A mass selective electron‐impact ionization detector for use in energy resolved thermal He scattering experiments is presented. The detector combines a short effective ionization region (approximately 3 mm FWHM) with a high detection efficiency of 10−5 at 2 mA electron emission. An efficiency of 10−4 at 10 mA emission current can be achieved if time resolution below 20 μs is dispensable. The design is discussed and measurements of detection efficiency and time resolution are presented. The ionizer can likewise be employed for an efficient detection of other types of atoms or molecules.
REFERENCES
1.
He detectors for different applications are the Pirani gauge, x-ray ionization, and detection of metastable atoms excited by electron bombardment [
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7.
Or, equivalently, the length over which the average electron trajectory intersects with the beam of neutrals.
8.
The emission current is defined as the electron current leaving the filament and ending up on any part of the device except on the filament itself.
9.
H. S. W. Massey, E. H. S. Burhop, and H. B. Gilbody, Electron and Ionic Impact Phenomena, 2nd ed. (Oxford University, New York, 1969), Vol. I, p. 128.
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The formula on page 153 in
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For model No. 4-324-9 with high-Q head model 11 from the former Extranuclear Laboratories, Inc.
12.
13.
The QMA electrode potentials oscillate with an ac amplitude of 500 V. An ion leaving the QMA experiences the ground potential outside the QMA either as repulsive or attractive. Ions leaving the QMA close enough to an electrode can thus be reflected from the exit, can be slowed down or they can gain a significant additional kinetic energy.
14.
The ions oscillate in the QMA with amplitudes of several millimeters at frequencies above 2.2 MHz. Thus the ion trajectory can be at a significant angle with respect to the QMA axis at the instant when the ion leaves the QMA.
15.
See, for example,
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16.
For example, O. Klemperer, Electron Optics (Cambridge University, New York, 1971).
17.
The gradient along the axis is about 2 V/mm, while the potential 1 mm from the axis is more than 1.5 V higher than on the axis (see Appendix).
18.
the effect is also described in the Appendix.
19.
From the simplified geometry on which Eq. (A1) is based, one obtains, for example, a decrease of 5.7 V over the ionization length of 3 mm.
20.
F.
Nakao
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The grid potential is often incorrectly called “ion energy” although these two potentials differ by the potential bending resulting from space charges in the INR.
22.
We have operated the filament, which is simply a ring of 0.1 mm Ta wire, at 15 mA for several days without problems. In long term operation at 2–5 mA emission the filament will work for more than one year.
23.
C. Félix, Diploma thesis EPF Lausanne Switzerland, 1991.
24.
Leybold-Heraeus, Grundlagen der Vakuumtechnik, Berechnungen und Tabellen, No. 70/A0 15.1.1. der Werksgruppe Vakuumseriengeräte.
25.
Even for detector settings in which simply the total counting rate was maximized, the tail never provided the dominating contribution to the peak height in time-of-flight spectra, although the integrated intensity in the tail may dominate the total counting rate.
26.
K.
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1994
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