The counting efficiency of a channel electron multipier is measured for H+and Hminus;impact with the input of the Channeltron grounded and for impact energies between 0.3 and 5 keV. The counting efficiency is found to be about 30percnt; greater for Hminus;than for H+at energies below 1 keV. This difference and an observed difference between Hminus;and H+in the variation of counting efficiency across the Channeltron opening are attributed to the surface potential of the Channeltron near the opening, which can accelerate or decelerate the incident particles.
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