Faraday Cup and Secondary-Emission Monitor Calibration

Two small Faraday cups, one shielded and one unshielded, have been built and calibrated for efficiency vs electron energy. Measurements with a pulse beam transformer gave the absolute efficiency by means of a unique method: direct measurement of the ratio of the "lost" current to the incident beam current. Efficiencies for the unshielded cup are in the range 93 to 99 percent with uncertainties of 0.5 percent for electron energies from 10 to 50 MeV. The shielded Faraday cup has a measured efficiency of 97.8 ± 0.2 percent over the electron energy range from 10 to 60 MeV. This cup was used to calibrate the secondary emission produced by electrons or positrons incident on a 0.4 mm Be target. The resulting positive current was 4.3 ± 0.3 percent of the incident beam current over the energy range of 15 to 40 MeV.