The accurate measurement of small electrical currents is essential for the measurement of ionizing radiation as generated by radionuclides. The Dutch Metrology Institute (VSL) has been involved in the current measurement part of the development of a new well-type ionization chamber at IRMM. This well-chamber is meant to be able to replace the ‘Système International de Référence’ (SIR), the BIPM facility used for international comparisons of radioactivity measurements. If constructed according to specifications, this well-chamber should give reproducible results with an uncertainty of 0.1 % for radionuclides emitting gamma rays in the range from 20 keV to 2000 keV.
To achieve this goal, accurate and traceable measurements of the ionization current are extremely important. For the prototype ionization chamber, background radiation typically produces a current of about 45 fA, whereas a relatively strong source results in a 1 nA current. IRMM contacted VSL to support the development in this part of their project. A generator equivalent to the VSL primary standard for small currents was duplicated and brought to IRMM. This generator in fact generates a very stable and linear voltage ramp dV/dt that is fed to a capacitor C to convert it to a current I = C∙dV/dt. After some initial testing, two-day training course was organized to discuss the details of high-precision small current measurements.
For small current related issues, VSL will continue to be involved in the further development of the new realization of the becquerel at IRMM. This way, the VSL expertise in electrical current measurement gives a significant contribution to the future international traceability of the becquerel.
For more information on this subject, please contact Helko van den Brom, hvdbrom@vsl.nl.
