Operation Characteristics of the Bennett-Type Radio-Frequency Mass Spectrometer

Abstract

Operation characteristics of the7-5-3-1cycle5-stage Bennett-type radio-frequency mass spectrometer(Bennett-type MS)with the secondary electron multiplier, designed for the square wave operation, was investigated in the pressure range from10-5to10-9Torr, simultaneously using the omegatron mass spectrometer as a reference. As the Bennett-type MS has many electrodes, the determination of relative electrode potentials for nice operation has been said difficult. In this investigation, however, getting the reference data with the omegatron mass spectrometer made it easy to determine the relative electrode potentials for obtaining good operation characteristics. The important factors of the Bennett-type MS operation are the voltage and the wave form of the applied radio-frequency, the drift voltages and the stability of the barrier voltage. As a result of the ion accelerating-voltage scanning method at a fixed radio-frequency, the ion background current wasincreased gradually in the high mass range. The reproducibility of the mass spectra obtained with the Bennett-type MS was satisfactory under the same conditions throughout the experiment. Even in the ultra-high vacuum conditions(-10-9Torr), Bennett-type MS can be used in the low mass range(1-10)without any significant ghost peaks though the operation in this mass range was difficult in the past, and can also be used up to the high mass range(-200). The minimum detectable partial pres sure was about10-13Torr and obtaining the resolution of50was relatively easy. The mass spectrum was also observed on the synchroscope screen in the range from10-5to10-9Torr. A so-called broad surface peak appeared around M=15was gradually predominant in the ultra-high vacuum region, and this peak could probably be due to the O ions produced from adsorbed CO on the tungsten grid surface under electron bombardment because of the inadequate structure of the ion source.