Measurement of low currents in an external beam set-up

Abstract

It is well known that an accurate measurement of beam currents in an external set-up is not a straightforward task. Charge collection in a Faraday cup can be altered by ionisation along the beam path in atmosphere. These problems are more and more relevant when weaker currents are used, as in the ion beam analysis of precious objects where beam-induced damage should be strictly avoided. We performed a calibration of the integrated charge values obtained with a Faraday cup in an external set-up, at different beam current intensities, versus the “true” values. Effects of wrong charge integration have been detected, with increasing relevance at currents below 3 nA. The measurement of ultra-weak currents in atmosphere adds further problems for their integration. By measuring the X-ray yield obtained from a rotating chopper made of thick nickel in front of the target we obtain a sufficiently accurate beam current monitoring and charge-equivalent normalisation down to below 1 pA.