A type of the omegatron with an additional electrode inserted between the shielding-box and the electron target was developed for the purpose of the improvement on sensitivity according to the earlier experimental results and a new trapping method was tested.
Positive potentials were applied to the beam-forming and the additional electrodes in order to produce an electric field in the shielding-box available for the prevention of the loss of resonant ions in the axial direction. Two holes made in the shielding-box for electron beam were widened to 2×2 mm in size for the easier penetration of the field.
This new trapping method produced good result on sensitivity as the hf (high frequency) voltage decreased. Most ions could be collected under the weak hf field so that the sensitivity varied slightly with the increase of the hf voltage from a certain value of it. Such a tendency is desirable one for the qualitative analysis of partial pressure.
The bad effect on resolution as had always originated from the other trapping method was scarcely found by the application of the new trapping method, the higher resolution was therefore attainable without rapid drop in sensitivity and masses up to M=42 could be separated completely from the neigh-boring masses. A typical spectrum showing mass selectivity is illustrated in this paper.

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The pumping speed was measured for a pumping system which consists of three traps, a mercury diffusion pump and a rotary pump in series, using a flow-pipe of conductance S_f. The correct value of the speed S can be obtained if the sample gas partial pressures at the inlet and outlet side of the pipe. are used forP_s and P_e in the formular S = S_f (P_s/P_e-1). On the contrary the use of the total pressures as indicated by the ionization gauges will give rise to smaller values of S, especially when theresidual gases begin to predominate at lower pressures. Consistent values of S have been obtained by using for P_s and P_e the pressure increments P_s and P_e as indicated by the ionization gauges each time when the flow rate of the sample gas was changed stepwise from zero or vice verse. This procedure is free from the interference due to the back diffusion of the sample gas. The result is that in the pressure range from 1×10^-5 to 1×10^-10mmHg the pumping speed of the present system for the sample gas maintains the same value. This is reasonable because no change of pumping mechanism is to be possible over the whole range of pressure.

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