Angular response
The IEC 60846-1:2009 §8.4.2 evaluation of how response changes with the direction radiation arrives from — the two-plane rotation scheme, the minimum rated range of use, and when angles may be omitted.
Last reviewed
Calibration is performed with the instrument facing the source. Field measurements are not. A survey meter used for an area survey receives photons over a wide range of incidence angles, and if the response falls at 60° off-axis the indication underestimates the dose equivalent rate — the direction of error that understates the hazard.
The test is optional in the standard. For instruments used for area survey rather than directed measurement it is among the most informative of the optional tests.
Measurement procedure
Start at the reference direction, 0°, and establish the reference response there. That row plays the same role the reference quality plays in the energy test: everything else is normalised against it.
Then rotate. The scheme uses two mutually perpendicular planes through the reference point, taking readings at ±30°, ±45°, ±60°, ±75° and ±90° in each, and at the manufacturer’s stated maximum angle if there is one. Two planes matter because detectors are not radially symmetric — a cylindrical GM tube behaves quite differently when you rotate about its long axis than when you tip it end-on, and testing only one plane can miss the bad case entirely.
The standard allows some angles to be omitted where the response is similar and varies monotonically between the measured points. Use that allowance sparingly, and only after you have enough points to know the behaviour is monotonic. Omitting angles to save time and then discovering a dip between two measured points is a wasted campaign.
Reference point and rotation axis
The instrument has a marked reference point and reference direction, and the rotation must be about that point. If you rotate about the case centre when the detector sits at one end, the detector translates through the field as it turns, and you have measured a distance effect dressed up as an angular effect. Get the geometry right before the first reading; it is not recoverable afterwards.
Keep the dose rate the same across all angles. Unlike the energy test there is no correction mechanism here — the whole measurement assumes the only thing changing is direction.
Acceptance criterion and rated range of use
Every angle’s relative response must fall inside the acceptance band.
But there is a subtlety worth knowing. The standard’s minimum rated range of use for angle is 0° to 45°. Beyond that, out to ±90°, the comparison is against what the manufacturer specified. An instrument may legitimately be declared as having a restricted angular range — and if it is, the angles beyond that range are assessed against the manufacturer’s own statement rather than against the general band.
This means an angular result is only interpretable alongside the instrument’s specification. Record the declared rated range in the report; a table of relative responses without it cannot be judged by a later reader.
Failure patterns
The pattern of the deviation identifies the cause.
Symmetric fall towards ±90°
Geometric. The detector presents a smaller effective area edge-on. This is expected behaviour and is what the acceptance band is sized to tolerate.
Asymmetry between +θ and −θ
Attenuation by a component on one side of the detector — a battery pack, a display board, a metal bracket. This is a design characteristic rather than a fault, but it determines how the instrument should be oriented in use and belongs in the report.
Deviation only at low photon energy
Interaction between angle and energy. At 50 keV a few millimetres of case material attenuates significantly; at 662 keV the same material has little effect. For instruments intended for low-energy fields, running the angular test at a low-energy quality in addition to the reference is not required by the standard but frequently yields the most useful result.
Applicability
The test adds little for an instrument always directed at a known source. It is informative for any instrument used for area survey, boundary monitoring or contamination searching, and for a new model before site-wide deployment: orientation in routine use is determined by handling habit rather than by the manual, so the angular dependence needs to be known in advance.