Ii7 situ absolute calibration of a channel electron multipkr for detection of positive ions

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 C?+ were created by charge transfer of C? on H, in the thin target regime. By adjusting the Hz pressure in the beam scattering chamber from lo-” to 10Y6 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 C?’ current, scaling by the change in H2 pressure, and also scaling by the incident C?+ current for each C? 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 C? ions was determined to be 96%. 0 I995 American Institute ofPhysics.