Portable faraday cup for nonvacuum proton beams

A portable Faraday cup design is described for the accurate measurement of large-diameter, low-current, and high-energy proton beams traveling in air. The unit has been tested with protons from 4 to 300 MeV. The unit has an accuracy of 10% for beams of 1 pA, improving to about 2% accuracy for ion currents of 20 pA to 1 μA.