Detecting Radiation and Radiation Safety

Radiation is detected with a Geiger-Müller tube and counter, and you stay safe by cutting dose through distance, time and shielding. This subtopic also supplies the background-radiation correction that Extended half-life questions depend on, plus the precaution questions that appear almost every year. The marks are pure preparation: lists and one subtraction.

What is background radiation and where does it come from?

Background radiation is the low-level ionising radiation present everywhere, all the time. A detector reads counts even with no source in the room. The main sources:

Source	Type
Radon gas from rocks and soil	Natural (largest contributor in many regions)
Cosmic rays from space	Natural
Rocks, soil and building materials	Natural
Food and drink (e.g. potassium-40 in bananas)	Natural
Medical procedures (X-rays, radiotherapy)	Artificial
Nuclear industry and historic weapons testing	Artificial (small share)

Detection uses a Geiger-Müller (GM) tube connected to a counter or ratemeter. Radiation entering the tube ionises the gas inside; the ions create a pulse of current that the counter records. Count rate is counts per second or counts per minute. Photographic film also detects radiation: it darkens with exposure, the principle behind film badges worn by radiographers.

Whenever you measure a source, the reading includes background. In words: corrected count rate equals measured count rate minus background count rate. Measure the background first, with the source absent, then subtract it from every reading.

How do you stay safe around radioactive sources?

All safety methods reduce the radiation dose a person absorbs. Cambridge accepts these precautions, each tied to a reason:

• Distance: handle sources with tongs and stand back. Intensity falls rapidly with distance.
• Time: keep exposure short. Dose grows with exposure time.
• Shielding: store sources in lead-lined containers; use lead screens. Absorbing material cuts the radiation reaching you.
• Monitoring: wear a film badge or dosimeter to record accumulated dose.
• Never point a source at anyone, and never touch it directly.

Each precaution scores only when paired with its reason. “Use tongs” alone is half an answer; “use tongs to increase distance from the source, reducing the dose” is the full mark.

Worked Exam Question

A student measures a count rate of 6 counts/minute with no source present. With a sealed source 10 cm from the GM tube, the counter records 1,440 counts in 4.0 minutes. (a) Calculate the corrected count rate of the source. (3 marks) (b) State two precautions the student should take, giving a reason for each. (2 marks)

Solution. (a) Measured rate $= 1440 \div 4.0 = 360$ counts/minute. Background = 6 counts/minute. Corrected rate $= 360 - 6 = 354$ counts/minute. (b) Hold the source with tongs, increasing distance and so reducing the dose received. Return the source to its lead-lined box immediately after use, so shielding absorbs the radiation when it is not needed.

Mark scheme:

• M1: converts total counts to a rate ($1440 / 4.0 = 360$).
• M1: subtracts background ($360 - 6$).
• A1: 354 counts/minute with unit.
• B1: first precaution with matching reason.
• B1: second different precaution with matching reason.

Common Mistakes

• Forgetting to subtract background. Fix: scan the question for a “no source” reading; if one is given, the examiner expects subtraction.
• Subtracting background from total counts instead of count rate. Fix: convert to a rate first, then subtract rate from rate.
• Listing only artificial background sources. Fix: natural sources dominate, so lead with radon, rocks and cosmic rays.
• Precautions without reasons. Fix: pair every precaution with “to reduce the dose by…” distance, time or shielding.
• “Wear gloves to block radiation.” Fix: gloves prevent contamination transfer; they do not stop gamma. Choose lead shielding for absorption marks.

Exam Technique Tip

For “state precautions” questions, run the mnemonic D-T-S: Distance, Time, Shielding. Pick as many as the marks demand, write one sentence each in the form action + how it cuts dose. This template fits every variant of the question Cambridge has set, including handling, storage and disposal versions.

How This Is Examined

This is Core content sat by all candidates. Papers 1 and 2 ask one MCQ on background sources or GM-tube detection. Papers 3 and 4 set 2-4 mark questions on precautions and background correction; on Paper 4 the corrected count rate usually feeds directly into a half-life calculation, so an error here costs marks twice. Paper 6 sometimes presents count-rate tables to process. Malaysia’s background levels are unremarkable globally, but radon from granite bedrock (relevant to hillside developments) has appeared as a comprehension context in regional practice papers.