Detection efficiency of electron multipliers for low-energy electrons

Pulse-height spectra have been obtained for electron multipliers with discrete dynodes of CuBe. A peak in the spectrum shows up at low pulse heights (≈ 0.1 pC for a multiplier with a dc gain of about 106), but a long tail extends out to very large pulses. These observations can be understood in terms of secondary emission processes. They have provided a good basis for the design and calibration of a soft particle spectrometer for the ISIS-2 satellite. The detection efficiency for 300-eV electrons, with a 14-stage venetian blind multiplier having a total gain of 106, is about 80%. Below 100 eV the detection efficiency falls off, but in practice this can be overcome by postacceleration. At higher energies it also decreases to about 30% at 4 keV. This efficiency is reduced at very high counting rates, above 107 sec−1, owing partly to amplifier bandwidth limitations related to noise immunity requirements, and partly to the finite rise time of the multiplier pulses.