Detecting Radiation: Geiger Counters and Dosimetry
Radiation is detected through the ionization or excitation it causes in matter, for example in a Geiger–Müller counter, a scintillation detector or photographic film, and its effect on people is measured by dosimetry, using the gray (absorbed dose) and the sievert (equivalent dose). In brief: 1 gray is 1 joule absorbed per kilogram, and the equivalent dose in sieverts is the absorbed dose multiplied by a weighting factor that is 1 for gamma and beta radiation, but 20 for alpha particles; the natural background dose is about 2.4 mSv per year.
We cannot see, smell or feel radioactivity, so instruments are needed to find and measure it. Detectors are used in medicine, nuclear power, geology and in homes with radon problems. Dosimeters tell workers how much radiation they have absorbed. This article explains how the main types of detectors work, the units of dose, typical doses in a table, and how to calculate the dose and dose rate, with worked examples.
How does a Geiger–Müller counter work?
A Geiger–Müller (GM) tube is a sealed metal cylinder filled with a low-pressure gas, usually argon with a little quenching vapor, with a thin wire (the anode) along its axis kept at a high voltage of about 400–1000 V relative to the wall. When ionizing radiation enters the tube through a thin window, it ionizes some gas atoms. The electrons are accelerated toward the central wire, and they ionize more atoms on the way, causing an avalanche that spreads along the whole wire. This produces a pulse of current of the same size for any particle, which is counted and may be heard as a click. After each discharge the tube is insensitive for a short time, the dead time of about 100 µs, so counts are lost at high rates.
A GM counter shows how many particles or photons have entered the tube, but not their energy, and so it cannot identify the isotope or give the dose accurately. It detects beta particles and gamma rays easily, and alpha particles only with a very thin window. It always gives a background count, from cosmic rays and natural radioactivity, of about 10–30 counts per minute, which must be subtracted from the reading.
What other detectors are used?
- Scintillation counters: a crystal such as sodium iodide doped with thallium, NaI(Tl), emits flashes of light when radiation is absorbed, and a photomultiplier tube converts them to electrical pulses with a size proportional to the energy. They are used in gamma spectroscopy and in medical imaging.
- Semiconductor detectors: germanium and silicon diodes give the best energy resolution, so that individual gamma lines can be identified. Germanium detectors must be cooled with liquid nitrogen.
- Ionization chambers: similar to a GM tube, but operated at a lower voltage so that the current is proportional to the dose rate. They are used for accurate measurements.
- Cloud and bubble chambers: they show the tracks of charged particles as lines of droplets or bubbles. Used in research.
- Film badges and thermoluminescent dosimeters (TLD): worn by workers; film is blackened by radiation, and TLD crystals store energy that is released as light on heating, proportional to the dose.
- Electronic personal dosimeters: small silicon detectors with a digital reading and an alarm.
Which units measure radiation?
| Quantity | Meaning | SI unit | Older unit |
|---|---|---|---|
| Activity | Decays per second | becquerel (Bq) | curie (1 Ci = 3.7 × 1010 Bq) |
| Absorbed dose | Energy absorbed per kg | gray (Gy = 1 J/kg) | rad (1 rad = 0.01 Gy) |
| Equivalent dose | Absorbed dose × radiation weighting factor | sievert (Sv) | rem (1 rem = 0.01 Sv) |
| Effective dose | Equivalent dose weighted for the sensitivity of organs | sievert (Sv) | rem |
The radiation weighting factor takes into account that the same absorbed dose causes more damage with some types of radiation: photons and electrons 1, protons 2, alpha particles 20, and neutrons 2.5 to 20, depending on the energy. The effective dose adds a tissue weighting factor, since organs differ in sensitivity.
