The operating characteristics of field ionizing tungsten tips used as detectors of low temperature beams of 3He and 4He are presented. These include typical I‐V characteristics, effective detection area, conditions for fast ionization time, and details necessary for reliable detector performance. In addition, we report that, under our operating conditions the detection efficiency was independent of the incident atom velocity. This is in contradiction to predictions from current field ionization theory.
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The output of the electron multiplier was amplified and fed to a 128 channel scaler. The scaler’s counting sequency was triggered at each opening of the chopper gate by a photocell not shown in Fig. 1. The period between successive chopper gate openings was typically 25 msec with a duty cycle of 0.0455. Typical counting channel widths were 125 μsec. Counting times varied from 10 sec to 20 min.
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W. D. Johnston, Ph.D. thesis, “An Atomic Beam Study of Helium at Temperatures Below 4.2 K” MIT, 1966.
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© 1974 The American Institute of Physics.
1974
The American Institute of Physics
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