Pilot study to verify the calibration of electrometers

Full text: The main detector used for standardization of the quantities used in measurements of ionizing radiation is the ionization chamber. The interaction of the radiation with this detector produces electrical charge, usually, in the range of pC to nC. The instrument used to measure such small charges (or currents) is the electrometer. As part of a good practice, the measured charge (current) must be traceable to a primary or secondary standard. Some calibration laboratories can only provide a system calibration coefficient, i.e. a calibration coefficient for the combination of electrometer plus ionization chamber (Gy/scale division). This practice is acceptable, but it can impose a limitation to the automation of their calibration procedures (using computerized application for the acquisition of current/charge). Not all models have the possibility of a connection to a computer and in the case of those that don't have this capability, automation is not possible without the development of a specific interface. In addition, end-users receive a calibration coefficient, which is only valid for the set ion chamber and electrometer. In case of a broken chamber, the end-user cannot connect another chamber to their electrometer without knowing its calibration coefficient. If the calibration laboratories had the capability of calibrating the chamber separately from the electrometer, for example, using an electrometer calibrated in terms of charge, all the chambers could be calibrated using this electrometer. The laboratory can also benefit from the automation of the measurements. This requires that the laboratory must be able to cross-calibrate the electrometers (associated to the chambers) also in terms of charge (Coulombs). Electrical charge is standardized by the use of a standard air capacitor and a standard voltage source (Q=CV) and the National Laboratory for Metrology of Ionizing Radiation (IRD) in Brazil has also adopted this procedure. Since the Brazilian National Laboratory for Electrical Measurements has not yet developed its capability for the standardization of small electrical charge produced by DC, the IRD is trying to verify its standardization procedures of the electrical charge through a comparison programme. This subject was discussed with a major electrometer manufacturer that has offered to provide free of charge, three of their electrometer calibration standards for a pilot run. The model to be provided consists of four calibrated resistors and two calibrated capacitors, covering the charge/current range of interest. For producing charge or current a standard DC voltage must be applied to these components. Since practically all-modern electrometers measure using virtual ground, this methodology is viable. The IRD, in collaboration with the IAEA, wishes to invite interested laboratories to participate in this pilot comparison programme. This exercise is expected to be useful for all participants and will hopefully open the way for the establishment of routine comparisons in this area. The results will be discussed and published in an appropriate journal. Interested institutions should contact directly Mr. Paulo H. B. Becker through e-mail (pbecker at ird.gov.br) or fax +55 21 24421950 informing him of the model and manufacturer of the electrometer to be used for the pilot study and discuss all practical details. (author)