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THE MEASUREMENT OF ENVIRONMENTAL GAMMA DOSES

Objective

The ambient background radiation is typically measured by active high-pressure ionization chambers, plastic scintillators, GM counters, proportional counters and passive TL dosemeters. To ensure that measurements of the environmental photon radiation can be made with sufficient accuracy, it is necessary to determine the response of the monitoring instruments and dosemeters (eg to cosmic and terrestrial radiation) and to take into account the inherent instrument (dosemeter) background. Calibration also poses problems since it is usually not possible to use a calibration facility within which the dose rate is low enough to permit calibration of the instruments and dosemeters at the level to be measured.

The intention is to improve calibration methods at low dose rates with the scope of recommending internationally standardized procedures to obtain reliable and comparable measurement results of the ambient background radiation.
A Monte Carlo code (MCNP) has been used to calculate the air kerma rate from a certificated caesium-137 source for different source detector distances in a free field geometry. Uncollided photons contribute approximately 85% of the measured kerma rate and scattered photons the remaining 15%. The MCNP code has also been used to calculate the dose rate to a high pressure ionization chamber in a shadow shield geometry. It appears that the scatter contribution is around 30% of which the source material is contributing about 4%. The 0.03% scatter due to the shield is considered negligible.

Long term measurement studies at Hinkley Point Nuclear Power Station of the variation in the ambient radiation due to argon-41 plumes and the nitrogen-16 6 MeV radiation have been completed. The 4 monitors used different types of detector: a proportional counter and a plastic scintillation detector system; a high pressure ionization chamber; and a Geiger-Mueller detector system. In addition to the air kerma rake measurements, coincident records of the wind speed, wind direction and reactor operation history were obtained. A preliminary comparison of the results show that, although the detectors all closely followed the variations in air kerma rate with time, they did not all give agreement on the magnitude of the different components. In particular the scintillator detector measured lower values.

Free field and shadow shield calibrations and field measurements to determine different detector responses from terrestrial and cosmic radiations were carried out. 4 groups of active detector types were studied and the measurements are now being analyzed.

A laboratory with an ultra low background was established in the Asse salt mine. The inherent background of the proportional counter FHZ 600A was determined in this laboratory as 14 nSv/hour.
2 recently developed commercially available doserate meters for environmental radiation measurements have been positioned near the Hinkley Point Nuclear Power Station. These were the proportional counter FHZ 600A equipped with a special count rate meter and a Funksonde scintillation doserate meter. For 4 months the instruments measured the variation in ambinet radiation due to argon-41plumes and the nitrogen-16, 6 MeV radiation. The Funksonde had technical problems and requires accurate calibration.
Free field and shadow shield calibration measurements were performed and detector responses for terrestrial and cosmic radiations determined.

Different analysis codes for different TL materials and heating modes have been developed. These codes fall into two main catagories: complete analysis and simplified analysis. For the complete analysis the mathematical expressions for thermally activated kinetic processes are employed to fit the experimental curve. The various peaks that may be present in a complex glow curve are resolved obtained the best fittred values for their characteristic parameters. The simplified analysis does not attempt to resolve peaks. The global shape of the glow curve is analysed looking for specific and characteristic points associated to singular mathematical points. The detection in the experimental curve of these points permits identification of the region of the curve containing adequate dosimetric information, rejecting other luminescence signals also produced during the measurement.

Several computer programmes implementing the different analysis approaches have been tested in practical working conditions with excellent results. The complete glow curve analysis has proved to be superior to conventional evaluation methods in radotherepy dosimetry, in the Gy range, permitting a substantial improvement in the measurement uncertainty This is particularly useful for TL mailed dosimetry systems for quality control serving radiotherepy services.
Instrument calibration studies will include free-field and shadow-shield calibrations where ground albedo, air buildup and room scatter components for a variety of source and detector geometries will be calculated using Monte Carlo calculations.

Collimated beam calibrations and the determination of instrument linearity, angle and temperature dependence and inherent background will be performed in the Asse salt mine facility (925 metre depth) having an ultralow radiation background. The energy response will be determined including the measurements of 4 MeV and 6 MeV photons.

Environmental monitoring based on TL dosemeters will be improved by introducing a new evaluation method based on numerical analysis of the TL glow curve. This method has proved to be useful especially in assessing the individual dosemeter background from the same readout from which the radiation dose is evaluated.

Emphasis will be laid on long term measurements of the ambient radiation around a nuclear installation to assess how environmental monitors and TL dosemeters respond to small variations of the background radiation and how to differentiate between natural and man made radiation.

Work Programme:
calibration experiments and field measurements;
measurements in an ultralow radiation environment;
improvement of TL methods for environmental monitoring.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

RISOE NATIONAL LABORATORY
Address
Frederiksborgvej 399
Roskilde
Denmark

Participants (2)

CENTRO DE INVESTIGACIONES ENERGETICAS, MEDIAMBIENTALES Y TECNOLOGICAS
Spain
Address
22,Avenida Complutense 22
28040 Madrid
PHYSIKALISCH-TECHNISCHE BUNDESANSTALT
Germany
Address
Bundesallee 100
Braunschweig