Detecting Radiation, Background Count, and Safety

Ionizing radiation is detected by its effects on matter, using instruments such as the Geiger-Müller tube, scintillation counters, cloud chambers and photographic film; measured count rates must be corrected for the natural background radiation, and safety relies on limiting time, increasing distance and using shielding. In brief: the net count rate is the gross count rate minus the background. Because decay is random, a count N has a statistical uncertainty of about √N, so the fractional uncertainty falls as the count increases. Radiation dose is measured in grays (Gy, energy absorbed per kg) and the biologically weighted equivalent dose in sieverts (Sv). The average background dose is about 2.4 mSv per year worldwide.

We cannot see, hear or feel ionizing radiation, so instruments are needed to detect it. This guide describes the main detectors and how they work, explains background radiation and count-rate corrections, units of dose and the safety principles, compares the detectors in a table, works through numerical examples and corrects common mistakes.

How is radiation detected?

• Geiger-Müller (GM) tube: a gas-filled tube with a thin window and a high voltage (about 400 to 900 V) between a central wire and the wall. Radiation ionizes the gas; the ions are accelerated and produce an avalanche, giving a pulse of current for each particle (the characteristic click). It counts particles but does not give their energy or type. Alpha particles need a very thin window.
• Scintillation counter: a crystal (sodium iodide with thallium) or plastic emits flashes of light when radiation passes; a photomultiplier tube converts them into electrical pulses of size proportional to the energy deposited, so the energy spectrum of gamma rays can be measured and the isotopes identified.
• Semiconductor detectors (silicon, germanium): radiation creates electron-hole pairs, giving a high-resolution energy measurement.
• Cloud chamber and bubble chamber: tracks of droplets or bubbles show the paths of charged particles: alpha tracks are short and thick, beta tracks are long and thin and wiggly, and gamma rays leave sparse tracks of secondary electrons. In a magnetic field, the curvature shows charge and momentum.
• Photographic film and dosimeters (film badges, thermoluminescent dosimeters) record the cumulative dose of workers.

What is background radiation?

A detector without any source nearby still registers counts, because of natural radiation: cosmic rays from space, gamma rays from rocks and soil (uranium, thorium, potassium-40), radon gas accumulating in buildings (usually the largest part), radioactive isotopes in food and in the body (potassium-40, carbon-14), and small amounts of artificial radiation (medical procedures, fallout from tests). The background count rate varies from place to place (higher at altitude and over granite). In experiments, the background count is measured with no source present, over a long time, and subtracted from the measured count rate: net count rate = gross count rate − background count rate. The distance dependence of a point gamma source follows the inverse square law once the background has been subtracted.

How are dose and risk measured?

• Activity: becquerel (Bq), decays per second, a property of the source.
• Absorbed dose: gray (Gy) = 1 J of energy deposited per kg of tissue.
• Equivalent dose: sievert (Sv) = absorbed dose × radiation weighting factor (1 for gamma and beta, 20 for alpha particles, which cause more biological damage per unit of energy). The effective dose also takes into account the sensitivity of the organs.

Typical doses: natural background about 2.4 mSv per year; chest X-ray about 0.02 mSv; return flight from London to New York about 0.05 mSv; CT scan of the abdomen 5 to 10 mSv; occupational limit in many countries 20 mSv per year (averaged); acute radiation sickness above about 1 Sv; a dose of 4 to 5 Sv received quickly is fatal in about half of untreated cases. At low doses, the risk is assumed to be proportional to the dose, with a risk of fatal cancer of about 5 percent per sievert (a conservative estimate).

How is exposure controlled?

The three basic protections are time (a shorter exposure means a smaller dose), distance (the intensity from a small source falls as 1/r²; use tongs, not fingers) and shielding (paper or plastic for alpha and beta, lead or concrete for gamma). Sealed sources are kept in lead containers; unsealed sources need gloves, labeling and washing to avoid contamination (radioactive material on or in the body, as distinct from irradiation). The general principle is ALARA, as low as reasonably achievable, and the practice must be justified by its benefits.

Detectors compared

Worked example: count rates and uncertainty

1. Background correction. A GM counter records 150 counts in one minute with a sample present, and 20 counts per minute from background. The net count rate = 150 − 20 = 130 counts/min.
2. Statistical uncertainty. A total of 900 counts gives an uncertainty of √900 = 30 counts, or 3.3 percent. With 90,000 counts, the uncertainty is 300 counts, or 0.33 percent. A longer counting time (or a stronger source) reduces the fractional error.
3. Inverse square law. A gamma source gives a net count rate of 400 counts/min at 10 cm. At 20 cm: 400 ÷ 4 = 100 counts/min. At 40 cm: 25 counts/min.
4. Half-life from measurements. The net count rate falls from 800 to 100 counts/min in 30 minutes. 800 → 100 is a factor of 8 = 2³, so 3 half-lives in 30 minutes: T_½ = 10 minutes.

5. Shielding. A gamma source has a net rate of 1,600 counts/min. A lead sheet with a half-value thickness of 1.0 cm: with 3.0 cm, the rate becomes 1,600 ÷ 8 = 200 counts/min.
6. Dose. A worker is exposed to a dose rate of 5.0 µSv/h for 8 hours: 40 µSv. At this rate, working 2,000 hours in a year would give 10 mSv, below the occupational limit of 20 mSv per year. At 1.25 µSv/h (twice the distance), the dose is a quarter.

How were detectors developed?

Becquerel used photographic plates (1896). C. T. R. Wilson developed the cloud chamber in 1911 (Nobel Prize 1927). Hans Geiger and Walther Müller developed the Geiger-Müller counter in 1928, building on Geiger’s earlier counter (1908). Scintillation detectors with photomultipliers were developed in the late 1940s, and bubble chambers by Donald Glaser in 1952. The sievert was adopted in 1979 and the becquerel in 1975 as SI units.

Which mistakes are common?

Mistake 1: Forgetting to subtract the background

Measure it separately and correct the readings.

Mistake 2: Confusing contamination and irradiation

Irradiation is exposure to radiation. Contamination means radioactive material on or inside the body or objects.

Mistake 3: Mixing up the units of activity and dose

Bq measures decays per second of the source; Gy and Sv measure the energy absorbed by tissue.

Mistake 4: Assuming that a Geiger counter identifies the radiation type or energy

It counts particles. Other detectors (scintillation, semiconductor) or absorption tests are needed to identify the radiation.