Typical radiation doses
| Source | Dose |
|---|---|
| Eating a banana (potassium-40) | about 0.1 µSv |
| Dental X-ray | about 5–10 µSv |
| Flight from London to New York | about 40–50 µSv |
| Chest X-ray | about 0.1 mSv |
| Natural background (world average, per year) | about 2.4 mSv |
| Abdominal CT scan | about 8–10 mSv |
| Annual limit for a radiation worker (averaged over 5 years) | 20 mSv per year |
| Annual limit for the general public (above background) | 1 mSv |
| Acute dose that would kill about half of those exposed without treatment | about 4 Sv |
The values depend on the circumstances, and are given as orders of magnitude. The largest part of the natural dose is from radon (about half) and the rest from cosmic rays, the ground, food and the body itself.
What are the biological effects?
Ionizing radiation can damage DNA, directly or through reactive radicals formed from water. High acute doses (above about 1 Sv) cause radiation sickness: nausea, a fall in blood cell counts and, above several sieverts, death. Low doses spread over time cause no immediate effects, but increase the risk of cancer by a small amount, which is assumed to be proportional to the dose (the linear no-threshold model, which is still debated for very low doses). The risk is about 5 % per sievert for the whole population, which means that a dose of 10 mSv would increase the lifetime risk of fatal cancer by about 0.05 %.
How can exposure be reduced?
- Time: the dose is proportional to the time of exposure.
- Distance: the dose rate from a point source decreases with the square of the distance.
- Shielding: paper for alpha, aluminum or plastic for beta, lead and concrete for gamma and X-rays.
- Containment: sealed sources and ventilation prevent the inhalation or ingestion of alpha and beta emitters.
Worked examples
Example 1: absorbed dose and equivalent dose of gamma rays
A 70 kg person absorbs 0.35 J of gamma radiation. D = 0.35 J / 70 kg = 5.0 × 10−3 Gy = 5.0 mGy. For gamma radiation, the weighting factor is 1, so the equivalent dose is 5.0 mSv.
Example 2: alpha radiation
A 1.0 kg organ absorbs 2.0 mJ of energy from alpha particles. D = 2.0 × 10−3 J / 1.0 kg = 2.0 mGy. With a weighting factor of 20, the equivalent dose is 20 × 2.0 = 40 mSv. The same absorbed dose from gamma rays would have given 2.0 mSv.
Example 3: the inverse-square law
The dose rate from a gamma source is 4.0 mSv/h at 1.0 m. At 2.0 m the rate is 4.0 / 22 = 1.0 mSv/h, and at 5.0 m it is 4.0 / 25 = 0.16 mSv/h. A worker who needs to work for 30 minutes at 5.0 m receives 0.16 × 0.5 = 0.08 mSv.
Example 4: the net count rate
A Geiger counter gives 380 counts in 2.0 min near a sample, and 40 counts in 2.0 min without the sample. The net count rate is (380 − 40) / 2.0 = 170 counts per minute. The statistical uncertainty is about the square root of the counts: √380 ≈ 19.5, so the relative uncertainty is about 5 %.
Example 5: annual dose from the background
The background dose rate is 0.27 µSv per hour. In one year (8760 h) the dose is 0.27 × 8760 = 2365 µSv = 2.4 mSv, close to the world average.
How did the instruments develop?
Hans Geiger and Ernest Rutherford built a gas counter for alpha particles in 1908, and Geiger and Walther Müller developed the sealed tube that bears their names in 1928. The Wilson cloud chamber (1911, Charles T. R. Wilson, Nobel Prize in Physics 1927) made the tracks of particles visible. The unit of dose was defined by the International Commission on Radiological Protection. The gray is named for Louis Harold Gray, a pioneer of radiobiology, and the sievert for Rolf Sievert, a Swedish medical physicist; both were adopted by the SI in 1975 and 1979.
Which mistakes are common?
- Confusing becquerel, gray and sievert: activity, absorbed dose and equivalent dose are different things.
- Ignoring the weighting factor: the same dose of alpha radiation does 20 times more damage.
- Forgetting the background: subtract it from Geiger counter readings.
- Assuming that a Geiger counter measures the dose or identifies isotopes: it only counts events.
- Thinking that low radiation levels are zero risk, or that natural radiation does not matter: both are over-simplifications; the risk is small but not zero.