7.4.1 Surface measurements

One of the more common radiation hazards arise from external radiation exposure when population access the premises of the site. Therefore, dose rate measurements are usually made over the surface of such areas. It is recommended to cover the whole area as to verify to which extent the dose rate levels do not exceed the values accepted for a given purpose of use.

The measurements can be made with dose rate monitors based on either Geiger Muller detectors or ionization chambers. Many models of scintillator detectors offer not only the possibility of radionuclide identification, but also incorporate the calculation of the dose rate based on the activity of registered photons. Two basic approaches are used for dose calculation: a) the use of coefficients to convert the measured NORM activity to dose or, b) calculation of the dose based in the actual spectrum measured. The measurement usually accounts for photons with energies above 200 keV, as to avoid the inaccuracies arising from differences in attenuation of low energy photons across different surface matrices and assuming that equilibrium is kept within the disintegration series. Therefore, the calculated dose might be overestimated when measuring tailings from uranium processing, or when other causes have lead to breach of the equilibrium.

It is advisable to make measurements at locations that can be later remembered and allowing verifying the accuracy in the GPS readings. One possibility is to start by walking along the perimeter of the area, as to 'frame' the area later when representing the pathways in the maps. Transect lines can be followed to cover the area, and the survey can be speeded up if more instruments are used. The distance between these lines shall be no less than that arising from the accuracy of the GPS readers and the effective volume measured by the detectors (ca. 10 m radii). It is recommended to keep a distance of about 10 meters between the transects.