Dose rate dependence

This test applies only to instruments that integrate — that accumulate a total dose rather than indicating a rate. It establishes whether the accumulated value is independent of the rate at which the dose was delivered.

It should be: one millisievert delivered over ten minutes and one millisievert delivered over ten hours are the same accumulated dose. A discrepancy indicates a rate-dependent effect in the detector or the electronics — recombination in an ionisation chamber, dead time in a counting detector, leakage in an integrator — which makes the indication depend on conditions not visible to the user.

Reference value derivation

You do not set a dose directly. You set a dose rate and an exposure time, and the reference dose follows:

The tool computes it from the two values you enter, and it keeps the unit consistent with the row’s dose rate unit. Entering a dose directly is the most frequent error in this test: the rate and the time are the independent variables and the dose is their product.

Test condition selection

Aim for a dose near 80 % of each decade of the effective dose measuring range, and use one dose rate per decade. This produces a set of points at comparable total dose with the rate varying over the instrument’s rated range, which is the comparison the test requires.

Exposure times are bounded: from ten seconds to ten hours, with something in the range of ten seconds to an hour being the practical working band. The tool flags rows outside ten seconds to an hour as worth reconsidering.

• Below ten seconds, the step transitions at the start and end of the exposure become a significant fraction of the measurement, and the result characterises the shutter as much as the detector.
• Above ten hours, drift, background accumulation and stability of the field start to dominate, and the result characterises the laboratory rather than the instrument.

For a digital integrating instrument, a single reading of about a hundred seconds per dose rate is normally sufficient; the variation that would justify repetition is not present.

Acceptance criterion

The relative response at each condition — the response divided by the response at the reference condition — must lie inside the acceptance band, which for this test is asymmetric: roughly thirteen percent low to eighteen percent high.

The asymmetry reflects the physical failure modes, which are not symmetric. Recombination and dead time both cause the instrument to under-read at high dose rate, which is the direction that understates a dose actually received. The band is tighter on that side.

Failure patterns

Under-reading at high dose rate

The most common result: recombination in an ionisation chamber, or dead-time losses in a pulse-counting detector. This is a physical limit of the detector and normally means the usable rate range for integration is narrower than the rate range for rate measurement.

Over-reading at low dose rate over long exposures

This points to the integrator rather than the detector — leakage current, or a background subtraction that is not tracking. Repeat the measurement with the source removed for the same duration: accumulation with no field present confirms the cause.

Failure only at a unit-decade change

Not a dose rate effect. This is a range-switching artefact, and the correction is to record the per-row unit correctly and re-check the arithmetic before attributing it to the detector.

Applicability

The test does not apply to a rate meter with no integrating function.

It applies to any instrument whose accumulated-dose indication is used for recording operator exposure during a task, demonstrating compliance, or comparison against a passive dosimeter. That last comparison is where a rate dependence typically appears, as an unexplained disagreement between two instruments that are individually within specification.