Apparatus (1) for detecting and/or measuring radioactivity comprising sensor means (5, 6), radiation detector means (4) and signal processing means (10, 11), the sensor means being adapted to provide a measure of one or more characteristics of a medium between the radiation detector means and a source of radioactivity, the arrangement being such that, in use, the signal processing means is configured to compensate for varying degrees of interference from unwanted isotopes by taking into account information received from the sensor means.