Low background-rate detector for 40-keV ions using a conversion dynode and a microchannel-plate electron multiplier to reject low-energy ions, electrons, and photons

We have developed and tested an inexpensive ion detector with dark-count rates below 10−4 s−1 and excellent background rejection. Each 40-keV ion that strikes a grazing-incidence Al2O3 conversion dynode ejects a few tens of secondary electrons, which enter separate pores of a microchannel plate and are independently multiplied to saturated pulse height. The pulses are summed to produce an output signal that is proportional to the number of secondary electrons. This permits pulse-height discrimination against backgrounds of low-energy ions, electrons, and photons. We have successfully tested the detector with C+, Na+, K+, Rb+, and Cs+ at 5–40 keV, and with 36-keV C− and CH−. It should detect ions and neutrals of all species, at energies above 5–10 keV, with good efficiency and excellent background discrimination. Counting efficiency and background discrimination improve with higher ion energy. The detector can be operated at pressures up to at least 2×10−7 Torr and be exposed to air repeatedly without requiring reactivation. The maximum ion count rate is 3×106 s−1 in pulse-counting mode and 6×109 s−1 in current-integrating mode. Detection of 40-keV ions at rates below 10−2 s−1 was required in development of a 10-cm radius 40-keV cyclotron dedicated to high-sensitivity radioisotope dating by accelerator mass spectrometry (AMS). In the cyclotron, the detector suppresses to 6×10−4 s−1 the background count rate from the presence of low-energy ions, photons, and electrons.