Reference radiation qualities

The ISO 4037 radiation qualities used for characteristic evaluation — the narrow-spectrum N series, the radionuclide gamma qualities, mean energy versus spectrum, and the conventional true value assigned by the calibration laboratory.

Last reviewed 7 September 2026

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A characteristic evaluation is performed in reference radiation fields established by a calibration laboratory. The instrument standard specifies what to measure and what passes; the ISO 4037 series specifies the fields themselves — how they are produced, how they are characterised, and how an instrument is calibrated in them. That division holds for every instrument standard in the series, which is why the fields are treated here rather than under any one of them.

The validity of an evaluation is bounded by the validity of these fields.

Radiation covered by each instrument standard

Which qualities are relevant follows from the radiation the instrument standard covers. A neutron survey meter and a contamination monitor are not evaluated in the same fields as a photon dose rate meter, and ISO 4037 does not supply the fields for all of them.

Read from the title of each standard as published. ISO 4037 covers the photon and beta fields; neutron and contamination work is referred to other reference-field standards.

Narrow-spectrum X-ray qualities

The N series is produced by an X-ray tube with added filtration that narrows the emitted spectrum. Each quality is designated by its generating potential in kilovolts: N-10, N-15, N-20 and so on up to N-400.

Designation and mean energy

Each quality has a characteristic mean energy, Ē(Φ), which is lower than the generating potential because the spectrum is continuous. N-60 has a mean energy near 48 keV; N-100 near 83 keV.

Record the designation, not the mean energy. The designation identifies the quality completely: the generating potential, the filtration, and therefore the spectrum. A mean energy does not — two different spectra can share a mean energy and produce different responses in the same instrument, because the response depends on the distribution rather than on its first moment.

A report stating “48 keV” is not reproducible. A report stating “N-60” is.

Filtration

The added filtration is part of the quality definition. A tube operated at 60 kV without the specified filtration does not produce N-60; it produces a broader, softer spectrum with a lower mean energy. Confirm from the calibration certificate that the filtration was as specified, rather than inferring the quality from the tube potential alone.

Radionuclide gamma qualities

Three are used routinely in this evaluation:

These are effectively monoenergetic, which makes them useful anchors: a response measured at S-Cs is attributable to one photon energy rather than to a distribution. ⁶⁰Co emits two lines close together and is treated as a single quality at their mean.

Conventional true value

The conventional true value is the dose equivalent rate the calibration laboratory assigns to the field at the measurement position. It is not measured by the instrument under test; it is the reference the instrument is compared against, and it is what the term reference value denotes in the tool.

It is derived from the laboratory’s own reference instrument, corrected for distance, air density and field non-uniformity, and it carries an expanded uncertainty — normally quoted at k = 2. That uncertainty is a property of the field, not of the instrument under test, and it enters the evaluation in two places:

• as a component of the reported uncertainty of the result;
• where the acceptance rule is stated relative to a set field value, as in the IEC 60846-1 alarm accuracy test, where it widens the exposure conditions.

Selection of test points

Coverage

Points must span the instrument’s rated energy range. The response of a photon detector varies most steeply below about 100 keV, where photoelectric absorption in the detector and its housing rises sharply. A set of points spread evenly across the full range will under-sample exactly the region where the response changes fastest.

Reference point

One point must be at the reference quality determined by the instrument’s declared operational quantity — see the guide on that distinction. Every other point is normalised against it.

Dose rate

Where practical, irradiate every energy point at the same dose rate as the reference point. Where that is not achievable, the difference is corrected using the linearity result. The mechanism is worked through for IEC 60846-1 in the linearity correction note.

Distance and field uniformity

The conventional true value applies at a defined position. Moving the instrument changes the value, and at short distances the inverse-square relationship is sensitive to small displacements.

Two conditions apply:

• the instrument’s reference point — marked on the case — must be at the calibration position, not the case centre or the front face;
• the field must be uniform across the detector volume. At short source-to-detector distances a large detector may span a significant gradient, and the assigned value no longer represents the field the detector integrates over.

Both are recorded in the calibration certificate. Both are frequent causes of an evaluation that disagrees with a previous one on the same instrument.