In situ absolute calibration of a channel electron multiplier for detection of positive ions

Abstract

A technique has been developed to determine the absolute single particle counting efficiency of a channel electron multiplier (CEM) for positively charged ions at keV energies. The calibration technique is applicable to positive ion detectors in general. Ion beam currents of C²⁺ were created by charge transfer of C³⁺ on H₂ in the thin target regime. By adjusting the H₂ pressure in the beam scattering chamber from 10⁻¹⁰ to 10⁻⁶ Torr, C²⁺ currents could be created either low enough to be detected by the CEM in the particle counting mode or high enough to be measured as a current using the CEM as a Faraday cup. The CEM counting efficiency was determined by comparing the C²⁺ count rate to the C²⁺ current, scaling by the change in H₂ pressure, and also scaling by the incident C³⁺ current for each C²⁺ measurement. This method, which effectively uses the charge transfer cross section as a ‘‘transfer standard,’’ allows the CEM counting efficiency to be determined in situ and to be monitored accurately over extended periods of time. The calibration requires that only the relative, not the absolute, pressure change be known. The technique does not require the value of the charge transfer cross section to be known. The accuracy of the calibration technique for the present results was 10% at a confidence level considered to be equivalent to a statistical 90% confidence level. For the present work, the peak counting efficiency of a Galileo 4039 CEM for 32.5 keV C²⁺ ions was determined to be 96%.