True and spurious anomalies in ambient dose rate monitoring.

Ambient dose rate surveys reveal zones of an elevated dose rate. As the observation results from two physical processes-the 'true' process in nature that one wants to assess and the observation process that consists of measuring under certain conditions-an observed anomaly can have its cause in either process. Anomalous effects rooted in the observation process are called spurious. Distinction between true and spurious anomalies may not be straightforward in many cases. As an example, dose rate surveying using a specific instrument is discussed, whose analysis shows the presence of both types of anomalies. Examples of both types are shown. The physical cause of a true anomaly can often be identified by inspecting the environment or by more sophisticated investigation, e.g. by gamma spectrometry. Identification of spurious anomalies is more difficult and requires investigation of count time series.