Energy response
The IEC 60846-1:2009 §8.4.2 evaluation of how an instrument's response varies with photon energy — reference qualities, the ISO 4037 narrow-spectrum series, and the linearity correction that keeps the result honest.
Last reviewed
A dose rate meter does not measure dose equivalent directly. It measures a detector signal — ionisation, scintillation light, collected charge — and converts it to dose equivalent using a conversion factor established at one photon energy. At other energies the conversion carries an error. This test quantifies that error across the energy range the instrument is rated for.
For most survey instruments it is the largest single contribution to the measurement error in routine use, because the spectrum encountered in the field differs from the calibration spectrum.
Reference radiation quality
Before any measurement, settle which operational quantity the instrument is declared for, because that fixes the reference radiation quality you normalise against:
- for ambient dose equivalent H*(10), the reference is the ¹³⁷Cs gamma quality;
- for directional dose equivalent H′(0,07), the reference is a narrow-spectrum X-ray quality around 80 kV.
Everything in this test is expressed relative to the response at that reference. Normalising to the wrong reference does not produce an obviously incorrect result; it produces a plausible relative response curve for the wrong quantity. Confirm which quantity the instrument declares before booking beam time — see the guide on the distinction.
Radiation qualities for the test points
The energy points come from the ISO 4037 series: the narrow-spectrum X-ray qualities, denoted N-10 through N-400 by their generating potential, plus radionuclide gamma qualities — ¹³⁷Cs at 662 keV, ⁶⁰Co at about 1,25 MeV, and ²⁴¹Am at 59,5 keV where a low-energy gamma point is wanted.
Choose points that actually span the instrument’s rated energy range, and put enough of them at the low end. The interesting behaviour is almost always below 100 keV, where photoelectric absorption in the detector and its housing rises steeply and a few tens of keV can change the response by a large factor. Three points spread evenly from 50 keV to 1,25 MeV will not resolve that region.
Dose rate mismatch and linearity correction
The problem
Ideally every energy point is irradiated at the same dose rate as the reference point, so that the only thing changing between rows is energy. In a real calibration laboratory that is often impossible — the available output at N-40 is nothing like the available output from a caesium source, and matching them may mean an impractical distance or an attenuator you do not have.
Irradiating at whichever dose rate is available makes the computed relative response a combination of two effects: the energy dependence being measured and the dose rate dependence. The result is not incorrect but it is not separable.
The correction
The correction uses the linearity result. If you know how the response varies with dose rate — which is exactly what the linearity curve tells you — you can correct the energy point back to the reference dose rate:
where L is the linearity curve evaluated at the reference dose rate and at the dose rate actually used. The tool applies this automatically to any row whose dose rate differs from the reference row, and shows the correction factor on that row so the correction is visible rather than hidden.
The correction has a precondition. The linearity curve must actually characterise the dose rate region you are correcting across, which needs at least two linearity points bracketing it in compatible units. Where it cannot, the tool says so on the row and judges the uncorrected relative response instead — a result that is still reportable, but with a caveat you must carry into the report rather than quietly drop.
Acceptance criterion
Every quality’s relative response must lie inside the acceptance band. As with linearity, one point outside fails the test.
A response that falls below 100 keV is characteristic of an energy-compensated Geiger–Müller tube at the limit of its compensation. A response that rises sharply at low energy indicates an uncompensated detector. Neither is a defect in itself: each defines a limit on the energy range in which the instrument may be used, and the appropriate outcome may be a restriction recorded in the report rather than a failure.
Reporting requirements
Record the quality designation, not just the mean energy: “N-60” is reproducible, “48 keV” is not, because two laboratories can produce quite different spectra with the same mean energy. Record the dose rate used at each point too, so that anyone reading the report can see which rows carried a linearity correction and why.