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Date: 22 October 2021

Time: 13:17

Contamination monitor tests

Tests are carried out in accordance with the industry Good Practice Guide, GPG14, “The examination, testing and calibration of portable radiation protection instruments”.  The range of tests offered is outlined below. Some tests may not be applicable to all types of contamination monitors.  Any deviations from type test values (where available) and any significant change in an instrument’s response is reported clearly on the certificate.

Light leakage test

Scintillation based detectors are exposed to a source of bright light to confirm they are light sealed and ambient light conditions have no effect on the instrument readings.

Scale linearity

Checks are carried out to confirm the instrument performs satisfactorily over its entire scale.

Detector uniformity

For large area detectors, the uniformity of the detection area is confirmed with a very small area source (usually 14C).

Response to contamination

A range of large area reference sources are available. Each instrument is calibrated against those appropriate for each instruments type. The calibration sources are all constructed in accordance with ISO8769. They are constructed to provide a known surface emission rate for each radiation type (e.g. alpha, beta or photon) and energy. They are not designed in any way to replicate what the user might encounter in practice.  Their purpose is to provide calibration services with a reliable, well constructed, long lived source with accurately known surface emission rates so that comparisons with manufacturer's type test data may be made. The price of the calibration remains the same and is independent of how many sources are used to calibrate a monitor.

Examples of monitors calibrated and their associated sources used for calibration

The examples listed are not exhaustive but give an indication of some of the most common radionuclides/monitors encountered at the RRPPS Calibration Service.

Example monitorReference source (ISO 8769)Radiation/energySuitable for users using radionuclides
NE AP range of probesBerthold α/β detector 241Am Alpha 5.5 MeV Alpha emitters 238U

Berthold α/β detector

Mini/Thermo EP15

NE BP range of probes

14C Beta 156 keV (max.) Low energy beta emitters: 14C, 35S, 147Pm, 33P
36Cl Beta 710 keV (max.) Medium energy beta emitters: 131I, 192Ir, 204Tl
90Sr/Y Beta 2.27 MeV (max.) High energy beta emitters: 60Co, 32P

Mini/Thermo 900 44B

Berthold β/g detector

NE Sodium iodide probes

55Fe Photon/gamma 5.9 keV (mean) Very low energy photon emitters: 99mTc, X-ray emissions from some display screens

Mini/Thermo 44A/B

Berthold b/g detector

NE sodium iodide probes

129I with Al filter Photon/gamma 32 keV (mean) Low-medium energy photon emitters: 125I, 123I, 109Cd
57Co with stainless steel filter Photon/gamma 124 keV (mean) Medium/high energy photon emitters: 57Co, 99mTc, 18F

Geometry of calibration

The response is normally measured at a source - detector distance of 3mm.  When assessing contamination in practice, this distance may not be achievable. Corrections will need to be made to account for this and other factors that are explained in the National Physical Laboratory (NPL) Good Practice Guide GPG30, "Practical Radiation Monitoring".

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