A particle detector designed for the high-sensitivity small cyclotron mass spectremetry was made and tested. The detector consists of an Al2O3 conversion dynode

a microchannel-plate e’etron multiplier and an anode. Each ion that strikes the dynode at 80° angle ejects a few tens of secondary electrons

which enter separate channels of the microchanne’-piate and are then independently multiplied to a saturated pulse height. The charge collected by the anode is proportional to the ion energy. This permits a pulse-height discrimination against background. Data taken in an isotope separator are extrapolated and the background rate is found to be lower than 10-3/s at 90% efficiency for 40keV K or Ar ions. The energy resolution of the detector is 6.5keV

suitable for the spectrum measurement of very low energy ions.