Testing of environmental radiation monitors using the Risø low-level radiation measurement stations

To harmonise the measurement of environmental dose rates from photon radiation within the EU countries. an EU sponsored intercomparison of environmental dose rate meters used for early warning of nuclear accidents was performed in June 1999. The intercomparison was organised by the EURADOS, Physikalisch Technische Bundesanstalt (Germany) and the Riso National Laboratory (Denmark).

This paper describes the free-field irradiation facilities at Rise and some details of the field intercomparison experiments performed at the Riso Natural Environmental Radiation Measurement Station during 3rd and 4th June, 1999. The chief aims of such experiments are to allow the participants to check their home calibrations of their detectors and to compare the responses of the individual environmental radiation measurement systems used in the different EU member states and making a link between the different reference standards used.

Photon radiation fields, including ground and air scatter components, were determined for certified Co-60, Cs-137 and Ra-226 gamma sources using Monte Carlo calculations. It is shown that an agreement between the measured and the calculated values is typically within 1%. Also a simulation of a radioactive plume passing over the monitors during a certain time was made using a specially designed source set up in the field that was able to vary the air kerma rate from low activity Cs-137 sources additional to the natural air kerma rate.

It is demonstrated that a typical environmental radiation detector can distinguish an increase of the ambient air kerma rate of less than 3%.