Draw a simple diagram of a Geiger tube with labels to show the ionisation process.
Practice converting between units: e.g., 1 kBq = 1000 decays s⁻¹.

Key Takeaway 💡

A detector is like a smart phone that records every “call” (decay) it receives. The counter is the phone’s dialer that keeps a running total. By understanding the detector’s efficiency, we can translate the raw counts into the true activity of the radioactive source.

Know that ionising nuclear radiation can be measured using a detector connected to a counter