NORM Monitor-IS. Performance data

Transcription

1 NORM Monitor-IS Performance data

2 Contents 1. Introduction... 3 Scintillator probe... 3 GM probe Energy response... 4 Scintillator probe gamma energy response... 4 Scintillator probe alpha/beta response... 4 GM probe beta energy response... 4 GM probe response to common NORM nuclides Angular response... 5 Scintillator probe... 5 GM probe Scintillator probe gamma dose rate linearity Effect of sludge/water... 7 Scintillator probe... 7 GM probe Typical background... 8 Scintillator probe... 8 GM Probe

3 1. Introduction The Tracerco NORM Monitor-IS is an intrinsically safe radiation measurement instrument that has been designed for the measurement of NORM in all typical environments. Further technical information can be found in the user manual and on the Tracerco web site ( The NORM Monitor-IS is compatible with two ruggedized probes: Scintillator probe This probe is based on highly sensitive scintillation crystal and photomultiplier technology. It is optimised for detection of NORM gamma emissions and is relatively insensitive to the presence of sludge and moisture. It is particularly useful when assessing NORM through metal walls. In addition to standard count rate output (CPS or CPM) a dose rate output (μr/h or μsv/h) is provided for compliance with North American NORM regulations. The dose rate output is not suited to area dose rate metering. GM probe This probe incorporates a pancake Geiger Muller (GM) tube that is optimised for alpha and beta detections. It is highly sensitive to all NORM emissions and is of particular use for lead-210 monitoring. 3

4 2. Energy response Scintillator probe gamma energy response Gamma energy (kev) Source Response (normalized against Cs-137) 59 Americium-241* Caesium-137* # Radium-226** 0.7 * Tracerco calibration sources. Traceable to national standards. ** Public Health England source. Secondary standard facility. # Ra-226 and daughters has a mixture of gamma emissions. Average energy is estimated at 740 kev ( Scintillator probe alpha/beta response The scintillator probe has been designed to maximise gamma response, whilst minimising alpha and beta response. This design feature has been implemented to reduce the effect of wet, sludgy material on instrument response. The scintillator probe response to alpha and beta particles will be negligible. See section 5 for further information on the effect of material consistency on scintillator response. GM probe beta energy response The GM probe efficiency to beta emitters was determined against three different radionuclide test plaques. The entire detector face was evenly irradiated from a distance of 2mm. Beta energy (kev) Source Efficiency (%) 200 Carbon-14* Promethium-147* Strontium/Yttrium-90* 56.5 # * Nationally traceable and compliant with ISO 8769 # Includes significant Bremsstrahlung contribution from the metal backing of the plaque 4

5 GM probe response to common NORM nuclides Source Typical response (CPS/Bq/cm 2 ) Radium -226* 10 Lead-210* 6 * Traceable to national standard 3. Angular response Scintillator probe The angular response data was generated with Tracerco calibration sources that are traceable to national standards. These are sealed sources that produce a homogenous gamma field. The scintillator probe is positioned some distance away from the source Cs-137 Am-241 5

6 Because of the shape of the scintillation crystal the angular response will vary as the probe is positioned more closely to the source of radiation. Tests against flat calibration plaques have indicated that at a separation of 2mm the angular response can reduce by 20% when moving from end-on to side-on geometry (90 o ). However, for measurements inside tubulars/pipes the near 4π geometry will provide additional sensitivity. Scintillator probe positioned inside tubulars can accept counts from all areas of the internal surface GM probe The GM probe is optimised for alpha and beta detection and as such will have a very low response if the detector face is angled away from the surface under investigation. 4. Scintillator probe gamma dose rate linearity The following chart shows dose rate linearity data for the scintillator probe. 6

7 The linearity data was generated with Tracerco caesium-137 calibration sources that are traceable to national standards. 5. Effect of sludge/water Scintillator probe To assess the effect of material consistency the scintillator probe response was tested on a Ra-226 plaque with various thicknesses of polymer film inserted between the detector and the plaque. The polymer film has a density of 1.2g/cm 3 and is a reasonable simulator of moisture or sludgy build up. Percentage change in response Film thickness (mm) The effect on the scintillator probe is small because it is primarily sensitive to gamma emissions, which are not heavily attenuated by moisture or sludge. GM probe As the GM probe is optimised for alpha and beta detection it is relatively sensitive to the effect of moisture. Test plaque Typical dry response (CPS/Bq/cm 2 ) Typical wet response # (CPS/Bq/cm 2 ) Percentage change Radium -226* % Lead-210* % * Traceable to national standard # wet conditions simulated by insertion of 0.17mm film of density 1.2g/cm 3 between the detector and test plaque 7

8 6. Typical background Scintillator probe The NORM Monitor-IS scintillator probe is highly sensitive to gamma radiation. Background radiation levels will therefore be dependent on factors such as local geology, building materials and proximity to man-made sources of radiation, such as nucleonic level gauges. For guidance the following table describes the background radiation measured by the NORM Monitor-IS scintillator probe in various locations. Location Measured background (counts per second) Offshore (North Sea) 2-6 Onshore (Middle East) 6-8 Onshore (Tracerco UK Service Centre, north England) Open lead castle (designed to eliminate ground radiation 1 whilst retaining the contribution from cosmic radiation) GM Probe As the GM probe is relatively insensitive to gamma radiation the background tends to remain very low and is not greatly affected by location. Background measurements of less than one count per second are typical for the GM probe. 8