Dose rate, contamination or nuclide identification – which instrument do you need?

Start with the question you need answered

A common mistake is to choose an instrument by detector type rather than by measurement task. A dose rate meter, a contamination monitor and a nuclide identifier may contain similar detectors but are designed and calibrated for entirely different quantities.

Dose rate – how much radiation is someone exposed to?

Radiation protection dose rate meters normally display ambient dose equivalent rate, H*(10), in µSv/h or mSv/h. The quantity is used to estimate the dose a person would receive at the location and is the basis for radiation protection work, cordons and area monitoring.

• GM tubes are rugged and sensitive but give no energy information; energy-compensated tubes give a flatter response across the energy range.
• Ionisation chambers measure the ionisation current directly and have a good energy response, which makes them common for X-rays and pulsed radiation fields.
• Scintillators give high sensitivity at low dose rates, for example in environmental measurements.
• Look for: measuring range, energy range, energy response and the standard the instrument has been tested against – for portable dose rate meters this is often IEC 60846-1.

Contamination – is there radioactive material on the surface?

Contamination monitors have large detectors with thin windows that are held close to the surface and register the alpha and beta particles that a dose rate meter cannot see. The result is usually shown as counts per second (cps). Converting to surface activity in Bq/cm² requires the instrument’s efficiency for the nuclide in question, determined with reference sources according to ISO 8769 and evaluated according to ISO 7503-1.

• Pancake GM probes are a simple and rugged choice for beta and gamma, with some sensitivity to alpha as well.
• ZnS(Ag) is used for alpha and plastic scintillator for beta; dual-phosphor detectors combine both and measure alpha and beta simultaneously.
• Proportional counters separate alpha from beta by pulse height.
• Look for: the detector’s active area, the window thickness (mg/cm²) and the efficiency for the nuclides you expect.

Nuclide identification – which nuclide is it?

A radionuclide identification device (RIID) measures a gamma spectrum and compares the peak energies with a nuclide library. It needs a spectrometric detector – NaI(Tl), LaBr₃(Ce), CZT or HPGe – and the better the energy resolution, the more reliable the identification when several nuclides are present. Many RIIDs also show dose rate. Performance requirements for handheld instruments are given in IEC 62327 and ANSI N42.34.

Read more about the materials in the guide Detector materials for gamma spectrometry.

Neutrons and personal dose

Neutrons need their own detectors, such as ³He or BF₃ counters, and for neutron dose rate a moderator around the detector that slows down fast neutrons. Personal dosimeters measure personal dose equivalent – Hp(10) and Hp(0.07) – where they are worn on the body and are used to follow up the dose each person actually receives.

Sample analysis

To determine the activity of a sample – for example Bq/kg in food or environmental samples – gamma spectrometry with an HPGe detector is used. For alpha emitters such as uranium, plutonium and americium, alpha spectrometry with silicon detectors in vacuum is used after radiochemical preparation of the sample.

Not sure? The instrument finder starts from the task and shows which products fit – or contact us and we will help you choose.