ODL values: Impact of external factors
The ODL measuring probes are used to record the ambient radiation at a height of one metre above the ground. The intensity of the external radiation is indicated in μSv/h (microsievert per hour).
Impact of external factors on the measured ODL values
The natural radiation caused by radioactive substances at a specific place depends in particular on the altitude and the geological formation and remains largely constant over long periods of time.
Impact of the weather
The intensity of the local dose rate is sometimes affected by external factors. These include:
Radioactive decay products of radon, a natural radioactive noble gas, are washed out of the atmosphere by precipitation and are deposited on the ground. This may cause the local dose rate (ODL) to increase, so that it temporarily reaches twice the normal value. However, the local dose rate will return to its normal level within a few hours.
As in the case of rain, snowfall may lead to an increase of the local dose rate, in particular if the ground was free of snow before. If the ground remains covered by snow, the snow will shield the terrestrial radiation so that it does not reach the probe. The local dose rate will decrease and remain at a lower level until the snow begins to thaw. Then the local dose rate will return to its former level.
Strong winds can cause oscillations in the entire probe including the standpipe. This may trigger electric pulses within the counter and lead to an increase in the measured values.
The local dose rate is not only subject to short-term phenomena but also to seasonal variations when considered over longer periods of time. These variations may amount to several 0.01 μSv/h. The principle reasons are:
- ongoing changes in soil moisture,
- the geology of the ground, the surface properties of the ground, and
- the local microclimate.
Maintenance work, reconstruction of the site, malfunctions of transmitters, malfunctions of probes and regular radiological testing of the probes may lead to a short-term or long-term downtime of measuring stations.