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

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 🧪

DetectorPrincipleTypical UseExample
Geiger–Müller TubeIonisation of gas → spark → pulseGeneral radiation checksGeiger counter
Scintillation DetectorParticle excites crystal → light flash → photomultiplierMedical imaging, high‑energy physicsNaI(Tl) detector
Semiconductor DetectorCharge carriers generated in a solid → current pulseHigh‑resolution spectroscopySi(Li) detector

How a Geiger Counter Works 🔬

  1. Radioactive particle enters the Geiger–Müller tube.

  2. It ionises the gas inside, creating an electron‑ion pair.

  3. The electric field accelerates the electrons, causing an avalanche of further ionisations.

  4. A brief electrical pulse is produced and sent to the counter.

  5. 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.