Patent ID: 9576776
Date: 2017-02-21
CPC Classifications: G01T,H01J

Claim:
1. A charged particle sensor for detecting and measuring ionic current generated by charged particles resulting from ionisation processes, comprising: a housing, a detection electrode enclosed within the housing for collecting the charged particles, and an electrometer having an input connected to the detection electrode for receiving a DC input signal therefrom and an output for supplying a DC measurement signal as output, wherein the electrometer comprises an amplifier circuit having a DC bias resistor for providing an input bias current to the amplifier circuit, and feedback circuits for increasing the input impedance to the amplifier circuit and enhancing the sensitivity thereof, the feedback circuits including a guard circuit providing a guard surrounding the detection electrode, and a bootstrapping arrangement for bootstrapping the bias resistor, the housing comprising an electrostatic screen for screening the detection electrode from external electric fields, whereby to reduce the sensitivity of the detection electrode to such fields, the electrostatic screen including an electrically conducting screening sheet provided as a second electrode facing the detection electrode and formed with interstices to allow the entry of radiation into the housing, wherein 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 ionisation processes and thereby produce an ionic current impinging on the detection electrode.