The invention provides a charged particle sensor (10) for detecting and measuring ionic current generated by charged particles resulting from ionization processes, comprising: a housing (16), a detection electrode (14) enclosed within the housing for collecting the charged particles, and an electrometer (12) having an input connected to the detection electrode for receiving a DC input signal therefrom and an output (18) for supplying a DC measurement signal as output. The housing comprises an electrostatic screen (16) for screening the detection electrode from external electric fields, whereby to reduce the sensitivity of the detection electrode to such fields. The electrostatic screen includes an electrically conducting screening sheet (26) provided as a second electrode facing the detection electrode and formed with interstices to allow the entry of radiation into the housing, and the second electrode and the detection electrode are arranged to be maintained in use at a bias voltage with respect to one another so as to effect charge separation amongst charged particles resulting from ionization processes and thereby produce an ionic current impinging on the detection electrode.